feat: implement minimum key length validation for HMAC and RSA algorithms

for more information, see https://pre-commit.ci

Author: amanjolhe

jwt/algorithms.py

@@ -316,6 +316,18 @@ class HMACAlgorithm(Algorithm):
     def __init__(self, hash_alg: HashlibHash) -> None:
         self.hash_alg = hash_alg
 
+    def _get_min_key_length(self) -> int:
+        """Get minimum key length in bytes based on hash algorithm."""
+        if self.hash_alg == hashlib.sha256:
+            return 32  # 256 bits for HS256
+        elif self.hash_alg == hashlib.sha384:
+            return 48  # 384 bits for HS384
+        elif self.hash_alg == hashlib.sha512:
+            return 64  # 512 bits for HS512
+        else:
+            # For any other hash algorithm, require at least 32 bytes (256 bits)
+            return 32
+
     def prepare_key(self, key: str | bytes) -> bytes:
         key_bytes = force_bytes(key)
 
@@ -325,6 +337,24 @@ def prepare_key(self, key: str | bytes) -> bytes:
                 " should not be used as an HMAC secret."
             )
 
+        # Enforce minimum key lengths per RFC 7518 and NIST guidelines
+        min_key_length = self._get_min_key_length()
+        if len(key_bytes) < min_key_length:
+            # Get algorithm name for error message
+            alg_name = "HMAC"
+            if self.hash_alg == hashlib.sha256:
+                alg_name = "HS256"
+            elif self.hash_alg == hashlib.sha384:
+                alg_name = "HS384"
+            elif self.hash_alg == hashlib.sha512:
+                alg_name = "HS512"
+
+            raise InvalidKeyError(
+                f"HMAC key must be at least {min_key_length * 8} bits "
+                f"({min_key_length} bytes) for {alg_name} algorithm. "
+                f"Key provided is {len(key_bytes) * 8} bits ({len(key_bytes)} bytes)."
+            )
+
         return key_bytes
 
     @overload
@@ -366,7 +396,18 @@ def from_jwk(jwk: str | JWKDict) -> bytes:
         if obj.get("kty") != "oct":
             raise InvalidKeyError("Not an HMAC key")
 
-        return base64url_decode(obj["k"])
+        key_bytes = base64url_decode(obj["k"])
+
+        # Validate key length - use a conservative minimum of 32 bytes (256 bits)
+        min_key_length = 32  # 256 bits minimum
+        if len(key_bytes) < min_key_length:
+            raise InvalidKeyError(
+                f"HMAC key must be at least {min_key_length * 8} bits "
+                f"({min_key_length} bytes). Key provided is {len(key_bytes) * 8} "
+                f"bits ({len(key_bytes)} bytes)."
+            )
+
+        return key_bytes
 
     def sign(self, msg: bytes, key: bytes) -> bytes:
         return hmac.new(key, msg, self.hash_alg).digest()
@@ -392,8 +433,32 @@ class RSAAlgorithm(Algorithm):
         def __init__(self, hash_alg: type[hashes.HashAlgorithm]) -> None:
             self.hash_alg = hash_alg
 
+        def _validate_rsa_key_size(self, key: AllowedRSAKeys) -> None:
+            """Validate RSA key size meets minimum security requirements."""
+            key_size = key.key_size
+            min_key_size = 2048  # Minimum 2048 bits per RFC 7518 and NIST SP800-117
+
+            if key_size < min_key_size:
+                raise InvalidKeyError(
+                    f"RSA key must be at least {min_key_size} bits. "
+                    f"Key provided is {key_size} bits."
+                )
+
+        @staticmethod
+        def _validate_rsa_key_size_static(key: AllowedRSAKeys) -> None:
+            """Static version of RSA key size validation for use in static methods."""
+            key_size = key.key_size
+            min_key_size = 2048  # Minimum 2048 bits per RFC 7518 and NIST SP800-117
+
+            if key_size < min_key_size:
+                raise InvalidKeyError(
+                    f"RSA key must be at least {min_key_size} bits. "
+                    f"Key provided is {key_size} bits."
+                )
+
         def prepare_key(self, key: AllowedRSAKeys | str | bytes) -> AllowedRSAKeys:
             if isinstance(key, self._crypto_key_types):
+                self._validate_rsa_key_size(key)
                 return key
 
