NODE 02bc2280ECB, CBC, CFB, OFB
Anonymous <nobody@REPLAY.COM>Sun, 21 Dec 1997 06:44:15 +0800
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]
NODE 1a42871fRe: ECB, CBC, CFB, OFB
Bruce Schneier <schneier@counterpane.com>Mon, 22 Dec 1997 01:41:28 +0800
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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101 E Minnehaha Parkway, Minneapolis,MN 55419 Fax: 612-823-1590
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NODE 9b554ee0Re: ECB, CBC, CFB, OFB
David Honig <honig@otc.net>Tue, 23 Dec 1997 02:18:23 +0800
At 11:35 PM 12/20/97 +0100, 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]
>
>
A partial answer on why feedback is better than codebook mode:
consider a video signal, with large amounts of uniform background. Encrypted
with a codebook mode cipher, silhouettes will be visible since the uniform
background
will map to the same cipher value. NB: the "uniformity" must span a block,
e.g., 64 bits for BF.
The tradeoff is 1. slightly increased latency & complexity 2. an error in
your output stream mangles
more data than in ECB mode.
Feeding a stream of zeros into a feedback-mode cipher produces a
pseudo-random output stream...in a good cipher.
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