diff --git a/include/mbedtls/ctr_drbg.h b/include/mbedtls/ctr_drbg.h index 86d546a10..2b0c61712 100644 --- a/include/mbedtls/ctr_drbg.h +++ b/include/mbedtls/ctr_drbg.h @@ -22,6 +22,9 @@ * - 256 bits if AES-256 is used, #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set * to 32 or more, and the DRBG is initialized with an explicit * nonce in the \c custom parameter to mbedtls_ctr_drbg_seed(). + * - 256 bits if AES-256 is used, #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set + * to 32 or more, and mbedtls_ctr_drbg_set_nonce_len() is called to set + * an entropy nonce length of 16 bytes or more. * - 128 bits if AES-256 is used but #MBEDTLS_CTR_DRBG_ENTROPY_LEN is * between 24 and 47 and the DRBG is not initialized with an explicit * nonce (see mbedtls_ctr_drbg_seed()). @@ -29,6 +32,9 @@ * and #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set to 24 or more (which is * always the case unless it is explicitly set to a different value * in config.h). + * - 128 bits if AES-128 is used (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY enabled) + * to 16 or more, and mbedtls_ctr_drbg_set_nonce_len() is called to set + * an entropy nonce length of 8 bytes or more. * * Note that the value of #MBEDTLS_CTR_DRBG_ENTROPY_LEN defaults to: * - \c 48 if the module \c MBEDTLS_SHA512_C is enabled and the symbol @@ -172,7 +178,11 @@ typedef struct mbedtls_ctr_drbg_context int reseed_counter; /*!< The reseed counter. * This is the number of requests that have * been made since the last (re)seeding, - * minus one. */ + * minus one. + * Before the initial seeding, this field + * contains the amount of entropy in bytes + * to use as a nonce for the initial seeding. + */ int prediction_resistance; /*!< This determines whether prediction resistance is enabled, that is whether to systematically reseed before @@ -222,43 +232,45 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx ); * The entropy length is #MBEDTLS_CTR_DRBG_ENTROPY_LEN by default. * You can override it by calling mbedtls_ctr_drbg_set_entropy_len(). * - * You can provide a personalization string in addition to the + * You can provide a nonce and personalization string in addition to the * entropy source, to make this instantiation as unique as possible. + * See SP 800-90A §8.6.7 for more details about nonces. + * + * The _seed_material_ value passed to the derivation function in + * the CTR_DRBG Instantiate Process described in NIST SP 800-90A §10.2.1.3.2 + * is the concatenation of the following strings: + * - A string obtained by calling \p f_entropy function for the entropy + * length. + * - A string obtained by calling \p f_entropy function for the nonce + * length set with mbedtls_ctr_drbg_set_nonce_len(). If the entropy + * nonce length is \c 0, this function does not make a second call + * to \p f_entropy. + * - The \p custom string. + * + * \note To achieve the nominal security strength permitted + * by CTR_DRBG, the entropy length must be: + * - at least 16 bytes for a 128-bit strength + * (maximum achievable strength when using AES-128); + * - at least 32 bytes for a 256-bit strength + * (maximum achievable strength when using AES-256). + * + * In addition, if you do not pass a nonce in \p custom, + * the sum of the entropy length + * (#MBEDTLS_CTR_DRBG_ENTROPY_LEN unless overridden with + * mbedtls_ctr_drbg_set_entropy_len()) + * and the entropy nonce length (\c 0 unless overridden + * with mbedtls_ctr_drbg_set_nonce_len()) must be: + * - at least 24 bytes for a 128-bit strength + * (maximum achievable strength when using AES-128); + * - at least 48 bytes for a 256-bit strength + * (maximum achievable strength when using AES-256). * - * \note The _seed_material_ value passed to the derivation - * function in the CTR_DRBG Instantiate Process - * described in NIST SP 800-90A §10.2.1.3.2 - * is the concatenation of the string obtained from - * calling \p f_entropy and the \p custom string. - * The origin of the nonce depends on the value of - * the entropy length relative to the security strength. - * - If the entropy length is at least 1.5 times the - * security strength then the nonce is taken from the - * string obtained with \p f_entropy. - * - If the entropy length is less than the security - * strength, then the nonce is taken from \p custom. - * In this case, for compliance with SP 800-90A, - * you must pass a unique value of \p custom at - * each invocation. See SP 800-90A §8.6.7 for more - * details. - */ -#if MBEDTLS_CTR_DRBG_ENTROPY_LEN < MBEDTLS_CTR_DRBG_KEYSIZE * 3 / 2 -/** \warning When #MBEDTLS_CTR_DRBG_ENTROPY_LEN is less than - * #MBEDTLS_CTR_DRBG_KEYSIZE * 3 / 2, to achieve the - * maximum security strength permitted by CTR_DRBG, - * you must pass a value of \p custom that is a nonce: - * this value must never be repeated in subsequent - * runs of the same application or on a different - * device. - */ -#endif -/** * \param ctx The CTR_DRBG context to seed. * \param f_entropy The entropy callback, taking as arguments the * \p p_entropy context, the buffer to fill, and the * length of the buffer. * \p f_entropy is always called with a buffer size - * equal to the entropy length. + * less than or equal to the entropy length. * \param p_entropy The entropy context to pass to \p f_entropy. * \param custom The personalization string. * This can be \c NULL, in which case the personalization @@ -320,11 +332,35 @@ void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx, * * \param ctx The CTR_DRBG context. * \param len The amount of entropy to grab, in bytes. - * This must be at most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT. + * This must be at most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT + * and at most