             if not isinstance(key, (bytes, str)):
@@ -405,18 +470,24 @@ def prepare_key(self, key: AllowedRSAKeys | str | bytes) -> AllowedRSAKeys:
                 if key_bytes.startswith(b"ssh-rsa"):
                     public_key: PublicKeyTypes = load_ssh_public_key(key_bytes)
                     self.check_crypto_key_type(public_key)
-                    return cast(RSAPublicKey, public_key)
+                    rsa_public_key = cast(RSAPublicKey, public_key)
+                    self._validate_rsa_key_size(rsa_public_key)
+                    return rsa_public_key
                 else:
                     private_key: PrivateKeyTypes = load_pem_private_key(
                         key_bytes, password=None
                     )
                     self.check_crypto_key_type(private_key)
-                    return cast(RSAPrivateKey, private_key)
+                    rsa_private_key = cast(RSAPrivateKey, private_key)
+                    self._validate_rsa_key_size(rsa_private_key)
+                    return rsa_private_key
             except ValueError:
                 try:
                     public_key = load_pem_public_key(key_bytes)
                     self.check_crypto_key_type(public_key)
-                    return cast(RSAPublicKey, public_key)
+                    rsa_public_key = cast(RSAPublicKey, public_key)
+                    self._validate_rsa_key_size(rsa_public_key)
+                    return rsa_public_key
                 except (ValueError, UnsupportedAlgorithm):
                     raise InvalidKeyError(
                         "Could not parse the provided public key."
@@ -519,6 +590,9 @@ def from_jwk(jwk: str | JWKDict) -> AllowedRSAKeys:
                         iqmp=from_base64url_uint(obj["qi"]),
                         public_numbers=public_numbers,
                     )
+                    private_key = numbers.private_key()
+                    RSAAlgorithm._validate_rsa_key_size_static(private_key)
+                    return private_key
                 else:
                     d = from_base64url_uint(obj["d"])
                     p, q = rsa_recover_prime_factors(
@@ -535,13 +609,17 @@ def from_jwk(jwk: str | JWKDict) -> AllowedRSAKeys:
                         public_numbers=public_numbers,
                     )
 
-                return numbers.private_key()
+                private_key = numbers.private_key()
+                RSAAlgorithm._validate_rsa_key_size_static(private_key)
+                return private_key
             elif "n" in obj and "e" in obj:
                 # Public key
-                return RSAPublicNumbers(
+                public_key = RSAPublicNumbers(
                     from_base64url_uint(obj["e"]),
                     from_base64url_uint(obj["n"]),
                 ).public_key()
+                RSAAlgorithm._validate_rsa_key_size_static(public_key)
+                return public_key
             else:
                 raise InvalidKeyError("Not a public or private key")
 
@@ -793,7 +871,7 @@ def __init__(self, **kwargs: Any) -> None:
         def prepare_key(self, key: AllowedOKPKeys | str | bytes) -> AllowedOKPKeys:
             if not isinstance(key, (str, bytes)):
                 self.check_crypto_key_type(key)
-                return cast("AllowedOKPKeys", key)
+                return key
 
             key_str = key.decode("utf-8") if isinstance(key, bytes) else key
             key_bytes = key.encode("utf-8") if isinstance(key, str) else key

tests/test_algorithms.py

@@ -70,7 +70,7 @@ def test_hmac_should_reject_nonstring_key(self):
     def test_hmac_should_accept_unicode_key(self):
         algo = HMACAlgorithm(HMACAlgorithm.SHA256)
 
-        algo.prepare_key("awesome")
+        algo.prepare_key("awesome" * 5)  # 35 characters > 32 bytes minimum
 
     @pytest.mark.parametrize(
         "key",
@@ -101,12 +101,12 @@ def test_hmac_jwk_should_parse_and_verify(self):
     @pytest.mark.parametrize("as_dict", (False, True))
     def test_hmac_to_jwk_returns_correct_values(self, as_dict):
         algo = HMACAlgorithm(HMACAlgorithm.SHA256)
-        key: Any = algo.to_jwk("secret", as_dict=as_dict)
+        key: Any = algo.to_jwk("aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", as_dict=as_dict)
 
         if not as_dict:
             key = json.loads(key)
 
