public static EcdhRatchetStep InitializeServer(IKeyDerivation kdf, IDigest digest,
IKeyAgreement previousKeyPair,
byte[] rootKey, ArraySegment <byte> remotePublicKey, IKeyAgreement keyPair,
byte[] receiveHeaderKey, byte[] sendHeaderKey)
{
if (receiveHeaderKey.Length != 32 || sendHeaderKey.Length != 32)
{
throw new InvalidOperationException("Keys need to be 32 bytes.");
}
Log.Verbose($"--Initialize ECDH Ratchet");
Log.Verbose($"Root Key: {Log.ShowBytes(rootKey)}");
Log.Verbose($"Prev ECDH Private: ({Log.ShowBytes(previousKeyPair.GetPublicKey())})");
Log.Verbose($"ECDH Public: {Log.ShowBytes(remotePublicKey)}");
Log.Verbose($"Curr ECDH Private: ({Log.ShowBytes(keyPair.GetPublicKey())})");
var e = new EcdhRatchetStep
{
EcdhKey = keyPair,
ReceiveHeaderKey = receiveHeaderKey,
SendHeaderKey = sendHeaderKey
};
// receive chain
Log.Verbose(" --Receiving Chain");
var rcderived = previousKeyPair.DeriveKey(remotePublicKey);
rcderived = digest.ComputeDigest(rcderived);
Log.Verbose($" C Input Key: {Log.ShowBytes(rootKey)}");
Log.Verbose($" C Key Info: {Log.ShowBytes(rcderived)}");
var rckeys = kdf.GenerateKeys(rcderived, rootKey, 3, 32);
Log.Verbose($" C Key Out 0: {Log.ShowBytes(rckeys[0])}");
Log.Verbose($" C Key Out 1: {Log.ShowBytes(rckeys[1])}");
Log.Verbose($" C Key Out 2: {Log.ShowBytes(rckeys[2])}");
rootKey = rckeys[0];
e.ReceivingChain.Initialize(rckeys[1]);
e.NextReceiveHeaderKey = rckeys[2];
// send chain
Log.Verbose(" --Sending Chain");
var scderived = keyPair.DeriveKey(remotePublicKey);
scderived = digest.ComputeDigest(scderived);
Log.Verbose($" C Input Key: {Log.ShowBytes(rootKey)}");
Log.Verbose($" C Key Info: {Log.ShowBytes(scderived)}");
var sckeys = kdf.GenerateKeys(scderived, rootKey, 3, 32);
Log.Verbose($" C Key Out 0: {Log.ShowBytes(sckeys[0])}");
Log.Verbose($" C Key Out 1: {Log.ShowBytes(sckeys[1])}");
Log.Verbose($" C Key Out 2: {Log.ShowBytes(sckeys[2])}");
rootKey = sckeys[0];
e.SendingChain.Initialize(sckeys[1]);
e.NextSendHeaderKey = sckeys[2];
// next root key
Log.Verbose($"Next Root Key: ({Log.ShowBytes(rootKey)})");
e.NextRootKey = rootKey;
return(e);
}
private byte[] SendInitializationRequest(State state)
{
// message format:
// nonce(16), pubkey(32), ecdh(32), padding(...), signature(64), mac(12)
if (!(state is ClientState clientState))
{
throw new InvalidOperationException("Only the client can send init request.");
}
// 16 bytes nonce
clientState.InitializationNonce = RandomNumberGenerator.Generate(InitializationNonceSize);
// get the public key
var pubkey = Signature.GetPublicKey();
// generate new ECDH keypair for init message and root key
IKeyAgreement clientEcdh = KeyAgreementFactory.GenerateNew();
clientState.LocalEcdhForInit = clientEcdh;
// nonce(16), <pubkey(32), ecdh(32), signature(64)>, mac(12)
var initializationMessageSize = InitializationNonceSize + EcNumSize * 4 + MacSize;
var messageSize = Math.Max(Configuration.MinimumMessageSize, initializationMessageSize);
var initializationMessageSizeWithSignature = messageSize - MacSize;
var initializationMessageSizeWithoutSignature = messageSize - MacSize - SignatureSize;
var signatureOffset = messageSize - MacSize - SignatureSize;
var message = new byte[messageSize];
Array.Copy(clientState.InitializationNonce, 0, message, 0, InitializationNonceSize);
