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>

scripts/mbedtls_dev/crypto_knowledge.py

@@ -211,7 +211,7 @@ class KeyType:
 
         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():
@@ -423,28 +423,41 @@ class Algorithm:
         """
         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_CBC_MAC': 16, # actually the block cipher length
-        'PSA_ALG_CMAC': 16,
+        'PSA_ALG_CMAC': 16, # actually the block cipher length
-        'PSA_ALG_HMAC(PSA_ALG_MD5)': 16,
-        'PSA_ALG_HMAC(PSA_ALG_SHA_1)': 20,
     }
-    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))
@@ -464,7 +477,7 @@ class Algorithm:
         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

tests/scripts/generate_psa_tests.py

@@ -516,6 +516,32 @@ class StorageFormat:
         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.
 
@@ -544,7 +570,8 @@ class StorageFormat:
             # 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')

tests/suites/test_suite_psa_crypto_storage_format.v0.data

@@ -7218,7 +7218,7 @@ key_storage_read:0x0001:PSA_KEY_TYPE_RSA_KEY_PAIR:1024:PSA_KEY_USAGE_DECRYPT | P
 
 read type: RSA_PAIR 1024-bit, RSA_OAEP(SHA_512)
 depends_on:PSA_WANT_ALG_RSA_OAEP:PSA_WANT_ALG_SHA_512:PSA_WANT_KEY_TYPE_RSA_KEY_PAIR
-key_storage_read:0x0001:PSA_KEY_TYPE_RSA_KEY_PAIR:1024:PSA_KEY_USAGE_DECRYPT | PSA_KEY_USAGE_ENCRYPT | PSA_KEY_USAGE_EXPORT:PSA_ALG_RSA_OAEP(PSA_ALG_SHA_512):0x0000:"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":"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":TEST_FLAG_EXERCISE
+key_storage_read:0x0001:PSA_KEY_TYPE_RSA_KEY_PAIR:1024:PSA_KEY_USAGE_DECRYPT | PSA_KEY_USAGE_ENCRYPT | PSA_KEY_USAGE_EXPORT:PSA_ALG_RSA_OAEP(PSA_ALG_SHA_512):0x0000:"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":"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":0
 
 read type: RSA_PAIR 1024-bit, RSA_PKCS1V15_CRYPT
 depends_on:PSA_WANT_ALG_RSA_PKCS1V15_CRYPT:PSA_WANT_KEY_TYPE_RSA_KEY_PAIR
@@ -7530,7 +7530,7 @@ key_storage_read:0x0001:PSA_KEY_TYPE_RSA_PUBLIC_KEY:1024:PSA_KEY_USAGE_ENCRYPT |
 
 read type: RSA_PUB 1024-bit, RSA_OAEP(SHA_512)
 depends_on:PSA_WANT_ALG_RSA_OAEP:PSA_WANT_ALG_SHA_512:PSA_WANT_KEY_TYPE_RSA_PUBLIC_KEY
-key_storage_read:0x0001:PSA_KEY_TYPE_RSA_PUBLIC_KEY:1024:PSA_KEY_USAGE_ENCRYPT | PSA_KEY_USAGE_EXPORT:PSA_ALG_RSA_OAEP(PSA_ALG_SHA_512):0x0000:"30818902818100af057d396ee84fb75fdbb5c2b13c7fe5a654aa8aa2470b541ee1feb0b12d25c79711531249e1129628042dbbb6c120d1443524ef4c0e6e1d8956eeb2077af12349ddeee54483bc06c2c61948cd02b202e796aebd94d3a7cbf859c2c1819c324cb82b9cd34ede263a2abffe4733f077869e8660f7d6834da53d690ef7985f6bc30203010001":"505341004b455900000000000100000001400004010100000b030007000000008c00000030818902818100af057d396ee84fb75fdbb5c2b13c7fe5a654aa8aa2470b541ee1feb0b12d25c79711531249e1129628042dbbb6c120d1443524ef4c0e6e1d8956eeb2077af12349ddeee54483bc06c2c61948cd02b202e796aebd94d3a7cbf859c2c1819c324cb82b9cd34ede263a2abffe4733f077869e8660f7d6834da53d690ef7985f6bc30203010001":TEST_FLAG_EXERCISE
+key_storage_read:0x0001:PSA_KEY_TYPE_RSA_PUBLIC_KEY:1024:PSA_KEY_USAGE_ENCRYPT | PSA_KEY_USAGE_EXPORT:PSA_ALG_RSA_OAEP(PSA_ALG_SHA_512):0x0000:"30818902818100af057d396ee84fb75fdbb5c2b13c7fe5a654aa8aa2470b541ee1feb0b12d25c79711531249e1129628042dbbb6c120d1443524ef4c0e6e1d8956eeb2077af12349ddeee54483bc06c2c61948cd02b202e796aebd94d3a7cbf859c2c1819c324cb82b9cd34ede263a2abffe4733f077869e8660f7d6834da53d690ef7985f6bc30203010001":"505341004b455900000000000100000001400004010100000b030007000000008c00000030818902818100af057d396ee84fb75fdbb5c2b13c7fe5a654aa8aa2470b541ee1feb0b12d25c79711531249e1129628042dbbb6c120d1443524ef4c0e6e1d8956eeb2077af12349ddeee54483bc06c2c61948cd02b202e796aebd94d3a7cbf859c2c1819c324cb82b9cd34ede263a2abffe4733f077869e8660f7d6834da53d690ef7985f6bc30203010001":0
 
 read type: RSA_PUB 1024-bit, RSA_PKCS1V15_CRYPT
 depends_on:PSA_WANT_ALG_RSA_PKCS1V15_CRYPT:PSA_WANT_KEY_TYPE_RSA_PUBLIC_KEY