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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

 

efficiency

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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