Server should not require a MAC algorithm when AEAD cipher is used.

[tmds]

Version

2.11.0

Bug description

When the client and server agree on an AEAD ciper (like aes256-gcm@openssh.comor chacha20-poly1305@openssh.com) the server disconnects when the client and server have no common MAC algorithm.

Actual behavior

The server disconnects when there is no common algorithm MAC algorithm between the client and server.

Expected behavior

With an AEAD cipher, no separate MAC algorithm is needed, so the server should not require a common MAC algorithm. The key exchange should continue.

Relevant log output

Unable to negotiate key exchange for mac algorithms (client to server) (client:  / server: hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1)

Other information

Reported by @pandabytes when using Tmds.Ssh .NET client in tmds/Tmds.Ssh#356.

[tmds]

cc @tomaswolf