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Document that the minimum truncated MAC length is implementation-defined

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Gilles Peskine 2018-08-21 14:55:08 +02:00
parent e1f2d7d1ac
commit 6d72ff9e79
2 changed files with 8 additions and 2 deletions
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4
include/psa/crypto.h
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@ -788,6 +788,10 @@ typedef uint32_t psa_algorithm_t;
* is true). This may be a truncated or untruncated * is true). This may be a truncated or untruncated
* MAC algorithm. * MAC algorithm.
* \param mac_length Desired length of the truncated MAC in bytes. * \param mac_length Desired length of the truncated MAC in bytes.
* This must be at most the full length of the MAC
* and must be at least an implementation-specified
* minimum. The implementation-specified minimum
* shall not be zero.
* *
* \return The corresponding MAC algorithm with the specified * \return The corresponding MAC algorithm with the specified
* length. * length.

6
library/psa_crypto.c
View file

@ -1600,8 +1600,10 @@ static psa_status_t psa_mac_setup( psa_mac_operation_t *operation,
} }
else if( truncated < 4 ) else if( truncated < 4 )
{ {
/* Too small to make any sense. Reject. 4 bytes is too small for /* A very short MAC is too short for security since it can be
* security but ancient protocols with 32-bit MACs do exist. */ * brute-forced. Ancient protocols with 32-bit MACs do exist,
* so we make this our minimum, even though 32 bits is still
* too small for security. */
status = PSA_ERROR_NOT_SUPPORTED; status = PSA_ERROR_NOT_SUPPORTED;
} }
else if( truncated > operation->mac_size ) else if( truncated > operation->mac_size )