/**
* \file ctr_drbg.h
*
- * \brief CTR_DRBG is based on AES-256, as defined in <em>NIST SP 800-90A:
- * Recommendation for Random Number Generation Using Deterministic
- * Random Bit Generators</em>.
- *
+ * \brief The CTR_DRBG pseudorandom generator.
+ *
+ * CTR_DRBG is a standardized way of building a PRNG from a block-cipher
+ * in counter mode operation, as defined in <em>NIST SP 800-90A:
+ * Recommendation for Random Number Generation Using Deterministic Random
+ * Bit Generators</em>.
+ *
+ * The Mbed TLS implementation of CTR_DRBG uses AES-256
+ * as the underlying block cipher, with a derivation function.
+ * The initial seeding grabs #MBEDTLS_CTR_DRBG_ENTROPY_LEN bytes of entropy.
+ * See the documentation of mbedtls_ctr_drbg_seed() for more details.
+ *
+ * Based on NIST SP 800-90A §10.2.1 table 3 and NIST SP 800-57 part 1 table 2,
+ * here are the security strengths achieved in typical configuration:
+ * - 256 bits under the default configuration of the library,
+ * with #MBEDTLS_CTR_DRBG_ENTROPY_LEN set to 48 or more.
+ * - 256 bits if #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().
+ * - 128 bits if #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()).
+ *
+ * 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
+ * \c MBEDTLS_ENTROPY_FORCE_SHA256 is not enabled at compile time.
+ * This is the default configuration of the library.
+ * - \c 32 if the module \c MBEDTLS_SHA512_C is disabled at compile time.
+ * - \c 32 if \c MBEDTLS_ENTROPY_FORCE_SHA256 is enabled at compile time.
*/
/*
- * Copyright (C) 2006-2018, Arm Limited (or its affiliates), All Rights Reserved
+ * Copyright (C) 2006-2019, Arm Limited (or its affiliates), All Rights Reserved
* SPDX-License-Identifier: GPL-2.0
*
* This program is free software; you can redistribute it and/or modify
* \{
*/
+/** \def MBEDTLS_CTR_DRBG_ENTROPY_LEN
+ *
+ * \brief The amount of entropy used per seed by default, in bytes.
+ */
#if !defined(MBEDTLS_CTR_DRBG_ENTROPY_LEN)
#if defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256)
+/** This is 48 bytes because the entropy module uses SHA-512
+ * (\c MBEDTLS_ENTROPY_FORCE_SHA256 is not set).
+ */
#define MBEDTLS_CTR_DRBG_ENTROPY_LEN 48
-/**< The amount of entropy used per seed by default:
- * <ul><li>48 with SHA-512.</li>
- * <li>32 with SHA-256.</li></ul>
+
+#else /* defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256) */
+
+/** This is 32 bytes because the entropy module uses SHA-256
+ * (the SHA512 module is disabled or
+ * \c MBEDTLS_ENTROPY_FORCE_SHA256 is enabled).
+ *
+ * \warning To achieve a 256-bit security strength, you must pass a nonce
+ * to mbedtls_ctr_drbg_seed().
*/
-#else
#define MBEDTLS_CTR_DRBG_ENTROPY_LEN 32
-/**< Amount of entropy used per seed by default:
- * <ul><li>48 with SHA-512.</li>
- * <li>32 with SHA-256.</li></ul>
- */
-#endif
-#endif
+#endif /* defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256) */
+#endif /* !defined(MBEDTLS_CTR_DRBG_ENTROPY_LEN) */
#if !defined(MBEDTLS_CTR_DRBG_RESEED_INTERVAL)
#define MBEDTLS_CTR_DRBG_RESEED_INTERVAL 10000
#if !defined(MBEDTLS_CTR_DRBG_MAX_SEED_INPUT)
#define MBEDTLS_CTR_DRBG_MAX_SEED_INPUT 384
-/**< The maximum size of seed or reseed buffer. */
+/**< The maximum size of seed or reseed buffer in bytes. */
#endif
/* \} name SECTION: Module settings */
* \brief This function seeds and sets up the CTR_DRBG
* entropy source for future reseeds.
