crypto: ansi_cprng - fix inverted DT increment routine

The ANSI X9.31 PRNG docs aren't particularly clear on how to increment DT,
but empirical testing shows we're incrementing from the wrong end. A 10,000
iteration Monte Carlo RNG test currently winds up not getting the expected
result.

From http://csrc.nist.gov/groups/STM/cavp/documents/rng/RNGVS.pdf :

# CAVS 4.3
# ANSI931 MCT
[X9.31]
[AES 128-Key]

COUNT = 0
Key = 9f5b51200bf334b5d82be8c37255c848
DT = 6376bbe52902ba3b67c925fa701f11ac
V = 572c8e76872647977e74fbddc49501d1
R = 48e9bd0d06ee18fbe45790d5c3fc9b73

Currently, we get 0dd08496c4f7178bfa70a2161a79459a after 10000 loops.

Inverting the DT increment routine results in us obtaining the expected result
of 48e9bd0d06ee18fbe45790d5c3fc9b73. Verified on both x86_64 and ppc64.

Signed-off-by: Jarod Wilson <[email protected]>
Acked-by: Neil Horman <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>

crypto/ansi_cprng.c

@@ -161,7 +161,7 @@ static int _get_more_prng_bytes(struct prng_context *ctx)
 	/*
 	 * Now update our DT value
 	 */
-	for (i = 0; i < DEFAULT_BLK_SZ; i++) {
+	for (i = DEFAULT_BLK_SZ - 1; i >= 0; i--) {
 		ctx->DT[i] += 1;
 		if (ctx->DT[i] != 0)
 			break;