Re: 160-bit key limit

On Jul 12, 3:01 am, Simon Tatham <ana...@xxxxxxxxx> wrote:
yawnmoth <terra1...@xxxxxxxxx> wrote:
I thought that the maximum key size was whatever the modulo for the
diffie-hellman key exchange was. If you're using diffie-hellman-
group1-sha1, that'd be 1024 bits. I don't see where SHA-1 factors
into it.

SHA-1 is used _after_ the key exchange, to convert the output of the
key exchange into the session keys used to do the actual bulk
symmetric data encryption. (This is the meaning of `sha1' in the key
exchange method name you quote.)
Hmmm. What, then, is the difference between aes256-cbc or aes192-
cbc? My guess would be that the 160 bits of the SHA-1 hash are
repeated in both cases.

I'd look in the relevant RFC (4253), but didn't see anything about
this at all. Maybe it was deleted with a newer revision? Such things
wouldn't be unprecedented, as this post elaborates:

http://groups.google.com/group/comp.security.ssh/msg/7e7e121da0dddd53

.

Relevant Pages

  • Re: 160-bit key limit
    ... SHA-1 is used _after_ the key exchange, to convert the output of the ... the algorithm for generating symmetric keys is in section 7.2. ... their entropy is limited is a consequence of that algorithm. ...
    (comp.security.ssh)
  • Re: 160-bit key limit
    ... diffie-hellman key exchange was. ... group1-sha1, that'd be 1024 bits. ... SHA-1 is used _after_ the key exchange, to convert the output of the ...
    (comp.security.ssh)