/// <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>
/// Tries to decrypt the given <paramref name="cipherContent"/>.
/// </summary>
/// <param name="authMsg">The <see cref="OmemoAuthenticatedMessage"/> that should be used for decrypting the <paramref name="cipherContent"/>.</param>
/// <param name="session">The <see cref="OmemoSessionModel"/> that should be used for decryption.</param>
/// <param name="cipherContent">The cipher text that should be decrypted.</param>
/// <returns>On success the plain text for the given <paramref name="cipherContent"/>.</returns>
public byte[] DecryptMessage(OmemoAuthenticatedMessage authMsg, OmemoSessionModel session, byte[] cipherContent)
{
byte[] keyHmac = DecryptKeyHmacForDevice(authMsg, session);
byte[] key = new byte[32];
Buffer.BlockCopy(keyHmac, 0, key, 0, key.Length);
byte[] hmacRef = new byte[16];
Buffer.BlockCopy(keyHmac, key.Length, hmacRef, 0, hmacRef.Length);
// 32 byte (256 bit) of salt. Initialized with 0s.
byte[] hkdfOutput = CryptoUtils.HkdfSha256(key, new byte[32], "OMEMO Payload", 80);
CryptoUtils.SplitKey(hkdfOutput, out byte[] encKey, out byte[] authKey, out byte[] iv);
byte[] hmac = CryptoUtils.HmacSha256(authKey, cipherContent);
hmac = CryptoUtils.Truncate(hmac, 16);
if (!hmacRef.SequenceEqual(hmac))
{
throw new OmemoException("Failed to decrypt. HMAC does not match.");
}
return(CryptoUtils.Aes256CbcDecrypt(encKey, iv, cipherContent));
}