*
- * \note Personalization data can be provided in addition to the more generic
- * entropy source, to make this instantiation as unique as possible.
- *
+ * A typical choice for the \p f_entropy and \p p_entropy parameters is
+ * to use the entropy module:
+ * - \p f_entropy is mbedtls_entropy_func();
+ * - \p p_entropy is an instance of ::mbedtls_entropy_context initialized
+ * with mbedtls_entropy_init() (which registers the platform's default
+ * entropy sources).
+ *
+ * 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
+ * entropy source, to make this instantiation as unique as possible.
+ *
+ * \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
+ * 48, to achieve a 256-bit security strength,
+ * 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.
+ * It must have been initialized with
+ * mbedtls_ctr_drbg_init().
+ * After a successful call to mbedtls_ctr_drbg_seed(),
+ * you may not call mbedtls_ctr_drbg_seed() again on
+ * the same context unless you call
+ * mbedtls_ctr_drbg_free() and mbedtls_ctr_drbg_init()
+ * again first.
* \param f_entropy The entropy callback, taking as arguments the
* \p p_entropy context, the buffer to fill, and the
- length of the buffer.
- * \param p_entropy The entropy context.
- * \param custom Personalization data, that is device-specific
- identifiers. Can be NULL.
- * \param len The length of the personalization data.
+ * length of the buffer.
+ * \p f_entropy is always called with a buffer size
+ * 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
+ * string is empty regardless of the value of \p len.
+ * \param len The length of the personalization string.
+ * This must be at most
+ * #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT
+ * - #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
*
* \return \c 0 on success, or
* #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on failure.
* The default value is off.
*
* \note If enabled, entropy is gathered at the beginning of
- * every call to mbedtls_ctr_drbg_random_with_add().
+ * every call to mbedtls_ctr_drbg_random_with_add()
+ * or mbedtls_ctr_drbg_random().
* Only use this if your entropy source has sufficient
* throughput.
*
/**
* \brief This function sets the amount of entropy grabbed on each
- * seed or reseed. The default value is
- * #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
+ * seed or reseed.
+ *
+ * The default value is #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
+ *
+ * \note The security strength of CTR_DRBG is bounded by the
+ * entropy length. Thus \p len must be at least
+ * 32 (in bytes) to achieve a 256-bit strength.
*
* \param ctx The CTR_DRBG context.
- * \param len The amount of entropy to grab.
+ * \param len The amount of entropy to grab, in bytes.
+ * This must be at most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
*/
void mbedtls_ctr_drbg_set_entropy_len( mbedtls_ctr_drbg_context *ctx,
size_t len );
/**
* \brief This function sets the reseed interval.
- * The default value is #MBEDTLS_CTR_DRBG_RESEED_INTERVAL.
+ *
+ * The reseed interval is the number of calls to mbedtls_ctr_drbg_random()
+ * or mbedtls_ctr_drbg_random_with_add() after which the entropy function
+ * is called again.
+ *
+ * The default value is #MBEDTLS_CTR_DRBG_RESEED_INTERVAL.
*
* \param ctx The CTR_DRBG context.
* \param interval The reseed interval.
* extracts data from the entropy source.
*
* \param ctx The CTR_DRBG context.
- * \param additional Additional data to add to the state. Can be NULL.
+ * \param additional Additional data to add to the state. Can be \c NULL.
* \param len The length of the additional data.
+ * This must be less than
+ * #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - \c entropy_len
+ * where \c entropy_len is the entropy length
+ * configured for the context.
*
* \return \c 0 on success, or
* #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on failure.
* \brief This function updates the state of the CTR_DRBG context.
