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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>
This commit is contained in:
Joachim Vandersmissen 2023-08-06 14:19:03 -05:00 committed by Herbert Xu
parent 9e47a758b7
commit 91cb1e1432
1 changed files with 7 additions and 3 deletions
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8
crypto/jitterentropy.c
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@ -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 */
/* 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 */