/// <summary>
/// Decrypts the key and HMAC concatenation (<see cref="OmemoMessage.cipherText"/>) with the given <paramref name="session"/>.
/// </summary>
/// <param name="authMsg">The <see cref="OmemoAuthenticatedMessage"/> containing the key and HMAC.</param>
/// <param name="session">The <see cref="OmemoSessionModel"/> that should be used for decryption.</param>
/// <returns>key || HMAC</returns>
private byte[] DecryptKeyHmacForDevice(OmemoAuthenticatedMessage authMsg, OmemoSessionModel session)
{
OmemoMessage msg = new OmemoMessage(authMsg.OMEMO_MESSAGE);
byte[] plainText = TrySkippedMessageKeys(msg, authMsg.HMAC, session);
if (!(plainText is null))
{
return(plainText);
}
if (session.dhR is null || !msg.DH.Equals(session.dhR.pubKey))
{
SkipMessageKeys(session, msg.PN);
session.InitDhRatchet(msg);
}
SkipMessageKeys(session, msg.N);
// If no receive chain has been initialized yet, initialize it now.
// This happens in case we received a key exchange message with a new session and now would like to send our first message.
if (session.ckR is null)
{
session.InitReceiverKeyChain();
}
byte[] mk = LibSignalUtils.KDF_CK(session.ckR, 0x01);
session.ckR = LibSignalUtils.KDF_CK(session.ckR, 0x02);
++session.nR;
return(DecryptKeyHmacForDevice(mk, msg, authMsg.HMAC, session.assData));
}
public void Test_OmemoKeyExchangeMessage()
{
Random rand = new Random();
byte[] hmac = new byte[16];
rand.NextBytes(hmac);
byte[] sessionKey = new byte[32];
rand.NextBytes(sessionKey);
IdentityKeyPairModel identityKey = KeyHelper.GenerateIdentityKeyPair();
SignedPreKeyModel signedPreKey = KeyHelper.GenerateSignedPreKey(0, identityKey.privKey);
OmemoSessionModel refOmemoSession = new OmemoSessionModel(new Bundle()
{
identityKey = identityKey.pubKey,
preKeys = KeyHelper.GeneratePreKeys(0, 10),
preKeySignature = signedPreKey.signature,
signedPreKey = signedPreKey.preKey.pubKey,
signedPreKeyId = signedPreKey.preKey.keyId
}, 0, KeyHelper.GenerateIdentityKeyPair())
{
ek = KeyHelper.GenerateKeyPair().pubKey
};
OmemoMessage refOmemoMessage = new OmemoMessage(refOmemoSession);
OmemoAuthenticatedMessage refOmemoAuthenticatedMessage = new OmemoAuthenticatedMessage(new byte[16], refOmemoMessage.ToByteArray());
OmemoKeyExchangeMessage refOmemoKeyExchangeMessage = new OmemoKeyExchangeMessage((uint)rand.Next(), (uint)rand.Next(), KeyHelper.GenerateKeyPair().pubKey, KeyHelper.GenerateKeyPair().pubKey, refOmemoAuthenticatedMessage);
byte[] data = refOmemoKeyExchangeMessage.ToByteArray();
Assert.IsTrue(data.Length > 0);
OmemoKeyExchangeMessage omemoKeyExchangeMessage = new OmemoKeyExchangeMessage(data);
Assert.IsTrue(omemoKeyExchangeMessage.Equals(refOmemoKeyExchangeMessage));
}
/// <summary>
/// Decrypts the key and HMAC concatenation (<see cref="OmemoMessage.cipherText"/>) with the given <paramref name="mk"/> and returns the result.
/// </summary>
/// <param name="mk">The message key that should be used for decryption.</param>
/// <param name="msg">The <see cref="OmemoMessage"/> containing the key and HMAC concatenation (<see cref="OmemoMessage.cipherText"/>).</param>
/// <param name="msgHmac">The HMAC of the <paramref name="msg"/>.</param>
/// <param name="assData">Encode(IK_A) || Encode(IK_B) => Concatenation of Alices and Bobs public part of their identity key.</param>
/// <returns>key || HMAC</returns>
private byte[] DecryptKeyHmacForDevice(byte[] mk, OmemoMessage msg, byte[] msgHmac, byte[] assData)
{
// 32 byte (256 bit) of salt. Initialized with 0s.
