Re: ECB, CBC, CFB, OFB

[Bruce Schneier <schneier@counterpane.com>]

At 4:35 PM -0600 12/20/97, Anonymous wrote:
>Can somebody more experienced than I am explain the strengths and weaknesses
>of these encryption modes as applied to CAST, IDEA, DES, and Blowfish?
>
>>       -m mode
>>              Set the transfer mode.
>>
>>              Valid ENCRYPTION modes are:
>>
>>              ecb    Electronic codebook mode
>>                     c[i] = f1(K, p[i])
>>                     p[i] = f2(K, c[i])
>>
>>              cbc    Ciphertext block chaining mode
>>                     c[i] = f1(K, p[i] ^ c[i-1])
>>                     p[i] = f2(K, c[i]) ^ c[i-1]
>>
>>              cfb    Ciphertext feeback mode
>>                     c[i] = f1(K, c[i-1]) ^ p[i]
>>                     p[i] = f2(K, c[i-1]) ^ c[i]
>>
>>              ofb    Output feeback mode
>>                     h[i] = f1(K, h[i-1])
>>                     c[i] = p[i] ^ h[i]
>>                     p[i] = c[i] ^ h[i]

There is a pretty good explanation of the various block cipher modes in
Applied Cryptography.

Bruce

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