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 <jarod@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
diff --git a/crypto/ansi_cprng.c b/crypto/ansi_cprng.c
index 1b3b1da..0fac8ff 100644
--- a/crypto/ansi_cprng.c
+++ b/crypto/ansi_cprng.c
@@ -161,7 +161,7 @@
/*
* 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;