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DRBG: make reseed_counter not be off by 1

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Change `reseed_counter` to be the number of requests made since the last
reseed, rather than this number minus 1. Thus, reseed when
`reseed_counter >= reseed_interval` rather than
`reseed_counter > reseed_interval`. The field `reseed_counter` is private so
this is not an API change.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine
2026-01-29 13:01:06 +01:00
parent 48e37275ec
commit bd57d52490
3 changed files with 5 additions and 6 deletions
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3
include/mbedtls/ctr_drbg.h
View File

@@ -186,8 +186,7 @@ typedef struct mbedtls_ctr_drbg_context {
unsigned char MBEDTLS_PRIVATE(counter)[16]; /*!< The counter (V). */
int MBEDTLS_PRIVATE(reseed_counter); /*!< The reseed counter.
* This is the number of requests that have
* been made since the last (re)seeding,
* minus one.
* been made since the last (re)seeding.
* Before the initial seeding, this field
* contains the amount of entropy in bytes
* to use as a nonce for the initial seeding,

4
library/ctr_drbg.c
View File

@@ -494,7 +494,7 @@ static int mbedtls_ctr_drbg_reseed_internal(mbedtls_ctr_drbg_context *ctx,
if ((ret = ctr_drbg_update_internal(ctx, seed)) != 0) {
goto exit;
}
ctx->reseed_counter = 1;
ctx->reseed_counter = 0;
exit:
mbedtls_platform_zeroize(seed, sizeof(seed));
@@ -629,7 +629,7 @@ int mbedtls_ctr_drbg_random_with_add(void *p_rng,
memset(locals.add_input, 0, MBEDTLS_CTR_DRBG_SEEDLEN);
if (ctx->reseed_counter > ctx->reseed_interval ||
if (ctx->reseed_counter >= ctx->reseed_interval ||
ctx->prediction_resistance) {
if ((ret = mbedtls_ctr_drbg_reseed(ctx, additional, add_len)) != 0) {
return ret;

4
library/hmac_drbg.c
View File

@@ -196,7 +196,7 @@ static int hmac_drbg_reseed_core(mbedtls_hmac_drbg_context *ctx,
}
/* 3. Reset reseed_counter */
ctx->reseed_counter = 1;
ctx->reseed_counter = 0;
exit:
/* 4. Done */
@@ -326,7 +326,7 @@ int mbedtls_hmac_drbg_random_with_add(void *p_rng,
/* 1. (aka VII and IX) Check reseed counter and PR */
if (ctx->f_entropy != NULL && /* For no-reseeding instances */
(ctx->prediction_resistance == MBEDTLS_HMAC_DRBG_PR_ON ||
ctx->reseed_counter > ctx->reseed_interval)) {
ctx->reseed_counter >= ctx->reseed_interval)) {
if ((ret = mbedtls_hmac_drbg_reseed(ctx, additional, add_len)) != 0) {
return ret;
}