Network & Security

Question 1

7

What is Computer Security?

Answer 1

Correctnes and Efficient algorithms against an Attacker

Question 2

How can data we send over wifi be intercepted or stolen?

Answer 2

• Data is sent over wifi is transmitted through radiowaves
• Someone with a radio reciever or radio can retrieve and steal and catch this data.

Question 3

What helps protect your data over the Wifi?

Answer 3

Making sure it is encrytped

Question 4

What do we safeguard?

Answer 4

Decide on Assests : Information or Infrastructure:
- Sensitive Data
- Control Systems
- Hardware Devices

Question 5

How do you safegaurd?

Answer 5

• Security goal
• Estimate impacts of attacks
• Design mitigation
• Analyse system
• Spot Vulnerabilities
• Build Protection

Question 6

Information Security Aims:

Answer 6

• Confidentiality: Attacker should not retrieve any info
• Integrity & Authenticity: Received data is authentic & sender is genuine
• Avaliabilty: Data should be accessible on demand

Question 7

Information Security

Who are Potential Attackers?

Answer 7

ANYONE & EVERYONE
-Hackers: Potentially learning by running known attacks, exploting vunlnerabilities.
-Criminals: Take control of computers via bugs in software. Phising attacks or Denial of Service (DoS Attacks)
-Governments: Extreme computing powers. control on resources (wiretaps)
-Business House like ISPS Spying to sell your data

Question 8

What are the Known Attacks?

Answer 8

• Ransomeware
• Phising

Question 9

Examples of Ransomware:

Answer 9

• Malware: Trojan disguised as legitimate file
• Malware encrypted data on computer and ask for payments in bitcoin

**REAL LIFE **
- Wannacry 2017 move automatically via unpatched vulnerablities in Microsoft Windows
- Widespread impacts, NHS & Nissan among affected

Question 10

Examples of Phising:

Answer 10

• Emails pretneding to be from Known people
• Emails ask for username & password
• Emails ask for software instillation (includes word macros)
• Install malware to spread within networks and downloads further malware

Question 11

Unix Commands & Shell Scripts

What are Shell Commands?

Answer 11

Operations on Files

Question 12

Unix Commands & Shell Scripts

Why do we Utilise OS Commands?

Answer 12

Potenitally Faster

Question 13

What is Batch Proecessing?

Answer 13

Sequentially Running Programs

Question 14

Modular Arithmetic

How to do simplify modular arithmetic for large (NON POWER) numbers

Answer 14

• Split it up by adding 2 easier number that make the original number
• Split it up by multiplying 2 easier number that make the original number

Question 15

Modular Arithmetic

What is the rule for adding 2 easy numbers?

Answer 15

x = (t1)z + r1 => x mod z = r1
y = (t2)x + r2 => y mod z = r2

p = x + y
p mod z =
(x + y) mod z = (r1 + r2) mod z

Question 16

Modular Arthmetic

Why do you need to do mod z in (r1 + r2) mod z & (r1 * r2) mod z?

Answer 16

-Because r1+r2 can be larger than z
- mod make the remainder smaller than z

Question 17

Modular Arthimetic

What is the simple rule modular arithmetic for addition?

Answer 17

x + y mod z = (x mod z + y mod z ) mod z

Question 18

What is the simple rule modular arithmetic for multiplication?

Answer 18

(x * y) mod z = (x mod z * y mod z) mod z

Question 19

Modular Arithmetic

What is the algorithm for power mod?

where time complexity = n

Answer 19

product = 1
for i = 1 to n
product = (prodcut * 2) mod m
end for

m is the value you want to mod by

if n = 2^10 it will take 2^10

Question 20

Modular Arithmetic

What’s a better algorithm where T(n) = 2T(n/2) + 1

Answer 20

find(n)
product = 1
product = (find(n/2) * find(n/2)) mod m
return product

m is the value you want to mod by

T(n) = 2T(n/2)+ 1 => log n + something

Question 21

Modular Arithmetic

What algorithm gives Total Complexity of log n?

Answer 21

find(n)
product = 1
product = (find(n/2))^2 mod m
return product 

**IN ORDER TO COMPUTE **
2 ^n mod 57
I need to compute log n many powers
So Total Complexity ceil (log n)

This gets rid of the extra find call as you just get the same num twice

Question 22

Linux Shell

If you type echo "Hello, welcome to $SHELL" what is the output, in a Linux Shell?

Answer 22

Hello, welcome to /bin/bash

Question 23

Shell Commands

How to create a directories?

Answer 23

FORMAT: mkdir directoryname

EXAMPLE CODE:

mkdir folder1

Question 24

Shell Commands

How to create Multiple Directories?

Only 2 Directories

Answer 24

FORMAT: mkdir directoryname1 direcotryname2

EXAMPLE CODE:

mkdir folder1 folder2

Question 25

# Shell Commands How to make n directories? | 2+ Directories

Answer 25

**FORMAT:** mkdir directoryname1{0..n} ***EXAMPLE CODE:*** *Make 10 directories * ``` mkdir folder1{0..9} ```

Question 26

# Shell Commands what does `cd ` do?

Answer 26

Moves inside the directory of the address given *EXAMPLE:* ``` cd folder1 // Moves into folder1 ```

Question 27

# Shell Commands What does `cd ..` do?

Answer 27

Moves back to the parent directory

Question 28

# Shell Commands What are the two commands to find the content of a directory?

Answer 28

**`ls `** - show all the files and folders in the directory **`ls -l`** - shows the same as ls with extra info *(user, date and time)*

Question 29

# Shell Commands What is the command to find all the content of a directory including hidden files?

Answer 29

**`ls -a`** - Just **`ls`** with hidden files For more details use **`ls -la`** - Same as **`ls -l`** with hidden files

Question 30

# Shell Commands What feature do hidden files have?

Answer 30

A period in the front of / start of their name | example : .filename or (1+ periods (.))

Question 31

# Shell Commands How to Create a File in the Shell?

Answer 31

``` echo "hello" > test ``` - puts "hello" in test - '>' means to write into file

Question 32

# Shell Commands How to Append to a File?

Answer 32

``` echo "Welcome to the shell" >> test ``` - appends "Welcome to the shell" - '>>' means to append into the file

Question 33

# Shell Command How to View the content in a file?

Answer 33

``` cat test ``` - outputs all the content in the test file - Main purpose of this is to concatenate files

Question 34

# Shell Commands What does the command `less` allow?

Answer 34

- allows you to view the file one page at a time **EXAMPLE CODE:** ```less test ```

Question 35

# Shell Command What can you use to get n line from the file?

Answer 35

``` head -n test ``` - gets the first n lines from the test file

Question 36

# Shell Command What is the code to Copy Files in a new directory in the same location?

Answer 36

**FORMAT:** ```cp ``` **EXAMPLE CODE:** ```cp test copiedtest``` - copies all the content in test to copiedtest

Question 37

# Shell Command How to copy files into a directory in another location?

Answer 37

- You need to use the path of the different directory to copy the file from the current one **EXAMPLE CODE:** ``` cp test ./folder1/copiedtest ```

Question 38

# Shell Commands What is the format for copying a file to/from remote computer?

Answer 38

`scp [[user]@source:]sourcefile [[user]@destination:]destinationfile`

Question 39

# Shell Commander What are the 3 Classes that the OS keeps track of permission access rights for?

Answer 39

- Owner - Group - Others

Question 40

# Shell Commander Does Linux consider everything a file?

Answer 40

Yes

Question 41

# Shell Commander What are the 3 permissions types for each class of user?

Answer 41

- read - write - execute

Question 42

# Shell Commander What do File Permissions Describe?

Answer 42

- Describes who, which & what activites could be performed with a specific file - Each file has an owner & a group

Question 43

# Access Control What does Access Control Allow?

Answer 43

Authorised users only have access to what they need

Question 44

# Access Control What are the components of the Access Control Model?

Answer 44

- Principal *(Subject e.g. user/program)* - Action *(e.g. read)* - Resource Monitor - Object *(resource e.g. file)*

Question 45

# Access Control Describe the Access Control Model?

Answer 45

- Principal performs an action on an object - Resource Monitor checks if the action is legal or NOT

Question 46

# Access Control Give an Example of Principal:

Answer 46

Password Program

Question 47

# Access Control Give an Example of Action:

Answer 47

Reading

Question 48

# Access Control Give an Example of Object:

Answer 48

Password FIle

Question 49

# Access Control Give an Example of Resource Monitor:

Answer 49

Operating System

Question 50

# Access Control What is the Access Control Matrix?

Answer 50

Who is allowed to do what is captured in this matrix - What permission do each principal have on each Object

Question 51

# Access Controls What does the permission `x` mean?

Answer 51

execute

Question 52

# Access Control What does the permission `r` mean?

Answer 52

read

Question 53

# Access Control What does the permission `w` mean?

Answer 53

write

Question 54

# Access Control What limits does the Matrix model have?

Answer 54

System Admin can bypass these permission and re-write the os to gain access to specific files - Very large matrix can be difficult to maintain - Corrupted Matrix, all control is lost

Question 55

# Access Control Summary of Access Control Matrix:

Answer 55

- Matric of all principals & objects - Matrix entries describe the permissions - Problem: Maintaining such a matric can be difficult*** (VERY LARGE)*** - If Matrix gets corrupted then all control is lost

Question 56

# Access Control What is the Access Control List (ACL)?

Answer 56

- Don't want to store one massove matrix - Instead we store each column of the matrix with the object it refers to

Question 57

# Access Control Give an example of ALC:

Answer 57

(Account data, [(sam,r),(Bob,r),(Account Program, rw)]) (object,[principal a actions, pricipal b actions]) actions can be a combo of {r,w,x}

Question 58

# Access Control What are Permissions and give an exmaple of what it looks like in Unix Access Control List??

Answer 58

Permission different users have on the file e.g.```-rw-r----- ```

Question 59

# Access Control What is the Link counter in Unix Access Control List & Give example?

Answer 59

File can appear with a different name in different directories. The - **- Link Counter Identitfies this Frequency ** e.g.```1 ``` (Can be any number)

Question 60

# Access Control What is Owner in Unix Access Control List & example?

Answer 60

Owner of the Files e.g.```root OR Bob ```

Question 61

# Access Control What is Size in in Unix Access Control List & example?

Answer 61

How big the file is ```e.g. 2150 ```

Question 62

# Access Control What is Group in in Unix Access Control List & example?

Answer 62

Each user can belong to several groups & a file to only one group ```e.g. Staff ```

Question 63

# Access Control What is File name in in Unix Access Control List & example?

Answer 63

Name of the file ```e.g. text.txt ```

Question 64

# Access Control What is Date in in Unix Access Control List & example?

Answer 64

Indicates when file was last modifed ```e.g. 26 Jan 12:45 ```

Question 65

# Access Control Give an example of Unix Access Control List:

Answer 65

``` |-rw-r-----|1|bob|staff|2150|26 Jan 12:45|test.txt| ```

Question 66

# Access Control Give an example of UNIX File Permissions:

Answer 66

**```drwxrwxrwx ```** - 1st char `(d)` represents ** File Type** - char 2 -4 `(rwx)` represent **Owner Permissions** - char 5-7 `(rwx)` represent **Group Permissions** - char 8-10 `(rwx)` represent **Other Permissions** *(Anyone else)*

Question 67

# Access Control (ACL) Give all the File Types in File Permission :

Answer 67

- `-` => file - `d` => directory - `b/c` => device file

Question 68

# Access Control (ACL) Give all the permissions in File Permission :

Answer 68

- `r` => Read Permission - `w` => Write Permission - `x` => Execute Permission - `-` => No Permissions

Question 69

# Access Control What is Access Control for directories?

Answer 69

- `r`: Read Only for directory contents - `x`: permission to traverse *e.g. switch to, run * - **NO** `x`: Can't run away commands inside the directory - **NO** `r`: Can't list files in the directory

Question 70

# Access Control What is Access Control for Programs?

Answer 70

```e.g: -r-sr-xr-x 1 root wheel 70352 19 Jun 2009 passwd ``` - `x` => Controls who can run a program* (Above example anyone)* - `s` => Indicates that the program runs with permission of its owner* (Root/Owner shoulf be allowed to write to the file)*

Question 71

# Access Control What are the Different User Identifiers (uids)?

Answer 71

- **Real uid (ruid)** - **Effective uid (euid)** - **File System uid (fsuid)** - **Saved User uid (suid)**

Question 72

# Access Control What is **Real uid (ruid)**?

Answer 72

Owner of proccess (*Whoever starts the program*)

Question 73

What is **Effective uid (euid)**?

Answer 73

Used for access checks (*except filesystem*)

Question 74

What is **File System uid (fsuid)**?

Answer 74

Used for access checks & ownership of files (*equal to euid*)

Question 75

What is **Saved User uid (suid)**?

