private byte[] Decrypt(SessionState sessionState, SignalMessage ciphertextMessage)
{
if (!sessionState.HasSenderChain())
{
throw new InvalidMessageException("Uninitialized session!");
}
if (ciphertextMessage.GetMessageVersion() != sessionState.GetSessionVersion())
{
throw new InvalidMessageException($"Message version {ciphertextMessage.GetMessageVersion()}, but session version {sessionState.GetSessionVersion()}");
}
uint messageVersion = ciphertextMessage.GetMessageVersion();
IEcPublicKey theirEphemeral = ciphertextMessage.GetSenderRatchetKey();
uint counter = ciphertextMessage.GetCounter();
ChainKey chainKey = GetOrCreateChainKey(sessionState, theirEphemeral);
MessageKeys messageKeys = GetOrCreateMessageKeys(sessionState, theirEphemeral,
chainKey, counter);
ciphertextMessage.VerifyMac(messageVersion,
sessionState.GetRemoteIdentityKey(),
sessionState.GetLocalIdentityKey(),
messageKeys.GetMacKey());
byte[] plaintext = GetPlaintext(messageVersion, messageKeys, ciphertextMessage.GetBody());
sessionState.ClearUnacknowledgedPreKeyMessage();
return(plaintext);
}