*
* \param ctx The CTR_DRBG context.
- * \param additional The data to update the state with.
+ * \param additional The data to update the state with. This must not be
+ * \c NULL unless \p add_len is \c 0.
* \param add_len Length of \p additional in bytes. This must be at
* most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
*
* The remaining Bytes are silently discarded.
*
* \param ctx The CTR_DRBG context.
- * \param additional The data to update the state with.
- * \param add_len Length of \p additional data.
+ * \param additional The data to update the state with. This must not be
+ * \c NULL unless \p add_len is \c 0.
+ * \param add_len Length of \p additional data. This must be at
+ * most #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT.
*/
void mbedtls_ctr_drbg_update( mbedtls_ctr_drbg_context *ctx,
const unsigned char *additional,
* \brief This function updates a CTR_DRBG instance with additional
* data and uses it to generate random data.
*
- * \note The function automatically reseeds if the reseed counter is exceeded.
+ * This function automatically reseeds if the reseed counter is exceeded
+ * or prediction resistance is enabled.
*
* \param p_rng The CTR_DRBG context. This must be a pointer to a
* #mbedtls_ctr_drbg_context structure.
* \param output The buffer to fill.
- * \param output_len The length of the buffer.
- * \param additional Additional data to update. Can be NULL.
- * \param add_len The length of the additional data.
- *
- * \return \c 0 on success, or
- * #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
+ * \param output_len The length of the buffer in bytes.
+ * \param additional Additional data to update. Can be \c NULL, in which
+ * case the additional data is empty regardless of
+ * the value of \p add_len.
+ * \param add_len The length of the additional data
+ * if \p additional is not \c NULL.
+ * This must be less than #MBEDTLS_CTR_DRBG_MAX_INPUT
+ * and less than
+ * #MBEDTLS_CTR_DRBG_MAX_SEED_INPUT - \c entropy_len
+ * where \c entropy_len is the entropy length
+ * configured for the context.
+ *
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
* #MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG on failure.
*/
int mbedtls_ctr_drbg_random_with_add( void *p_rng,
/**
* \brief This function uses CTR_DRBG to generate random data.
*
- * \note The function automatically reseeds if the reseed counter is exceeded.
+ * This function automatically reseeds if the reseed counter is exceeded
+ * or prediction resistance is enabled.
+ *
*
* \param p_rng The CTR_DRBG context. This must be a pointer to a
* #mbedtls_ctr_drbg_context structure.
* \param output The buffer to fill.
- * \param output_len The length of the buffer.
+ * \param output_len The length of the buffer in bytes.
*
- * \return \c 0 on success, or
- * #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
* #MBEDTLS_ERR_CTR_DRBG_REQUEST_TOO_BIG on failure.
*/
int mbedtls_ctr_drbg_random( void *p_rng,
* \param ctx The CTR_DRBG context.
* \param path The name of the file.
*
- * \return \c 0 on success,
- * #MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR on file error, or
- * #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR on file error.
+ * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on reseed
* failure.
*/
int mbedtls_ctr_drbg_write_seed_file( mbedtls_ctr_drbg_context *ctx, const char *path );
* \param ctx The CTR_DRBG context.
* \param path The name of the file.
*
- * \return \c 0 on success,
- * #MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR on file error,
- * #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED or
- * #MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG on failure.
+ * \return \c 0 on success.
+ * \return #MBEDTLS_ERR_CTR_DRBG_FILE_IO_ERROR on file error.
+ * \return #MBEDTLS_ERR_CTR_DRBG_ENTROPY_SOURCE_FAILED on
+ * reseed failure.
+ * \return #MBEDTLS_ERR_CTR_DRBG_INPUT_TOO_BIG if the existing
+ * seed file is too large.
*/
int mbedtls_ctr_drbg_update_seed_file( mbedtls_ctr_drbg_context *ctx, const char *path );
#endif /* MBEDTLS_FS_IO */
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