Answer 75

**euid** changed, old **euid** is saved as **suid** - unprivalleged processes may change **euid** only to **ruid** or **suid**

Question 76

# Access Control What do **uids** allow?

Answer 76

Provides flexibility for granting Higher Privileges Temporarily **Example:** - Start as root (*bind to prots < 1024*) - Set **ruid, euid & suid** to unprivileged values - Cannot gain root privileges afterwards Process run as privileged user may set euid to priviledged value then execute non-privileged operations & gain roof privileges afterwards

Question 77

# Access Control What are security Issues with granting Higher Privleges?

Answer 77

- User can run programs with more privileges - Mistakes in program we could use it to do root actions - *Particular Problems:* **Race Condition in code** e.g. `if can_access file then peform_operations on file` - Make sure processes have a low level as possible

Question 78

# Access Control How should passwords be stored?

Answer 78

- **NOT** in Cleat text - Hash the password and store that

Question 79

# Access Control What is wrong with storing password in Clear Text?

Answer 79

Anyone with access to the sytem or has used access can see all the passwords

Question 80

# Access Control What can you do for further Security of Passwords?

Answer 80

- Store pair (salt, Hash) - Hash is the hash of salt & password - Same password for 2 users give rise to different entries in password file - Makes cracking passwords harder *(Crack one user password, won't help with the other users )*

Question 81

# Access Control What is Salt?

Answer 81

Salt is a random bitstring

Question 82

# Access Control How does Windows Hash Passwords?

Answer 82

- **Stores hashes in: system32/config/SAM** - **Need admin level to read** - **Locked & Encrypted with a key, based on other key values ** *(Adds no real security because if you are admin then you need to find out what that key is and decrypt it with a key you can read)*

Question 83

# Access Control What are the uses for Password Hashes in Windows Domain?

Answer 83

- Password hashes are used to authenticate users on host in the domain - Passwords hashes are cached to avoid asking for the password

Question 84

# Access Control What are the devestating Attacks Password hashes give rise too? | **pass the hash**

Answer 84

- Obtain user credential for one host in the domain *(Phising)* - Exploit vulnerability to become local admin - Install process which waits for domain admin to login - Extract cached hash for domain admin - Login as domain admin - Has defence mechanism, but painful to use

Question 85

# Access Control Why is SSH much better in protecting against **pass-the-hash**?

Answer 85

- Public key on untrusted machine *Attacker can get this but cannot get private key* - Private key on trusted machine *Only used by domain admin*

Question 86

# Access Control How to get windows password Hashes?

Answer 86

- Boot into Linux - Get SAM file *Very difficult to defend against attack*

Question 87

# Access Control Give examples of password crackers?

Answer 87

**John the Ripper:** - Most common brute force cracker - open source **Hashcat:** - Claims to be the fastest/best **Ohacrack:** - State of the art, free, rainbow table software

Question 88

# Access Control How are passwords captured?

Answer 88

**Phishing:** Username & password captured by attackers via malicious links *e.g. Fake bank email* - used to login & then for attacks *Ransomware, theft of details, IP...*

Question 89

# Access Control What are the best protection against passwords capture?

Answer 89

**Multi-factor authentication *Besides Username & Password*** - SMS codes - One-time passwords - App approval **SSH with public key authentication ONLY protects against phishing** (*As long as private key is not given out, there is not much the attacker can do*)

Question 90

# Access Control What is Password Injection?

Answer 90

- Want access to the system without cracking passwords - Have access to hard disk - Add your own account ot replace the hash wiht one you know

Question 91

# Access Control What is better security that will help prevent Password injection?

Answer 91

**BIOS:** - Set a password in BIOS to stop the computer booting from anything but the hard disk - Very hard to brute force BIOS *(Cannot Automate)* - *Workaround*: Remove the hard disk from the computer or reset BIOS password

Question 92

# Access Control How to reset BIOS password?

Answer 92

**Opening the Box**: - Applying Jumpers - Removing the battery

Question 93

# Access Control What is the Best Security that will not be affected in any BIOS workarounds?

Answer 93

- Encrypt important files - Whole disk Encryption: - Encrypt the whole drive (*Resetting BIOS psswrd won't work*) - Key can be brute forced - Not safe if the computer is in sleep mode - E.g. *BitLocker (Windows) , FIlevault (IOS) , Luks (Linux)*

Question 94

# Hashes & Encryption What are Hashes?

Answer 94

**- Hash of any message is a short string generated from that message - Hash of a message always the same (*Deterministic*) - Small Change = New Hash - Very Hard to go from Hash to message - Very Unlikely 2 message have the same hash ** -*Cannot say impossible due to pigeonhole principal* - *But it is computationally Infeasible to fins 2 strings that map to same hash*

Question 95

# Hashes & Encryption What is Pigeon Hole Principal?

Answer 95

If you are using a hash function that generates 10 possible hash values (pigeonholes) but have 11 input keys (pigeons), at least one hash value will be shared by two or more input keys. ## Footnote It states that if you place more items into fewer containers than the number of items, at least one container must hold more than one item.

Question 96

# Hashing & Encryption What are the uses of Hashing?

Answer 96

- Verification of download message - Tying parts of a message together (*Hash whole Message*) - Hash the message, then sign the hash *Electronic signature* - Reduces file size & not as computationally expensive as public key encryption - Protect Passwords: - store hash , not passwords - Attacker can get access, but passwords are not clear)

Question 97

# Hashing & Encryption Give the types of Hash Attacks

Answer 97

- Collision Attack - Preimage Attack - Prefix Collision Attack

Question 98

# Hashing & Encryption What is Preimage Attack?

Answer 98

FInd a message for a given hash : VERY HARD - Attacker gets to hash cannot go back and find original password OR Given a random y ∈ {0,1}^n, if the adversary could find a message m such that H(m) = y **the adversary can produce a message 𝑚 that, when hashed by H, results in exactly the binary string 𝑦*

Question 99

# Hashing & Encryption What is an Adversary?

Answer 99

an entity (often an attacker or an opponent) that tries to break or exploit a system.

Question 100

# Hashing & Encryption What does it mean if there is a successful Preimage attack on a Hash?

Answer 100

It would indicate a weakness in the hash function.

Question 101

# Hashing & Encryption What is Collision Attack?

Answer 101

Adversary could find distinct messages m & m' such that H(m) = H(m') *Represents collsions*

Question 102

# Hashing & Encyption What problem does Collision Attack present?

Answer 102

- Weakness beacuse it means that the hash fucntion is not unique for the same hash - **E.g of these Weaknesses:** - Digital Signatures can be forged using a different message - File Integrity/ password Storage, mean differnt inputs could be viewed as identical

Question 103

# Hashing & Encryption What is Prefix Collision Attack?

Answer 103

A collsion where the attacker can pick a prefix for the message - Easier for the attacker *(can pick a prefix for the message & produce a hash, which is a hash only for the prefix of the message )* - Imagine beginning of the message contains all the impritant info then this attack is enough for the attacker.

Question 104

# Hashing & Encryption How many people do you need to ask before 2 people have the same Birthday with a probablity of 0.5? | Birthday Paradox

Answer 104

- 23 people gives (23 * 22)/ 2 = 253 pairs - Probablilty 2 people have different Birthday is 364/365 - Probability is (364/365)^253 = 0.4995

Question 105

# Hashing & Encryption What does H : {0,1} * => {0,1}^n mean?

Answer 105

- Hash Function (H) is random - ∀x ∈ {0,1} * ∀y ∈ {0,1}^n - Pr[H(x) = y] = (1/2^n) - Every point was equally there

Question 106

# Hashing & Encryption What is the Hash Algorithm & Explain it?

Answer 106

**ALGORITHM:** x = 0^n while (collision not found){ compute H(x) & store check if collison else x ++ } **Explanation:** Take a number, hash it and store it then repeat it for the next number unto collision found

Question 107

# Hashing & Collison What is the probability of getting a collision at collision(1)?

Answer 107

Pr[collision(1)] = 0

Question 108

# Hashing & Collison What is the probability of getting a collision at collision(2)?

Answer 108

Pr[collison(2)|¬collision(1)] = 2/(2^n)

Question 109

# Hashing & Collison What is the probability of getting a collision at collision(n)?

Answer 109

Pr[collison(n)|¬collision(1) ||...|| ¬collision(n-1)] = (i-1)/(2^n)

Question 110

# Hashing & Collison

Question 111

# Hashing & Collison | Π

Question 112

# Hashing & Collison

Question 113

# Hashing & Collison

Question 114

# Hashing & Collison

Question 115

# Hashing & Collison

Question 116

# Symmetric Key Cryptography What is the Total Computation for 1 Year?

Answer 116

2^(82)

Question 117

# Symmetric Key Cryptography What is Cryptography?

Answer 117

Describes how to transfer messages between particapants without anyone else being able to read/modify them

Question 118

# Symmetric Key Cryptography What is **code** & give examples?

Answer 118

**Any way to represent data** E.g ASCII , HEX , MORSE CODE

Question 119

# Symmetric Key Cryptography What is a **Cipher**?

Answer 119

Code where it is difficult to derive data from

Question 120

# Symmetric Key Cryptography What are features of Cipher?

Answer 120

- Always uses a key - Data for cipher is **Plaintext** - Encoded Plaintext is **Ciphertext**

Question 121

# Symmetric Key Cryptography What is Encryption?

Answer 121

Function From Plaintext to Ciphertext

Question 122

# Symmetric Key Cryptography What is Decryption?

Answer 122

Function From Ciphertext to Plaintext

Question 123

# Symmetric Key Cryptography What is HEX?

Answer 123

**- Chars from 0 to F - Encode 4 Bits - Easiest way to write binary as text** *- Just use Binary values of 0-14 to represent numbers. i.e 34 would be 0011 0100 (Binary 3 & 4)*

Question 124

# Symmetric Key Cryptography Give 27 as Hex?

Answer 124

0010 0011

Question 125

# Symmetric Key Cryptography What is a Ceaser Cipher?

Answer 125

- One of the oldest Ciphers - Shifts every char in the plaintext 3 spaces to the right (in the alphabet)

Question 126

# Symmetric Key Cryptography Why is the Ceaser Cipher easy to Break?

Answer 126

- As soon as you know the scheme you can decrypt the message. - This is due to the Algorithms to decrypt & encrypt are public.

Question 127

# Symmetric Key Cryptography What is Kerckhoff's Principal:

Answer 127

Cipher should be secure even if the attacker knows everything about it apart from the key. Rule 2: System should not require secrecy

Question 128

# Symmetric Key Cryptography What is the problem with have n rotations Ceaser Cipher and give a better modle?

Answer 128

- 26 rotations, therefore 26 possible key, so you can try them all. **Better Scheme:** - Replace each letter with anothe - thus 26! = 4 * 10^(26) possible keys

Question 129

# Symmetric Key Cryptography How can the Ceaser cipher were it replaces each letter with another be broken?

Answer 129

-**Cannot be broken** by Brute Force - Replacing each letter with another can be broken by **Frquency Analysis**

Question 130

# Symmetric Key Cryptography What does Frequency Analysis count the number of?

Answer 130

- Each Symbol - Each pair of Symbol Then it draws a conclusion

Question 131

# Symmetric Key Cryptography How does Frequency Analysis work?

Answer 131

- Use the most frequently used character , e - Then do the analysis - the most frequent char is what e has been ecrypted to - Find the difference between e and new char and use that difference for the other chars. - If makes no sense use the next most frequent chars

Question 132

# Symmetric Key Cryptography What is XOR?

Answer 132

**Binary Addition MOD 2** - XOR on bit string of same length defined by applying XOR to corresponfing bitd

Question 133

# Symmetric Key Cryptography Solve the following: 1) 0⊕0 2) 1⊕0 3) 0⊕1 4) 1⊕1

Answer 133

1) 0 2) 1 3) 1 4) 0

Question 134

# Symmetric Key Cryptography What is XOR properties?

Answer 134

**-Associative & Commutative - All bit string M , M⊕0 = M - All bit string M , M⊕M = 0 ** *- Where 0 is bit string of 0's of the appropriate length*

Question 135

# Symmetric Key Cryptography What are **One Time Pads**?

Answer 135

Each CHAR is shifted by random offset

Question 136

# Symmetric Key Cryptography How does one time pads work?

Answer 136

- Keys are long as message - Add/XOR key & message - For each char a random is chosen

Question 137

# Symmetric Key Cryptography What is the formula for creating the Cipher for One Time Pad?

Answer 137

**Cipher = Message + Key** -*Mod 26 the value to get the correct one *

Question 138

# Symmetric Key Cryptography What is the formula for finding out the message given Cipher and key?

Answer 138

**Message = Cipher - Key** - *if it negative +26*

Question 139

# Symmetric Key Cryptography What are the problems with One TIme Pads?

Answer 139

- Key needs to be as long as the message - Key must only be used once

Question 140

# Symmetric Key Cryptography Is One Time Pad Perfect Encryption?

Answer 140

Yes, but one when the key is Random

Question 141

# Symmetric Key Cryptography What is Perfect Encryption?

Answer 141

You Learn nothing about plaintext grom ciphertext

Question 142

# Symmetric Key Cryptography What is the Theorem of Perfect Encryption?

Answer 142

**Given any ciphertext of certain length withiut knowing the key the probability of ciphertext being the encryption of a plaintext of same length is the same for all plaintexts of the same length as ciphertext.**

Question 143

# Symmetric Key Cryptography What are Block Ciphers? | More Detail

Answer 143

- Modern Ciphers work on blocks of plaintext, not just single symbol - Key controls the exact nature permutaions & substitution - Made up of a series of permutations & substitution repeated on each block

Question 144

# Symmetric Key Cryptography What is Block Cipher? | Simple

Answer 144

Takes 128-bit & Returns 128-bit - Encrypts n-bit block via a randomly chosen permutation

Question 145

# Symmetric Key Cryptography What is a Stream Cipher?

Answer 145

- Small msg can give a cipher as long as you want - Genreate a random looking bit-stream from a smaller key and XOR the message with the stream

Question 146

# Symmetric Key Cryptography What is AES (Advanced Encryption Standard)?

Answer 146

- Works on Blcoks of 128-bit - Generates 10 round keys from a single 128-bit key

Question 147

# Symmetric Key Cryptography How is 128-bits represented?

Answer 147

4 * 4 matrix - Where each element is a byte (8 bits) - 16 elements all together

Question 148

# Symmetric Key Cryptography What is S-Box (SubBytes)?

Answer 148

- An Operation on byte using a finite field arithmetic - Has a Look up table - Picks a bit from one matrix and using the lookup table matches it to the corresponding value in the other matrix.

Question 149

What is ShiftRows?

Answer 149

Move the : -2nd row 1 byte to the left - 3rd row 2 bytes - 4th row 3 bytes

Question 150

# Symmetric Key Cryptography What is MixColumn?

Answer 150

Substitution of each column such that: (m1(x^3) + m2(x^2) + m3(x) + m4) * (3x^3 + x^2 + x + 2) mod (x^4 + 1) = b0(x^3) + b1(x^2) + b2(x) = b3 | mn are elemnts from matrix m and bn is outupt matrix

Question 151

# Symmetric Key Cryptography What does Adding Round Key do?

Answer 151

-Applies XOR to the Block & the 128-bit round key *(generated by main key) *

Question 152

# Symmetric Key Cryptography How Secure is AES?

Answer 152

- NO FORMAL PROOF - Best Know cyrptographic attack require 2^126 key guesses. - An improvement of factor 4 compared to 2^128 key guesses for brute force attack.

Question 153

# Symmetric Key Cryptography What are the Side Channel Attacks?

Answer 153

- Measuring Power Consumption - Execution Time

Question 154

# Symmetric Key Cryptography What the Key aspects of Security?

Answer 154

- Shuffle rows & colums to ensure small change in input causes very big output change - Requires at least one non linear operation on the data **(Given by SubByte Operation)**

Question 155

# Symmetric Key Cryptography What is DES (DATA ENCRYPTION STANDARD)

Answer 155

- Previous Standard before AES - Designed by IBM in 70s - Before accepted, NSA adddes S-Boxes & fixed key length at 56 bits

Question 156

# Symmetric Key Cryptography What are S-Boxes | DES

Answer 156

- Type of Substituion - Unknown why NSA added it - Thought to be a backdoor for NSA

Question 157

# Symmetric Key Cryptography What is 3-DES?

Answer 157

-Triple DES, was a stop gap unit AES - Takes 3 keys (k1,k2,k3) - Expected to be good unitl 2030 (Computing power won't increase quickly) - Used in Bank card & RFID Chips - Setting the keys equal to each other you get DES.

