Re: [PATCH net-next v5 03/20] zinc: ChaCha20 generic C implementation and selftest

On Wed, Sep 19, 2018 at 3:08 AM Eric Biggers [off-list ref] wrote:

Does this consistently perform as well as an implementation that organizes the
operations such that the quarterrounds for all columns/diagonals are
interleaved?  As-is, there are tight dependencies in QUARTER_ROUND() (as well as
in the existing chacha20_block() in lib/chacha20.c, for that matter), so we're
heavily depending on the compiler to do the needed interleaving so as to not get
potentially disastrous performance.  Making it explicit could be a good idea.

It does perform as well, and the compiler outputs good code, even on
older compilers. Notably that's all a single statement (via the comma
operator).

quoted

+}
+
+static void chacha20_generic(u8 *out, const u8 *in, u32 len, const u32 key[8],
+                          const u32 counter[4])
+{
+     __le32 buf[CHACHA20_BLOCK_WORDS];
+     u32 x[] = {
+             EXPAND_32_BYTE_K,
+             key[0], key[1], key[2], key[3],
+             key[4], key[5], key[6], key[7],
+             counter[0], counter[1], counter[2], counter[3]
+     };
+
+     if (out != in)
+             memmove(out, in, len);
+
+     while (len >= CHACHA20_BLOCK_SIZE) {
+             chacha20_block_generic(buf, x);
+             crypto_xor(out, (u8 *)buf, CHACHA20_BLOCK_SIZE);
+             len -= CHACHA20_BLOCK_SIZE;
+             out += CHACHA20_BLOCK_SIZE;
+     }
+     if (len) {
+             chacha20_block_generic(buf, x);
+             crypto_xor(out, (u8 *)buf, len);
+     }
+}

If crypto_xor_cpy() is used instead of crypto_xor(), and 'in' is incremented
along with 'out', then the memmove() is not needed.

Nice idea, thanks. Implemented.

Jason