Cryptography

1. ECB
2. CBC
3. CFB
4. OFB
5. CTR
6. 3DES
7. Cipher notation
8. HMAC

ECB

• electronic code book
• does not use IV ⇒ same data + single key ≡ same cryptoblocks ⇒ statistically vulnerable
• suitable for small data: PIN, challenge-response, keys
• can be implemented concurrently
• independent blocks ⇒ suitable for DB encryption
• does not spread error in encrypting/decrypting

CBC

• cipher block chaining
• uses IV to increase entropy for the 1st block
• not suitable for streaming

CFB

• cipher feedback
• emulates stream cipher with block cipher

OFB

• output feedback
• does not spread error ⇒ can be used for audio/video

CTR

• counter
• OFB-like, but uses counter instead of Oi
• no chaining ⇒ can be implemented concurrently
• useful for encrypting data that might be out of order
  • ATM cell
  • IPsec packet
  • 802.11i frame

3DES

• modes
  1. EEE3: 3 different keys, 3 encryptions
  2. EDE3: 3 different keys, encrypt-decrypt-encrypt
  3. EEE2: 2 different keys, I and III are equal
  4. EDE2

Cipher notation

cipher–block/rounds/key

RC5-32/12/16:

• 32 words = 64bit block
• 12 rounds
• 16 bytes key

HMAC

• origin authC because of key (not provided by hash)