Week 9 - Cryptography

Question 1

What is the fundamental idea behind cryptography?

Answer 1

1. Confidentiality (encryption and decryption)
2. Data integrity (data has a has, to ensure data has not been tampered with)
3. Authentication and non-repudiation (digital signature)

Question 2

What is an example of a simple algorithm for encryption? What are the ways to decrypt it?

Answer 2

The Caesar cipher.
1. Find a pattern
2. Brute force guess and check

Question 3

What is the limitation behind this ceasar cipher?

Answer 3

Relies on security by obscurity. The algorithm is obscuring it.

Question 4

What is the kerchoff’s principle

Answer 4

A crypto system should be secure even if the attacker knows all the details about the system with the exception of the secret key. In particular, the system should be secure even when the attacker knows the encryption and decryption algorithm

Question 5

What is a cryptographic key?

Answer 5

A key is a strong of characters used within an encryption algorithm for altering data so it appears random

Question 6

Symmetric encryption, what are the two core components?

Answer 6

1. A secure secret key
2. Strong encryption algorithm

Question 7

What is a secure secret key?

Answer 7

Both the sender and receiver must have obtained copies of the secret key in a secure fashion and they must keep the key secure.

Question 8

What defines a strong encryption algorithm?

Answer 8

The opponent should be unable to decrypt ciphertext or discover the key even if he/she is in possession of a number of cipher texts together with the plaintext that produced each ciphertext.

Question 9

What is the limitation behind a symmetric encryption?

Answer 9

How are you going to share the key?

Question 10

Number theory

Answer 10

It is computationally hard to find the prime number of a large number.

Question 11

How is this useful for cryptography?

Answer 11

It forms the one way relationship: easy to form the prime number but difficult to reverse engineer it. The adversary wont be able to brute force it.

Question 12

Public key encryption: key generation

Answer 12

Since its hard to derive the prime numbers forming the public key (p and q)
We can add an additional level of security by using some mathematical formula to form a private key out of the prime numbers.

Question 13

How to decode the message. If lets say the message is represented by the number 200.

Answer 13

We use the public key to encrypt the secret message (200 —> 61)
The person with the private key can successfully convert 61 —> 200

Question 14

Why should we use asymmetric encryption?

Answer 14

Public keys are always available on the public domain (to share). You are relying on the unique mathematical relationship to decode it. This is very hard to guess.

Question 15

How does asymmetric key encryption work?

Answer 15

1. Alice generates a pair of keys using an asymmetric key algorithm - private key is always kept secret by the user and the public key is broadcasted out to the network
2. Bob has access to Alice’s public key
3. He uses Alice’s public key to encrypt his message to her [hexadecimal]
4. Alice is able to decrypt using her private key

Question 16

How does cryptography assist with digital signatures?

Answer 16

You can prove that the person with the private key signed and sent the message. You are using someone’s private key to append the information

Question 17

How do digital signatures work?

Answer 17

1. Alice writes a message
2. Message is hashed
3. Hash is encrypted with Alice’s private key
4. Message and encrypted hash are sent
5. Hash is decrypted by Bob using Alice’s public key

Note: message is not encrypted

Question 18

What is a cryptographic hash?

Answer 18

String of text - put through a function - fixed length number

Question 19

What are the features of a cryptographic hash?

Answer 19

Deterministic, one way, diffusion, non-predictable, collision resistant (DODNC)

Question 20

What’s the difference between hash and key encryption?

Answer 20

Hash is irreversible. It’s to check to see if the file has been tampered with.
Key encryption is reversible. It’s for security reasons.