Question 158

# Symmetric Key Cryptography Explain 3 DES encryption:

Answer 158

**E{k1,k2,k3}(M) = E{k3}(E{K2}(E{k1}(M)))** - Encrypt M with K1 First - Then Decrypt with K2 - Then Encrypt with K3

Question 159

# Symmetric Key Cryptography What is Padding ?

Answer 159

- Block Ciphers only work on fixed size blocks - If message isn't of the right block need to pad it - Reciever need to tell the difference between padding & message

Question 160

# Symmetric Key Cryptography What is used as Padding?

Answer 160

PKCS 5 / PKCS 7

Question 161

# Symmetric Key Cryptography Explain how PK5 / PK7 Work:

Answer 161

- 1 Byte of space write **01** - 2 Bytes of Space write **0202** - 3 Bytes of Space write **030303** *LEFT OVER BYTES* - If message goes to the end of the block add a new block of **16161616...** (*Message exactly fits the block*) ## Footnote If you have something that takes 5 bytes and block has 8 you need 3 bytes to fill it so use **030303** to fill it

Question 162

# Symmetric Key Cryptography What is the block size for PKCS 5?

Answer 162

8 Byte Blocks

Question 163

# Symmetric Key Cryptography What is the block size for PKCS 7?

Answer 163

16 Byte Blocks

Question 164

# Symmetric Key Cryptography Give the Block Cipher Modes:

Answer 164

- Electronic Codebook Mode (ECB) - Cipher Block Chaning Mode (CBC) - Counter Mode (CTR)

Question 165

# Symmetric Key Cryptography What is Electronic Codebook Mode (ECB)?

Answer 165

- Each block is encrypted indvidually - Encrypted blocks are assembled in the same order as plaintext blocks - Blocks are repeated in plaintext, revealed by Ciphertext

Question 166

# Symmetric Key Cryptography What is CBC

Answer 166

- Each Block is XOR'd with previous one - Start with a random *Initialisation Vector* **IV** - Helps overcome relay attacks

Question 167

# Symmetric Key Cryptography Give format of CBC Encryption

Answer 167

*Plaintext is B1,...,Bn. Ciphertext is C1,...,Cn Where:* - IV => RANDOM NUM (SET IN CLEAR) - C1 => Encrypt(B1 ⊕ IV) - C2 => Encrypt(B2 ⊕ C1) **...** - Cn => Encrypt(Bn ⊕ C(n-1))

Question 168

# Symmetric Key Cryptography Give format of CBC Decryption:

Answer 168

*Recive IV & Ciphertext. Plaintext is B1,..,Bn Where:* - B1 = decrypt(C1) ⊕ IV - B2 = decrypt(C2) ⊕ C1 **...** - B3 = decrypt(Cn) ⊕ C(n-1)

Question 169

# Symmetric Key Cryptography What is Probabilistic Encryption?

Answer 169

Schemes use random Elements to make every encryption different

Question 170

# Symmetric Key Cryptography What is an Example of a Good Probabilistic Encryption & Benefit ?

Answer 170

CBC with **RANDOM IV** is a good way to make encryption probabilistic *Benefit* - Using both allows us to encrypt same message & key without attacker realising

Question 171

# Symmetric Key Cryptography How can IV be Misused?

Answer 171

By Choosing IV as it can have devastating Attacks - IV must be random and unique for each encrypted block

Question 172

# Symmetric Key Cryptography What happend in ZEROLOGON?

Answer 172

- IV was set as 0 - Encryption on All-Zero Plaintext will provide All-Zero Ciphertext - Can be used to bypass Authentication completely & set domain controller bypass - Only requires network access to domain controller (*Avaliable from any machine*)

Question 173

# Symmetric Key Cryptography What is Counter Mode (CTR)

Answer 173

Mode used with block ciphers like AES to turn them into a stream cipher. It works by encrypting a counter value and XOR-ing it with the plaintext.

Question 174

# Symmetric Key Cryptography Give the format of CTR Encryption:

Answer 174

*Plaintext: B1,...,Bn. IV => RANDOM (Set In Clear)* - C1 = B1 ⊕ encrypt(IV) - C2 = B2 ⊕ encrypt(IV+1) **...** - Cn = Bn ⊕ encrypt(IV + (n-1)) COUNTER = NONCE + COUNTER | IV / Counter are interchangable

Question 175

# Symmetric Key Cryptography What is XOR symbol

Answer 175

⊕

Question 176

# Symmetric Key Cryptography What Happens if the Message > 128 bits?

Answer 176

**- Break the message into Chunks of 128** - m0 = m1 , so C0 = C1 - Inputs are the same then the outputs are the same therefore the message may still be visible & not encrypted fully. ## Footnote Mode is ECB : Parallel Applications of Block Cipher

Question 177

# Symmetric Key Cryptography Explain the Counter Mode:

Answer 177

- Same messages map to diiferent ciphertext - m0 = m1 but C0 != C1 - as Counter0 != Counter1 - As we XOR m0 & m1 with different Counter values the ciphertext will be different

Question 178

# Symmetric Key Cryptography What is a Caution for CTR?

Answer 178

**Ciphertexts are malleable** - If you flip bit of C0 to make C0' then C0'||C1 is a ciphertext of message m0'm1 where m0' is m0 with last bit flipped (GET DIFFERENT MESSAGE BUT CHANGE IS CIPHERTEXT & PLAINTEXT IS THE SAME)

Question 179

# Symmetric Key Cryptography Summaries AES:

Answer 179

-** Encryption (128/256 bit block) & Decryption ** with a specific key maps any 128-bit block to a 128-bit block - Decrypt can run on any block ( just NOT encryption)

Question 180

# Symmetric Key Cryptography What is the Know Plaintext Attacks?

Answer 180

**I know the plaintext, I can change the CTR encrypted Message**

Question 181

# Symmetric Key Cryptography Give an Example of *Known Plaintext Attacks*

Answer 181

*I Know Enc{CTR}(M1) & M1 I can make a ciphertext that decrypts to any message* **NEW CIPHERTEXT:** - Enc{CTR}(M1) ⊕ (M1 ⊕ M2) **Decrypt It:** - Dec{CTR} (Enc{CTR}(M1) ⊕ (M1 ⊕ M2)) - = Dec{CTR} ((Enc(N||Ctr) ⊕ (M1)) ⊕ (M1 ⊕ M2)) - = Enc(N||Ctr) ⊕ ((Enc(N||Ctr) ⊕ (M1)) ⊕ (M1 ⊕ M2)) - = M2 ## Footnote N = Nonce Ctr = Counter M1 = Message1 M2 = Message 2 Dec = Decrypt Enc = Encrypt CTR = Countrer Mode

Question 182

# Symmetric Key Cryptography & MACs What are Authenticated Encryption Modes?

Answer 182

- They stop Plaintext Attacks - Can ONLY form valid ciphertext if you know the key - Common way to do this is by Adding MAC to ciphertext

Question 183

# Symmetric Key Cryptography & MACs Give an Example of Authenticated Encryption Modes?

Answer 183

CCM mode Encryption

Question 184

# Symmetric Key Cryptography & MACs Explain CCM mode Encryption process

Answer 184

- 1st calculates an AES CBC-MAC on Data - Encrypts message followed by MAC using same key & CTR mode - Proven to be secure * fully defined as RFC 3610*

Question 185

# Message Authentication Codes What are MACs (Message Authentication Codes)?

Answer 185

**- Used for authentication** e.g. Alice & Bank share Key k Alice sends "Pay Bob £10" to bank, MAC{k} ("Pay Bob £10") MAC & k allows bank to authnticate the request | - NOT RELATED TO MAC ADDRESS

Question 186

# Message Authentication Codes What are Possible Attack with MACs?

Answer 186

**LENGTH EXTENSION** - Adding data to MAC without knowing the key | IF ATTACKER DOESN'T HAVE SHARED KEY WON'T BE ABLE TO DO ANYTHING

Question 187

# Message Authentication Codes How do we Make MAC?

Answer 187

- Making a CBC MAC: - Need to make it determininstic - Set IV to always be 0 - Keep the key - Only use last block of plaintext | LOOK AT NOTES FOR PICTURE / DIAGRAM ## Footnote CAN USE an Ineffcient hash function where we set the key to 0

Question 188

# Message Authentication Codes How to go From Broken Hash to MAC?

Answer 188

Have a hash try and make a MAC by: **MAC{key}(M) = H (Key,M)** - Use the output from the previous block as the input for the next block until the last block which is final output | LOOK AT NOTES FOR IMAGE

Question 189

# Message Authentication Codes What is the Problem with Broken Hash to MAC?

Answer 189

**- Can lead to Length Extension Attacks** *e.g.*: - Alice tells bank to "Pay Bob £10" - Adversary can easily add another step "Pay Eve £10" - Takes Output of "Pay Bob £10" as input & XORs it with "Pay Eve £10" & the encrypts it -*Bank is none the wiser to this attack when attacker adds/extend the length of MAC*

Question 190

# Message Authentication Codes / TAGS How to Make a Tag?

Answer 190

Tag = MAC (key, message)

Question 191

# Message Authentication Codes & Tags What are TAGS?

Answer 191

- Fixed length - They are generated alongside ciphertext to provide confidentiality + integrity - Decryption verifies the tag to detect tampering - Part of encrypted output

Question 192

# Message Authentication Codes & Tags Can Adversary Generate a Valid Tag?

Answer 192

**Not without knowing the Key** - They can modify ciphertext but without valid Tag it will not change, as decryption would reject it.

Question 193

# Public-Key Cryptography What are the 4 Objective / Direction Cryptography aim to achieve?

Answer 193

- **Confidentiality** - **Message Integrity** - **Sender Authentication** - **(soft) Sender Undenialbilty (NON-REPUDIATION)**

Question 194

# Public-Key Cryptography What is Confidentiality?

Answer 194

Transmitted message should remain hidden to adversary

Question 195

# Public-Key Cryptography What is Message Integrity?

Answer 195

Any modifications made by Adversary should be detected

Question 196

# Public-Key Cryptography Where Sender Authentication?

Answer 196

Ensure sender of messages before accepting it

Question 197

# Public-Key Cryptography What is (soft) Sender Undeniability?

Answer 197

Sender of a message should not be able to deny they sent a message

Question 198

# Public-Key Cryptography What is Symmetic Key Cryptography?

Answer 198

- 2 Users hold Identical keys - Encryption / Decryption use the same keys - Message Authentication Keys must be used to indentify the MAC

Question 199

# Public-Key Cryptography How are Symmetric Keys Shared?

Answer 199

Secret Communication to share it

Question 200

# Public-Key Cryptography What are the main Issues with Symmetric Key Encryption?

Answer 200

- Every pair needs a separate key - Everyone needs n-1 many different key (one for each person) This is insufficent & Probibitively Expensive

Question 201

# Public-Key Cryptography What is Private Key Cryptography?

Answer 201

- Each person has 2 keys: One Private & One Public - Keys are Asymmetric (Related but not identical) - Public Keys are Known to everyone - Private Keys are Secret

Question 202

# Public-Key Cryptography Give an example of Public Key Encryption:

Answer 202

- Sender encrypts with reciever's public key - Reciever decrypts with their private key ## Footnote No one will be able to decrypt message even if they know the public key

Question 203

# Public-Key Cryptography How many keys does Public Key Encryption need?

Answer 203

- No longer require pairwise distinct key - For secret communication amoung n people, we need n secret keys & n public keys

Question 204

# Public-Key Cryptography How are Public Keys Authenticated?

Answer 204

- Signatures ## Footnote Receiver need to be convinced message is not tampered & was sent by correct sender

Question 205

# Public-Key Cryptography How to check integrity?

Answer 205

We have signatures

Question 206

# Public-Key Cryptography What are Public Key Signatures?

Answer 206

- Like Physical Signature: Public key signature a signer creates a signature message pair (*Signed Message*)

Question 207

# Public-Key Cryptography How does a signing algorithm ensure that a signature is unforgeable?

Answer 207

- Signer uses sign Algorithms, for security the signature needs to be unforgeable - The sign Algorithms would take a Private key & message as input to make signed document

Question 208

# Public-Key Cryptography How can the Signature / Signed Message be Authenticated?

Answer 208

- Authenticity can be verified by anyone - Use Verify Algorithm

Question 209

# Public-Key Cryptography Explain the Verify Algorithm:

Answer 209

- Takes a public key of signer & signed message & Algo will accept or reject the input - Authenticated Signer the Algorithm will accept the inputs - Forger does not know the secret key, the Algorithm can reject it.

Question 210

# Public-Key Cryptography What are the steps for Public Key Infrastructure?

Answer 210

- User registers to system & obtain public/private key pair 1) Register with Registration Authority 2) Registration Authority requests certificate Authority to generate public/private key pair 3) Certificate Authority provide user a certificate Authority for public key, with public & private key 4) Certifcate Authority also put user certificate of public key in the directory (PUBLIC)

Question 211

# Public-Key Cryptography Give an Example of 2 User Communication:

Answer 211

1) User A obtians & verfies User B's public key from the public directory 2) Then User A sends User B a meesage & encrypts it using verified public key obtianed

Question 212

# Public-Key Cryptography What is the Requirement for the Public Key Infrastructure?

Answer 212

Trusted 3rd Party {Certificate Authority}

Question 213

# Public-Key Cryptography How to Secure Key Exchange?

Answer 213

User needs to agree on a secret key

Question 214

# Public-Key Cryptography What is the MultiRound Solution?

Answer 214

- User needs to transfer a set of keys in the physical world | Solution: Postal Service

Question 215

# Public-Key Cryptography What is the idea of MultiRound solution steps?

Answer 215

1) User A obtains a 2 sided lockbox 2) User A puts secret key in the box & lock using their own lock & key 3) User A ships box to User B 4) User B recives box with User A's lock 5) User B adds their own lock & key 6) User B sends it back to User A, who takes the lock and sends it back to User B 7) User B opens their lock and takes the shared key

Question 216

# Public-Key Cryptography Why is the lockbox solution secure?

Answer 216

- Secure if the padlocks on box cannot be opened without the key - Not impossible, just assume it difficult to do so - Assumption: Anyone carrying box has no intrest in opening it

Question 217

# Public-Key Cryptography What are the parameter in the Deffie Hellman (DH) Key Exchange Protocal?

Answer 217

- *p*=>a prime - *g*=>an integer, *g* < *p* , such that gcd(*g*,*p*-1) = 1 | gcd => greatest common divisor A.K.A HCF

Question 218

# Public-Key Cryptography How does DH protcol Work?

Answer 218

1) User A & B publicly announce *g* & *p* 2) User A & B choose their corresponding Secret (e.g. a/b∈R {2,...,*p*-2}) 3) User A computes X = *g*^a mod *p* User B computes Y = *g*^b mod *p* 4) User A sends X to User B & User B sends Y to User A 5) User A derives Key as: k = (B)^a mod *p* = *g*^(ba) mod *p* User A derives Key as: k = (A)^b mod *p* = *g*^(ab) mod *p* ## Footnote Adversary knows *g* & *p*, X, Y

Question 219

# Public-Key Cryptography Why is the shared key in Diffie-Hellman secure against brute-force attacks?

Answer 219

- B = *g*^b mod *p* (similarly A = *g*^a mod *p*) is not an increasing function as mod cycles through 0 - *p-1*. - Small changes in b do not lead to small, predictable changes in B. - It is resistant to Brute force approaches like aylor series approximation because it assumes continuity and differentiability. - No polytime algorithms to solve discrete log problem, therefore an adversary cannot compute the shared secret key.

