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mbedtls_rsa_rsassa_pss_*: improve documentation

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Hashes used in RSA-PSS encoding (EMSA-PSS-ENCODE, see §9.1.1 in RFC
8017):

- H1: Hashing the message (step 2)
- H2: Hashing in the salt (step 6)
- H3: Mask generation function (step 9)

According to the standard:

- H1 and H2 MUST be done by the same hash function
- H3 is RECOMMENDED to be the same as the hash used for H1 and H2.

According to the implementation:

- H1 happens outside of the function call. It might or might not happen
and the implementation might or might not be aware of the hash used.
- H2 happens inside the function call, consistency with H1 is not
enforced and might not even be possible to detect.
- H3 is done with the same hash as H2 (with the exception of
mbedtls_rsassa_pss_verify_ext(), which takes a dedicated parameter for
the hash used in the MGF).

Issues with the documentation:

- The comments weren't always clear about the three hashes involved and
often only mentioned two of them (which two varied from function to
function).
- The documentation was giving the impression that the standard
recommends aligning H2 and H1 (which is not a recommendation but a
must).

Signed-off-by: Janos Follath <janos.follath@arm.com>
This commit is contained in:
Janos Follath 2021-04-01 14:44:17 +01:00
parent 8cad2e22fc
commit b795332401
1 changed files with 30 additions and 25 deletions
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55
include/mbedtls/rsa.h
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@ -857,12 +857,13 @@ int mbedtls_rsa_rsassa_pkcs1_v15_sign( mbedtls_rsa_context *ctx,
* \brief This function performs a PKCS#1 v2.1 PSS signature
* operation (RSASSA-PSS-SIGN).
*
* \note The \p hash_id in the RSA context is the one used for the
* encoding. \p md_alg in the function call is the type of hash
* that is encoded. According to <em>RFC-3447: Public-Key
* \note The \c hash_id set in \p ctx by calling
* mbedtls_rsa_set_padding() selects the hash used for the
* encoding operation and for the mask generation function
* (MGF1). For more details on the encoding operation and the
* mask generation function, consult <em>RFC-3447: Public-Key
* Cryptography Standards (PKCS) #1 v2.1: RSA Cryptography
* Specifications</em> it is advised to keep both hashes the
* same.
* Specifications</em>.
*
* \note This function enforces that the provided salt length complies
* with FIPS 186-4 §5.5 (e) and RFC 8017 (PKCS#1 v2.2) §9.1.1
@ -910,12 +911,13 @@ int mbedtls_rsa_rsassa_pss_sign_ext( mbedtls_rsa_context *ctx,
* \brief This function performs a PKCS#1 v2.1 PSS signature
* operation (RSASSA-PSS-SIGN).
*
* \note The \p hash_id in the RSA context is the one used for the
* encoding. \p md_alg in the function call is the type of hash
* that is encoded. According to <em>RFC-3447: Public-Key
* \note The \c hash_id set in \p ctx by calling
* mbedtls_rsa_set_padding() selects the hash used for the
* encoding operation and for the mask generation function
* (MGF1). For more details on the encoding operation and the
* mask generation function, consult <em>RFC-3447: Public-Key
* Cryptography Standards (PKCS) #1 v2.1: RSA Cryptography
* Specifications</em> it is advised to keep both hashes the
* same.
* Specifications</em>.
*
* \note This function always uses the maximum possible salt size,
* up to the length of the payload hash. This choice of salt
@ -934,7 +936,7 @@ int mbedtls_rsa_rsassa_pss_sign_ext( mbedtls_rsa_context *ctx,
* \param md_alg The message-digest algorithm used to hash the original data.
* Use #MBEDTLS_MD_NONE for signing raw data.
* \param hashlen The length of the message digest.
* Ths is only used if \p md_alg is #MBEDTLS_MD_NONE.
* This is only used if \p md_alg is #MBEDTLS_MD_NONE.
* \param hash The buffer holding the message digest or raw data.
* If \p md_alg is #MBEDTLS_MD_NONE, this must be a readable
* buffer of length \p hashlen Bytes. If \p md_alg is not
@ -1021,16 +1023,15 @@ int mbedtls_rsa_rsassa_pkcs1_v15_verify( mbedtls_rsa_context *ctx,
* \brief This function performs a PKCS#1 v2.1 PSS verification
* operation (RSASSA-PSS-VERIFY).
*
* The hash function for the MGF mask generating function
* is that specified in the RSA context.
*
* \note The \p hash_id in the RSA context is the one used for the
* verification. \p md_alg in the function call is the type of
* hash that is verified. According to <em>RFC-3447: Public-Key
* \note The \c hash_id set in \p ctx by calling
* mbedtls_rsa_set_padding() selects the hash used for the
* encoding operation and for the mask generation function
* (MGF1). For more details on the encoding operation and the
* mask generation function, consult <em>RFC-3447: Public-Key
* Cryptography Standards (PKCS) #1 v2.1: RSA Cryptography
* Specifications</em> it is advised to keep both hashes the
* same. If \p hash_id in the RSA context is unset,
* the \p md_alg from the function call is used.
* Specifications</em>. If the \c hash_id set in \p ctx by
* mbedtls_rsa_set_padding() is #MBEDTLS_MD_NONE, the \p md_alg
* parameter is used.
*
* \param ctx The initialized RSA public key context to use.
* \param md_alg The message-digest algorithm used to hash the original data.
@ -1059,13 +1060,11 @@ int mbedtls_rsa_rsassa_pss_verify( mbedtls_rsa_context *ctx,
* \brief This function performs a PKCS#1 v2.1 PSS verification
* operation (RSASSA-PSS-VERIFY).
*
* The hash function for the MGF mask generating function
* is that specified in \p mgf1_hash_id.
*
* \note The \p sig buffer must be as large as the size
* of \p ctx->N. For example, 128 Bytes if RSA-1024 is used.
*
* \note The \p hash_id in the RSA context is ignored.
* \note The \c hash_id set in \p ctx by mbedtls_rsa_set_padding() is
* ignored.
*
* \param ctx The initialized RSA public key context to use.
* \param md_alg The message-digest algorithm used to hash the original data.
@ -1077,7 +1076,13 @@ int mbedtls_rsa_rsassa_pss_verify( mbedtls_rsa_context *ctx,
* buffer of length \p hashlen Bytes. If \p md_alg is not
* #MBEDTLS_MD_NONE, it must be a readable buffer of length
* the size of the hash corresponding to \p md_alg.
* \param mgf1_hash_id The message digest used for mask generation.
* \param mgf1_hash_id The message digest algorithm used for the
* verification operation and the mask generation
* function (MGF1). For more details on the encoding
* operation and the mask generation function, consult
* <em>RFC-3447: Public-Key Cryptography Standards
* (PKCS) #1 v2.1: RSA Cryptography
* Specifications</em>.
* \param expected_salt_len The length of the salt used in padding. Use
* #MBEDTLS_RSA_SALT_LEN_ANY to accept any salt length.
* \param sig The buffer holding the signature. This must be a readable