From 1d2a9e88c3817469e32a1efb248700f78eea88ba Mon Sep 17 00:00:00 2001 From: Gilles Peskine Date: Fri, 4 Oct 2019 11:47:35 +0200 Subject: [PATCH] HMAC_DRBG: support set_entropy_len() before seed() 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(). --- include/mbedtls/hmac_drbg.h | 19 ++++++------------- library/hmac_drbg.c | 25 ++++++++++++++----------- 2 files changed, 20 insertions(+), 24 deletions(-) diff --git a/include/mbedtls/hmac_drbg.h b/include/mbedtls/hmac_drbg.h index 95aac3b23..eec05e471 100644 --- a/include/mbedtls/hmac_drbg.h +++ b/include/mbedtls/hmac_drbg.h @@ -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. diff --git a/library/hmac_drbg.c b/library/hmac_drbg.c index 82703b0a3..dd895be42 100644 --- a/library/hmac_drbg.c +++ b/library/hmac_drbg.c @@ -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 ) {