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Chapter: Cryptography and Network Security

Modes of Operation

1 Electronic Codebook Book (ECB) 2 Cipher Block Chaining (CBC) 3 Message Padding 4 Cipher FeedBack (CFB) 5 Output FeedBack (OFB) 6 Counter (CTR)

Modes of Operation


block ciphers encrypt fixed size blocks


eg. DES encrypts 64-bit blocks with 56-bit key


need some way to en/decrypt arbitrary amounts of data in practise


ANSI X3.106-1983 Modes of Use (now FIPS 81) defines 4 possible modes


subsequently 5 defined for AES & DES


have block and stream modes


1 Electronic Codebook Book (ECB)


message is broken into independent blocks which are encrypted


each block is a value which is substituted, like a codebook, hence name


each block is encoded independently of the other blocks


Ci = DESK1(Pi)


uses: secure transmission of single values

Advantages and Limitations of ECB


message repetitions may show in ciphertext

if aligned with message block


particularly with data such graphics


or with messages that change very little, which become a code-book analysis problem


weakness is due to the encrypted message blocks being independent


main use is sending a few blocks of data


2 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


Ci = DESK1(Pi  XOR Ci-1)


C-1 = IV


uses: bulk data encryption, authentication

3 Message Padding


at end of message must handle a possible last short block

which is not as large as blocksize of cipher


pad either with known non-data value (eg nulls)


or pad last block along with count of pad size o 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


4 Cipher FeedBack (CFB)


message is treated as a stream of bits


added to the output of the block cipher


result is feed back for next stage (hence name)


standard allows any number of bit (1,8, 64 or 128 etc) to be feed back

ü denoted CFB-1, CFB-8, CFB-64, CFB-128 etc


most efficient to use all bits in block (64 or 128)


Ci = Pi XOR DESK1(Ci-1 )


C-1 = IV


uses: stream data encryption, authentication

Advantages and Limitations of CFB


appropriate when data arrives in bits/bytes


most common stream mode


limitation is need to stall while do block encryption after every n-bits


note that the block cipher is used in encryption mode at both ends


errors propogate for several blocks after the error


5 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


Advantages and Limitations of OFB


bit errors do not propagate


more vulnerable to message stream modification


a variation of a Vernam cipher


hence must never reuse the same sequence (key+IV)

sender & receiver must remain in sync


originally specified with m-bit feedback


subsequent research has shown that only full block feedback (ie CFB-64 or CFB-128) should ever be used


6 Counter (CTR)


a “new” mode, though proposed early on


similar to OFB but encrypts counter value rather than any feedback value


must have a different key & counter value for every plaintext block (never reused)


Ci = Pi XOR Oi


Oi = DESK1(i)


uses: high-speed network encryptions


Advantages and Limitations of CTR



can do parallel encryptions in h/w or s/w


can preprocess in advance of need


good for bursty 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)

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