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HMAC_DRBG: support set_entropy_len() before seed()

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mbedtls_hmac_drbg_seed() always set the entropy length to the default,
so a call to mbedtls_hmac_drbg_set_entropy_len() before seed() had no
effect. Change this to the more intuitive behavior that
set_entropy_len() sets the entropy length and seed() respects that and
only uses the default entropy length if there was no call to
set_entropy_len().
This commit is contained in:
Gilles Peskine 2019-10-04 11:47:35 +02:00 committed by Simon Butcher
parent 6e2cb64a97
commit 1d2a9e88c3
2 changed files with 20 additions and 24 deletions
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19
include/mbedtls/hmac_drbg.h
View File

@ -139,13 +139,11 @@ void mbedtls_hmac_drbg_init( mbedtls_hmac_drbg_context *ctx );
* Note that SHA-256 is just as efficient as SHA-224.
* The security strength can be reduced if a smaller
* entropy length is set with
* mbedtls_hmac_drbg_set_entropy_len() afterwards.
* mbedtls_hmac_drbg_set_entropy_len().
*
* \note The entropy length for the initial seeding is
* the security strength (converted from bits to bytes).
* You can set a different entropy length for subsequent
* seeding by calling mbedtls_hmac_drbg_set_entropy_len()
* after this function.
* \note The default entropy length is the security strength
* (converted from bits to bytes). You can override
* it by calling mbedtls_hmac_drbg_set_entropy_len().
*
* \note During the initial seeding, this function calls
* the entropy source to obtain a nonce
@ -224,14 +222,9 @@ void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx
/**
* \brief This function sets the amount of entropy grabbed on each
* reseed.
* seed or reseed.
*
* The default value is set by mbedtls_hmac_drbg_seed().
*
* \note mbedtls_hmac_drbg_seed() always sets the entropy length
* to the default value based on the chosen MD algorithm,
* so this function only has an effect if it is called
* after mbedtls_hmac_drbg_seed().
* See the documentation of mbedtls_hmac_drbg_seed() for the default value.
*
* \param ctx The HMAC_DRBG context.
* \param len The amount of entropy to grab, in bytes.

25
library/hmac_drbg.c
View File

@ -320,16 +320,19 @@ int mbedtls_hmac_drbg_seed( mbedtls_hmac_drbg_context *ctx,
ctx->reseed_interval = MBEDTLS_HMAC_DRBG_RESEED_INTERVAL;
/*
* See SP800-57 5.6.1 (p. 65-66) for the security strength provided by
* each hash function, then according to SP800-90A rev1 10.1 table 2,
* min_entropy_len (in bits) is security_strength.
*
* (This also matches the sizes used in the NIST test vectors.)
*/
ctx->entropy_len = md_size <= 20 ? 16 : /* 160-bits hash -> 128 bits */
md_size <= 28 ? 24 : /* 224-bits hash -> 192 bits */
32; /* better (256+) -> 256 bits */
if( ctx->entropy_len == 0 )
{
/*
* See SP800-57 5.6.1 (p. 65-66) for the security strength provided by
* each hash function, then according to SP800-90A rev1 10.1 table 2,
* min_entropy_len (in bits) is security_strength.
*
* (This also matches the sizes used in the NIST test vectors.)
*/
ctx->entropy_len = md_size <= 20 ? 16 : /* 160-bits hash -> 128 bits */
md_size <= 28 ? 24 : /* 224-bits hash -> 192 bits */
32; /* better (256+) -> 256 bits */
}
if( ( ret = hmac_drbg_reseed_core( ctx, custom, len, HMAC_NONCE_YES ) ) != 0 )
{
@ -349,7 +352,7 @@ void mbedtls_hmac_drbg_set_prediction_resistance( mbedtls_hmac_drbg_context *ctx
}
/*
* Set entropy length grabbed for reseeds
* Set entropy length grabbed for seeding
*/
void mbedtls_hmac_drbg_set_entropy_len( mbedtls_hmac_drbg_context *ctx, size_t len )
{