mirror of
https://github.com/yuzu-emu/mbedtls.git
synced 2024-12-23 17:25:38 +00:00
mbedtls_ctr_drbg_set_entropy_len() only matters when reseeding
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
parent
7e27936767
commit
dddda81fbc
|
|
@ -10,9 +10,13 @@
|
|||
* Bit Generators</em>.
|
||||
*
|
||||
* 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)
|
||||
* 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
|
||||
* 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
|
||||
|
|
@ -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_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
|
||||
* (\c MBEDTLS_ENTROPY_FORCE_SHA256 is disabled).
|
||||
*
|
||||
* \note See mbedtls_ctr_drbg_set_entropy_len() regarding what values are
|
||||
* acceptable.
|
||||
*/
|
||||
#define MBEDTLS_CTR_DRBG_ENTROPY_LEN 48
|
||||
#else
|
||||
/** The amount of entropy used per seed by default.
|
||||
*
|
||||
* This is 32 bytes because the entropy module uses SHA-256
|
||||
* (the SHA512 module is disabled or #MBEDTLS_ENTROPY_FORCE_SHA256 is enabled).
|
||||
*
|
||||
* \note See mbedtls_ctr_drbg_set_entropy_len() regarding what values are
|
||||
* acceptable.
|
||||
|
||||
#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).
|
||||
*/
|
||||
#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
|
||||
#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
|
||||
|
|
@ -209,7 +215,10 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
|
|||
* entropy sources).
|
||||
*
|
||||
* \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
|
||||
* 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.
|
||||
* The origin of the nonce depends on the value of
|
||||
* 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
|
||||
* security strength then the nonce is taken from the
|
||||
* 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
|
||||
* 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
|
||||
|
|
@ -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
|
||||
* seed or reseed.
|
||||
* subsequent reseed.
|
||||
*
|
||||
* The default value is #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
|
||||
*
|
||||
* \note For compliance with NIST SP 800-90A, the entropy length
|
||||
* (\p len bytes = \p len * 8 bits)
|
||||
* must be at least the security strength.
|
||||
* Furthermore, if the entropy input is used to provide
|
||||
* 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.
|
||||
* \note mbedtls_ctr_drbg_seed() always sets the entropy length
|
||||
* to #MBEDTLS_CTR_DRBG_ENTROPY_LEN, so this function
|
||||
* only has an effect when it is called after
|
||||
* mbedtls_ctr_drbg_seed().
|
||||
*
|
||||
* To achieve 256-bit security,
|
||||
* you must use AES-256 and
|
||||
* the entropy input must be at least:
|
||||
* - 48 bytes if the \p custom argument to
|
||||
* mbedtls_ctr_drbg_seed() may repeat (for example
|
||||
* because it is empty, or more generally constant);
|
||||
* - 32 bytes if the \p custom argument to
|
||||
* mbedtls_ctr_drbg_seed() includes a nonce.
|
||||
*
|
||||
* To achieve 128-bit security,
|
||||
* whether AES-128 or AES-256 is used,
|
||||
* 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.
|
||||
* \note The security strength of CTR_DRBG is bounded by the
|
||||
* entropy length. Thus:
|
||||
* - When using AES-256
|
||||
* (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is disabled,
|
||||
* which is the default),
|
||||
* \p len must be at least 32 (in bytes)
|
||||
* to achieve a 256-bit strength.
|
||||
* - When using AES-128
|
||||
* (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is enabled)
|
||||
* \p len must be at least 16 (in bytes)
|
||||
* to achieve a 128-bit strength.
|
||||
*
|
||||
* \param ctx The CTR_DRBG context.
|
||||
* \param len The amount of entropy to grab, in bytes.
|
||||
|
|
|
|||
Loading…
Reference in a new issue