Re: [PATCH v2 2/2] random: use BLAKE2s instead of SHA1 in extraction

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Tue, Jan 11, 2022 at 1:28 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> If you're really quite concerned about m68k code size, I can probably
> do some things to reduce that. For example, blake2s256_hmac is only
> used by wireguard and it could probably be made local there. And with
> some trivial loop re-rolling, I can shave off another 2300 bytes. And
> I bet I can find a few other things too. The question is: how
> important is this to you?

And with another trick (see below), another extra 1000 bytes or so
shaved off. Aside from moving blake2s256_hmac, I'm not really super
enthusiastic about making these changes, but depending on how important
this is to you, maybe we can make something work. There are probably
additional possibilities too with the code.

====

diff --git a/lib/crypto/blake2s-generic.c b/lib/crypto/blake2s-generic.c
index 75ccb3e633e6..8e3c6372363a 100644
--- a/lib/crypto/blake2s-generic.c
+++ b/lib/crypto/blake2s-generic.c
@@ -46,7 +46,7 @@ void blake2s_compress_generic(struct blake2s_state *state, const u8 *block,
 {
 	u32 m[16];
 	u32 v[16];
-	int i;
+	int i, j;
 
 	WARN_ON(IS_ENABLED(DEBUG) &&
 		(nblocks > 1 && inc != BLAKE2S_BLOCK_SIZE));
@@ -76,29 +76,11 @@ void blake2s_compress_generic(struct blake2s_state *state, const u8 *block,
 	b = ror32(b ^ c, 7); \
 } while (0)
 
-#define ROUND(r) do { \
-	G(r, 0, v[0], v[ 4], v[ 8], v[12]); \
-	G(r, 1, v[1], v[ 5], v[ 9], v[13]); \
-	G(r, 2, v[2], v[ 6], v[10], v[14]); \
-	G(r, 3, v[3], v[ 7], v[11], v[15]); \
-	G(r, 4, v[0], v[ 5], v[10], v[15]); \
-	G(r, 5, v[1], v[ 6], v[11], v[12]); \
-	G(r, 6, v[2], v[ 7], v[ 8], v[13]); \
-	G(r, 7, v[3], v[ 4], v[ 9], v[14]); \
-} while (0)
-		ROUND(0);
-		ROUND(1);
-		ROUND(2);
-		ROUND(3);
-		ROUND(4);
-		ROUND(5);
-		ROUND(6);
-		ROUND(7);
-		ROUND(8);
-		ROUND(9);
-
+		for (i = 0; i < 10; ++i) {
+			for (j = 0; j < 8; ++j)
+				G(i, j, v[j % 4], v[((j + (j / 4)) % 4) + 4], v[((j + 2 * (j / 4)) % 4) + 8], v[((j + 3 * (j / 4)) % 4) + 12]);
+		}
 #undef G
-#undef ROUND
 
 		for (i = 0; i < 8; ++i)
 			state->h[i] ^= v[i] ^ v[i + 8];