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mbedtls_ctr_drbg_set_entropy_len() only matters when reseeding

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The documentation of CTR_DRBG erroneously claimed that
mbedtls_ctr_drbg_set_entropy_len() had an impact on the initial
seeding. This is in fact not the case: mbedtls_ctr_drbg_seed() forces
the initial seeding to grab MBEDTLS_CTR_DRBG_ENTROPY_LEN bytes of
entropy. Fix the documentation and rewrite the discussion of the
entropy length and the security strength accordingly.
This commit is contained in:
Gilles Peskine 2019-10-03 14:22:04 +02:00
parent e215a4d05e
commit cc74872ba9
1 changed files with 56 additions and 50 deletions
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106
include/mbedtls/ctr_drbg.h
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@ -10,9 +10,13 @@
* Bit Generators</em>. * Bit Generators</em>.
* *
* The Mbed TLS implementation of CTR_DRBG uses AES-256 (default) or AES-128 * The Mbed TLS implementation of CTR_DRBG uses AES-256 (default) or AES-128
* as the underlying block cipher, with a derivation function. The security
* strength is:
* (if \c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is enabled at compile time) * (if \c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is enabled at compile time)
* 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 AES-256 * - 256 bits under the default configuration of the library, with AES-256
* and with #MBEDTLS_CTR_DRBG_ENTROPY_LEN set to 48 or more. * and with #MBEDTLS_CTR_DRBG_ENTROPY_LEN set to 48 or more.
* - 256 bits if AES-256 is used, #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set * - 256 bits if AES-256 is used, #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set
@ -102,29 +106,31 @@
* \{ * \{
*/ */
/** \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_CTR_DRBG_ENTROPY_LEN)
#if defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256) #if defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256)
/** The amount of entropy used per seed by default. /** This is 48 bytes because the entropy module uses SHA-512
*
* This is 48 bytes because the entropy module uses SHA-512
*
* \note See mbedtls_ctr_drbg_set_entropy_len() regarding what values are
* acceptable.
* (\c MBEDTLS_ENTROPY_FORCE_SHA256 is disabled). * (\c MBEDTLS_ENTROPY_FORCE_SHA256 is disabled).
*/ */
#define MBEDTLS_CTR_DRBG_ENTROPY_LEN 48 #define MBEDTLS_CTR_DRBG_ENTROPY_LEN 48
#else
/** The amount of entropy used per seed by default. #else /* defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256) */
*
* This is 32 bytes because the entropy module uses SHA-256 /** This is 32 bytes because the entropy module uses SHA-256
* (the SHA512 module is disabled or #MBEDTLS_ENTROPY_FORCE_SHA256 is set). * (the SHA512 module is disabled or
* * \c MBEDTLS_ENTROPY_FORCE_SHA256 is enabled).
* \note See mbedtls_ctr_drbg_set_entropy_len() regarding what values are
* acceptable.
*/ */
#if !defined(MBEDTLS_CTR_DRBG_USE_128_BIT_KEY)
/** \warning To achieve a 256-bit security strength, you must pass a nonce
* to mbedtls_ctr_drbg_seed().
*/
#endif /* !defined(MBEDTLS_CTR_DRBG_USE_128_BIT_KEY) */
#define MBEDTLS_CTR_DRBG_ENTROPY_LEN 32 #define MBEDTLS_CTR_DRBG_ENTROPY_LEN 32
#endif #endif /* defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256) */
#endif #endif /* !defined(MBEDTLS_CTR_DRBG_ENTROPY_LEN) */
#if !defined(MBEDTLS_CTR_DRBG_RESEED_INTERVAL) #if !defined(MBEDTLS_CTR_DRBG_RESEED_INTERVAL)
#define MBEDTLS_CTR_DRBG_RESEED_INTERVAL 10000 #define MBEDTLS_CTR_DRBG_RESEED_INTERVAL 10000
@ -209,7 +215,10 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
* entropy sources). * entropy sources).