Question 220

# Public-Key Cryptography What are the Assumptions in DH?

Answer 220

- No Polytime solutions to solve B = *g*^b mod *p* (similarly A = *g*^a mod *p*) - The Adversary does not tamper with X & Y ## Footnote -2nd assumption may not be true in practice (Courier may have adversary that causes Man-in-the-Middle Attack)

Question 221

# Public-Key Cryptography What is the Man-in-the-Middle Attack?

Answer 221

- Advesary sits in between User A & B (pretends to be User A to B & User B to A) - User A sends X, the adversary recieves it & Y from User B - User A & B secret keys don't match as they didn't recieve message from each other only from adversary - Adversary will know secret key for both user, so when they send messages the adversary can easily retrieve the message.

Question 222

# Public-Key Cryptography What are the solutions to Man-in-the-middle?

Answer 222

- Authenticating Public Key (Authenticate Sender) - Requires a trusted 3rd party like certifcation Authority - ## Footnote Authenticate that User A is sending message to User B & vice versa

Question 224

# Public-Key Cryptography What are the 3 RSA Algorithms?

Answer 224

- Gen (key Generation): Produces a Public & Private key Pair - Enc (Encryption) - Dec (Decryption)

Question 225

# Public-Key Cryptography What are the Steps of for Gen Algorithm in RSA?

Answer 225

On a security parameter λ (No. of bits public & private keys will have) 1) Generate 2 distinct primes p & q of bit-size λ 2) Compute N = pq & Ø(N) = (p-1)(q-1) 3) Choose a random int e where e is (1< e< Ø(N)) s.t gcd(e, Ø(N)) = 1 4) Let Z{n} * = {x|0

Question 226

# Public-Key Cryptography what is Z{n} * ?

Answer 226

- All Ints between 0 & N s.t gcd(x,N) = 1 - Basic Num Theory tell use their are Ø(N) integers

Question 227

# Public-Key Cryptography Give an example of RSA:

Answer 227

1) p = 3 & q = 11 2) N = 3 * 11 & Ø(N) = (3-1)(11-1) = 2 * 10 = 20 3) Let e = 7 (gdc(7,20) = 1) 4) Find d = 3 as (7 * 3 ) + (-1) * 20 = 1 (Find a value to times e with to get 1 when you subtract Ø(N) from it) 5) PK = (e = 7 , N = 33) 6) SK = (e =7 , d = 3 , N = 33)

Question 228

# Public-Key Cryptography What does the Encryption Algorithm do?

Answer 228

Enc(PK,m): On input an element (m∈Z{N} * ) & public PK = (e,N) compute - c = (m^e) (mod N) - c is an element (m∈Z{N} * )

Question 229

# Public-Key Cryptography Give an Example of Encryption in RSA:

Answer 229

Z{N} * = {1,2,4,5,7,8,10,13,14,16,17,19,20,23,25,26,28,29,31,32} 1) m =4 2) c = m^e (mod N) = 4^7 (mod 33) = 16 (mod 33) 3) c = 16 | Z{N} * => Ints between 0-N, that are Prime to N(Not factor of N)

Question 230

# Public-Key Cryptography What does the Decryption Algorithm do?

Answer 230

DEC(SK,c): On input an element (c∈Z{N} * ) & private key SK = (e,d,N) compute - m = c^d (mod N) | m is an element of Z{N} *

Question 231

# Public-Key Cryptography Give an Example of Decryption:

Answer 231

- Recover m = c^d (mod N) - 16^3 (mod 33) = 4 (mod 33) - m = 4

Question 232

# Public-Key Cryptography What are Digital Signatures?

Answer 232

- Primary tool to achieve Authentication in public key setting - Aims to achieve what hand-written signatures achieve in real world - Ensures hardness of forgery (signature prodcued by fraud, should be easily detected)

Question 233

# Public-Key Cryptography What is the Hand-written Signature Function?

Answer 233

- Bind a statement/message to its Author(i.e Job offer should have signature of employer) - Verification is Public (Against a Prior Authenticated one)

Question 234

# Public-Key Cryptography What are the Properties of Digital Signature?

Answer 234

- **Correctness**: A correct signature should always be verified as true - **Security**: Hard to forge (s.t the bank doesn't pass a fake as valid)

Question 235

# Public-Key Cryptography What are the Applications of Digital Signatures?

Answer 235

- **Public Key Infrastructure**: Provide authentication of Public Keys (*Used in certificate to certify validity of public key*) - **Software Authentication**: Boot loader authentication, software updates (*Verifies signature before booting OS/ installing Update*) - **Contract Signing**: Legal documents, smart contracts, Authorise assets transfers in blockchain

Question 236

# Public-Key Cryptography What are the 3 Algorithms of Signature Scheme?

Answer 236

- Gen (Key Generation) - Sign - Verify

Question 237

# Public-Key Cryptography What is Gen Algorithm in Digital Scheme?

Answer 237

- Takes security parameters & creates & secret key - Secret key is used for signing, so called Signing Key ## Footnote (1^k) => Gen => (PK,SK)

Question 238

# Public-Key Cryptography What is the Sign Algorithm in Digital Signature?

Answer 238

- Takes signing key & public key & message, to produce a signature σ (sigma) ## Footnote M => Sign(PK,SK) => σ

Question 239

# Public-Key Cryptography What is the Verify Algorithm in Digital Algorithm?

Answer 239

- Takes message & candidate signature x, with public key & either accept or rejects the message, signature pair ## Footnote (M,x) => Verify{PK} => Accept/Reject

Question 240

# Public-Key Cryptography What is Correctness?

Answer 240

If a message, signature pair was generated by Sign Algorithm, with the public key & secret key pair, then the verification algorithm wiht same public key & message, signature pair should ALWAYS ACCEPT ## Footnote If message signature pair was created honestly should always accept it

Question 241

# Public-Key Cryptography What is Unforgeability?

Answer 241

Adversary would not be able to produce a message, signature that will be accepted - Verify the algo will always reject the adversary's message & signature pair

Question 242

# Public-Key Cryptography What is RSA-Full Domain Hash?

Answer 242

Based on the idea of applying a cryptographic hash function to a message before signing it with RSA.

Question 243

# Public-Key Cryptography What is involved in RSA-Full Domain Hash?

Answer 243

- **Public Function**: A Hash Function H (*Standard hash function e.g. SHA-3*) - **keygen**: Runs RSA.keygen.PK = (e,N) , SK = (d,N) - **Sign**: Input: SK,M Output: σ = RSA.Des(SK,H(M)) = H(M)^d mod N - **Verify**: Input: PK, M , σ => if RSA.Enc(PK,σ) = H(M) => Output: Accepted else Rejected *** (If σ^e mod N = H(M) => ouput accepted else rejected)***

Question 244

# Public-Key Cryptography How to Prove Correctness?

Answer 244

CORRECTNESS: verify(PK,M,Sign(SK,M)) = Accept RSA.Enc(PK,Sign(SK,M)) = RSA.Enc(PK,H(M)^d mod N) = (H(M)^d) mod N = H(M) => by correctness of RSA

Question 245

# Public-Key Cryptography Can Textbook RSA be Practical for Encryption?

Answer 245

NO

Question 246

# Public-Key Cryptography The RSA security depends on...

Answer 246

...Hardness of finding *d* from *e,n*

Question 247

# Public-Key Cryptography Is RSA Deterministic?

Answer 247

- Textbook algorithms are - In practice some random padding is used

Question 248

# Public-Key Cryptography When will m < N work?

Answer 248

- Requires gcd(m,n) =1 - However, finding a m < n s.t gcd(m,n) >1, leads to finding *p* or *q* & break the system - NO

Question 249

# Public-Key Cryptography What Algorithm can solve factoring in polytime?

Answer 249

- Shor's Quantum Algorithm - Quantum Computer of required capacity is still quite far away in the future

Question 250

# Internet & Sockets What is the Internet? | Dictionary Definiton

Answer 250

A global computer network providing a variety of info & connumication facilities, comsisting of interconnected networks using standardized communication protocols

Question 251

# Internet & Sockets What is the Interent?

Answer 251

- Internet is not a single entity but a network of networks - Uses protocols to enable devices worldwide to exchange data - Packet of data travel through different routes & devices, making intenet decentralised & flexible

Question 252

# Internet & Sockets What does decentralised mean in Interent definition?

Answer 252

- Packets do not rely on a single path or central hub to tavel from one place to another. - Takes multiple paths, making system flexible & resilient to failures

Question 253

# Internet & Sockets What did we have before the Internet?

Answer 253

- Isolated local networkd with minimal external communications - Telephone-based connection (SLOW & COSTLY) - Dedicated leased lines for exclusive, high-cost connections - File sharing required physical transfers (floppy disk)

Question 254

# Internet & Sockets What are the challenges with the methods before the internet?

Answer 254

**Limited Interoperability:** - Different systems couldn't easily connect. - Need to know exact IP of device you were connecting to **Expensive & Low-speed Data exchange:** - Due to dedicated connections **No Universal communication Standard **

Question 255

# Internet & Sockets What is Leased Line Connection?

Answer 255

- Big cables underground - Decictaed Lines between Computers - High Cost but reliable for direct communication - Limited Scaliability as more node join | (More device = cost rises exponetially )

Question 256

# Internet & Sockets What is the Problem with Leased Line Connection?

Answer 256

- Bottle Necks leads to slow communciation **E.G: ** *A talks to D, B cannot communicate with C Simultaneously *

Question 257

# Internet & Sockets What is the solution to Bottlenecks in Leased Line Connection?

Answer 257

Packet Switching

Question 258

# Internet & Sockets What is Packet Switching?

Answer 258

- Data is plit inot packets rather than continuos communication - Packets travel separately allowing multiple conversation at the same time

Question 259

# Internet & Sockets What is a Packet and What does it contain?

Answer 259

- A Packet is a Message/Data split into small chunks Contains Packets: - **HEADER:** Source, destination, size stored here - **Actual Data:** The message being sent

Question 260

# Internet & Sockets How does data travel (IP PACKETS)?

Answer 260

Each Packet: - Travels Independently - Reassembles at the destination - Use IP for Routing ## Footnote Helps diagnose slow connections ?& network issues

Question 261

# Internet & Sockets What does Traceroute allow?

Answer 261

Allows us to trace the route our packets takes to gain access to a website

Question 262

# Internet & Sockets What was the issue with packets before the TCP was introduced?

Answer 262

Many packets were getting lost & arriving out of order

Question 263

# Internet & Sockets How did the TCP solve the Issues?

Answer 263

- Ensuring lost packets are retransmitted - Guaranteed ordered,complete & error-checked data transfer - User Acknowledgment (Recievers confirms recieved data) ## Footnote Leads to Reliable file download, web browsing & video streaming

Question 264

# Internet & Sockets What are Domain Names?

Answer 264

- IP is too hard to remember - We use Domain names, a name to match IP which will be easier to remeber

Question 265

# Internet & Sockets What is DNS & Hierarchy?

Answer 265

- DNS translates names into IP address via a hierarchial system

Question 266

# Internet & Sockets What are the steps for DNS?

Answer 266

- Request first go to the ISP's DNS resolver, then higher level DNS Servers - Authoritative DNS server provides the correct IP

Question 267

# Internet & Sockets Why do Ports matter in TCP?

Answer 267

- Allows multiple connections - Each connection is uniquely identified by a TCP socket

Question 268

# Internet & Sockets What is TCP connection defined by?

Answer 268

- Destination IP - Destination Port - Source IP - Source Port

Question 269

# Internet & Sockets What does the Destination Port depend on?

Answer 269

The Service ## Footnote HTTP => 80 SSH => 22 DNS => 53

Question 270

# Internet & Sockets How is Source port chosen

Answer 270

Chosen Randomly

Question 271

# Internet & Sockets What is Netcat (nc)?

Answer 271

- A versatile tool for making TCP & UDP connections - Used to SEND, RECIECE, & LISTEN on network ports

Question 272

# Internet & Sockets What is Nmap?

Answer 272

- A tool for discovering hosts & open doors - Scans system for open ports

Question 273

# Internet & Sockets How does Internet Communication Work?

Answer 273

- Follows a Stack of protocols - Each layer depends on the layer below to send & recieve data - Layers focus on a specific task

Question 274

# Internet & Sockets What are the 4 Layer ihe Internet Protocol Stack?

Answer 274

- Application - Transport - Internet - Link

Question 275

# Internet & Sockets What is the Function of the Application Layer?

Answer 275

- Handles user-facing applications - Formats data for application - Decides what to send | e.g Web, Email, FTP, DNS ## Footnote Adds the protocol the application will follow i.e HTTP,HTTPS,FTP

Question 276

# Internet & Sockets What is the Function of the Transport Layer?

Answer 276

- Manages end-to-end connections - Adds port Number to direct the packets to travel across different networks **TCP**: Ensures data is recieved completely & in the correct order **UDP**: Sends data quickly without checking for errors

Question 277

# Internet & Sockets What is the function of the Internet Layer?

Answer 277

- Routes packets across networks - Handles the addressing & Routing using IP - Determining the best path for packets to travel across | IP, ICMP, Routing

Question 278

# Internet & Sockets What is the function of the Link Layer?

Answer 278

- Manages physical network connections - Converts data in signals - Manges the physical transmission & error decetion in the communication ## Footnote Signal Sent over : Ethernet (CABLES) , WIFI (WIRELESS) ARP is also done here

Question 279

# Internet & Sockets Why does the Internet Protocol Matter?

Answer 279

- **Scalability**: Network can grow without needign to change everything - **Flexibility**: New technologies can be added at specific layers without breaking everything -** Interoperability**: Different hardware & structure can work together as long as they follow te protol rules

Question 280

# Internet & Sockets What is MAC (Media Access Control) Address ?

Answer 280

- Unique to each device - Assigned by manufactuer - Identifies a user - Can be changes quite easily (Negative) | E.g 48:D7:05:D6:7A:51

Question 281

# Internet & Sockets What is the IP (Internet Protocol) Address?

Answer 281

Identifies devices golabally across networks | E.g. 147.133.123.15

Question 282

# Internet & Sockets What are NAT (Network Adress Translations)?

Answer 282

- Private IPs - Not Unique - Enable multiple devices to share a single public IP

Question 283

# Internet & Sockets What is DHCP?

Answer 283

- Assigns an IP to machine based on MAC address - IP assignment is temporary & not stored long term

Question 284

# Internet & Sockets What is ARP?

Answer 284

-Helps routers map IP addresses to MAC adresses - Discovers who is using a specific IP

Question 285

# Internet & Sockets What is the Process for ARP?

Answer 285

- Send a Request to fins who has specific IP - Person who has it replies back with IP

Question 286

# Internet & Sockets What is ARP Spoofing (Network Attack)?

Answer 286

- Malicious machin can inpersonate another by faking ARP response - Allows Man-in-the-Middle Attacks

Question 287

# Internet & Sockets What is Wireshark?

Answer 287

Powerful network protocol analyzer that records & inspects networks

Question 288

# Internet & Sockets What happens in ARP spoof Attacks?

Answer 288

- Attacker send fake ARP reply, pretending to be gateway - Tricks victim into sending traffic to them - Attacker can now intercept, modify or drop packets ## Footnote Lack of Authentication

Question 289

# Internet & Sockets What are the Assumptions for modern Security?

Answer 289

- All modles assume the attacker controls the network - Strong encryption is **only** protection (**True Defense**) - Data is unencrypted, should be considered compromised

Question 290

# Internet & Sockets What was the Internet Designed for?

Answer 290

- Connectivity, not security - Traffic can be monitored,intercepted or altered

Question 291

# Internet & Sockets When can Network attacks Occur?

Answer 291

Any point along the path | Path of the packets / communication

Question 292

# Cryptographic Protocols What is a Simple Protocol?

Answer 292

**A => B** - A sends a messgae to B - Message can be "I am Alice" It has basic authentication but lack security

Question 293

# Cryptographic Protocols What is the Impersonation Attack in a Simple Protocol?

Answer 293

**E(A) => B** - Attacker/ Adversary pretends to be a user - No way to check of sender is who they say they are. | E(A) => Adversary impersonating A

Question 294

# Cryptographic Protocols Give a Solution for Impersonation on a Simple Protocol?

Answer 294

Symmetric Key Encryption: - Message encrypted & decrypted by the shared key Steps: - User A encrypts message using shared key kab - User B know its User A becaue they only share a key kab

Question 295

# Cryptographic Protocols Eventhough the Replay Attack can occur with symemetric key encryption is the message still secure?

