SFTP connection fails - Negotiation failed. The client and the server have no common key exchange algorithm

Question

The full error message is:
Negotiation failed. The client and the server have no common key exchange algorithm. Server supports 'diffie-hellman-group1-sha1' which is weak and not enabled at the client.

How do I enable 'diffie-hellman-group1-sha1' ?

Answer 1

This legacy algorithm can be enabled using Sftp object's Settings.SshParameters.KeyExchangeAlgorithms property:

var client = new Sftp();
client.Settings.SshParameters.KeyExchangeAlgorithms |= SshKeyExchangeAlgorithm.DiffieHellmanGroup1SHA1;
client.Connect(serverName);
...

For additional information, see SSH ciphers and Setting connection options.

Also, please be aware that the diffie-hellman-group1-sha1 cipher is not secure.