Remove SHA-1 Signature Support

Problem Statement

This item is to track the removal of SHA-1 as a cryptographic signature (HMAC) option from SSH.NET. Nist formally deprecated SHA-1 in 2011 (source). The OpenSSH project also officially dropped support for SHA-1 in 2020 (source).

TL;DR: SHA-1 is considered insecure and we should remove it as a supported signature.

Proposal

The following would be removed from the codebase:

Key Exhange Methods

• diffie-hellman-group1-sha1
• diffie-hellman-group14-sha1
• diffie-hellman-group-exchange-sha1

Host Key Algorithms

• ssh-dss
• ssh-rsa

Impact

I don't see a compatibility concern with this change because even very old servers would still support more modern algorithms than those being removed. So, it would really only affect someone connecting to a really, really old SSH server (i.e. running pre-2010s era algorithms). Those who have that need should 1) realize that their connection is not secure and stop doing that immediately, and 2) can continue to use one of our older library versions.

Additional Considerations

The SHA-1 (and MD5) algorithms are also used in message authentication codes, however, those are not listed above because they play a much lesser role in security and are used only after a secure connection has been established. However, if we think those should be in scope also, that's fine too.

Feedback welcome.

Start by locating the SSH.NET registrations and tests for diffie-hellman-group1-sha1, diffie-hellman-group14-sha1, diffie-hellman-group-exchange-sha1, ssh-dss, and ssh-rsa. Confirm the intended scope with maintainers, then verify that these algorithms are no longer offered and that the relevant test suite passes.