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cs6035 Test 2 > Cryptography, Encryption, Hashes Cont. > Flashcards

Cryptography, Encryption, Hashes Cont. Flashcards

1
Q

Symmetric Encryption Ingredients

A

Plaintext message

Encryption algorithm

Secret Key

Ciphertext (scrambled message)

Decryption algorithm

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2
Q

Requirements for secure use of symmetric encryption

A

Strong encruption algorithm

Sender and receive have obtained copies of secret key in secure fashion and keep key secure

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3
Q

Encryption/Decryption services

A

Integrity checking:
–no tampering

Authenticity:
–verified authorship

Authentication:
–not an imposter

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4
Q

Attacks on Encryption

A

Break a cipher:
Uncovering plaintext p from ciphertext c, or, alternatively, discovering the key

Brute-force attack
E.g., try all possible keys

Cryptanalysis
Analysis of the algorithm and data characteristics

Implementation attacks
E.g., side channel analysis
Social-engineering attacks

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5
Q

Block ciphers

A

Most common symmetric encryption algorithms

Processes plaintext input in fixed size blocks and produces a block of ciphertext of equal size for each plaintext block

Most common:

  • -Data Encryption Standard (DES)
  • -Triple DES
  • -Advanced Encryption Standard (AES)
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6
Q

Asymmetric Encryption

A

Plaintext

Encryption algorithm

Public and private key

Ciphertext

Decryption key

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7
Q

Digital Encryption Standard

A

Adopted 1977, most widely used until recently

Plaintext block of 64 bits, key of 56 bits, produces ciphertext block of 64 bits

Algorithm hasn’t been exploited

Key length is too short, so can be guessed

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8
Q

Triple DES (3DES)

A

repeats DES three times

Advantages:

  • -168 bit key length, so more secure, overcomes vulnerability of brute force attack
  • -Underlying encryption is same as DES, so already subjected to scrutiny

Drawbacks:

  • -sluggish due to three times as many calculations
  • -64 bit block size, need larger for efficiency and security
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9
Q

Advanced Encryption Standard

A

128 bit block, key lengths of 128, 192, 256

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10
Q

Stream ciphers

A

Processes input elements continuously, producing output one element at a time

Output called keystream

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11
Q

Authentication using symmetric encryption

A

symmetric encryption alone not sufficient for authentication even though only sender/receive share the key because the message may be reordered

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12
Q

Message Authentication Method

A

(Note: these don’t encrypt messages so confidentiality isn’t guaranteed)

Message Authentication Code: generated with secret key

Secure Hash Functions aka one way hash

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13
Q

Hash Function Requirements

A

Compute message digest of data of any size

Fixed length output: 128-512 bits

Easy to compute H(m)

Given H(m), no easy way to find m
--One-way function

Given m1, it is computationally infeasible to find m2≠m1 s.t. H(m2) = H(m1)
–Weak collision resistant

Computationally infeasible to find m1≠m2 s.t. H(m1) = H(m2)
–Strong collision resistant

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14
Q

Public key cryptography:

A

Asymmetric: two keys

  • -Public for encryption, private for decryption
  • -Private for signing and public for verification

Not necessarily more secure than symmetric encryption

Has not made symmetric encryption obsolete
—-due to overhead from public key encryption

Protocol to distribute public key still required and is no simpler than handshaking required for symmetric encryption key distribution

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15
Q

Security of encryption scheme depends on 2 factors

A

Length of the key
–not the number of keys

Computational work involved in breaking cipher

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16
Q

Public Key encryption ingredients

A

Plaintext: Readable message or datathat is fed into the algorithm

Encryption algorithm: Performs transformations on the plaintext

Public and private key: Pair of keys, one for encryption, one for decryption

Ciphertext: Scrambled message produced as output

Decryption key: Produces the original plaintext

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17
Q

Steps of public key encryption

A

Each user generates a pair of keys

Each user places one of the keys in a public register (public key) and keeps other key private

To send message, user encrypts with other user’s public key

When other user receives message, decrypts using private key

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18
Q

Asymmetric Encryption Algorithms

A

RSA

Diffie-Hellman Key Agreement

Digital Signature Standard

Elliptic Curve Cyrptography

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19
Q

Digital Signature

A

Encrypts hash code for authentication

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20
Q

Public key certificates

A

Solves problem of impersonators sending out public announcement of public key

Certificate consists of public key plus user ID of key owner, whole block signed by trusted third pary

