crypto: jitter - Add clarifying comments to Jitter Entropy RCT cutoff values

The RCT cutoff values are correct, but they don't exactly match the ones
one would expect when computing them using the formula in SP800-90B. This
discrepancy is due to the fact that the Jitter Entropy RCT starts at 1. To
avoid any confusion by future reviewers, add some comments and explicitly
subtract 1 from the "correct" cutoff values in the definitions.

Signed-off-by: Joachim Vandersmissen <git@jvdsn.com>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/crypto/jitterentropy.c b/crypto/jitterentropy.c
index c7d7f2c..fe9c233 100644 (file)
--- a/crypto/jitterentropy.c
+++ b/crypto/jitterentropy.c
@@ -89,10 +89,14 @@ struct rand_data {
        unsigned int rct_count;                 /* Number of stuck values */
 
        /* Intermittent health test failure threshold of 2^-30 */
-#define JENT_RCT_CUTOFF                30      /* Taken from SP800-90B sec 4.4.1 */
-#define JENT_APT_CUTOFF                325     /* Taken from SP800-90B sec 4.4.2 */
+       /* From an SP800-90B perspective, this RCT cutoff value is equal to 31. */
+       /* However, our RCT implementation starts at 1, so we subtract 1 here. */
+#define JENT_RCT_CUTOFF                (31 - 1)        /* Taken from SP800-90B sec 4.4.1 */
+#define JENT_APT_CUTOFF                325                     /* Taken from SP800-90B sec 4.4.2 */
        /* Permanent health test failure threshold of 2^-60 */
-#define JENT_RCT_CUTOFF_PERMANENT      60
+       /* From an SP800-90B perspective, this RCT cutoff value is equal to 61. */
+       /* However, our RCT implementation starts at 1, so we subtract 1 here. */
+#define JENT_RCT_CUTOFF_PERMANENT      (61 - 1)
 #define JENT_APT_CUTOFF_PERMANENT      355
 #define JENT_APT_WINDOW_SIZE   512     /* Data window size */
        /* LSB of time stamp to process */