byte[] hkdfOutput = CryptoUtils.HkdfSha256(mk, new byte[32], "OMEMO Message Key Material", 80);
CryptoUtils.SplitKey(hkdfOutput, out byte[] encKey, out byte[] authKey, out byte[] iv);
byte[] hmacInput = CryptoUtils.Concat(assData, msg.ToByteArray());
byte[] hmacResult = CryptoUtils.HmacSha256(authKey, hmacInput);
byte[] hmacTruncated = CryptoUtils.Truncate(hmacResult, 16);
// Debug trace output:
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(mk) + ": " + CryptoUtils.ToHexString(mk));
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(msgHmac) + ": " + CryptoUtils.ToHexString(msgHmac));
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(assData) + ": " + CryptoUtils.ToHexString(assData));
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(hkdfOutput) + ": " + CryptoUtils.ToHexString(hkdfOutput));
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(encKey) + ": " + CryptoUtils.ToHexString(encKey));
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(authKey) + ": " + CryptoUtils.ToHexString(authKey));
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(iv) + ": " + CryptoUtils.ToHexString(iv));
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(hmacInput) + ": " + CryptoUtils.ToHexString(hmacInput));
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(hmacResult) + ": " + CryptoUtils.ToHexString(hmacResult));
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(hmacTruncated) + ": " + CryptoUtils.ToHexString(hmacTruncated));
if (!hmacTruncated.SequenceEqual(msgHmac))
{
throw new OmemoException("Failed to decrypt. HMAC of OmemoMessage does not match.");
}
return(CryptoUtils.Aes256CbcDecrypt(encKey, iv, msg.cipherText));
}
/// <summary>
/// Decrypts the key and HMAC concatenation (<see cref="OmemoMessage.cipherText"/>) with the given <paramref name="session"/>.
/// </summary>
/// <param name="authMsg">The <see cref="OmemoAuthenticatedMessage"/> containing the key and HMAC.</param>
/// <param name="session">The <see cref="OmemoSessionModel"/> that should be used for decryption.</param>
/// <returns>key || HMAC</returns>
public byte[] DecryptKeyHmacForDevice(OmemoAuthenticatedMessage authMsg, OmemoSessionModel session)
{
OmemoMessage msg = new OmemoMessage(authMsg.OMEMO_MESSAGE);
byte[] plainText = TrySkippedMessageKeys(msg, authMsg.HMAC, session);
if (!(plainText is null))
{
return(plainText);
}
if (session.dhR is null || !msg.DH.Equals(session.dhR))
{
SkipMessageKeys(session, msg.PN);
session.InitDhRatchet(msg);
}
SkipMessageKeys(session, msg.N);
byte[] mk = LibSignalUtils.KDF_CK(session.ckR, 0x01);
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(session.ckR) + ": " + CryptoUtils.ToHexString(session.ckR));
session.ckR = LibSignalUtils.KDF_CK(session.ckR, 0x02);
++session.nR;
byte[] result = DecryptKeyHmacForDevice(mk, msg, authMsg.HMAC, session.assData);
// Update the session state:
if (session.state == SessionState.SEND)
{
session.state = SessionState.READY;
}
Logger.Trace("[" + nameof(DecryptKeyHmacForDevice) + "] " + nameof(session.state) + ": " + session.state);
return(result);
}
/// <summary>
/// Encrypts the given key and HMAC concatenation and returns the result.
/// </summary>
/// <param name="keyHmac">The key, HMAC concatenation result.</param>
/// <param name="session">The <see cref="OmemoSessionModel"/> between the sender and receiver.</param>
/// <param name="assData">Encode(IK_A) || Encode(IK_B) => Concatenation of Alices and Bobs public part of their identity key.</param>
private OmemoAuthenticatedMessage EncryptKeyHmacForDevices(byte[] keyHmac, OmemoSessionModel session, byte[] assData)
{
byte[] mk = LibSignalUtils.KDF_CK(session.ckS, 0x01);
session.ckS = LibSignalUtils.KDF_CK(session.ckS, 0x02);
OmemoMessage omemoMessage = new OmemoMessage(session);
++session.nS;
// 32 byte (256 bit) of salt. Initialized with 0s.
