3DES key-length

Kevin Steves stevesk at pobox.com
Sat Dec 28 10:14:00 EST 2002

On Fri, Dec 27, 2002 at 05:09:51PM -0500, Hari-Isoft wrote:
> >From sshd manpage:
>      The rest of the session is encrypted using a symmetric cipher,
> currently
>      128 bit AES, Blowfish, 3DES, CAST128, Arcfour, 192 bit AES, or 256 bit
>      AES.  The client selects the encryption algorithm to use from those of-
>      fered by the server.  Additionally, session integrity is provided
> through
>      a cryptographic message authentication code (hmac-sha1 or hmac-md5).
> is this 128 bit applicable only to AES?
> if so, does 3DES use 192 bit keys.

i suppose it could be clearer.  i think the intention is to only
bit-length-qualify ciphers with variable key-lengths.

from draft-ietf-secsh-transport-14.txt:

   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]

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