the maximum length accepted by the + * entropy function that is set in the context. */ void mbedtls_ctr_drbg_set_entropy_len( mbedtls_ctr_drbg_context *ctx, size_t len ); +/** + * \brief This function sets the amount of entropy grabbed + * as a nonce for the initial seeding. + * + * Call this function before calling mbedtls_ctr_drbg_seed() to read + * a nonce from the entropy source during the initial seeding. + * + * \param ctx The CTR_DRBG context. + * \param len The amount of entropy to grab for the nonce, in bytes. + * This must be at most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT + * and at most the maximum length accepted by the + * entropy function that is set in the context. + * + * \return \c 0 on success. + * \return #MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG if \p len is + * more than #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT. + * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED + * if the initial seeding has already taken place. + */ +int mbedtls_ctr_drbg_set_nonce_len( mbedtls_ctr_drbg_context *ctx, + size_t len ); + /** * \brief This function sets the reseed interval. * diff --git a/library/ctr_drbg.c b/library/ctr_drbg.c index 92316dabb..85bd04f2b 100644 --- a/library/ctr_drbg.c +++ b/library/ctr_drbg.c @@ -86,6 +86,32 @@ void mbedtls_ctr_drbg_set_entropy_len( mbedtls_ctr_drbg_context *ctx, ctx->entropy_len = len; } +int mbedtls_ctr_drbg_set_nonce_len( mbedtls_ctr_drbg_context *ctx, + size_t len ) +{ + /* If mbedtls_ctr_drbg_seed() has already been called, it's + * too late. Return the error code that's closest to making sense. */ + if( ctx->f_entropy != NULL ) + return( MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED ); + + if( len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT ) + return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG ); +#if SIZE_MAX > INT_MAX + /* This shouldn't be an issue because + * MBEDTLS_CTR_DRBG_MAX_SEED_INPUT < INT_MAX in any sensible + * configuration, but make sure anyway. */ + if( len > INT_MAX ) + return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG ); +#endif + + /* For backward compatibility with Mbed TLS <= 2.19, store the + * entropy nonce length in a field that already exists, but isn't + * used until after the initial seeding. */ + /* Due to the capping of len above, the value fits in an int. */ + ctx->reseed_counter = (int) len; + return( 0 ); +} + void mbedtls_ctr_drbg_set_reseed_interval( mbedtls_ctr_drbg_context *ctx, int interval ) { @@ -319,7 +345,7 @@ void mbedtls_ctr_drbg_update( mbedtls_ctr_drbg_context *ctx, #endif /* MBEDTLS_DEPRECATED_REMOVED */ /* CTR_DRBG_Reseed with derivation function (SP 800-90A §10.2.1.4.2) - * mbedtls_ctr_drbg_reseed(ctx, additional, len) + * mbedtls_ctr_drbg_reseed(ctx, additional, len, nonce_len) * implements * CTR_DRBG_Reseed(working_state, entropy_input, additional_input) * -> new_working_state @@ -327,11 +353,14 @@ void mbedtls_ctr_drbg_update( mbedtls_ctr_drbg_context *ctx, * ctx contains working_state * additional[:len] = additional_input * and entropy_input comes from calling ctx->f_entropy + * for (ctx->entropy_len + nonce_len) bytes * and with output * ctx contains new_working_state */ -int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx, - const unsigned char *additional, size_t len ) +int mbedtls_ctr_drbg_reseed_internal( mbedtls_ctr_drbg_context *ctx, + const unsigned char *additional, + size_t len, + size_t nonce_len ) { unsigned char seed[MBEDTLS_CTR_DRBG_MAX_SEED_INPUT]; size_t seedlen = 0; @@ -339,7 +368,9 @@ int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx, if( ctx->entropy_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT ) return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG ); - if( len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len ) + if( nonce_len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len ) + return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG ); + if( len > MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - ctx->entropy_len - nonce_len ) return( MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG ); memset( seed, 0, MBEDTLS_CTR_DRBG_MAX_SEED_INPUT ); @@ -351,6 +382,16 @@ int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx, } seedlen += ctx->entropy_len; + /* Gather entropy for a nonce if requested. */ + if( nonce_len != 0 ) + { + if( 0 != ctx->f_entropy( ctx->p_entropy, seed, nonce_len ) ) + { + return( MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED ); + } + seedlen += nonce_len; + } + /* Add additional data if provided. */ if( additional != NULL && len != 0 ) { @@ -372,6 +413,12 @@ exit: return( ret ); } +int mbedtls_ctr_drbg_reseed( mbedtls_ctr_drbg_context *ctx, + const unsigned char *additional, size_t len ) +{ + return( mbedtls_ctr_drbg_reseed_internal( ctx, additional, len, 0 ) ); +} + /* CTR_DRBG_Instantiate with derivation function (SP 800-90A §10.2.1.3.2) * mbedtls_ctr_drbg_seed(ctx, f_entropy, p_entropy, custom, len) * implements @@ -403,16 +450,18 @@ int mbedtls_ctr_drbg_seed( mbedtls_ctr_drbg_context *ctx, ctx->entropy_len = MBEDTLS_CTR_DRBG_ENTROPY_LEN; ctx->reseed_interval = MBEDTLS_CTR_DRBG_RESEED_INTERVAL; - /* - * Initialize with an empty key - */ + /* Initialize with an empty key. */ if( ( ret = mbedtls_aes_setkey_enc( &ctx->aes_ctx, key, MBEDTLS_CTR_DRBG_KEYBITS ) ) != 0 ) { return( ret ); } - if( ( ret = mbedtls_ctr_drbg_reseed( ctx, custom, len ) ) != 0 ) + /* Do the initial seeding. + * ctx->reseed_counter contains the desired amount of entropy to + * grab for a nonce (see mbedtls_ctr_drbg_set_nonce_len()). */ + if( ( ret = mbedtls_ctr_drbg_reseed_internal( ctx, custom, len, + ctx->reseed_counter ) ) != 0 ) { return( ret ); }