private static CryptographicKey DeriveKey(CryptographicKey keyDerivationKey) { using (HMACSHA512 hmac = CryptoAlgorithms.CreateHMACSHA512(keyDerivationKey.GetKeyMaterial())) { byte[] label, context; GetKeyDerivationParameters(out label, out context); byte[] derivedKey = DeriveKeyImpl(hmac, label, context, keyDerivationKey.KeyLength); return(new CryptographicKey(derivedKey)); } }
/// <summary> /// Returns an IV that's based solely on the contents of a buffer; useful for generating /// predictable IVs for ciphertexts that need to be cached. The output value is only /// appropriate for use as an IV and must not be used for any other purpose. /// </summary> /// <remarks>This method uses an iterated unkeyed SHA256 to calculate the IV.</remarks> /// <param name="buffer">The input buffer over which to calculate the IV.</param> /// <param name="ivBitLength">The requested length (in bits) of the IV to generate.</param> /// <returns>The calculated IV.</returns> public static byte[] CreatePredictableIV(byte[] buffer, int ivBitLength) { byte[] output = new byte[ivBitLength / 8]; int bytesCopied = 0; int bytesRemaining = output.Length; using (SHA256 sha256 = CryptoAlgorithms.CreateSHA256()) { while (bytesRemaining > 0) { byte[] hashed = sha256.ComputeHash(buffer); int bytesToCopy = Math.Min(bytesRemaining, hashed.Length); Buffer.BlockCopy(hashed, 0, output, bytesCopied, bytesToCopy); bytesCopied += bytesToCopy; bytesRemaining -= bytesToCopy; buffer = hashed; // next iteration (if it occurs) will operate over the block just hashed } } return(output); }