SshConnectionException: MAC Error when using only hmac-sha2-512 with .NET 8 and SSH.NET 2024.1.0 #1660
Description
When connecting to a Cisco switch that only allows the hmac-sha2-512 MAC algorithm, the SSH.NET client throws a SshConnectionException with the message MAC Error.
This happens only when hmac-sha2-512 is the sole allowed MAC on the server. If we include hmac-sha2-256 in the server config, the connection succeeds.
Environment:
SSH.NET version: 2024.1.0
.NET version: .NET 8
Platform: Linux and Windows (tested on both)
SSH server: Cisco switch with the following settings:
ip ssh server algorithm mac hmac-sha2-512
Server-Side SSH Debug Output (Cisco):
Jun 25 10:31:45.083: kex: client->server enc:aes128-ctr mac:hmac-sha2-512
Jun 25 10:31:45.083: kex: server->client enc:aes128-ctr mac:hmac-sha2-512
Jun 25 10:31:47.932: SSH2 1: SSH2_MSG_NEWKEYS received
Jun 25 10:31:48.760: SSH1: Session disconnected - error 0x00
Client Stack Trace:
Renci.SshNet.Common.SshConnectionException: MAC Error
at Renci.SshNet.Session.ReceiveMessage(Socket socket)
at Renci.SshNet.Session.MessageListener()
at Renci.SshNet.Session.Connect()
at Renci.SshNet.BaseClient.CreateAndConnectSession()
at Renci.SshNet.BaseClient.Connect()
What Works:
When we change the server to:
ip ssh server algorithm mac hmac-sha2-512 hmac-sha2-256
Then the client connects successfully, negotiating hmac-sha2-256.
Expected Behavior:
SSH.NET should be able to successfully connect using hmac-sha2-512 as the MAC algorithm.
Notes:
This looks like a bug in the hmac-sha2-512 MAC handling implementation in SSH.NET.
Possibly related to incorrect handling of the MAC size (512 bits = 64 bytes) or key derivation.
Repro Steps (Pseudocode):
KeyboardInteractiveAuthenticationMethod kauth = new KeyboardInteractiveAuthenticationMethod(_user);
PasswordAuthenticationMethod pauth = new PasswordAuthenticationMethod(_user, _password);
//kauth.AuthenticationPrompt += new EventHandler<Renci.SshNet.Common.AuthenticationPromptEventArgs>(HandleKeyEvent);
kauth.AuthenticationPrompt += (a, b) => HandleKeyEvent(a, b, _password);
// Setup Credentials and Server Information
ConnectionInfo ConnNfo = new ConnectionInfo(_ipAddress, 22, _user,
[kauth, pauth]
);
ConnNfo.Timeout = TimeSpan.FromSeconds(30);
_sshClient = new SshClient(ConnNfo);
_sshClient.TrustKeyReceived();
_sshClient.Connect(); // throws SshConnectionException: MAC Error