public async Task <KeyExchangeOutput> TryExchangeAsync(SshConnection connection, KeyExchangeInput input, ILogger logger, CancellationToken ct)
{
var sequencePool = connection.SequencePool;
using ECDiffieHellman ecdh = ECDiffieHellman.Create(_ecCurve);
// Send ECDH_INIT.
using ECDiffieHellmanPublicKey myPublicKey = ecdh.PublicKey;
ECPoint q_c = myPublicKey.ExportParameters().Q;
{
using var ecdhInitMsg = CreateEcdhInitMessage(sequencePool, q_c);
await connection.SendPacketAsync(ecdhInitMsg, ct);
}
// Receive ECDH_REPLY.
Packet exchangeInitMsg = input.ExchangeInitMsg;
using Packet exchangeInitMsgDispose =
exchangeInitMsg.IsEmpty ? (exchangeInitMsg = await connection.ReceivePacketAsync(ct)) : default(Packet);
var ecdhReply = ParceEcdhReply(exchangeInitMsg);
// TODO: Verify received key is valid.
// TODO: Verify host key belongs to server.
// Compute shared secret.
// TODO: what types of exceptions can we get when creating the public key?
ECParameters parameters = new ECParameters
{
Curve = _ecCurve,
Q = ecdhReply.q_s
};
using ECDiffieHellman peerEcdh = ECDiffieHellman.Create(parameters);
using ECDiffieHellmanPublicKey peerPublicKey = peerEcdh.PublicKey;
BigInteger sharedSecret = DeriveSharedSecret(ecdh, peerPublicKey);
var publicHostKey = PublicKey.Read(ecdhReply.public_host_key, input.HostKeyAlgorithms);
// Generate exchange hash.
byte[] exchangeHash = CalculateExchangeHash(sequencePool, input.ConnectionInfo, input.ClientKexInitMsg, input.ServerKexInitMsg, ecdhReply.public_host_key, q_c, ecdhReply.q_s, sharedSecret);
// Verify the server's signature.
if (!publicHostKey.VerifySignature(exchangeHash, ecdhReply.exchange_hash_signature))
{
throw new KeyExchangeFailedException("Signature does not match host key.");
}
byte[] sessionId = input.ConnectionInfo.SessionId ?? exchangeHash;
byte[] initialIVC2S = Hash(sequencePool, sharedSecret, exchangeHash, (byte)'A', sessionId, input.InitialIVC2SLength);
byte[] initialIVS2C = Hash(sequencePool, sharedSecret, exchangeHash, (byte)'B', sessionId, input.InitialIVS2CLength);
byte[] encryptionKeyC2S = Hash(sequencePool, sharedSecret, exchangeHash, (byte)'C', sessionId, input.EncryptionKeyC2SLength);
byte[] encryptionKeyS2C = Hash(sequencePool, sharedSecret, exchangeHash, (byte)'D', sessionId, input.EncryptionKeyS2CLength);
byte[] integrityKeyC2S = Hash(sequencePool, sharedSecret, exchangeHash, (byte)'E', sessionId, input.IntegrityKeyC2SLength);
byte[] integrityKeyS2C = Hash(sequencePool, sharedSecret, exchangeHash, (byte)'F', sessionId, input.IntegrityKeyS2CLength);
return(new KeyExchangeOutput(exchangeHash,
initialIVS2C, encryptionKeyS2C, integrityKeyS2C,
initialIVC2S, encryptionKeyC2S, integrityKeyC2S));
}
private async static Task PerformDefaultExchange(SshConnection connection, Packet clientKexInitMsg, Packet serverKexInitMsg, ILogger logger, SshClientSettings settings, SshConnectionInfo connectionInfo, CancellationToken ct)
{
// Key Exchange: https://tools.ietf.org/html/rfc4253#section-7.
SequencePool sequencePool = connection.SequencePool;
var remoteInit = ParseKeyExchangeInitMessage(serverKexInitMsg);
// The chosen algorithm MUST be the first algorithm on the client's name-list
// that is also on the server's name-list.
Name encC2S = ChooseAlgorithm(settings.EncryptionAlgorithmsClientToServer, remoteInit.encryption_algorithms_client_to_server);
Name encS2C = ChooseAlgorithm(settings.EncryptionAlgorithmsServerToClient, remoteInit.encryption_algorithms_server_to_client);
Name macC2S = ChooseAlgorithm(settings.MacAlgorithmsClientToServer, remoteInit.mac_algorithms_client_to_server);
Name macS2C = ChooseAlgorithm(settings.MacAlgorithmsServerToClient, remoteInit.mac_algorithms_server_to_client);
Name comC2S = ChooseAlgorithm(settings.CompressionAlgorithmsClientToServer, remoteInit.compression_algorithms_client_to_server);
Name comS2C = ChooseAlgorithm(settings.CompressionAlgorithmsServerToClient, remoteInit.compression_algorithms_server_to_client);
if (encC2S.IsEmpty || encS2C.IsEmpty || macC2S.IsEmpty || macS2C.IsEmpty || comC2S.IsEmpty || comS2C.IsEmpty)
{
ThrowHelper.ThrowKeyExchangeFailed("No common encryption/integrity/compression algorithm.");
}
// Make an ordered list of host key algorithms. The key exchange algorithm will pick a compatible one.
