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cprng(9): Fix accidental 4x seed size. 

With SHA-256, NIST Hash_DRBG takes an preferred 440-bit/55-byte seed.
It's a weird number, and I'm not sure where it comes from (a quick
skim of SP800-90A doesn't turn anything up), but it's certainly
sufficient (256-bit/32-byte seed is almost certainly enough) so it's
not a problem to use something larger; Hash_DRBG can absorb seeds of
arbitrary lengths and larger seeds can't really hurt security (with
minor caveats like HMAC RO quirks that don't apply here).

Except -- owing to a typo, we actually used a 1760-bit/220-byte seed,
because I wrote `uint32_t seed[...]' instead of `uint8_t seed[...]'.
Again: not a problem to use a seed larger than needed.  But let's
draw no more than we need out of the entropy pool!

Verified with CTASSERT(sizeof(seed) == 55).  (Assertion omitted from
this commit because we might swap out Hash_DRBG for something else
with a different seed size like 32 bytes.)
This commit is contained in:
riastradh 2022-05-13 09:40:25 +00:00
parent ec85059186
commit fc15b5cfe6
1 changed files with 3 additions and 3 deletions
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6
sys/kern/subr_cprng.c
View File

@ -1,4 +1,4 @@
/* $NetBSD: subr_cprng.c,v 1.42 2022/03/16 23:56:33 riastradh Exp $ */
/* $NetBSD: subr_cprng.c,v 1.43 2022/05/13 09:40:25 riastradh Exp $ */
/*-
* Copyright (c) 2019 The NetBSD Foundation, Inc.
@ -52,7 +52,7 @@
*/
#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: subr_cprng.c,v 1.42 2022/03/16 23:56:33 riastradh Exp $");
__KERNEL_RCSID(0, "$NetBSD: subr_cprng.c,v 1.43 2022/05/13 09:40:25 riastradh Exp $");
#include <sys/param.h>
#include <sys/types.h>
@ -268,7 +268,7 @@ cprng_fini_cpu(void *ptr, void *cookie, struct cpu_info *ci)
size_t
cprng_strong(struct cprng_strong *cprng, void *buf, size_t len, int flags)
{
uint32_t seed[NIST_HASH_DRBG_SEEDLEN_BYTES];
uint8_t seed[NIST_HASH_DRBG_SEEDLEN_BYTES];
struct cprng_cpu *cc;
unsigned epoch;
int s;