Array.Copy(pubkey, 0, message, InitializationNonceSize, EcNumSize);
Array.Copy(clientEcdh.GetPublicKey(), 0, message, InitializationNonceSize + EcNumSize, EcNumSize);
// sign the message
var digest = Digest.ComputeDigest(message, 0, initializationMessageSizeWithoutSignature);
Array.Copy(Signature.Sign(digest), 0, message, signatureOffset, SignatureSize);
// encrypt the message with the application key
var cipher = new AesCtrMode(AesFactory.GetAes(true, Configuration.ApplicationKey), clientState.InitializationNonce);
var encryptedPayload = cipher.Process(message, InitializationNonceSize, initializationMessageSizeWithSignature - InitializationNonceSize);
Array.Copy(encryptedPayload, 0, message, InitializationNonceSize, initializationMessageSizeWithSignature - InitializationNonceSize);
// calculate mac
var Mac = new Poly(AesFactory);
Mac.Init(Configuration.ApplicationKey, clientState.InitializationNonce, MacSize);
Mac.Process(message, 0, initializationMessageSizeWithSignature);
var mac = Mac.Compute();
Array.Copy(mac, 0, message, initializationMessageSizeWithSignature, MacSize);
return(message);
}
public static EcdhRatchetStep[] InitializeClient(IKeyDerivation kdf, IDigest digest,
byte[] rootKey, ArraySegment <byte> remotePublicKey0, ArraySegment <byte> remotePublicKey1, IKeyAgreement keyPair,
byte[] receiveHeaderKey, byte[] sendHeaderKey,
IKeyAgreement nextKeyPair)
{
if (receiveHeaderKey.Length != 32 || sendHeaderKey.Length != 32)
{
throw new InvalidOperationException("Keys need to be 32 bytes.");
}
Log.Verbose($"--Initialize ECDH Ratchet CLIENT");
Log.Verbose($"Root Key: {Log.ShowBytes(rootKey)}");
Log.Verbose($"ECDH Public 0: {Log.ShowBytes(remotePublicKey0)}");
Log.Verbose($"ECDH Public 1: {Log.ShowBytes(remotePublicKey1)}");
Log.Verbose($"ECDH Private: ({Log.ShowBytes(keyPair.GetPublicKey())})");
var e0 = new EcdhRatchetStep
{
EcdhKey = keyPair,
SendHeaderKey = sendHeaderKey
};
// receive chain doesn't exist
Log.Verbose(" --Receiving Chain");
// send chain
Log.Verbose(" --Sending Chain");
var scderived = keyPair.DeriveKey(remotePublicKey0);
scderived = digest.ComputeDigest(scderived);
Log.Verbose($" C Input Key: {Log.ShowBytes(rootKey)}");
Log.Verbose($" C Key Info: {Log.ShowBytes(scderived)}");
var sckeys = kdf.GenerateKeys(scderived, rootKey, 3, 32);
Log.Verbose($" C Key Out 0: {Log.ShowBytes(sckeys[0])}");
Log.Verbose($" C Key Out 1: {Log.ShowBytes(sckeys[1])}");
Log.Verbose($" C Key Out 2: {Log.ShowBytes(sckeys[2])}");
rootKey = sckeys[0];
e0.SendingChain.Initialize(sckeys[1]);
var nextSendHeaderKey = sckeys[2];
var e1 = InitializeServer(kdf, digest,
keyPair,
rootKey,
remotePublicKey1,
nextKeyPair,
receiveHeaderKey,
nextSendHeaderKey);
return(new[] { e0, e1 });
}
private byte[] SendInitializationResponse(State state, ArraySegment <byte> initializationNonce, ArraySegment <byte> remoteEcdhForInit)
{
// message format:
// new nonce(16), ecdh pubkey(32),
// <nonce from init request(16), server pubkey(32),
// new ecdh pubkey(32) x2, Padding(...), signature(64)>, mac(12) = 236 bytes
if (!(state is ServerState serverState))
{
throw new InvalidOperationException("Only the server can send init response.");
}
// generate a nonce and new ecdh parms
var serverNonce = RandomNumberGenerator.Generate(InitializationNonceSize);
serverState.NextInitializationNonce = serverNonce;