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21
Q

Methods in which public key encryption is used to protect a message

A

Digital Signature

Public key certificates

Symmetric key exchange using public-key encryption

Digital Envelopes

  • -Protects a message without needing to first arrange for sender and receiver to have the same secret key
  • -Equates to the same thing as a sealed envelope containing an unsigned letter
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22
Q

Cryptanalysis

A

Analysis of the algorithm and data characteristics to discover the plaintext or key

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23
Q

Cryptographic systems classified along three independent dimensions

A

Type of operations used for transforming plaintext to ciphertext

The number of keys used

The way in which plaintext is processed

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24
Q

Feistel Cipher Structure

A

Structure for symmetric block encryption.

Half the data block is used to modify the other half and then halves are swapped.

Following parameters/design features:

  • -Block size: larger means greater security
  • -Key size: larger key size means greater security
  • -Number of rounds: single round inadequate security
  • -Subkey generation algorithm: greater complexity in algorithm leads to greater difficulty in cryptanalysis
  • -Round function: greater complexity greater resistence to cryptanalysis
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25
Q

DES

A

The process of decryption with DES is essentially the same as the encryption
process. The rule is as follows: Use the ciphertext as input to the DES
algorithm, but use the subkeys K i in reverse order. That is, use K 16 on the first
iteration, K 15 on the second iteration, and so on until K 1 is used on the sixteenth
and last iteration.

26
Q

AES

A

Not Feistel because processes entire data block in parallel using substitution and permutation

Key provided a input is expanded into an array of 44 32-bit words.

4 stages:

  • -substitute bytes
  • -shift rows (permutation)
  • -mix columns
  • -add round key

Structure:

  • -cipher begins with Add Round Key stage (only time key is used)
  • -9 rou ds that each include all four stages
  • -10th round of three stages
27
Q

AES generally

A

In 1997, the U.S. National Institute for Standards and Technology (NIST) put out a public call for a replacement to DES

It narrowed down the list of submissions to five finalists, and ultimately chose an algorithm (Rijndael) that is now known as the Advanced Encryption Standard (AES)

New (Nov. 2001) symmetric-key NIST standard, replacing DES
Processes data in 128 bit blocks

Key length can be 128, 192, or 256 bits

28
Q

Forward substitute byte transformation

A

SubBytes

simple table lookup

Each individual byte of state is mapped to a new byte.

The leftmost 4 bits of the byte are used as row value and the rightmost 4 bits are used as a column value

29
Q

Inverse substitute byte transformation

A

InvSubBytes

uses S box

Designed to be resistant simple mathematical function of the input

30
Q

Forward Shift Row Transformation

A

ShiftRows

First row of State is not altered

Second row, 1 byte circular left shift performed

Third row, 2 byte ciruclar left shift performed

Third roww, 3 byte circular left shift is performed

31
Q

Principle Elements of AES

A

Forward substitute byte transformation

Inverse substitute byte transformation

Forward Shift Row Transformation

Inverse row transformation

Forward Mix column transformation

Forward add round key transformation

Add round key transformation

AES key expansion

32
Q

Inverse row transformation

A

InvShiftRows

Circular shifts in opposite direction for each of last three rows, with 1 byte circular right shift for second row

33
Q

Forward Mix column transformation

A

MixColumns

34
Q

Forward add round key transformation

A

AddRoundKey

128 bits of State are bitwise XORed with 128 bits of round key

35
Q

Add round key transformation

A

Identical to forward add round key transformation

36
Q

AES key expansion

A

input 4 word key and produces linear array of 44 words

37
Q

Stream Cipher Structure

A

encrypts plaintext 1 byte at a time

key input into psuedorandom bit generator that produces a stream of 8 bit numbers that are random

Output of the generator is called a keystream

38
Q

RC4 algorithm

A

Stream cipher designed in 1987 by Ron Rivest for RSA Security

USed in Secure Socket Layer/Transport Layer Security

Variable length key from 1 to 256 bytes is used to initialize 256 byte vector S.

