Security (2)

Question 1

What is symmetric key cryptography?

Answer 1

Alice and Bob share the same (symmetric) key: K_s

Question 2

What is a Substitution Cipher?

Answer 2

Substitute one thing for another

Question 3

What are Block Ciphers?

Answer 3

• Message is broken into blocks
• Each block is encrypted / decrypted separately

Question 4

What is Cipher Block Chaining?

Answer 4

• First block is XOR’d with an arbitrary number known by both parties (initialization vector or IV)
• Following blocks are XOR’d with previous block
• Decryption: apply the substitution using K_s, then XOR with previous block

Question 5

What is DES?

Answer 5

Data Encryption Standard

Question 6

Is DES considered secure?

Answer 6

• No - can be brute forced in less than a day

Question 7

How does DES work?

Answer 7

• 56-bit symmetric key, 64-bit plaintext input
• Block cipher: substitution derived from symmetric key
• Cipher block chaining: initial number derived from symmetric key

Question 8

Which is more secure: AES or DES?

Answer 8

• AES
• Brute force decryption that takes 1 second for DES takes 149 trillion years for 128-bit AES

Question 9

What is AES?

Answer 9

Advanced Encryption Standard

Question 10

How does AES Work?

Answer 10

• Symmetric key
• 128 bit block cipher
• 128, 192, or 256-bit key