Answer 295

Yes, as long as the adversary does not know the key kab, which will be hard to retrieve the message is still secure.

Question 296

# Cryptographic Protocols How can the Adversary bypass the symmetric key assumption?

Answer 296

Replay Attack: - Even if a message is encrypted an attacker can record it and resend it later. How it works: - User A send User B a message - Attacker either steals it through man-in-the-midde or using wireshark and records it to send to User B. ## Footnote Attacker will not know messgae due to not knowing the Key

Question 297

# Cryptographic Protocols How to resolve Replay attack?

Answer 297

- Use a Nonce (a number that is only used once) Steps: - User A introduces themseleves to User B - User B sends a Random Num (Nonce) - User A sends the Nonce + 1 and the message

Question 298

# Cryptographic Protocols How does nonce resolve replay attacks?

Answer 298

- Nonce is used once and if message is replayed checked and finds that the nonce is the same it was sent by an attacker, therfore can reject the message/ don't send packet to them. - This is due to Attacker not knowing shared key so they don't know the nonce value and the update value from the other user.

Question 299

# Cryptographic Protocols What attack is still possible with nonces?

Answer 299

- Man-in-the-middle - Nothing is authenticated. - Due to the return message being split in 2 parts & the second part has no nonce therefore easy to change it. - If attacker knows ciphertext, they know the outcome, therfore can use previous message to swap with new message. ## Footnote Slide example, the attacker know the message gave him money last time so swap the new message with the old message will give him more money.

Question 300

# Cryptographic Protocols How to resolve Man-in-the-middle for nonce simple protocol? ## Footnote In slides this is the 'A better protocol'

Answer 300

- Merge the 2 part of the message / packts, so that the nonce and message are together and the adversary cannot change it.

Question 301

# Cryptographic Protocols What does the solution for MITM attack ensure in a better protocol?

Answer 301

- Users can be sure they are talking to the actual person they want to talk to. (B is talking to A - The message sent is the actual message (e.g send £5 to Bob) - The nonce & messages are fresh (NOT REPLAYED)

Question 302

# Cryptographic Protocols What is the Key Establishment Protocol?

Answer 302

- A way to generate and exchange keys. - It is neede because A & B shared a key - Creates a session key so not too much happens of session key is compromised as it it one time use.

Question 303

# Cryptographic Protocols What is a Session Key & how is it set up?

Answer 303

- A key used once to encrypted & secrypted messages. - Not too much if key is intecrpted only loss 1 message. Set up: The principals need to set up a session key using a Key establisment ## Footnote Principals are the entities involved in setting up the shared key.

Question 304

# Cryptographic Protocols How to ensure you communicating with the correct principal?

Answer 304

Must know either: - Each other's public key (AES Key Est) - Use a trusted Third Party (Symmettic Key Est.) | 3rd Party ~ cannot have cryptography without trusting someone ## Footnote Est = Establisment

Question 305

# Cryptographic Protocols What is the Needham-Schroder Public Key Protocol?

Answer 305

Used to establish a shared symmetric session key between two parties. The protocol itself relies on public-key encryption for the exchange.

Question 306

# Cryptographic Protocols What are the step for Needham-Schroeder & explaination?

Answer 306

- User A encrypts a nonce Na and their indentity A with User B's public Key [E{B}(Na,A)] - User B decrypts Na & send it back with their own Nonce Nb (They send back [E{A}(Na,Nb)]) - User A decrypts the message and sends back Nb [E{B}(Nb)] This ensure users are talking to each other & generate a shared key using Na & Nb ## Footnote E{x} => mean encrypted with x's public key

Question 307

# Cryptographic Protocols What are the goals of Needham-Schroeder?

Answer 307

- Users A & B are sure they are talking to each other - Whoever knows Na is the person to decrypt the first message. - Whoever Knows Nb is the person to decrypt the second message

Question 308

# Cryptographic Protocols Explain the MITM attack on Needham-Schroeder?

Answer 308

STEPS: - User A sends nonce Na & their Identity A to User B, But Attacker intercepts it and replaces the identity with theirs E. - User B sends Ne & Nb to Attacker who forwards a modified version Na & Ne to User A. - User A send Attacker Ne & the attacker decrypts it and re-encrypts Nb and send it to User B - Attacker can now impersonate A & B, due to having a key with both users, so they can read the messages and modify it.

Question 309

# Cryptographic Protocols Solution for MITM in Needham-Schreoder?

Answer 309

New Protocol called Needham-Schreoder-Lowe. - When User B sends back Na,Nb to user A they also send back their Identity. This is now secure because: - A & B mutually authenticate each other - Attacker can no longer impersonate User B (MITM protection) ## Footnote Needham-Schreider-Lowe ensures Mutual Authentication by including User B's identity

Question 310

# Cryptographic Protocols What attack can occur in Needham-Schroeder-Lowe?

Answer 310

- Eventhough adversary cannot see the identity message they can still do a replay attack

Question 311

# Cryptographic Protocols Can the Attacker read the Message encrypted with key (Na,Nb)?

Answer 311

No, the key is secure against : - Intecept , replay, delete & Alter attacks

Question 312

# Cryptographic Protocols Can Government read your message?

Answer 312

- By law they can legally force you to give up your key, after the protocol has ran.

Question 313

# Cryptographic Protocols What is Forward Secrecy?

Answer 313

Protocol has forward secrecy if it keep the messaf secrt from an attacker who has: - Recording of the protocol run - Long term keys (SHARED KEYS) of the principals They Protect Against: - Government forcing people to give up keys - Hacker that might steal private keys

Question 314

# Cryptographic Protocols What are the steps for Station-to-Station Protocol? ## Footnote A.K.A STS

Answer 314

STEPS: - User A sends User B random DH value g^x - User B sends User A a Random DH value g^y and a signed message (g^y , g^x) encrypted by g^xy - User A send a signed message (g^y,g^x) encrypted by g^xy - User B sends A a Message encrypted by g^xy ## Footnote The Signed messages are used to prove identity of the user, Signature computed by a Users Private Key ensure it can only be generated by that user. PREVENTS MITM ATTACKS Key Arrangement is based on DH exchange Well desinged protocls prevent key leaks from compromisng post conversation

Question 315

# Cryptographic Protocols How is STS Secure?

Answer 315

- x,y,g^xy are not stored after protocol run - A & B's keys don't let attacker read M - STS ensure Forward Secrecy as we don't store g^xy and keys are different everytime.

Question 316

# Cryptographic Protocols STS & Forward Secrecy?

Answer 316

STS ensures forward secrecy because it does not store g^xy (the shared key), and a new key pair is generated for each session. This means that even if the long-term private keys are compromised, past session keys (which are used to encrypt the data) cannot be decrypted.

Question 317

# Cryptographic Protocols How do user's verfiy each other without knowing the Public Key?

Answer 317

Possible Solutions: - Meet face to face & exchange keys - Use a pre-shared key mechanism Better Solutions: - Trusted 3rd Party (Signed Identies & public key, creating) - A certificate that verifies each other's keys ## Footnote This is important as it prevents MITM attacks that ensure only real users can decrypts other user messages

Question 318

# Cryptographic Protocols What are the steps for Full Station to Station Protocol?

Answer 318

- User A sends g^x to User B - User B sends User A : g^y, Certificate for B CertB & Signed message containing (g^x,g^y) encrypted by g^xy - User A sends User B: Certificate for A CertA & Signed Message containing (g^x,g^y) encrypted by g^xy ## Footnote Verify Signature b#using certificate to make sure sender is who they say they are

Question 319

# Cryptographic Protocols What is CertX ?

Answer 319

CertX is the certificate for user X - Contains public key X - Signed by a trusted Certificate Authority To authenticate X

Question 320

# Cryptographic Protocols

Question 321

# Cryptographic Protocols Why do we add a certificate?

Answer 321

- Full STS protocol includes certificates for A & B - Certificated contain public keys signed by Trusted 3rd Party - Don't nned to know Public key beforehand

Question 322

# Cryptographic Protocols What is the Needham-Schroeder Key Establishment Protocol Steps?

Answer 322

- User A send the Server its Identity A , Indentity B & Nonce Na - Server Sends User A, Nonce Na, Idnetity B & Session Key K{ab}, the Session Key K{ab} & A's Idneity encrypted with S's Session key K{bs} All of this is encrypted by K{as} - User A sends User B the Session Key K{ab} & A's Identity encrypted with B's Session key K{bs} - User B send User A a Nonce Nb encrypted with the session key K{ab} - User A sends User B Nb+1 encrypted in the session ket K{ab}

Question 323

# Cryptographic Protocols How does Needham-Schroeder Key Establishment Protocol Work?

Answer 323

- S is a Trusted 3rd Party that helps establish a shared key K{ab} - S enccrypts the session key for A & B - A & B mutulally authenticate using Nonce Na & Nb - Ensure that only A & B K{ab}

Question 324

# Cryptographic Protocols What are some Key Establishment Goals?

Answer 324

- **Key Freshness:** Don't want to use the same key all the time / Key estabished is new - **Key Exclusivity:** Only the intended users know it. - **Good Key:** A good key is both fresh & exclusive

Question 325

# Cryptographic Protocols What is Hierarchy of Goals?

Answer 325

Good Key / \ Fresh Key Key Exclusivity

Question 326

# Cryptographic Protocols What is Far-end Operative Authentication Goals?

Answer 326

- A knows that B exists / Currently active E.g.: - A sends B a Nonce Na - B sends A a signed message of Na

Question 327

# Cryptographic Protocols What is Once Authentication , Authentication Goals?

Answer 327

A knows B want to communicate with A E.g.: B sends A a message a Signed message A {B might have the name A is the message }

Question 328

# Cryptographic Protocols What is Entity Authentication ?

Answer 328

A Knows B is currently active & wants to communicate. - They will verify each other. E.g.: - A sends B Na - B sends A a signed message of A & Na

Question 329

# Cryptographic Protocols What do Strong Protocols provide all 3 forms of Authentication?

Answer 329

- Entity Aythentication - Far-End Operative - Once Authentication

Question 330

# Cryptographic Protocols What is the Highest Goal?

Answer 330

- Protocol Provides Mutual Belief in a Key K for A with respcet to B if, after running the protocol, B can be sure that: - K is a good key A - A can be sure B wishes to communicate with B using K - A knows that B believes that K is good key for B

Question 331

# Cryptographic Protocols Show the Heirarchy of Goals?

Answer 331

1) Mutual Beleif in Key 2) Good Key & Entity Authentication 3) Fresh key , Key Exclusivity, Far-end Operative & Once Authentication

Question 332

# Cryptographic Protocols What is the Ultimate Goal?

Answer 332

Mutual Belief in a Key - ensuring bith A & B trust the estabkished key & each other

Question 333

# TLS/SSL What is an X.509 Certificate

Answer 333

Contains: - Subject (entity Indentity) (Who the certificate is for) - Subject's public key - Issuer's name (Authority that signed the certificate)

Question 334

# TLS/SSL What is the SSL/TSL Protocol?

Answer 334

- SSL was renamed to TLS - Provides encrypted communication & authentication using public keys | SSL => Secure Sockets Layer TLS => Transport Layer Security

Question 335

# TLS/SSL What is TLS/SSL Encryption & Authentication?

Answer 335

TLS/SSL supports: - RSA - DES - DH Specific cipher suite is negotiated at the start of the session

Question 336

# TLS/SSL What were the X.509 Verification Process?

Answer 336

- Issuer signs the hash of all certificate data - To Verify a certificate: - Compute the hash of the data - Check the signature using the issuer's public key - If I can trust the issuer's public key, I can trust the subject's pulbic key

Question 337

# TLS/SSL What is the IP Stack?

Answer 337

- **Application:** Where your program will operate e.g. web browser - **Transport:** Uses TCP/UDP to manahe data transmission - **Network:** Uses IP for routing Data - **Link/Hardware:** Underlies phyiscal connections

Question 338

# TLS/SSL What is the IP with TLS?

Answer 338

- TLS layer runs inbetween the Application & Transport Layer - Encryption is transparent to the Application layer - Normal TCP & IP protocol etc. can be used at the low layers - TLS handles encryption/decryption before the data is passed to TCP/IP (Making secure communication 'invisible' to application)

Question 339

# TLS/SSL What is the TLS protocol?

Answer 339

**TLS Handshake: establishing a secure connection:** 1) C sends a message listing supported cipher & random Numbers to S 2) Server responds with its chosen cipher suite & its own random Nonce 3) Server sens its certificate, which contains public key 4) The client sends a pre-master secret encrypted with the server's public key 5) Both Parties exchange verification messages to confirm that the handshake was successful **Key Exchange Options:** - RSA: client encrypts pre-master secret with server's public key - Diffie-Hellman: Client & server derive a shared secret After the handshake, all communication is encrypted using the negotiated symmetric key

Question 340

# TLS/SSL What are the Simplified Steps for TLS?

Answer 340

1) Client send sever a nonce Nc 2) The server responds with its nonce NC & certificate CertS 3) Client sends an encrypted pre-master secret (denoted E{s}(kSeed)) plus a hash encrypted with a session key 4) Server relies with its own hash encrypted with K{CS}

Question 341

# TLS/SSL What is the Key Derivation?

Answer 341

- Both side then compute session as: K{CS} = f (Nc,Ns,Kseed) -Where f is a key derivation function ensuring bith parties use the same key

Question 342

# TLS/SSL What is ClientHello (C => S) ?

Answer 342

- Textual: Client sends a random nonce & support cipher list - Transmits a Random Nonce Nc & list of supported Ciphers - Avoid Repay Attacks | Mathematical: C => S: Nc ## Footnote Nc => client's Nonce & used in key derivations

Question 343

# TLS/SSL What is ServerHello + Certificate (S => C)?

Answer 343

- Textual: Server responds with a random nonce & chosen cipher suite - Server send its certificate (signed by CA) - The Server's response incldes its nonce Ns & certificate containing its public ket | Mathematical: S => C : Ns , Cert{S} ## Footnote Ns => Server's Random Nonce Cert{S} => Server Certificate containing its public key

Question 344

# TLS/SSL What is Key Exchange(C => S)?

Answer 344

- Textual: Client encrypts a pre-master with the server's public key - Client encrypts the pre-master secret with the server's public key - A hash is computed over the exchaged values to ensute integrity Mathematical: C => S: E{S}(Kseed),{Hash1}Kcs ## Footnote E{S}(Kseed) => Pre-master secret encrypted with the server's public key Hash1 => ensures the integrity & is encrypted using the session key Kcs

Question 345

# TLS/SSL What is Server Verfies & Responds (S=>C)?

Answer 345

- Textual: Server verfies handshake & confirms key agreement - Server verfies this Hash & sends back another hash to confirm mutual agreement Mathematical: S => C : {Hash2}Kcs ## Footnote Hash2 => Is computed over handshake date, confiriming mutual agreement

Question 346

# TLS/SSL What is Key Derivation?

Answer 346

- Textual: Both sides dervie the session key using nonces & exchnaged key material - Use a function of Nc, Ns & keySeed Mathematical: - Kcs = f (Nc,Ns,Kseed) - f is key derivation function KDF that combined Nc, Ns & Kseed to generate Kcs.