S contains a permutation of all 8 bit number from 0 through 255

For encryption and decryption, a byte k is generated from S by selecting one of the 255 entries

39
Q

Cipher Block 5 Modes of Operation

A

Electronic Code book ECB

Cipher Block Chaining

Cipher Feedback

Output Feedback

Counter

40
Q

Block Cipher Primitives

A

Confusion:

  • -An encryption operationwhere the relationship between the key and ciphertext is obscured
  • -Achieved with substitution

Diffusion:

  • -An encryption operation where the influence of one plaintext bit is spread over many ciphertext bits with the goal of hiding statistical properties of the plaintext
  • -Achieved with permutation
41
Q

Electronic Code book ECB

A

Each block of 64 plaintext bits is encoded independently using the same key

Used for secure transmission of single values

42
Q

Cipher Block Chaining

A

The input to the encryption algoritm is the XOR of the next 64 bits of plaintext and the preceding 64 bits of ciphertext

Used for

  • -general purpose block oriented transmission
  • -authentication
43
Q

Cipher Feedback

A

Input processed s bits at a time. Preceding ciphertet is used as input to the encryption algorithm to produce psuedorandom output whih is XORed with plaintext to produce next unit ciphertext

Used for

  • -General purpose stream oriented transmission
  • -Authentication
44
Q

Output Feedback

A

Similar to CFB, except that the input to the encryption algorithm is the preceding DES output

Used for
–stream oriented transmission over noise channel (satellite communication)

45
Q

Counter

A

Each block of plaintext is XORed with an encrypted counter. The counter is incremented for each subsequent block.

Used for

  • -General purpose block oriented transmission
  • -Useful for high speed requirements

Advantages

  • -hardware efficiency
  • -software efficiency
  • -preprocessing
  • -random access
  • -provable security
  • -simplicity
46
Q

End to end encryption

A

Two keys:

  • -Session key: all user data encrypted with one time session key for the duration of logical connection
  • -Permanent key: used between entities for the purpose of distributing session keys

Configuration:

  • -key distribution center: determines which systems allowed to communicate with each other
  • -security service module: performs end to end encryption and obtains session keys on behalf of users
47
Q

Modular Arithmetic

A

Public key algorithms are based on modular arithmetic

Modular addition

  • -Addition modulo (MOD) M
  • -E.g., M=10, for k=2, its inverse is k-1=8 because 2+8 MOD 10 = 0

Modular multiplication

  • -Multiplication modulo M
  • -E.g., M=10, 3 and 7 are inverse of each other because 3×7 MOD 10 = 1
  • -But 2, 5, 6, 8 do not have inverse when M=10
  • -Use Euclid’s algorithm to find inverse. Given x, n, it finds y such that x×y mod n = 1

Modular exponentiation

  • -x^y mod n = x^(y mod ø(n)) mod n
  • -if y = 1 mod ø(n) then x^y mod n = x mod n
48
Q

Simple Hash Function

A

Input viewed as sequence of n-bit blocks

Input processed one block at a time in iterative fashion to produce n-bit hash function

49
Q

SHA-1

A

Developed by NIST in 1993

Produces hash value of 160 bits

50
Q

SHA-2

A

Produced in 2002

Produces hash value lengths of 256, 384, 512 bits

For SH-512:

  • -1 Append padding bits to get length congruent to 896 modulo 1024
  • -2 Append length (block of 129 bits)
  • -3 Initialize Hash Buffer (512 bit buffer to hold intermediate and final results)
  • -4 Process message in 1023 bit (128 word) blocks
  • -5 Output
51
Q

SHA-3

A

Competition to develop announced in 2007

Must be possible to replace SHA-2 with SHA-3 in any application by drop in substitution. Hash value lengths 224, 256, 384, 512

Must preserve online nature of SHA-2

52
Q

HMAC

A

Hash code approach to message authentication

Design objectives:

  • -Use, without modification, available hash functions
  • -Allow for easy replaceability of embedded hash function
  • -Preserver original performance of hash functions
  • -Use and handle keys in a simple way
  • -Have a well-understood cryptographic analysis of the strength of the authentication mechanism based on reasonable assumptions on the embedded ash function
53
Q

RSA

A

Widely used, and one of the first (1977)

Support both public key encryption and digital signature

Assumption/theoretical basis:
–Factoring a very large integer is hard

54
Q

RSA Characteristics

A

Variable key length

Variable plaintext block size

  • -Plaintext treated as an integer, and must be “smaller” than the key
  • -Ciphertext block size is the same as the key length
55
Q

RSA Algorithm

A

This summarizes the RSA algorithm.

