public byte[] Unprotect(byte[] protectedData)
{
// The entire operation is wrapped in a 'checked' block because any overflows should be treated as failures.
checked
{
// We want to check that the input is in the form:
// protectedData := IV || Enc(Kenc, IV, clearData) || Sign(Kval, IV || Enc(Kenc, IV, clearData))
// Definitions used in this method:
// encryptedPayload := Enc(Kenc, IV, clearData)
// signature := Sign(Kval, IV || encryptedPayload)
// These SymmetricAlgorithm instances are single-use; we wrap it in a 'using' block.
using (SymmetricAlgorithm decryptionAlgorithm = _cryptoAlgorithmFactory.GetEncryptionAlgorithm())
{
decryptionAlgorithm.Key = _encryptionKey.GetKeyMaterial();
// These KeyedHashAlgorithm instances are single-use; we wrap it in a 'using' block.
using (KeyedHashAlgorithm validationAlgorithm = _cryptoAlgorithmFactory.GetValidationAlgorithm())
{
validationAlgorithm.Key = _validationKey.GetKeyMaterial();
// First, we need to verify that protectedData is even long enough to contain
// the required components (IV, encryptedPayload, signature).
int ivByteCount = decryptionAlgorithm.BlockSize / 8; // IV length is equal to the block size
int signatureByteCount = validationAlgorithm.HashSize / 8;
int encryptedPayloadByteCount = protectedData.Length - ivByteCount - signatureByteCount;
if (encryptedPayloadByteCount <= 0)
{
// protectedData doesn't meet minimum length requirements
return(null);
}
// If that check passes, we need to detect payload tampering.
// Compute the signature over the IV and encrypted payload
// computedSignature := Sign(Kval, IV || encryptedPayload)
byte[] computedSignature = validationAlgorithm.ComputeHash(protectedData, 0, ivByteCount + encryptedPayloadByteCount);
if (!CryptoUtil.BuffersAreEqual(
buffer1: protectedData, buffer1Offset: ivByteCount + encryptedPayloadByteCount, buffer1Count: signatureByteCount,
buffer2: computedSignature, buffer2Offset: 0, buffer2Count: computedSignature.Length))
{
// the computed signature didn't match the incoming signature, which is a sign of payload tampering
return(null);
}
// At this point, we're certain that we generated the signature over this payload,
// so we can go ahead with decryption.
// Populate the IV from the incoming stream
byte[] iv = new byte[ivByteCount];
Buffer.BlockCopy(protectedData, 0, iv, 0, iv.Length);
decryptionAlgorithm.IV = iv;
// Write the decrypted payload to the memory stream.
using (MemoryStream memStream = new MemoryStream())
{
using (ICryptoTransform decryptor = decryptionAlgorithm.CreateDecryptor())
{
using (CryptoStream cryptoStream = new CryptoStream(memStream, decryptor, CryptoStreamMode.Write))
{
cryptoStream.Write(protectedData, ivByteCount, encryptedPayloadByteCount);
cryptoStream.FlushFinalBlock();
// At this point
// memStream := clearData
byte[] clearData = memStream.ToArray();
return(clearData);
}
}
}
}
}
}
}
public byte[] Protect(byte[] clearData)
{
// The entire operation is wrapped in a 'checked' block because any overflows should be treated as failures.
checked
{
// These SymmetricAlgorithm instances are single-use; we wrap it in a 'using' block.
using (SymmetricAlgorithm encryptionAlgorithm = _cryptoAlgorithmFactory.GetEncryptionAlgorithm())
{
// Initialize the algorithm with the specified key and an appropriate IV
encryptionAlgorithm.Key = _encryptionKey.GetKeyMaterial();
if (_predictableIV)
{
// The caller wanted the output to be predictable (e.g. for caching), so we'll create an
// appropriate IV directly from the input buffer. The IV length is equal to the block size.
encryptionAlgorithm.IV = CryptoUtil.CreatePredictableIV(clearData, encryptionAlgorithm.BlockSize);
}
else
{
// If the caller didn't ask for a predictable IV, just let the algorithm itself choose one.
encryptionAlgorithm.GenerateIV();
}
byte[] iv = encryptionAlgorithm.IV;
using (MemoryStream memStream = new MemoryStream())
{
memStream.Write(iv, 0, iv.Length);
// At this point:
// memStream := IV
// Write the encrypted payload to the memory stream.
using (ICryptoTransform encryptor = encryptionAlgorithm.CreateEncryptor())
{
using (CryptoStream cryptoStream = new CryptoStream(memStream, encryptor, CryptoStreamMode.Write))
{
cryptoStream.Write(clearData, 0, clearData.Length);
cryptoStream.FlushFinalBlock();
// At this point:
// memStream := IV || Enc(Kenc, IV, clearData)
// These KeyedHashAlgorithm instances are single-use; we wrap it in a 'using' block.
using (KeyedHashAlgorithm signingAlgorithm = _cryptoAlgorithmFactory.GetValidationAlgorithm())
{
// Initialize the algorithm with the specified key
signingAlgorithm.Key = _validationKey.GetKeyMaterial();
// Compute the signature
ArraySegment <byte> segment;
memStream.TryGetBuffer(out segment);
byte[] signature = signingAlgorithm.ComputeHash(segment.Array, 0, (int)memStream.Length);
// At this point:
// memStream := IV || Enc(Kenc, IV, clearData)
// signature := Sign(Kval, IV || Enc(Kenc, IV, clearData))
// Append the signature to the encrypted payload
memStream.Write(signature, 0, signature.Length);
// At this point:
// memStream := IV || Enc(Kenc, IV, clearData) || Sign(Kval, IV || Enc(Kenc, IV, clearData))
// Algorithm complete
byte[] protectedData = memStream.ToArray();
return(protectedData);
}
}
}
}
}
}
}
