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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
e215a4d05e
commit
cc74872ba9
@ -10,9 +10,13 @@
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* Bit Generators</em>.
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*
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* The Mbed TLS implementation of CTR_DRBG uses AES-256 (default) or AES-128
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* as the underlying block cipher, with a derivation function. The security
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* strength is:
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* (if \c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is enabled at compile time)
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* as the underlying block cipher, with a derivation function.
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* The initial seeding grabs #MBEDTLS_CTR_DRBG_ENTROPY_LEN bytes of entropy.
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* See the documentation of mbedtls_ctr_drbg_seed() for more details.
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*
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* Based on NIST SP 800-90A §10.2.1 table 3 and NIST SP 800-57 part 1 table 2,
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* here are the security strengths achieved in typical configuration:
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* - 256 bits under the default configuration of the library, with AES-256
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* and with #MBEDTLS_CTR_DRBG_ENTROPY_LEN set to 48 or more.
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* - 256 bits if AES-256 is used, #MBEDTLS_CTR_DRBG_ENTROPY_LEN is set
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@ -102,29 +106,31 @@
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* \{
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*/
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/** \def MBEDTLS_CTR_DRBG_ENTROPY_LEN
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*
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* \brief The amount of entropy used per seed by default, in bytes.
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*/
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#if !defined(MBEDTLS_CTR_DRBG_ENTROPY_LEN)
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#if defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256)
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/** The amount of entropy used per seed by default.
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*
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* This is 48 bytes because the entropy module uses SHA-512
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*
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* \note See mbedtls_ctr_drbg_set_entropy_len() regarding what values are
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* acceptable.
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/** This is 48 bytes because the entropy module uses SHA-512
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* (\c MBEDTLS_ENTROPY_FORCE_SHA256 is disabled).
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*/
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#define MBEDTLS_CTR_DRBG_ENTROPY_LEN 48
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#else
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/** The amount of entropy used per seed by default.
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*
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* This is 32 bytes because the entropy module uses SHA-256
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* (the SHA512 module is disabled or #MBEDTLS_ENTROPY_FORCE_SHA256 is set).
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*
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* \note See mbedtls_ctr_drbg_set_entropy_len() regarding what values are
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* acceptable.
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#else /* defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256) */
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/** This is 32 bytes because the entropy module uses SHA-256
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* (the SHA512 module is disabled or
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* \c MBEDTLS_ENTROPY_FORCE_SHA256 is enabled).
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*/
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#if !defined(MBEDTLS_CTR_DRBG_USE_128_BIT_KEY)
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/** \warning To achieve a 256-bit security strength, you must pass a nonce
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* to mbedtls_ctr_drbg_seed().
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*/
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#endif /* !defined(MBEDTLS_CTR_DRBG_USE_128_BIT_KEY) */
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#define MBEDTLS_CTR_DRBG_ENTROPY_LEN 32
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#endif
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#endif
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#endif /* defined(MBEDTLS_SHA512_C) && !defined(MBEDTLS_ENTROPY_FORCE_SHA256) */
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#endif /* !defined(MBEDTLS_CTR_DRBG_ENTROPY_LEN) */
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#if !defined(MBEDTLS_CTR_DRBG_RESEED_INTERVAL)
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#define MBEDTLS_CTR_DRBG_RESEED_INTERVAL 10000
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@ -209,7 +215,10 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
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* entropy sources).
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*
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* \p f_entropy is always called with a buffer size equal to the entropy
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* length described in the documentation of mbedtls_ctr_drbg_set_entropy_len().
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* length. The entropy length is initially #MBEDTLS_CTR_DRBG_ENTROPY_LEN
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* and this value is always used for the initial seeding. You can change
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* the entropy length for subsequent seeding by calling
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* mbedtls_ctr_drbg_set_entropy_len() after this function.
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*
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* You can provide a personalization string in addition to the
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* entropy source, to make this instantiation as unique as possible.