Question 347

# TLS/SSL What are Cipher Suites for TLS?

Answer 347

- Some suites provide encryption & authenitcation - Others provide authentication OR just encryption

Question 348

# TLS/SSL What is the Negotiation with Cipher Suites for TLS?

Answer 348

Client & server agree on one suite from the list they both support to govern cryptographic methods used in the session ## Footnote Usually the Highest level of security, the both have

Question 349

# TLS/SSL What is Weakness in TLS?

Answer 349

** - Cipher Downgrading: ** - Forcing a session to use a weaker cipher than both parties are capable of ** - Self-Singed certificates:** - Accepting certificates that are not validated by a trusted Authority 3RD Party

Question 350

# TLS/SSL What is the Cipher Downgrading Attack?

Answer 350

How it Works: - Both the client & server list their supported cipher suites in order of prefrences. - Attacker can interfer with the handshake to force both parites to use the highest mutulally supported cipher. (Lie & say you only support the weaker ciphet, so not secure) (Reduce it by using latest / secure protocols)

Question 351

# TLS/SSL What is the Impact of Cipher Downgrading Attack?

Answer 351

- Weaker cioer may lacj robust authentication, can make session vulnerable to further attacks - Like MITM attack

Question 352

# TLS/SSL What are Self-signed Certificates?

Answer 352

- Maintiang a set of certificates is hard - Much easier just to accept any certificate Trade Off: - In enviroments like loT or certian apps, maintain a full set of trusted certificate us challenginh - Easier to acceots self-signed certificates Security Problems: - Accepting self-signed certificates can allow attackes to easily impersonate servers, facilitating MITM attacks

Question 353

# TLS/SSL What is Apple's 'go fail' Bug in TLS & What Happened? ## Footnote 2014

Answer 353

- Allowed attackers to bypass TLS/SSL certificate validation What Happened: - Found in Apple's Secure Transport Library - A misplaced goto statement caused the certificate validation chek to be skipped - Allows MITM attack agaonst secure connections

Question 354

# TLS/SSL What was the Impact of the Apple's 'goto fail' Bug?

Answer 354

Affected Safari, Mail, iCloud & other apple services using TLS. - These were all at risk as the bug meant that invalid certificates were accepted without proper verification

Question 355

# TLS/SSL How could the Attacker Expoit the Apple's 'goto fail'?

Answer 355

- Client blindly trusts any certificate (Valid or not) - MITM Attacker could impersonate trusted website - Encrypted Communication coul be intercepted

Question 356

# TLS/SSL What was the bug in the code for Apple's 'goto fail'?

Answer 356

in the SSLVerifySignedServerKey Exchange Function, in the if((err= SSLHashSHA1.final(...)...)), there was an additional goto final which skipped verification, and any certificate was trusted, making secure connections vulnerable

Question 357

# TLS/SSL What are Apple's 'goto fail' fix?

Answer 357

Remove the additional goto fail

Question 358

# TLS/SSL What was the Lesson Learned from Apple's 'goto fail'?

Answer 358

Need to have proper coding practices & through testing to avoid vulnerabilities - **Code Style matter:** Good Indentation & clear structure can prevent logic errors. - **Testing Matters: **Automated Tests can catch subtle bugs that mutual reviews miss. - **Open review in valuable:**Bug was unnoticed for years, wider security can help identify vulnerabilities early

Question 359

# TLS/SSL What if one side supports a weaker cipher suite but the other does not? ## Footnote Cipher Suites

Answer 359

- Genrealy considered safe. - Developer removed all weak ciphers, some remained in servers. - Depnends on different cioher suites being incompatible ## Footnote - Handshake will select the highest mutually supported cipher - Attacker forces the use of weak cipher, security maybe comprimised

Question 360

# TLS/SSL Give an example of incompatible cipher suites:

Answer 360

SSL_RSA_WITH_DES_CBC_SHA & TLS_DHE_DSS_WITH_AES_256_CBC_SHA

Question 361

# TLS/SSL What is the LogJam:Breaking TLS encryption? ## Footnote Give KeyInsight also

Answer 361

- Attacker can force a downgrade to these weak groups & then use precomputed discrete logarithms to break the encryption in real time Key Insight: Weak DH groups are compatible with strong groups (Stronger DH group can be decrypted by a weaker DH group) ## Footnote DH => Diffie Hellman History : Snowden leaks revealed NSA's large-scale TLS interception

Question 362

# TLS/SSL What are the Key Takeaways of LogJam?

Answer 362

- TLS Servers commonly support weak 512-bit 'export-grade' DH groups - MITM attacker can force TLS to use weak DH group, even if strong group is avaliable - Precomputed discrete logs allow attackers to break key in real-time *(Lets attacker passively decrypts TLS traffic in scale)*

Question 363

# TLS/SSL How is Diffie-Hellman done in TLS?

Answer 363

DH key exchange is widely used in TLS for forward secrecy: 1) C & S agree of a **prime** p & **generator** g 2) C picks a secret a, Computes A = g^a & sends it to S 3) S picks a secret b, Comutes B = g^b & sends it to C 4) S & C compute shared secret: K = B^a mod p = A^b mod p ## Footnote C => Client S => Server

Question 364

# TLS/SSL What is the problem in DH in TLS?

Answer 364

- server reuse the same small set of DH primes, making them vulnerable to precomputatuin attacks - Making Pre-computatuon attacks feasible if weak primes are used

Question 365

# TLS/SSL How does LogJams Works?

Answer 365

**Step 1 : MITM Attack** - Attacker intercepts the ClientHelllo message - Client proposes a strong DH group (2048-bit) - Attacker modifies this to request an export-grade 512-bit DH group **Step 2 :Server Accepts Weak DH Group** - Server allows the downgrade & responds with a weak DH group - Attacker can now easily compute the discrte log for the shared key

Question 366

# TLS/SSL What are the Steps for the Original Secure TLS handshake?

Answer 366

1) C => S: ClientHello (Strong DH group, e.g. 2048 bits) 2) S => C: ServerHello (Same DH group) 3) S => C: Certificate, DH Params 4) C => S: Key Exchange

Question 367

# TLS/SSL What are the Steps for Downgraded TLS Handshake (LogJam Attack)?

Answer 367

The Server, supporitng export-grade DH for legacy reason, accepts the weak group. 1) C => MITM => S: ClientHello (Requesting weak 512-bit) 2) S => MITM => C: ServerHello (Accepting weak 512-bit) 3) S => C: Certificte, Weak DH Param 4) Attacker quickly computes the shared secret due to precomputed discrete logs 5) Attackers decrypts & relays traffic in real-time RESULT: Attacker can derive the session & decrypt communications

Question 368

# TLS/SSL What is the comparison Betwenn normal Handshake & downgraded Handshake?

Answer 368

- Normal handshake, a strong DH is used, making it computationally hard to break. - Handshake is manipulated to use a weak size-bit group

Question 369

# TLS/SSL How is the Key Broken in LogJams?

Answer 369

With weak parameter, the attacker can compute the shared secret quicklt & decrypt TLS traffic in real time

Question 370

# TLS/SSL Why does LogJam Work?

Answer 370

1) **Many server reuse the same small set of DH primes:** - NSA can precompute discrete logs for these primes 2)**Export-grade cryptography is still widley supported:** - Weak DH groups weere left in for legacy reasons - Attackers can downgrade connections to force weaker groups 3) **No Authentication Required of DH Params** - TLS handshake does not authenticate the DH group selection - Attackers can ,odify the handshake without detection

Question 371

# TLS/SSL How to Defend Against LogJams?

Answer 371

**Mitigation Steps:** - Increase minimum DH key size (~2048 bits) - Disable export-grade ciphers completely - Use unique DH groups, instead of common shared primes - Prefer Elliptic Curve DH over Traditional DH **Industry Response:** - Browser vendors removed support for weak DH groups - TLS 1.3 removes support for static DH & export-grade cryptography

Question 372

# TLS/SSL What is the Heartbleed: A Critical OpenSSL Bug?

Answer 372

- Allowed Attackers to read up to 64kb of memory from a server potentially exposing private keys, session cookies, passwords & other sensitive data ## Footnote Affected OpenSSL versions 1.0.1 to 1.0.1f

Question 373

# TLS/SSL What went wrong with Heartbleed?

Answer 373

- OpenSSL implemented a Heartbeat extension to keep TLS conncetion alive. - Bound-checking failure allowed attacker to read up to 64 kb of memory - No authenitcation, so memonry can be requested by an attakcer without being a legitimate user

Question 374

# TLS/SSL What did TLS Heatbeat exist?

Answer 374

- Allows clients & server to check if connection is still open - Works by sending a small requests & expecting the same response - Meant to prevent unescessary re-negotiations in TLS

Question 375

# TLS/SSL How does the Heartbleed exploit work?

Answer 375

EXPLOT: ``` Client => Server: HeartbeatRequest("Hello", length = 64KB) Server => Client: HeartbeatResponse(Leaked memory up to 64Kb) ``` - An attacker send a HeartbeatRequest with a deiliberately inflated length field - The server, not verfiying the actual length of tha payload, responds by sending back extra data from its memory ## Footnote Attacker could repeatedly send malicious reauest to extract private keys, passwords or other data

Question 376

# TLS/SSL What were the Mistakes & Vulnerabilities in the code?

Answer 376

- Lack of proper bound checking is the root cause of the vulnerability - Memory beyond legitimate data is copied & sent back to attacker - Attackers could srnd small packets, but request large response, causing leaks - Server reads a 'length' value from the request & then copies that many bytes from memory without checking if the requests actually contained that much data

Question 377

# TLS/SSL What could be stolen? ## Footnote Heartbleed

Answer 377

Could Expose: - **Server private keys** Allowing attacker to decrypt past & future TLS traffic - **User Credentials** Including session cookies & passwords - **Sensitive session data** Any Info presents in the server's memory (**Internal API Keys, OAuth tokens, SSH Keys** ) - **Anything stored in memory** Since OpenSSL is widely used, many systems were affected

Question 378

# TLS/SSL Why is Heartbleed Bad?

Answer 378

- Steak session cookies without triggering alerts - Compromised Private keys meant attackers could decrypt past traffic - Attack left no logs - **Undetectable unless you were monitoring memory leaks**

Question 379

# TLS/SSL How did Heartbleed impact the real world? ## Footnote e.g services affected & damges/responses

Answer 379

**Affected Services:** - **Major websites:** Yahoo, Tumnlr, GitHub & Google - **VPNs & Firewalls:** Cisco, Juniper Networks - **Cloud Providers:** AWS, Google Cloud, Azure **Response & Damages:** - OpenSSL released a patch 1.0.1g (after discovery) - Websites scrambled to revoke & reissue TLS Certificates - Users were told to change their passwords - Attacker likely had access to sensitive data for years before discovery

Question 380

# TLS/SSL How was Heartbleed Fixed?

Answer 380

1) **Proper Bound Checking:** - Length field must be checked against actual input size - Length is greater thand provided data, it is rejected 2) **OpenSSL Patch:** - Introduced proper bounds checking - Disablrd heartbeat extension unless explicitly enabled 3) **Industry Response:** - Websites had to revoke compromised certificates - Users advised to change passwords - Organisations moved away from Open SSL

Question 381

# TOR What is a Proxy?

Answer 381

- Proxy server sit between you & the Internet - Hides your IP address from the webiste you visit

Question 382

# TOR What are VPNs?

Answer 382

- Securely connect you to another network over the internet - Provides privacy & security by encrypting your connection - promise "Anonymity" (to website not to the VPN company) - Access to private networks remotely - Single (ONE) proxy

Question 383

# TOR Can an Internet connection reveal your IP number?

Answer 383

YES

Question 384

# TOR What is Hotspot Shield VPN?

Answer 384

- VPN acts like a proxy by routing traffic through a differnet server - Recipent websites only sees the VPN's IP, not yours - VPN provider knows your original IP

Question 385

# TOR How does VPN connect to websites?

Answer 385

- Connection made via their servers - Intended recipent server never sees your IP

Question 386

# TOR Encryption in VPNs?

Answer 386

- Use encryption like TLS & IPSec to protect data - Ensures that your ISP & hackers cannot easily intercept your traffic

Question 387

# TOR VPNs for Anonymity? ## Footnote Limitations & Anonymity

Answer 387

Helps with Anonymity?: - Server thinks you are the VPN provider - ISP sees the connection to VPN - Global observar can probablu link your connections Limitiations: - Global observer may still link your VPN activity to your identity - VPN provider itself can see your traffic & could store logs (NOT ANONYMOUS)

Question 388

# TOR When using a VPN what does the WIFI provider Know?

Answer 388

- WIFI's Outgoing IP address - That you are connect to the VPN Means, public WIFI cannot monitor your browusing habits if you use a VPN

Question 389

# TOR When using a VPN what does the VPN provider Know?

Answer 389

- WIFI's Outgoing IP address - You are connected to VPN - VPNs Outgoing IP address - The website you are browisng VPN provider can track & log everything you do. If provider is compromised, then your privacy is at Risk.