* *
* \p f_entropy is always called with a buffer size equal to the entropy * \p f_entropy is always called with a buffer size equal to the entropy
* length described in the documentation of mbedtls_ctr_drbg_set_entropy_len(). * length. The entropy length is initially #MBEDTLS_CTR_DRBG_ENTROPY_LEN
* and this value is always used for the initial seeding. You can change
* the entropy length for subsequent seeding by calling
* mbedtls_ctr_drbg_set_entropy_len() after this function.
* *
* You can provide a personalization string in addition to the * You can provide a personalization string in addition to the
* entropy source, to make this instantiation as unique as possible. * entropy source, to make this instantiation as unique as possible.
@ -221,9 +230,6 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
* calling \p f_entropy and the \p custom string. * calling \p f_entropy and the \p custom string.
* The origin of the nonce depends on the value of * The origin of the nonce depends on the value of
* the entropy length relative to the security strength. * the entropy length relative to the security strength.
* See the documentation of
* mbedtls_ctr_drbg_set_entropy_len() for information
* about the entropy length.
* - If the entropy length is at least 1.5 times the * - If the entropy length is at least 1.5 times the
* security strength then the nonce is taken from the * security strength then the nonce is taken from the
* string obtained with \p f_entropy. * string obtained with \p f_entropy.
@ -233,7 +239,18 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
* you must pass a unique value of \p custom at * you must pass a unique value of \p custom at
* each invocation. See SP 800-90A §8.6.7 for more * each invocation. See SP 800-90A §8.6.7 for more
* details. * 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 ctx The CTR_DRBG context to seed.
* \param f_entropy The entropy callback, taking as arguments the * \param f_entropy The entropy callback, taking as arguments the
* \p p_entropy context, the buffer to fill, and the * \p p_entropy context, the buffer to fill, and the
@ -281,37 +298,26 @@ void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx,
/** /**
* \brief This function sets the amount of entropy grabbed on each * \brief This function sets the amount of entropy grabbed on each
* seed or reseed. * subsequent reseed.
* *
* The default value is #MBEDTLS_CTR_DRBG_ENTROPY_LEN. * The default value is #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
* *
* \note For compliance with NIST SP 800-90A, the entropy length * \note mbedtls_ctr_drbg_seed() always sets the entropy length
* (\p len bytes = \p len * 8 bits) * to #MBEDTLS_CTR_DRBG_ENTROPY_LEN, so this function
* must be at least the security strength. * only has an effect when it is called after
* Furthermore, if the entropy input is used to provide * mbedtls_ctr_drbg_seed().
* the nonce, the entropy length must be 1.5 times
* the security strength.
* Per NIST SP 800-57A table 2, the achievable security
* strength is 128 bits if using AES-128 and
* 256 bits if using AES-256.
* *
* To achieve 256-bit security, * \note The security strength of CTR_DRBG is bounded by the
* you must use AES-256 and * entropy length. Thus:
* the entropy input must be at least: * - When using AES-256
* - 48 bytes if the \p custom argument to * (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is disabled,
* mbedtls_ctr_drbg_seed() may repeat (for example * which is the default),
* because it is empty, or more generally constant); * \p len must be at least 32 (in bytes)
* - 32 bytes if the \p custom argument to * to achieve a 256-bit strength.
* mbedtls_ctr_drbg_seed() includes a nonce. * - When using AES-128
* * (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is enabled)
* To achieve 128-bit security, * \p len must be at least 16 (in bytes)
* whether AES-128 or AES-256 is used, * to achieve a 128-bit strength.
* the entropy input must be at least:
* - 24 bytes if the \p custom argument to
* mbedtls_ctr_drbg_seed() may repeat (for example
* because it is empty, or more generally constant);
* - 16 bytes if the \p custom argument to
* mbedtls_ctr_drbg_seed() includes a nonce.
* *
* \param ctx The CTR_DRBG context. * \param ctx The CTR_DRBG context.
* \param len The amount of entropy to grab, in bytes. * \param len The amount of entropy to grab, in bytes.