Chapter 4ii: Modes of Encryption

Question 1

Electronic Code Book Mode: ECB.
Explain the drawbacks.

Answer 1

• Ciphertext blocks do not have any connection with each other
• For this reason, an attacker can …
• … reorder blocks
• … repeat blocks
• … delete blocks
• Ciphertext can even be decrypted; possibly to reasonable plaintext
• Once more: Encryption does not protect integrity!

Question 2

Answer 2

Identical plaintext blocks are encrypted to non-identical cipher text, being a huge benefit.

Decryption of a block depends on the previous block’s ciphertext.

reordering, repeating or deleting blocks typically makes the decryption yield garbled data.

Question 3

Cipher Block Chaining Mode – CBC Initialization Vector (IV)
* c0 = IV
* IV must not be kept secret – the security depends on k
* IV must be fresh
* Why must IV be fresh?
…

Answer 3

• Identical plaintext messages are encrypted to non-identical ciphertexts
  as plaintext is “masked” with IV or previous ciphertext block → Hugebenefit!

Question 4

Answer 4

Question 5

Output Feedback Mode – OFB Decrypt. Explain the Pros

Answer 5

• Decryption does not depend on previous blocks.: transmission error in ci only affects ci.
• Reordering, repeating or deleting of ciphertext blocks impossible due to feedback

Question 6

Explain the pros and cons of CTR.

Answer 6

Pros and cons:
* Decryption does not depend on previous blocks.: a transmission error in ci only affects ci.
* Reordering, repeating or deleting of ciphertext blocks impossible due to counter
* If n is too small, the key stream repeats itself! → Use new IV for a fresh key stream.