/// <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) {
// Algorithm:
// T_0 = SHA256(buffer)
// T_n = SHA256(T_{n-1})
// output = T_0 || T_1 || ... || T_n (as many blocks as necessary to reach 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;
}
/// <summary>
/// Computes the SHA256 hash of a given segment in a buffer.
/// </summary>
/// <param name="buffer">The buffer over which to compute the hash.</param>
/// <param name="offset">The offset at which to begin computing the hash.</param>
/// <param name="count">The number of bytes in the buffer to include in the hash.</param>
/// <returns>The binary hash (32 bytes) of the buffer segment.</returns>
public static byte[] ComputeSHA256Hash(byte[] buffer, int offset, int count) {
using (SHA256 sha256 = CryptoAlgorithms.CreateSHA256()) {
return sha256.ComputeHash(buffer, offset, count);
}
}
