Don't exercise OAEP with small key and large hash
RSA-OAEP requires the key to be larger than a function of the hash size.
Ideally such combinations would be detected as a key/algorithm
incompatibility. However key/algorithm compatibility is currently tested
between the key type and the algorithm without considering the key size, and
this is inconvenient to change. So as a workaround, dispense
OAEP-with-too-small-hash from exercising, without including it in the
automatic operation-failure test generation.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/scripts/mbedtls_dev/crypto_knowledge.py b/scripts/mbedtls_dev/crypto_knowledge.py
index 55eb54d..1491a84 100644
--- a/scripts/mbedtls_dev/crypto_knowledge.py
+++ b/scripts/mbedtls_dev/crypto_knowledge.py
@@ -213,7 +213,7 @@
This function does not currently handle key derivation or PAKE.
"""
- #pylint: disable=too-many-return-statements
+ #pylint: disable=too-many-branches,too-many-return-statements
if alg.is_wildcard:
return False
if alg.is_invalid_truncation():
@@ -425,28 +425,41 @@
"""
return short_expression(self.expression, level=level)
+ HASH_LENGTH = {
+ 'PSA_ALG_MD5': 16,
+ 'PSA_ALG_SHA_1': 20,
+ }
+ HASH_LENGTH_BITS_RE = re.compile(r'([0-9]+)\Z')
+ @classmethod
+ def hash_length(cls, alg: str) -> int:
+ """The length of the given hash algorithm, in bytes."""
+ if alg in cls.HASH_LENGTH:
+ return cls.HASH_LENGTH[alg]
+ m = cls.HASH_LENGTH_BITS_RE.search(alg)
+ if m:
+ return int(m.group(1)) // 8
+ raise ValueError('Unknown hash length for ' + alg)
+
PERMITTED_TAG_LENGTHS = {
'PSA_ALG_CCM': frozenset([4, 6, 8, 10, 12, 14, 16]),
'PSA_ALG_CHACHA20_POLY1305': frozenset([16]),
'PSA_ALG_GCM': frozenset([4, 8, 12, 13, 14, 15, 16]),
}
MAC_LENGTH = {
- 'PSA_ALG_CBC_MAC': 16,
- 'PSA_ALG_CMAC': 16,
- 'PSA_ALG_HMAC(PSA_ALG_MD5)': 16,
- 'PSA_ALG_HMAC(PSA_ALG_SHA_1)': 20,
+ 'PSA_ALG_CBC_MAC': 16, # actually the block cipher length
+ 'PSA_ALG_CMAC': 16, # actually the block cipher length
}
- HMAC_WITH_NOMINAL_LENGTH_RE = re.compile(r'PSA_ALG_HMAC\(\w+([0-9])+\)\Z')
+ HMAC_RE = re.compile(r'PSA_ALG_HMAC\((.*)\)\Z')
@classmethod
- def mac_or_tag_length(cls, base: str) -> FrozenSet[int]:
+ def mac_or_tag_lengths(cls, base: str) -> FrozenSet[int]:
"""Return the set of permitted lengths for the given MAC or AEAD tag."""
if base in cls.PERMITTED_TAG_LENGTHS:
return cls.PERMITTED_TAG_LENGTHS[base]
max_length = cls.MAC_LENGTH.get(base, None)
if max_length is None:
- m = cls.HMAC_WITH_NOMINAL_LENGTH_RE.match(base)
+ m = cls.HMAC_RE.match(base)
if m:
- max_length = int(m.group(1)) // 8
+ max_length = cls.hash_length(m.group(1))
if max_length is None:
raise ValueError('Unknown permitted lengths for ' + base)
return frozenset(range(4, max_length + 1))
@@ -466,7 +479,7 @@
if m:
base = m.group('base')
to_length = int(m.group('length'), 0)
- permitted_lengths = self.mac_or_tag_length(base)
+ permitted_lengths = self.mac_or_tag_lengths(base)
if to_length not in permitted_lengths:
return True
return False
diff --git a/tests/scripts/generate_psa_tests.py b/tests/scripts/generate_psa_tests.py
index 27ee52d..6fd311e 100755
--- a/tests/scripts/generate_psa_tests.py
+++ b/tests/scripts/generate_psa_tests.py
@@ -518,6 +518,32 @@
self.version = version #type: int
self.forward = forward #type: bool
+ RSA_OAEP_RE = re.compile(r'PSA_ALG_RSA_OAEP\((.*)\)\Z')
+ @classmethod
+ def valid_key_size_for_algorithm(
+ cls,
+ key_type: psa_storage.Expr, bits: int,
+ alg: psa_storage.Expr
+ ) -> bool:
+ """Whether the given key type and size are valid for the algorithm.
+
+ Normally only the type and algorithm matter for compatibility, and
+ this is handled in crypto_knowledge.KeyType.can_do(). This function
+ exists to detect exceptional cases. Exceptional cases detected here
+ are not tested in OpFail and should therefore have manually written
+ test cases.
+ """
+ #pylint: disable=unused-argument
+ # OAEP requires room for two hashes plus wrapping
+ m = cls.RSA_OAEP_RE.match(alg.string)
+ if m:
+ hash_alg = m.group(1)
+ hash_length = crypto_knowledge.Algorithm.hash_length(hash_alg)
+ key_length = (bits + 7) // 8
+ # Leave enough room for at least one byte of plaintext
+ return key_length > 2 * hash_length + 2
+ return True
+
def make_test_case(self, key: StorageTestData) -> test_case.TestCase:
"""Construct a storage format test case for the given key.
@@ -546,7 +572,8 @@
# encodings of the attributes.
# Raw data keys have no useful exercise anyway so there is no
# loss of test coverage.
- if key.type.string != 'PSA_KEY_TYPE_RAW_DATA':
+ if key.type.string != 'PSA_KEY_TYPE_RAW_DATA' and \
+ self.valid_key_size_for_algorithm(key.type, key.bits, key.alg):
flags.append('TEST_FLAG_EXERCISE')
if 'READ_ONLY' in key.lifetime.string:
flags.append('TEST_FLAG_READ_ONLY')