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@ -221,9 +230,6 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
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* calling \p f_entropy and the \p custom string.
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* The origin of the nonce depends on the value of
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* the entropy length relative to the security strength.
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* See the documentation of
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* mbedtls_ctr_drbg_set_entropy_len() for information
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* about the entropy length.
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* - If the entropy length is at least 1.5 times the
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* security strength then the nonce is taken from the
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* string obtained with \p f_entropy.
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@ -233,7 +239,18 @@ void mbedtls_ctr_drbg_init( mbedtls_ctr_drbg_context *ctx );
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* you must pass a unique value of \p custom at
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* each invocation. See SP 800-90A §8.6.7 for more
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* details.
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*
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*/
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#if MBEDTLS_CTR_DRBG_ENTROPY_LEN < MBEDTLS_CTR_DRBG_KEYSIZE * 3 / 2
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/** \warning When #MBEDTLS_CTR_DRBG_ENTROPY_LEN is less than
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* #MBEDTLS_CTR_DRBG_KEYSIZE * 3 / 2, to achieve the
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* maximum security strength permitted by CTR_DRBG,
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* you must pass a value of \p custom that is a nonce:
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* this value must never be repeated in subsequent
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* runs of the same application or on a different
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* device.
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*/
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#endif
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/**
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* \param ctx The CTR_DRBG context to seed.
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* \param f_entropy The entropy callback, taking as arguments the
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* \p p_entropy context, the buffer to fill, and the
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@ -281,37 +298,26 @@ void mbedtls_ctr_drbg_set_prediction_resistance( mbedtls_ctr_drbg_context *ctx,
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/**
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* \brief This function sets the amount of entropy grabbed on each
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* seed or reseed.
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* subsequent reseed.
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*
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* The default value is #MBEDTLS_CTR_DRBG_ENTROPY_LEN.
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*
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* \note For compliance with NIST SP 800-90A, the entropy length
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* (\p len bytes = \p len * 8 bits)
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* must be at least the security strength.
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* Furthermore, if the entropy input is used to provide
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* the nonce, the entropy length must be 1.5 times
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* the security strength.
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* Per NIST SP 800-57A table 2, the achievable security
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* strength is 128 bits if using AES-128 and
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* 256 bits if using AES-256.
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* \note mbedtls_ctr_drbg_seed() always sets the entropy length
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* to #MBEDTLS_CTR_DRBG_ENTROPY_LEN, so this function
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* only has an effect when it is called after
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* mbedtls_ctr_drbg_seed().
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*
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* To achieve 256-bit security,
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* you must use AES-256 and
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* the entropy input must be at least:
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* - 48 bytes if the \p custom argument to
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* mbedtls_ctr_drbg_seed() may repeat (for example
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* because it is empty, or more generally constant);
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* - 32 bytes if the \p custom argument to
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* mbedtls_ctr_drbg_seed() includes a nonce.
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*
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* To achieve 128-bit security,
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* whether AES-128 or AES-256 is used,
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* the entropy input must be at least:
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* - 24 bytes if the \p custom argument to
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* mbedtls_ctr_drbg_seed() may repeat (for example
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* because it is empty, or more generally constant);
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* - 16 bytes if the \p custom argument to
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* mbedtls_ctr_drbg_seed() includes a nonce.
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* \note The security strength of CTR_DRBG is bounded by the
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* entropy length. Thus:
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* - When using AES-256
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* (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is disabled,
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* which is the default),
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* \p len must be at least 32 (in bytes)
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* to achieve a 256-bit strength.
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* - When using AES-128
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* (\c MBEDTLS_CTR_DRBG_USE_128_BIT_KEY is enabled)
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* \p len must be at least 16 (in bytes)
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* to achieve a 128-bit strength.
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*
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* \param ctx The CTR_DRBG context.
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* \param len The amount of entropy to grab, in bytes.
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