List <Name> hostKeyAlgorithms = new List <Name>(capacity: settings.ServerHostKeyAlgorithms.Count);
foreach (var hostKeyAlgorithm in settings.ServerHostKeyAlgorithms)
{
if (remoteInit.server_host_key_algorithms.Contains(hostKeyAlgorithm))
{
hostKeyAlgorithms.Add(hostKeyAlgorithm);
}
}
// The first algorithm MUST be the preferred (and guessed) algorithm. If
// both sides make the same guess, that algorithm MUST be used.
Name matchingKex = settings.KeyExchangeAlgorithms.Count == 0 ||
remoteInit.kex_algorithms.Length == 0 ||
settings.KeyExchangeAlgorithms[0] != remoteInit.kex_algorithms[0] ? default(Name) : settings.KeyExchangeAlgorithms[0];
KeyExchangeOutput?keyExchangeOutput = null;
Packet exchangeInitMsg = default;
try
{
if (remoteInit.first_kex_packet_follows)
{
exchangeInitMsg = await connection.ReceivePacketAsync(ct);
if (matchingKex.IsEmpty ||
settings.ServerHostKeyAlgorithms.Count == 0 ||
remoteInit.server_host_key_algorithms.Length == 0 ||
settings.ServerHostKeyAlgorithms[0] != remoteInit.server_host_key_algorithms[0])
{
// Silently ignore if guessed wrong.
exchangeInitMsg.Dispose();
exchangeInitMsg = default;
}
else
{
// Only accept the first hostKeyAlgorithm.
if (hostKeyAlgorithms.Count > 1)
{
hostKeyAlgorithms.RemoveRange(1, hostKeyAlgorithms.Count - 1);
}
}
}
EncryptionFactory.Default.GetKeyAndIVLength(encC2S, out int encryptionKeyC2SLength, out int initialIVC2SLength);
EncryptionFactory.Default.GetKeyAndIVLength(encS2C, out int encryptionKeyS2CLength, out int initialIVS2CLength);
int integrityKeyC2SLength = HMacFactory.Default.GetKeyLength(macC2S);
int integrityKeyS2CLength = HMacFactory.Default.GetKeyLength(macS2C);
var keyExchangeInput = new KeyExchangeInput(hostKeyAlgorithms, exchangeInitMsg, clientKexInitMsg, serverKexInitMsg, connectionInfo,
initialIVC2SLength, initialIVS2CLength, encryptionKeyC2SLength, encryptionKeyS2CLength, integrityKeyC2SLength, integrityKeyS2CLength);
foreach (var keyAlgorithm in settings.KeyExchangeAlgorithms)
{
if (remoteInit.kex_algorithms.Contains(keyAlgorithm))
{
logger.KeyExchangeAlgorithm(keyAlgorithm);
using (var algorithm = KeyExchangeAlgorithmFactory.Default.Create(keyAlgorithm))
{
keyExchangeOutput = await algorithm.TryExchangeAsync(connection, keyExchangeInput, logger, ct);
}
if (keyExchangeOutput != null)
{
connectionInfo.SessionId ??= keyExchangeOutput.ExchangeHash;
break;
}
// Preferred algorithm must be used.
if (!matchingKex.IsEmpty)
{
ThrowHelper.ThrowKeyExchangeFailed("Preferred key exchange algorithm failed.");
}
}
}
// If no algorithm satisfying all these conditions can be found, the
// connection fails, and both sides MUST disconnect.
if (keyExchangeOutput == null)
{
ThrowHelper.ThrowKeyExchangeFailed("Key exchange failed");
}
}
finally
{
exchangeInitMsg.Dispose();
}
logger.AlgorithmsServerToClient(encS2C, macS2C, comS2C);
logger.AlgorithmsClientToServer(encC2S, macC2S, comC2S);
// Send SSH_MSG_NEWKEYS.
{
using Packet newKeysMsg = CreateNewKeysMessage(sequencePool);
await connection.SendPacketAsync(newKeysMsg, ct);
}
// Receive SSH_MSG_NEWKEYS.
using Packet newKeysReceivedMsg = await connection.ReceivePacketAsync(ct);
ParseNewKeysMessage(newKeysReceivedMsg);
var encrypt = EncryptionFactory.Default.CreateEncryptor(encC2S, keyExchangeOutput.EncryptionKeyC2S, keyExchangeOutput.InitialIVC2S);
var macForEncoder = HMacFactory.Default.Create(macC2S, keyExchangeOutput.IntegrityKeyC2S);
var decrypt = EncryptionFactory.Default.CreateDecryptor(encS2C, keyExchangeOutput.EncryptionKeyS2C, keyExchangeOutput.InitialIVS2C);
var macForDecoder = HMacFactory.Default.Create(macS2C, keyExchangeOutput.IntegrityKeyS2C);
connection.SetEncoderDecoder(new PacketEncoder(encrypt, macForEncoder), new PacketDecoder(sequencePool, decrypt, macForDecoder));