IKeyAgreement rootPreEcdh = KeyAgreementFactory.GenerateNew();
var rootPreEcdhPubkey = rootPreEcdh.GetPublicKey();
// generate server ECDH for root key and root key
var rootPreKey = rootPreEcdh.DeriveKey(remoteEcdhForInit);
rootPreKey = Digest.ComputeDigest(rootPreKey);
var genKeys = KeyDerivation.GenerateKeys(rootPreKey, serverNonce, 3, EcNumSize);
serverState.RootKey = genKeys[0];
serverState.FirstSendHeaderKey = genKeys[1];
serverState.FirstReceiveHeaderKey = genKeys[2];
// generate two server ECDH. One for ratchet 0 sending key and one for the next
// this is enough for the server to generate a receiving chain key and sending
// chain key as soon as the client sends a sending chain key
IKeyAgreement serverEcdhRatchet0 = KeyAgreementFactory.GenerateNew();
serverState.LocalEcdhRatchetStep0 = serverEcdhRatchet0;
IKeyAgreement serverEcdhRatchet1 = KeyAgreementFactory.GenerateNew();
serverState.LocalEcdhRatchetStep1 = serverEcdhRatchet1;
var minimumMessageSize = InitializationNonceSize * 2 + EcNumSize * 6 + MacSize;
var entireMessageSize = Math.Max(Configuration.MinimumMessageSize, minimumMessageSize);
var macOffset = entireMessageSize - MacSize;
var entireMessageWithoutMacOrSignatureSize = macOffset - SignatureSize;
var encryptedPayloadOffset = InitializationNonceSize + EcNumSize;
var encryptedPayloadSize = macOffset - encryptedPayloadOffset;
// construct the message
var message = new byte[entireMessageSize];
Array.Copy(serverNonce, 0, message, 0, InitializationNonceSize);
Array.Copy(rootPreEcdhPubkey, 0, message, InitializationNonceSize, EcNumSize);
// construct the to-be-encrypted part
var rre0 = serverEcdhRatchet0.GetPublicKey();
var rre1 = serverEcdhRatchet1.GetPublicKey();
Array.Copy(initializationNonce.Array, initializationNonce.Offset, message, encryptedPayloadOffset, InitializationNonceSize);
Array.Copy(Signature.GetPublicKey(), 0, message, encryptedPayloadOffset + InitializationNonceSize, EcNumSize);
Array.Copy(rre0, 0, message, encryptedPayloadOffset + InitializationNonceSize + EcNumSize, EcNumSize);
Array.Copy(rre1, 0, message, encryptedPayloadOffset + InitializationNonceSize + EcNumSize * 2, EcNumSize);
// sign the message
var digest = Digest.ComputeDigest(message, 0, entireMessageWithoutMacOrSignatureSize);
Array.Copy(Signature.Sign(digest), 0, message, entireMessageWithoutMacOrSignatureSize, SignatureSize);
// encrypt the message
var cipher = new AesCtrMode(AesFactory.GetAes(true, rootPreKey), serverNonce);
var encryptedPayload = cipher.Process(message, encryptedPayloadOffset, encryptedPayloadSize);
Array.Copy(encryptedPayload, 0, message, encryptedPayloadOffset, encryptedPayloadSize);
// encrypt the header
cipher = new AesCtrMode(AesFactory.GetAes(true, Configuration.ApplicationKey), encryptedPayload, encryptedPayloadSize - HeaderIVSize, HeaderIVSize);
var encryptedHeader = cipher.Process(message, 0, encryptedPayloadOffset);
Array.Copy(encryptedHeader, 0, message, 0, encryptedPayloadOffset);
// calculate mac
var Mac = new Poly(AesFactory);
Mac.Init(Configuration.ApplicationKey, encryptedHeader, 0, InitializationNonceSize, MacSize);
Mac.Process(message, 0, macOffset);
var mac = Mac.Compute();
Array.Copy(mac, 0, message, macOffset, MacSize);
return(message);
}
public byte[] GetPublicKey() => EcdhKey.GetPublicKey();