crypto: lib/sha256 - Clear W[] in sha256_update() instead of sha256_transform()

The temporary W[] array is currently zeroed out once every call to
sha256_transform(), i.e. once every 64 bytes of input data. Moving it to
sha256_update() instead so that it is cleared only once per update can
save about 2-3% of the total time taken to compute the digest, with a
reasonable memset() implementation, and considerably more (~20%) with a
bad one (eg the x86 purgatory currently uses a memset() coded in C).

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/lib/crypto/sha256.c b/lib/crypto/sha256.c
index 099cd11..c6bfeac 100644 (file)
--- a/lib/crypto/sha256.c
+++ b/lib/crypto/sha256.c
@@ -43,10 +43,9 @@ static inline void BLEND_OP(int I, u32 *W)
        W[I] = s1(W[I-2]) + W[I-7] + s0(W[I-15]) + W[I-16];
 }
 
-static void sha256_transform(u32 *state, const u8 *input)
+static void sha256_transform(u32 *state, const u8 *input, u32 *W)
 {
        u32 a, b, c, d, e, f, g, h, t1, t2;
-       u32 W[64];
        int i;
 
        /* load the input */
@@ -200,15 +199,13 @@ static void sha256_transform(u32 *state, const u8 *input)
 
        state[0] += a; state[1] += b; state[2] += c; state[3] += d;
        state[4] += e; state[5] += f; state[6] += g; state[7] += h;
-
-       /* clear any sensitive info... */
-       memzero_explicit(W, 64 * sizeof(u32));
 }
 
 void sha256_update(struct sha256_state *sctx, const u8 *data, unsigned int len)
 {
        unsigned int partial, done;
        const u8 *src;
+       u32 W[64];
 
        partial = sctx->count & 0x3f;
        sctx->count += len;
@@ -223,11 +220,13 @@ void sha256_update(struct sha256_state *sctx, const u8 *data, unsigned int len)
                }
 
                do {
-                       sha256_transform(sctx->state, src);
+                       sha256_transform(sctx->state, src, W);
                        done += 64;
                        src = data + done;
                } while (done + 63 < len);
 
+               memzero_explicit(W, sizeof(W));
+
                partial = 0;
        }
        memcpy(sctx->buf + partial, src, len - done);