private static uint chacha_next32([NativeTypeName("mi_random_ctx_t*")] ref mi_random_ctx_t ctx)
{
if (ctx.output_available <= 0)
{
chacha_block(ref ctx);
ctx.output_available = 16; // (assign again to suppress static analysis warning)
}
uint x = ctx.output[16 - ctx.output_available];
ctx.output[16 - ctx.output_available] = 0; // reset once the data is handed out
ctx.output_available--;
return(x);
}
private static void chacha_block([NativeTypeName("mi_random_ctx_t*")] ref mi_random_ctx_t ctx)
{
// scramble into `x`
uint *x = stackalloc uint[16];
for (nuint i = 0; i < 16; i++)
{
x[i] = ctx.input[i];
}
for (nuint i = 0; i < MI_CHACHA_ROUNDS; i += 2)
{
qround(x, 0, 4, 8, 12);
qround(x, 1, 5, 9, 13);
qround(x, 2, 6, 10, 14);
qround(x, 3, 7, 11, 15);
qround(x, 0, 5, 10, 15);
qround(x, 1, 6, 11, 12);
qround(x, 2, 7, 8, 13);
qround(x, 3, 4, 9, 14);
}
// add scrambled data to the initial state
for (nuint i = 0; i < 16; i++)
{
ctx.output[i] = unchecked (x[i] + ctx.input[i]);
}
ctx.output_available = 16;
// increment the counter for the next round
ctx.input[12] += 1;
if (ctx.input[12] == 0)
{
ctx.input[13] += 1;
if (ctx.input[13] == 0)
{
// and keep increasing into the nonce
ctx.input[14] += 1;
}
}
}
private static void chacha_init([NativeTypeName("mi_random_ctx_t*")] out mi_random_ctx_t ctx, [NativeTypeName("const uint8_t*")] ReadOnlySpan <byte> key, [NativeTypeName("uint64_t")] ulong nonce)
{
// since we only use chacha for randomness (and not encryption) we
// do not _need_ to read 32-bit values as little endian but we do anyways
// just for being compatible :-)
ctx = default;
for (nuint i = 0; i < 4; i++)
{
ctx.input[i] = read32(sigma, i);
}
for (nuint i = 0; i < 8; i++)
{
ctx.input[i + 4] = read32(key, i);
}
ctx.input[12] = 0;
ctx.input[13] = 0;
ctx.input[14] = unchecked ((uint)nonce);
ctx.input[15] = (uint)(nonce >> 32);
}
// random.c
private static partial void _mi_random_init([NativeTypeName("mi_random_ctx_t*")] out mi_random_ctx_t ctx);
