Cipher Block Chaining (CBC) and Output Feedback (OFB) Modes in Cryptography, Lecture notes of Cryptography and System Security

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Cipher Block Chaining (CBC)

• message is broken into blocks
• (^) linked together in encryption operation
• each previous cipher blocks is chained with current plaintext block, hence name
• use Initial Vector (IV) to start process
• uses: bulk data encryption, authentication

• at end of message must handle a possible last short blockMessage Padding

• which is not as large as block size of cipher
• pad either with known non-data value (e.g. nulls)
• (^) or pad last block along with count of pad size
  • eg. [ b1 b2 b3 0 0 0 0 5]
  • means have 3 data bytes, then 5 bytes pad+count
• this may require an extra entire block over those in message
• there are other, more esoteric modes, which avoid the need for an extra block

Advantages and Limitations of

CBC

• a ciphertext block depends on^ all^ blocks before it
• any change to a block affects all following ciphertext blocks
• need Initialization Vector (IV)
  • which must be known to sender & receiver
  • if sent in clear, attacker can change bits of first block, and change IV to compensate
  • hence IV must either be a fixed value (as in EFTPOS)
  • or must be sent encrypted in ECB mode before rest of message

OFB as a Stream Cipher

Continued

Figure 8.7 Output feedback (OFB) mode as a stream cipher

Output FeedBack (OFB)

• message is treated as a stream of bits
• output of cipher is added to message
• output is then feed back (hence name)
• feedback is independent of message
• can be computed in advance Ci = Pi XOR Oi Oi = DESK1(Oi-1) O-1 = IV
• uses: stream encryption on noisy channels

In the counter (CTR) mode, there is no feedback. The

pseudorandomness in the key stream is achieved using a

counter.

Counter (CTR) Mode

Figure 8.8 Encryption in counter (CTR) mode

Continued

Figure 8.9 Counter (CTR) mode as a stream cipher

Advantages and Limitations of CTR

• efficiency
  • can do parallel encryptions in hardware or software
  • can preprocess in advance of need
  • good for burst high speed links
• random access to encrypted data blocks
• provable security (good as other modes)
• but must ensure never reuse key/counter values, otherwise could break (cf OFB)