SSH - Diffie-Hellman; prime & g exchange

Asked Feb 05 '21 at 11:43

Active Feb 05 '21 at 11:43

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The Diffie-Hellman key exchange algorithm can be used with SSH.

But how does SSH exchange the large safe prime p and the generator for a subgroup of GF(p) g? According to RFC 4253 the first steps are to send e & f which are calculated depending via:

 e = g^x mod p
 f = g^y mod p

Now x & y are randomly generated but how do both parties come to the same values of g and p? There seems to be no exchange happening of them.

Does anybody have an explanation for this?

asked Feb 05 '21 at 11:43

burn_stick

1

See section 6.5, there are the links for those. part of the protocol the DHKE parameters are exchanged, too. – kelalaka Feb 05 '21 at 11:52
Does this answer the question? – fgrieu Feb 05 '21 at 11:52
@kelalaka ahhh thanks didn't know that p and g are just defined. – burn_stick Feb 05 '21 at 12:07
@fgrieu thanks for the link to the indepth reply this solved it – burn_stick Feb 05 '21 at 12:08

SSH - Diffie-Hellman; prime & g exchange

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