/// <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>
/// 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>
/// Skippes receiving message keys <paramref name="until"/>.
/// </summary>
/// <param name="session">The <see cref="OmemoSessionModel"/> the keys should be skipped for.</param>
/// <param name="until">Until which key should be skipped.</param>
private void SkipMessageKeys(OmemoSessionModel session, uint until)
{
if (session.nR + OmemoSessionModel.MAX_SKIP < until)
{
throw new OmemoException("Failed to decrypt. Would skip to many message keys from " + session.nR + " to " + until + ", which is more than " + OmemoSessionModel.MAX_SKIP + '.');
}
if (!(session.ckR is null))
{
while (session.nR < until)
{
byte[] mk = LibSignalUtils.KDF_CK(session.ckR, 0x01);
Logger.Trace($"[{nameof(SkipMessageKeys)}] {nameof(mk)} - {session.nR}: {CryptoUtils.ToHexString(mk)}");
session.ckR = LibSignalUtils.KDF_CK(session.ckR, 0x02);
session.MK_SKIPPED.SetMessageKey(session.dhR, session.nR, mk);
++session.nR;
}
}
}
/// <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);
}