Re: working of des with ssh
From: Darren Tucker (dtucker@dodgy.net.au)
Date: 11/22/02
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From: dtucker@dodgy.net.au (Darren Tucker) Date: Fri, 22 Nov 2002 07:42:00 GMT
In article <697b8851.0211211851.3264691e@posting.google.com>,
Nik <nchopra@think3.com> wrote:
>i am a IS professional, have to write a paper on details of how ssh utilizes
>DES/3DES. Does anybody know of any good books/papers/websites/anything?
The RFCs would be a good place to start. They can be found in the ssh1 and
ssh2 tarballs from ssh.com.
The draft RFC for SSH v1 says:
"SSH_CIPHER_DES
The key is taken from the first 8 bytes of the session key. The
least significant bit of each byte is ignored. This results in
56 bits of key data. DES [DES] is used in CBC mode. The iv
(initialization vector) is initialized to all zeroes.
SSH_CIPHER_3DES
The variant of triple-DES used here works as follows: there are
three independent DES-CBC ciphers, with independent initializa-
tion vectors. The data (the whole encrypted data stream) is
first encrypted with the first cipher, then decrypted with the
second cipher, and finally encrypted with the third cipher. All
these operations are performed in CBC mode."
The relevant part of the ssh2 rfc draft says:
"The "3des-cbc" cipher is three-key triple-DES (encrypt-decrypt-
encrypt), where the first 8 bytes of the key are used for the first
encryption, the next 8 bytes for the decryption, and the following 8
bytes for the final encryption. This requires 24 bytes of key data
(of which 168 bits are actually used). To implement CBC mode, outer
chaining MUST be used (i.e., there is only one initialization
vector). This is a block cipher with 8 byte blocks. This algorithm
is defined in [SCHNEIER]"
--
Darren Tucker (dtucker at zip.com.au)
GPG key 8FF4FA69 / D9A3 86E9 7EEE AF4B B2D4 37C9 C982 80C7 8FF4 FA69
Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.
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