pkcs7: update documentation adding a note about rejecting weak hash algs

Signed-off-by: Valerio Setti <[email protected]>

include/mbedtls/pkcs7.h

@@ -31,6 +31,10 @@
  *    assumed these fields are empty.
  *  - The RFC allows for the signed Data type to contain contentInfo. This
  *    implementation assumes the type is DATA and the content is empty.
+ *  - The RFC doesn't put any constrain on the hash algorithm to be used, but
+ *    this implementation rejects weak hash algorithms (i.e. RIPEMD160, MD5,
+ *    SHA-1, SHA-224, SHA3-224). In general accepted hash and PK algorithms are
+ *    the ones belonging to `mbedtls_x509_crt_profile_default`.
  */
 
 #ifndef MBEDTLS_PKCS7_H
@@ -190,6 +194,9 @@ int mbedtls_pkcs7_parse_der(mbedtls_pkcs7 *pkcs7, const unsigned char *buf,
  * \note           This function internally calculates the hash on the supplied
  *                 plain data for signature verification.
  *
+ * \note           For limitation on the supported hash algorithms, please refer
+ *                 to the note at the top of this document.
+ *
  * \return         0 if the signature verifies, or a negative error code on failure.
  */
 int mbedtls_pkcs7_signed_data_verify(mbedtls_pkcs7 *pkcs7,
@@ -219,6 +226,9 @@ int mbedtls_pkcs7_signed_data_verify(mbedtls_pkcs7 *pkcs7,
  * \note           This function is different from mbedtls_pkcs7_signed_data_verify()
  *                 in that it is directly passed the hash of the data.
  *
+ * \note           For limitation on the supported hash algorithms, please refer
+ *                 to the note at the top of this document.
+ *
  * \return         0 if the signature verifies, or a negative error code on failure.
  */
 int mbedtls_pkcs7_signed_hash_verify(mbedtls_pkcs7 *pkcs7,