/// <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));
}
/// <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);
}
