[Curdle] draft-ietf-curdle-ssh-kex-sha2 and diffie-hellman-group1-sha1 (1024-bit DH)

[Tero Kivinen <[e-mail]>]

I think it is bad idea to go from MUST to implement algorithm to MUST
NOT implement in one step. Especially as this will make all current
ssh implementations non-conforming as they do still implement
diffie-hellman-group1-sha1 even when it might be disabled by default.

We are defining here a MUST implement and MUST not implement, not MUST
use and MUST NOT use recommendations.

In IPsec we moved from MUST to SHOULD NOT just because that reason,
i.e., we didn't want to make all implementations non-conforming, and
forbid backwards compatibility with old implementations which might
only support previous MUST implement algorithm.

Also I guess there is quite a lot of ssh implementations in routers
and other devices which might not get updates very quickly, and they
might only support diffie-hellman-group1-sha1, and with this change,
new implementations cannot talk to them, as new implementations MUST
NOT implement diffie-hellman-group1-sha1.

I would suggest changing it to SHOULD NOT and say that in future it
might be changed to MUST NOT.
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