Question 390

# TOR When using a VPN what does the website provider know about you?

Answer 390

- The VPNs Outgoing IP address - The Website you are browsing - The contents of your website - Shouldn't know you are using VPN (can be inferred) Website does not know your real IP when using VPN, but know your activity

Question 391

# TOR Where do you get the best anonymity?

Answer 391

Onion Routing (TOR)

Question 392

# TOR What is Onion Routing?

Answer 392

- Best Anonymity by routing traffic via multiple proxies - Ensures your message is **seurely encrypted in layers** & routed through multiple nodes - Each node only know the PREV & NEXT hop, thus ensuring no single enity can trace full path

Question 393

# TOR How does TOR Onion Routing work?

Answer 393

- Each proxy only learns the IP of the PREV & NEXT Proxy in circuit - Public Keys of each proxy is known - Soruce IP is visible only to entry node - Destination IP is visible to exit node - Use picks 3 Proxies & remain anonymous as long as at least one node is not compromised

Question 394

# TOR How does TOR work?

Answer 394

User Picks 3 Proxies (Nodes): - **Entry Node:** Knows user's IP - **Middle Node:** Acts as Relay - **Exit Node:** Knows the destination websites

Question 395

# TOR Why is TOR Secure?

Answer 395

As long as at least one node is not compromised, anonymity is maintained - Entry Node doesn't allow the destination - Exit Node doesn't know the Sender

Question 396

# TOR How are TOR Onion Routed? ## Footnote SIMPLE

Answer 396

- Each node only decrypts one layer, enough to know where to send it - Final Layer is decrypted at the exit node, and goes to the website/gets message - Prevent any single entity from tacking their connection

Question 397

# TOR TOR Routing Explained in Detail:

Answer 397

- Message is Encrypted 3 times (for 3 node) - Each node decrypt the relative layer & if it cannot decrypt it, it will send it ot the next node - Then the node will send it back to the user & the decrypted message sent to it & repeats until final node is reached - Final node, the message gets decrypted and send back a response being encrypted layer by layer by each node. To be decrypted by the User

Question 398

# TOR Problem of TOR?

Answer 398

Due to the constant decrypting and encrypting TOR is very slow (Dercypting to send website message & encrypting response back to user to decrypt)

Question 399

# TOR What are TOR Hidden Servers?

Answer 399

- Hide Server from you (USER) - Give websites .'onion' addresses (Only accessible through TOR network)

Question 400

# TOR How do Hidden Services work?

Answer 400

- Server dies not reveal its IP address - Instead, it connect through TOR relay to stay anonymous

Question 401

# TOR What are the Steps to set up a Hidden services & User to connect to it?

Answer 401

1) Service Provider picks some introduction points (IPs from TOR Cloud) & builds circuit to them 2) Service Procider puts their service name.onion in Database & uses the IPs of the nodes (information Points). DONT use your own 3) User hear that the name.onion exist and request more info from database & she sets up a rendezvous point (Can be done before) 4) User writes to privder an encrypted message using their PK, lisiting the rendezvous point & one time secret and ask the indroduction point to deliver it to the service provider 5) Provider connects to User's redezvous point & provides their one-time secret {if connection is accepted} (Connection can be rejected) 6) User & provider proceed to use their TOR circuits like normal (User never knows Provider IP & vice versa)

Question 402

# TOR Does TOR provide Security? ## Footnote Explain why/ why not?

Answer 402

NO - Protects anonmity but not security - If exit node is compromised/ is malicoius, it can see unecrypted data send to website/service - If you login to your account (Facebook), your identity is exposed

Question 403

# TOR What is the Summary of TOR?

Answer 403

**Final Takeaways:** PROS: - Provides strong anonymity by routing traffic through multiple nodes - Hides IP, making tracking difficult - TOR hidden services allow anonymous websites CONS: - Remains one of the best tools for online privacy - **Not foolproof:** - entry node sees your IP - Exit node sees your traffic (if unencrypted) - Government & adversaries try to break TOR

Question 404

# SQL Injection

Question 405

# Web Attacks

Question 406

# Web Security What is Cross-site Scripting XSS?

Answer 406

- Inject client-side code into pages viewed by other users - Attacker tricks web application to include malicious code

Question 407

# Web Security What are the goals of XSS?

Answer 407

- Display images, open popups - Change page contents - Session Hijacking: Steal Cookies

Question 408

# Web Security What is the Underlying Issues of XSS?

Answer 408

Input/Output Validation - NEVER TRUST USER'S INPUT (THEY CAN BE ATTACKER)

Question 409

# Web Security How does attacker perform Cross-site Scripting XSS?

Answer 409

Attacker sends script to application, that sends it to the web application to send database, so it gets sent to others users

Question 410

# Web Security What is Stored XSS?

Answer 410

Occurs when the attacker can inject malicious code into server-side & code is later displayed to other users.

Question 411

# Web Security Give Examples of Stored XSS Attacks?

Answer 411

In an area with little access control you can put: - HTML : `

comment

` - Script : `` WORMS: - Self Replicating Attack - (NO. of links/comments appear multiple times) - E.g. If user goes to malicious page with worm, everytime they access that page it will output the output the worm produces.

Question 412

# Web Security What is Reflected XSS?

Answer 412

Attacker sends user URL with malicious code already in it. Not stored on server, but is displayed on the visted page `e.g: https://web.site/?search=`

Question 413

# Web Security Give Examples of Reflected XSS Attacks?

Answer 413

- Edit Script to flip page 180 degrees - HTML edit code to show other values OR get other inputs

Question 414

# Web Security What are the Steps for Session Hijacking with XSS?

Answer 414

1) Attacker injects script on the victim server & waits for a victim 2) Server passes a session cookie & the Attacker's Script to Visitor 3) Scriot runs in the victim's browser, & passes the session cookie to the attacker 4) Attacker passes the stolen cookie, making the server think he is the victim ## Footnote - Redirects Victim's browser to attackers site , passing cookies - Might also pass currently visited web page - ALT , a Request, load an image

Question 415

# Web Security What are XSS Protections?

Answer 415

- **Validate User Input** - **Output Filtering** - **HttpOnly Cookies** - **Content Security Policy**

Question 416

# Web Security How does **Validate User Input** work? | XSS protection

Answer 416

It only allows a very strict subset of inputs e.g : alphanumeric Chars | Validation can be tricky, so you need to understand dataflow through app

Question 417

# Web Security How does **Output Filtering** work? | XSS Protection

Answer 417

**Plain Output: HTML Encoding** - Stored data values need to be encoded to represent HTML **Marked up output: Encoding + Domain Specific Language (DSL)** - Use a dedicated syntax & convert it tot a safe subset of HTML

Question 418

# Web Security How do **HttpOnly Cookies** work? | XSS Protection

Answer 418

Cookies with HttpOnly flag, so they are not accessible via Javascript, preventing theft via document.cookies

Question 419

# Web Security How does **Content Security Policy (CPS)** work? | XSS Protection

Answer 419

A Strict CSP can prevent inline scripts & limit which domians can be requested.

Question 420

# Broken Access Control What is Broken Access Control?

Answer 420

Attacker can act otuside their intended Permissions

Question 421

# Broken Access Control Give Examples of Broken Access Control:

Answer 421

- Violation of the principle of least privilege OR deny by default - Bypassing access control checks by modifying the URL - Permitting viewinf OR editing someone else's account - Elevation of Privilege

Question 422

# Broken Access Control What is Violation of the Principle of least privilege?

Answer 422

Access should be granted for particular capabilities, roles, or users, but is available to anyone | (HIDE LINK OF ADMIN AREA)

Question 423

# Broken Access Control What is Bypassing Access cotnrol check by modifying the URL?

Answer 423

Parameter tampering OR force browsing Internal application state, OR the HTML page , or by using an attack tool modifying API request | (PATH TRAVERSAL)

Question 424

# Broken Access Control How is Permitting viewing or editing someone else's account done?

Answer 424

Providin its unqiue identifier | (OBJECT REFRENCES )

Question 425

# Broken Access Control How is Elevation of Privilege done?

Answer 425

Acting as a user without being logged in OR acting as admin when logged in as user | (REDIRECTING)

Question 426

# Broken Access Control What is the issue with path traversal designed like this: **https://myblog.org/index.php?entry=2025-03-17.html**

Answer 426

- Format is not reliable as you can change URL to something else to open different file - If you know paths in the application you can access them - Remote user can potentially vist any file on the system

Question 427

# Broken Access Control What does index.php do in https://myblog.org/index.php?entry=2025-03-17.html?

Answer 427

- Reads a plain HTML file - Wraps it with navigation links. site style

Question 428

# Broken Access Control What are defenses against path traversal?

Answer 428

- http sever should not serve hust any file - Use internal web server config (Each webapp is separate) - Add External OS config (e.g. nobody users , chroot) - Use of allow-list (Filter inputs against Know / safe options)

Question 429

# Broken Access Control What is the Issue with Object reference where all HTML code is visible / personal info visible?

Answer 429

- Can Modify & re-run HTML on your devise - Can change HTML, Change account and access it ## Footnote Uses Post to protect sensitve data

Question 430

# Broken Access Control What are Solutions for Object Reference?

Answer 430

- **Re-validate**: Check authorisation again after every action from user, to make sure they have the access to perform the action - **Add a data Indirection**: Session specific server side array of account numbers - **Use of database/hashtables**: Use SQL to make sure users can access their accounts only

Question 431

# Broken Access Control What is the Issue with Too much information in Object references?

Answer 431

- Passing Potentially unnecessary info to the client - Expecting it unmodified - User can change types / HTML code/ User (email)

Question 432

# Broken Access Control How to protect Information in Object References?

Answer 432

- If the info is not needed, don't send it/ add it to frontend - If it needs to be in visible frontend, encrypt it - Add MAC constructed with server-side key

Question 433

# Broken Access Control Can Hiding a link in the navigation for unauthorsed users prevent them from visiting it?

Answer 433

NO

Question 434

# Broken Access Control How to prevent unauthorised users accessing hidden links?

Answer 434

- Manage Authorisation in a separate module - Single Route through Code - Trace to make sure authentication happens - Make authorisation checks for each function - Use deny-by-default policy *{if you forget it its not accessible}*

Question 435

# Broken Access Control What is Cross-site Request Forgery (CSRF)?

Answer 435

Exploit Browser's trust relationship with website *E.g. Existing login* ## Footnote Examples: home router admin, banking, email , browser is authorised to connet here

Question 436

# Broken Access Control How is CSRF Attack work?

Answer 436

- Attacker Triggers malicious action - get user to open malicious link - Browser undertakes action on target site on behalf of authorised user

Question 437

# Broken Access Control How is CSRF different from XSS?

Answer 437

- User goes to bad website then to the good website in CSRF - XSS Changes/Modifes good website, user goes to good then gets bad page sent back

Question 438

# Broken Access Control Explain Steps of CSRF?

Answer 438

1) Attacker Send user a link to a website, and user opens it 2) Website has an image which triggers a get request in the browser 3) The get request access the actual website, the get request will perform transaction without user's knowledge (INCLUDES COOKIES) 4) Using the Cookies a transactions is performed ## Footnote example of transactions: Email, banking

Question 439

# Broken Access Control How to prevent CSRF?

Answer 439

- Include a CRSF token that the attacker cannot load in the HTML code The token is Hidden and the value is randomly genreated, it gets checked by server to make sure CSRF doesn't occur

Question 440

# Broken Access Control What is Same-origin Policy?

Answer 440

- Standard browser-side mechanism to protect simultaneously running web apps from one another

Question 441

# Broken Access Control What does Same-origin Policy Restrict?

Answer 441

- DOM - APIs for web access - Cookies, HTML5 local storage APIs Prevents attacker website from reading data from other website To pages from the same domain, i.e., protocol-host-port { Different ports for website cannot access each other data} - TOO RESTRICTIVE for legitmate cases , e.g. APIs / 3rd party services ## Footnote SANDBOXING enhances this

Question 442

# Broken Access Control What is Access Control : Cross-Origin Resource Sharing?

Answer 442

- Modern Web Apps use Javascript APIs like fetch to send & recieve data asynchronously - Relaxes Same-origin Policy securely - Works by allowing servers to specify permitted origins using special HTTP Header

Question 443

# Broken Access Control Give examples of Specail HTTP Headers?

Answer 443

**Access-Control-Allow-Origin: http://www.example.com** OR **Access-Control-Allow-Origin: * **

Question 444

# Broken Access Control What are the Security Configuration for Deployment?

Answer 444

- Whole web app stack must be secure - Makes sure its up-to-date with security patches - Disable unnecessary features - Use minimum Privilege - Ensure Error handling does not expose info - Have repeatable security config process - Have automated checking process

Question 445

# Broken Access Control Why should Everything {OS, WEB, SERVER, DBMS, APP FRAMEWORK , LIBARIES} be kept up to date?

Answer 445

Most attacks are due to systems being outdated

Question 446

# Broken Access Control Give Examples of Unnecessary Features:

Answer 446

- Default Accounts - Demo pages - Debug Interfaces

Question 447

# Broken Access Control What are Minimum Privileges?

Answer 447

- Separate Concerns - ACLs per component/app

Question 448

# Broken Access Control What are repeatable security config process?

Answer 448

- App-specific checklist to work through - Unifrom config for development, QA & deployment

Question 449

# Broken Access Control What is the Purpose of having an automated checking process?

Answer 449

Ensure an automated checking process

Question 450

# Broken Access Control What are redirects & forwards?

Answer 450

- Apps often allow redirections - Sends user off-site with a polite message - Reroute them immediately OR Forwards them to different part of the same site

Question 451

# Broken Access Control What are the security concerns with Redirects?

Answer 451

- Attacker could redirect to their website to steal your data E.g. Bank website in email, when clicked can redirect you to the phishing website, the ultimate destination can be concelaed in URL encoding

Question 452

# Broken Access Control In Open Redirect links for phishing can the ultimate destination be concealed?

Answer 452

YES, by URL encoding

Question 453

# Broken Access Control What is a typical Example of a community-wide desirable security measure?

Answer 453

- Preventing open redirects - Open mail Relays - ICMP broadcast GOOD PRACTICE OF ALL PROVIDES SECURITY FOR OTHERS

Question 454

# Reverse Engineering What is Reverse engineering?

Answer 454

- Means of analysing software to figure out how it works, often without original code (LIKE TALING APART A TOY & SEEING HOW INSIDES WORK) - Examining low-level code, can remove protection & alter function - Good protection slows this down, NOT STOP IT

Question 455

# Reverse Engineering Issues with "Data can be Code":

Answer 455

Lots of attacks we have seen trick a program into accepting data that is really code: - SQL Injection - XSS

Question 456

# Reverse Engineering What is the Issue with "Code being data"?

Answer 456

Executable code can be written & edited - An attacker can do anything they want with the program, edit the code

Question 457

# Reverse Engineering Goals of Reverse Engineering:

Answer 457

- Security research - Debugging & Performance Optimiation - Learning how compliers & systems work

Question 458

# Reverse Engineering What is a Binary?

Answer 458

- File that computer can rin and understand. - Complier converts programs it into this binary format - Helps understand which tools are needed for reverse engineeing on each system

Question 459

# Reverse Engineering Can differnet Sytems translate or understand other system's binary files?

Answer 459

NO

Question 460

# Reverse Engineering What are different systems & Instruction Sets?

Answer 460

- Each OS has it own Binary format {Program compiled for one system won't run on another} - CPUs Understand specific Instruction

Question 461

# Reverse Engineering Why are different systems & instruction sets IMPORTANT for Reverse Enginerring?

Answer 461

Need to know the binary format & CPU type the program is built for (DETERMINES TE TOOLS YOU SHOULD USE)

Question 462

# Reverse Engineering What are Reverse Engineering Tools?

Answer 462

- Debugger - Disassembler - Decompiler

Question 463

# Reverse Engineering What is a Debugger?

Answer 463

- Allows you to pause & inspect our running program - Helps you follow the program step-by-step & see's what's happening - Modify Contents of memory, CPU Registers & stack Frames

Question 464

# Reverse Engineering What is a Disassembler?

Answer 464

- Converts machine code into assembly code. - Show the assembly Instructions Binary is Machine code Assembly is Low-Level Language

Question 465

# Reverse Engineering What is a Decompiler?

Answer 465

- Translates machine code back into High-Level code to make it even easier to understand - Tries to recreate the original source cose (NOT PERFECT)

Question 466

# Reverse Engineering What are the steps for using the different tools for to get High level code to Assembly?

Answer 466

- The complier converts the high level code into machine code (Like Lossy Compression removes Variable names, an other redundant data) (IN Assembly each luine corresponds to a part to complete the function) - The Disassembler converts the Machine code into Assembly.

