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Document and check the consistency of truncated MAC encodings

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Add comments noting that the maximum length of a MAC must fit in
PSA_ALG_MAC_TRUNCATION_MASK. Add a unit test that verifies that the
maximum MAC size fits.
This commit is contained in:
Gilles Peskine
2018-08-21 14:54:54 +02:00
parent 3111981d94
commit e1f2d7d1ac
4 changed files with 30 additions and 0 deletions
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11
include/psa/crypto.h
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@ -756,6 +756,13 @@ typedef uint32_t psa_algorithm_t;
(((alg) & (PSA_ALG_CATEGORY_MASK | PSA_ALG_MAC_SUBCATEGORY_MASK)) == \
PSA_ALG_HMAC_BASE)
/* In the encoding of a MAC algorithm, the bits corresponding to
* PSA_ALG_MAC_TRUNCATION_MASK encode the length to which the MAC is
* truncated. As an exception, the value 0 means the untruncated algorithm,
* whatever its length is. The length is encoded in 6 bits, so it can
* reach up to 63; the largest MAC is 64 bytes so its trivial truncation
* to full length is correctly encoded as 0 and any non-trivial truncation
* is correctly encoded as a value between 1 and 63. */
#define PSA_ALG_MAC_TRUNCATION_MASK ((psa_algorithm_t)0x00003f00)
#define PSA_MAC_TRUNCATION_OFFSET 8
@ -887,6 +894,10 @@ typedef uint32_t psa_algorithm_t;
#define PSA_ALG_CCM ((psa_algorithm_t)0x06001001)
#define PSA_ALG_GCM ((psa_algorithm_t)0x06001002)
/* In the encoding of a AEAD algorithm, the bits corresponding to
* PSA_ALG_AEAD_TAG_LENGTH_MASK encode the length of the AEAD tag.
* The constants for default lengths follow this encoding.
*/
#define PSA_ALG_AEAD_TAG_LENGTH_MASK ((psa_algorithm_t)0x00003f00)
#define PSA_AEAD_TAG_LENGTH_OFFSET 8

3
include/psa/crypto_sizes.h
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@ -79,6 +79,9 @@
*/
/* All non-HMAC MACs have a maximum size that's smaller than the
* minimum possible value of PSA_HASH_MAX_SIZE in this implementation. */
/* Note that the encoding of truncated MAC algorithms limits this value
* to 64 bytes.
*/
#define PSA_MAC_MAX_SIZE PSA_HASH_MAX_SIZE
/* The maximum size of an RSA key on this implementation, in bits.