Begin by selecting two prime numbers, p and q and calculating their product n, which is the modulus for encryption and decryption.

Next, we need the quantity totient n, which is (p-1)*(q-1)

Then select an integer e that is relatively prime to φ(n) [i.e., the greatest common divisor of e and φ(n) is 1].

Finally, calculate as the multiplicative inverse of e, modulo φ(n).

The public key is (e,n), and the private key is (d,n)

For encryption, suppose Alice has published its public key and Bob wishes to send the message M to Alice. Then B calculates C = Me (mod n) and transmits C.

For decryption, on receipt of this ciphertext, Alice decrypts by calculating M = Cd (mod n). The property of RSA guarantees that only Alice can decrypt the message she has the private key that is paired with the public key used to encrypt the message.

56
Q

RSA Example generating keys

A
  1. Select two prime numbers, p = 17 and q = 11.
  2. Calculate n = pq = 17 × 11 = 187.
  3. Calculate φ(n) = (p – 1)(q – 1) = 16 × 10 = 160.
  4. Select e s.t. e is relatively prime to φ(n) = 160 and less than φ(n); we choose e = 7.
  5. Determine d such that de mod 160 = 1 and d < 160. The correct value is d = 23, because 23 × 7 = 161 = (1 × 160) + 1.

The resulting keys are public key PU = {7, 187} and private key PR = {23, 187}. The example shows the use of these keys for a plaintext input of M = 88. For encryption, we need to calculate C = 887 mod 187

57
Q

4 Approaches to attacking RSA

A

Brute force: trying all possible private keys

Mathematical attacks: factoring the product of two primes

  • -factor n into its two prime factors
  • -determine φ(n) directly without p and q
  • -determine d directly without φ(n)

Timing attacks: depend on running time of algorithm
–countermeasures include constant exponentiation time, random delay, blinding

Chosen ciphertext attacks:

58
Q

Diffie and Hellman Key Exchange

A

First published public-key algorithm

The purpose of the algorithm is to enable two users to exchange a secret key securely that can then be used for subsequent encryption of messages. The algorithm itself is limited to the exchange of the keys.

The security of the Diffie-Hellman key exchange lies in the fact that, while it is relatively easy to calculate exponentials modulo a prime, it is very difficult to calculate discrete logarithms. For large primes, the latter task is considered infeasible.

By Diffie and Hellman in 1976 along with the exposition of public key concepts

Used in a number of commercial products

Practical method to exchange a secret key securely that can then be used for subsequent encryption of messages

Security relies on difficulty of computing discrete logarithms

59
Q

Diffie-Hellman Limitations

A

Expensive exponential operation
–DoS possible

The scheme itself cannot be used to encrypt anything – it is for secret key establishment

No authentication, so you cannot sign anything
–man-in-the-middle attack possible

60
Q

Digital Signature Standard:

A

Makes use of SHA-1 and the Digital Signature Algorithm (DSA)

Originally proposed in 1991, revised in 1993 due to security concerns, and another minor revision in 1996

Cannot be used for encryption or key exchange

Uses an algorithm that is designed to provide only the digital signature function

61
Q

Elliptic-Curve Cryptography (ECC)

A

Equal security for smaller bit size than RSA

Seen in standards such as IEEE P1363

Confidence level in ECC is not yet as high as that in RSA

Based on a mathematical construct known as the elliptic curve

62
Q

Hash Function Properties

A

Compute message digest of data of any size

Fixed length output: 128-512 bits

Easy to compute H(m)

Given H(m), no easy way to find m
--One-way function

Given m1, it is computationally infeasible to find m2≠m1 s.t. H(m2) = H(m1)
–Weak collision resistant

Computationally infeasible to find m1≠m2 s.t. H(m1) = H(m2)
–Strong collision resistant

Note:
a hash function that is strong collision resistant is automatically weak collision resistant.

cs6035 Test 2 (11 decks)

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