/// <summary>
/// Set up the ChaCha state with the given key. A 32-byte key is required
/// and enforced.
/// </summary>
/// <param name="key">
/// A 32-byte (256-bit) key, treated as a concatenation of eight 32-bit
/// little-endian integers
/// </param>
private void KeySetup(byte[] key)
{
if (key == null)
{
throw new ArgumentNullException("Key is null");
}
if (key.Length != 32)
{
throw new ArgumentException("Key length must be 32. Actual is " + key.Length.ToString());
}
// These are the same constants defined in the reference implementation
// see http://cr.yp.to/streamciphers/timings/estreambench/submissions/salsa20/chacha8/ref/chacha.c
int keyIndex = key.Length - 16;
fixed(uint *state = ctx.state)
fixed(byte *k = key)
{
state[0] = Chacha.Constants[0];
state[1] = Chacha.Constants[1];
state[2] = Chacha.Constants[2];
state[3] = Chacha.Constants[3];
state[4] = Chacha.U8To32Little(k, 0);
state[5] = Chacha.U8To32Little(k, 4);
state[6] = Chacha.U8To32Little(k, 8);
state[7] = Chacha.U8To32Little(k, 12);
state[8] = Chacha.U8To32Little(k, 16);
state[9] = Chacha.U8To32Little(k, 20);
state[10] = Chacha.U8To32Little(k, 24);
state[11] = Chacha.U8To32Little(k, 28);
}
}
/// <summary>
/// Set up the ChaCha state with the given nonce (aka Initialization Vector
/// or IV) and block counter. A 12-byte nonce and a 4-byte counter are
/// required and enforced.
/// </summary>
/// <param name="nonce">
/// A 12-byte (96-bit) nonce, treated as a concatenation of three 32-bit
/// little-endian integers
/// </param>
/// <param name="counter">
/// A 4-byte (32-bit) block counter, treated as a 32-bit little-endian integer
/// </param>
public void IVSetup(byte[] nonce, uint counter)
{
if (nonce == null)
{
throw new ArgumentNullException("Nonce is null");
}
if (nonce.Length != 12)
{
throw new ArgumentException(
"Nonce length should be 12. Actual is " + nonce.Length.ToString()
);
}
_iv = nonce;
fixed(byte *n = nonce)
{
ctx.state[12] = counter;
ctx.state[13] = Chacha.U8To32Little(n, 0);
ctx.state[14] = Chacha.U8To32Little(n, 4);
ctx.state[15] = Chacha.U8To32Little(n, 8);
}
}
private static void NextKeystreamBuffer(Context *ctx)
{
Chacha.ChachaBlockFunction(ctx->state, (uint *)ctx->keyStreamBuffer, 20 / 2);
ctx->state[12]++;
}