Question 467

# Reverse Engineering What are the steps for using the different tools for the Assembly to High Level?

Answer 467

- Assembler converts assembly to binary - - Decomplier converts the Binary file into higher level code (NOT THE SAME AS ORIGINAL CODE)

Question 468

# Reverse Engineering Why is it hard to Reverse Code from Assembly/Machine Code to High Level?

Answer 468

**Optimisation:** Compiler may simplify the code, removing same steps **Variables Names:** Compiled, meaningful names like num are replaced with vauge things like edi **Onfuscation:** Some program are intentionally make harder to to reverse engineer mixing up the instruction.

Question 469

# Reverse Engineering What happens when the High level code gets translated into Assembly?

Answer 469

- Optimisation remove parts - Removes Variable names - Other info can get lost

Question 470

# Reverse Engineering What happens when the Assembly code gets recompiled into High Level code?

Answer 470

- Program might not be the orginal code written in C - Complier migh be unkniwn - Onfuscation can make decomping impossible

Question 471

# Reverse Engineering What are Registers?

Answer 471

- Small, super-fast storage location inside the cpu - Holds data the CPU os currently working with - Registers are fast than normal memory because they are inside processor

Question 472

# Reverse Engineering What are General Purpose Registers?

Answer 472

Used for most calculations & Data movements

Question 473

# Reverse Engineering What are the 2 types of specalised Registers?

Answer 473

- **Instruction Pointer (RIP):** Keeps track of where the CPU it in the program - **Stack Pointer (RSP):** Points to the top of the stack (USED FOR FUN CALLS & VARIABLES)

Question 474

# Reverse Engineering What is RAM?

Answer 474

- Large data - Referenced by Address - Stores large amounts of data for longer periods while program runs

Question 475

# Reverse Engineering What does RAM hold?

Answer 475

- **Code** : Instruction the CPU executes - **Heap**: For Dynamic Memory - **Stack**: Temporary data like local Variables

Question 476

# Reverse Engineering What are the IMPORTANCE of MEMORY in Reverse Engineering?

Answer 476

Reverse Engineering, knowing where the data is stored helps you understand what the program is doing

Question 477

# Reverse Engineering What is Register Aliasing?

Answer 477

Registers that describe different parts of the same memory cell

Question 478

# Reverse Engineering Why does Register Aliasing matter?

Answer 478

- Understand these register is key to reading assembly - E.g: If a program multiple numbers, will probably use RAX

Question 479

# Reverse Engineering What are the 2 most inportant Registers?

Answer 479

- **RSP** in 64-bits *Stack Pointer AKA: ESP(32)/SPL(8)* - **RBP** in 64-bits *Base Pointer AKA: EBP(32)/BPL(8)*

Question 480

# Reverse Engineering Why are Registers important in RE?

Answer 480

- Key to reading assembly - Registers hold specific data for function

Question 481

# Reverse Engineering What are the Registers for Accumulator?

Answer 481

**- 64 bit:** RAX **- 32 bit:** EAX **- 8 high bits of lower 16 bits:** AH **- 8 bit:** AL

Question 482

# Reverse Engineering What are the Registers for Base?

Answer 482

**- 64 bit:** RBX **- 32 bit:** EBX **- 8 high bits of lower 16 bits:** BH **- 8 bit:** BL

Question 483

# Reverse Engineering What are the Registers for Counter?

Answer 483

**- 64 bit:** RCX **- 32 bit:** ECX **- 8 high bits of lower 16 bits:** CH **- 8 bit:** CL

Question 484

# Reverse Engineering What are Registers for Data?

Answer 484

**- 64 bit:** RDX **- 32 bit:** EDX **- 8 high bits of lower 16 bits:** DH **- 8 bit:** DL

Question 485

# Reverse Engineering What are Registers for Source Index for string Operations?

Answer 485

**- 64 bit:** RSI **- 32 bit:** ESI **- 8 bit:** SIL

Question 486

# Reverse Engineering What are the Registers for Destination index for string operations?

Answer 486

**- 64 bit:** RDI **- 32 bit:** EDI **- 8 bit:** DIL

Question 487

# Reverse Engineering What are the Registers for General Purpose?

Answer 487

**- 64 bit:** R8-R15

Question 488

# Reverse Engineering Give the Common Instructions for Data Movement:

Answer 488

- **move dst,scr** => puts the value in scr into dst - **push scr** => stores the current value scr onto the stack - **pop dst** => Loads data from dst to scr *Loads last stores value into scr* ## Footnote scr (SOURCE) dst (DESTINATION)

Question 489

# Reverse Engineering Give the Common Instructions for Arithmetic:

Answer 489

- **add dst,scr** => adds scr to dst *e.g. add rax, 10 => Adds 10 to the rax register* - **sub dst,scr** => subs scr from dst *e.g. sub rbx, 10 => subs 10 to the rbx register* - **imul dst, scr** => multiplies dst to sct *e.g. imul rax, 10 => multiplies the value in rax by 10*

Question 490

# Reverse Engineering Give the Flags for Rev Eng:

Answer 490

- **ZF: Zero flag** set to 1 if the result is 0 - **SF: Sign flag** set 1 if the result is -VE - **OF: Overflow flag** set to 1 if operation overflowed

Question 491

# Reverse Engineering Why do flags matter?

Answer 491

- Important for decisions in code - In Rev Eng, watch the flag change to understand the program flow

Question 492

# Reverse Engineering Give the Common Instructions for Control Flow:

Answer 492

- **jmp label** jumps to label *e.g. jmp exit => Jumps to section labelled exit* - **call fn** calls a function, pushes current* IP* onto stack, and jumps to the function *e.g. call myFun => calls function named myFun* - **ret** Pop* IP* from the stack & continues from saved positon on stack *e.g. ret (Goes back to point from the stack)* - **cmp a,b** calculates b-a & sets flags *e.g. cmp rax,5 => compares rax with 5* - **test a,b** calcuates a&b & sets flags - **je label** Jumps to label if zero flag is set *e.g. je done => Jumps to done if the prev cmp was equal* - **jne label** Jumps to label if zero flag not set *e.g. Jumps if not equal (Opposite of jn)* - **nop** No-op instructions *DON'T DO ANYTHING*

Question 493

# Reverse Engineering Why does Control Flows Matter?

Answer 493

- Instruction control program's logic - When REV ENG, follow these jumps to trace how the program works

Question 494

# Reverse Engineering What are Common Instructions with Bitwise Operations?

Answer 494

- **and dst,scr** Performs a bitwise AND *e.g. and rax,0xFF keeps ont the loest 8 bits* (Both Bits must be 1) - **or dst,scr** Performs a bitwise OR *e.g. or rax,0x1 keeps ont hte loest 8 bits* (Either Bits can be 1) - **xor dst,scr** Performs a bitwise XOR *e.g. xor rax,rax sets rax to 0* (Flips bits where one is 1)

Question 495

# Reverse Engineering Why is Memory Addressing Important?

Answer 495

- Memory Access helps you track vatiables in a program - Crucial for understanding function arguments & local variables [] => REPRESENT that operand is Mem Address

Question 496

# Reverse Engineering What is Direct Memory Access: [address]?

Answer 496

- Use a specific memory address -e.g.:**mov eax,[0x1234]** *moves the value at 0x1234 into eax*

Question 497

# Reverse Engineering What is Register-based Memory Access [register] :

Answer 497

- Use a register holding an address - e.g.: **mov eax, [ebp]** *Moves the value at the address in ebp itno eax*

Question 498

# Reverse Engineering What is Register-based Memory Access [register + offset] :

Answer 498

- Access memory relative to a register - e.g.: **mov eax, [rbp-4]** *Loads data 4 bytes before rbp*

Question 499

# Reverse Engineering Why are Patterns useful in Rev Eng?

Answer 499

- **Find important Vals:** Looks for mov instruction moving data to register - **Trace Decisions:** Id cmp by conditional jumps - **Modify Behaviour:** Bypass checks by relpacing jump with nop (Do nothing instructions)

Question 500

# Reverse Engineering Explain this common pattern: **mov eax, [esp+1] add [esp+18], eax**

Answer 500

**What is Happening:** - ***mov eax, [esp+1]* ** Loads a value from memory location 1 byte after esp inot eax register. esp hold the current position on the stack, so this reads data from stack - ** *add [esp +1]* ** This takes the value in eax & adds it to the value located 18 bytes after esp **Interpretation:** - Pattern typically moves data from mem to a register, modify & write it back - Common calculations for handling variables stored on stack **Importance:** -Pattern means the program is manipulating local variables in a function - In Rev Eng, the iding the pattern allow you to identify where importance data is stored & changed

Question 501

# Reverse Engineering Explain this common pattern: **cmp [esp+1], 3 je lable1**

Answer 501

**What is Happening:** - *** cmp [esp+1],3* ** Compare the value 1 byte after esp with 3 Doesn't change data, only updates flags - ** *je label1* ** Jumps to a specific part o the code if the 2 values are equal **Interpretation:** - Conditional checks; if the program jumps to another part of the code - Condition is false, program continues to next instruction **Importance:** - Used for if statements, loops & error checking - Understanding these jumps is critical for Rev Eng beacuse modifying the jumo can alter program behaviour

Question 502

# Reverse Engineering Describe the Stack, where CPU stores temporary data?

Answer 502

- Last-in-First-Out - Grows from high address to Low address - Top is always the lowest address - Bottom of stack is the oldest item *(i.e first function call)* - Used to manage & organise information for functions (func args, return address & local variables)

Question 503

# Reverse Engineering What does RSP (Stack Pointer) do?

Answer 503

- RSP always points to top of stack - Data added, RSP values decreases - Data Removed, RSP value increases

Question 504

# Reverse Engineering How to push rax instruction equivalent to?

Answer 504

**sub rsp,8** (subs 8 from stack pointer) *beacause 64 bit register holds 8 bytes* **mov [rsp], rax** (move rax into new position ontop of stack) (stores the rax value in the new mem location)

Question 505

# Reverse Engineering What does **push** do?

Answer 505

- Saves a value onto the stack - Pushes the value onto the lowest adrees in the stack

Question 506

# Reverse Engineering What happens when you push mutliple values?

Answer 506

Multiple push instructions decreases stack pointer further

Question 507

# Reverse Engineering What is ***pop eax*** instruction is equivalent to?

Answer 507

**mov rax, [rsp]** Get the value from the top of the stack **add rsp,8** Move stack pointer up

Question 508

# Reverse Engineering What does **pop** do?

Answer 508

- Returns the top of the stack - Then moves the Stack pointer to the next lowest position

Question 509

# Reverse Engineering What happens to the data, when pointer is changed during pop?

Answer 509

- Data does not overwrite memory - Data is not erased (No longer referenced) - Value still exist until a new push, puts new data onto the stcak

Question 510

# Reverse Engineering What are Calling Conventions?

Answer 510

- How data is passed to & from functions - Ensures that both caller & callee agree on how to handle the stack - First 6 integers & pointers arguments should be passed by registers ***Example:*** 1. rdi (1st Arg) 2. rsi (2nd Arg) 3. rdx (3rd Arg) 4. rcx (4th Arg) 5. r8 (5th Arg) 6. r9 (6th Arg) - Extra Args go on Stack - Return val is stored in rax

Question 511

# Reverse Engineering In Assembly what happens in the function call: **long a = myFunc(1,2,3,4,5,6,7,8);**

Answer 511

1) First, args 7 & 8 are pushed on the stack 2) Other args go in registers (rdi,rsi,etc) 3) Call Instruction is used to jump to myFunc 4) returns value stored in rax

Question 512

# Reverse Engineering What happens during a function call?

Answer 512

- Register hold the first 6 args - Extra arguments go on the stack - **RIP** is saved to return after the call

Question 513

# Reverse Engineering What is the Return Address?

Answer 513

- Location the program returns to after finishing a function - **call** instruction saves this address on the stack - **ret** instruction pops it to resume execution **Importance:** - Changing the return address can redirect the program

Question 514

# Reverse Engineering What is a Stack Frame?

Answer 514

- Is the organised setion of the stack where the function's: - Args - return Address - Local Variables Are stored

Question 515

# Reverse Engineering What is the Standard Fun Prologue? ## Footnote A.K.A Start Function

Answer 515

**push rbp, 24;** save old base pointer **move rbp,rsp;** set new base pointer **sub rsp,24;** reverse space for local vars

Question 516

# Reverse Engineering What is the Function Epilogue? ## Footnote A.K.A End of Function

Answer 516

**add rsp,24;** clean up local vars **pop rbp;** restore old base pointer **ret;** return to caller

Question 517

# Reverse Engineering What are the Common techniques for Reverse Engineering?

Answer 517

- Look for human-readable text (*e.g. error message*) - Instruction Manipluataion (*e.g. Swap je & jne*) - Id key tests & check the vals in the register using debugger - Replace the instruction that perform checks with a nop (*e.g. can bypass security attacks*)

Question 518

# Reverse Engineering What are the defenses for Reverse Engineering?

Answer 518

**Dynamically Construct Code:** - *Attacker can run the code * **Encrypt the Binary:** - Program must include the key for it to be executable (Not readable until its ran) **Obfuscation:** - Mix data & code so its harder to read - Can slow down attacks by months/years **Online Activation:** - Force the program to connect to a server - *Completely disabled by an attacker* **Require Online Content** **Require a hardware dongle** **Hardware-based protection:**store & run part of the code in tamper-resistant hardware