byte[] hkdfOutput = CryptoUtils.HkdfSha256(mk, new byte[32], "OMEMO Message Key Material", 80);
CryptoUtils.SplitKey(hkdfOutput, out byte[] encKey, out byte[] authKey, out byte[] iv);
omemoMessage.cipherText = CryptoUtils.Aes256CbcEncrypt(encKey, iv, keyHmac);
byte[] omemoMessageBytes = omemoMessage.ToByteArray();
byte[] hmacInput = CryptoUtils.Concat(assData, omemoMessageBytes);
byte[] hmacResult = CryptoUtils.HmacSha256(authKey, hmacInput);
byte[] hmacTruncated = CryptoUtils.Truncate(hmacResult, 16);
return(new OmemoAuthenticatedMessage(hmacTruncated, omemoMessageBytes));
}
/// <summary>
/// Encrypts the given key and HMAC concatenation and returns the result.
/// </summary>
/// <param name="keyHmac">The key, HMAC concatenation result.</param>
/// <param name="session">The <see cref="OmemoSessionModel"/> between the sender and receiver.</param>
/// <param name="assData">Encode(IK_A) || Encode(IK_B) => Concatenation of Alices and Bobs public part of their identity key.</param>
private OmemoAuthenticatedMessage EncryptKeyHmacForDevices(byte[] keyHmac, OmemoSessionModel session, byte[] assData)
{
byte[] mk = LibSignalUtils.KDF_CK(session.ckS, 0x01);
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(session.ckS) + ": " + CryptoUtils.ToHexString(session.ckS));
session.ckS = LibSignalUtils.KDF_CK(session.ckS, 0x02);
OmemoMessage omemoMessage = new OmemoMessage(session);
++session.nS;
// 32 byte (256 bit) of salt. Initialized with 0s.
byte[] hkdfOutput = CryptoUtils.HkdfSha256(mk, new byte[32], "OMEMO Message Key Material", 80);
CryptoUtils.SplitKey(hkdfOutput, out byte[] encKey, out byte[] authKey, out byte[] iv);
omemoMessage.cipherText = CryptoUtils.Aes256CbcEncrypt(encKey, iv, keyHmac);
byte[] omemoMessageBytes = omemoMessage.ToByteArray();
byte[] hmacInput = CryptoUtils.Concat(assData, omemoMessageBytes);
byte[] hmacResult = CryptoUtils.HmacSha256(authKey, hmacInput);
byte[] hmacTruncated = CryptoUtils.Truncate(hmacResult, 16);
// Update the session state:
if (session.state == SessionState.RECEIVED)
{
session.state = SessionState.READY;
}
// Debug trace output:
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(mk) + ": " + CryptoUtils.ToHexString(mk));
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(assData) + ": " + CryptoUtils.ToHexString(assData));
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(hkdfOutput) + ": " + CryptoUtils.ToHexString(hkdfOutput));
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(encKey) + ": " + CryptoUtils.ToHexString(encKey));
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(authKey) + ": " + CryptoUtils.ToHexString(authKey));
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(iv) + ": " + CryptoUtils.ToHexString(iv));
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(hmacInput) + ": " + CryptoUtils.ToHexString(hmacInput));
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(hmacResult) + ": " + CryptoUtils.ToHexString(hmacResult));
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(hmacTruncated) + ": " + CryptoUtils.ToHexString(hmacTruncated));
Logger.Trace("[" + nameof(EncryptKeyHmacForDevices) + "] " + nameof(session.state) + ": " + session.state);
return(new OmemoAuthenticatedMessage(hmacTruncated, omemoMessageBytes));
}
/// <summary>
/// Tries a skipped message key for decrypting the key and HMAC concatenation (<see cref="OmemoMessage.cipherText"/>).
/// </summary>
/// <param name="msg">The <see cref="OmemoMessage"/> containing the key and HMAC concatenation (<see cref="OmemoMessage.cipherText"/>).</param>
/// <param name="msgHmac">The HMAC of the <paramref name="msg"/>.</param>
/// <param name="session">The <see cref="OmemoSessionModel"/> that should be used for decryption.</param>
/// <returns>key || HMAC on success, else null.</returns>
private byte[] TrySkippedMessageKeys(OmemoMessage msg, byte[] msgHmac, OmemoSessionModel session)
{
byte[] mk = session.MK_SKIPPED.GetMessagekey(msg.DH, msg.N);
return(!(mk is null) ? DecryptKeyHmacForDevice(mk, msg, msgHmac, session.assData) : null);
}