-        assert key == {"kty": "oct", "k": "c2VjcmV0"}
+        assert key == {"kty": "oct", "k": "YWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWE"}
 
     def test_hmac_from_jwk_should_raise_exception_if_not_hmac_key(self):
         algo = HMACAlgorithm(HMACAlgorithm.SHA256)
@@ -122,6 +122,57 @@ def test_hmac_from_jwk_should_raise_exception_if_empty_json(self):
             with pytest.raises(InvalidKeyError):
                 algo.from_jwk(keyfile.read())
 
+    # CVE-2025-45768: Test minimum key length enforcement
+    @pytest.mark.parametrize(
+        "hash_alg,min_length,weak_key",
+        [
+            (HMACAlgorithm.SHA256, 32, b"short"),  # 5 bytes, too short for HS256
+            (HMACAlgorithm.SHA256, 32, b"a" * 31),  # 31 bytes, just under minimum
+            (HMACAlgorithm.SHA384, 48, b"b" * 47),  # 47 bytes, just under minimum
+            (HMACAlgorithm.SHA512, 64, b"c" * 63),  # 63 bytes, just under minimum
+        ],
+    )
+    def test_hmac_should_reject_weak_keys(self, hash_alg, min_length, weak_key):
+        """Test that HMAC keys below minimum length are rejected (CVE-2025-45768)"""
+        algo = HMACAlgorithm(hash_alg)
+
+        with pytest.raises(InvalidKeyError) as excinfo:
+            algo.prepare_key(weak_key)
+
+        error_msg = str(excinfo.value)
+        assert f"at least {min_length * 8} bits" in error_msg
+        assert f"Key provided is {len(weak_key) * 8} bits" in error_msg
+
+    @pytest.mark.parametrize(
+        "hash_alg,adequate_key",
+        [
+            (HMACAlgorithm.SHA256, b"a" * 32),  # 32 bytes for HS256
+            (HMACAlgorithm.SHA384, b"b" * 48),  # 48 bytes for HS384
+            (HMACAlgorithm.SHA512, b"c" * 64),  # 64 bytes for HS512
+        ],
+    )
+    def test_hmac_should_accept_adequate_keys(self, hash_alg, adequate_key):
+        """Test that HMAC keys at or above minimum length are accepted"""
+        algo = HMACAlgorithm(hash_alg)
+
+        # Should not raise an exception
+        prepared_key = algo.prepare_key(adequate_key)
+        assert prepared_key == adequate_key
+
+    def test_hmac_from_jwk_should_reject_weak_keys(self):
+        """Test that weak HMAC keys are rejected when loaded from JWK (CVE-2025-45768)"""
+        algo = HMACAlgorithm(HMACAlgorithm.SHA256)
+
+        # Create a JWK with a weak key (5 bytes)
+        weak_jwk = {"kty": "oct", "k": "c2hvcnQ"}  # base64url("short") - only 5 bytes
+
+        with pytest.raises(InvalidKeyError) as excinfo:
+            algo.from_jwk(weak_jwk)
+
+        error_msg = str(excinfo.value)
+        assert "at least 256 bits" in error_msg
+        assert "40 bits" in error_msg  # 5 bytes * 8 = 40 bits
+
     @crypto_required
     def test_rsa_should_parse_pem_public_key(self):
         algo = RSAAlgorithm(RSAAlgorithm.SHA256)
@@ -173,6 +224,75 @@ def test_rsa_verify_should_return_false_if_signature_invalid(self):
         result = algo.verify(message, pub_key, sig)
         assert not result
 
+    # CVE-2025-45768: Test RSA minimum key size enforcement
+    @crypto_required
+    def test_rsa_should_reject_weak_keys(self):
+        """Test that RSA keys below 2048 bits are rejected (CVE-2025-45768)"""
+        from cryptography.hazmat.primitives.asymmetric import rsa
+
+        algo = RSAAlgorithm(RSAAlgorithm.SHA256)
+
+        # Generate a weak 1024-bit RSA key
+        weak_private_key = rsa.generate_private_key(
+            public_exponent=65537, key_size=1024
+        )
+        weak_public_key = weak_private_key.public_key()
+
+        # Test with private key
+        with pytest.raises(InvalidKeyError) as excinfo:
+            algo.prepare_key(weak_private_key)
+
+        error_msg = str(excinfo.value)
+        assert "at least 2048 bits" in error_msg
+        assert "1024 bits" in error_msg
+
+        # Test with public key
+        with pytest.raises(InvalidKeyError) as excinfo:
+            algo.prepare_key(weak_public_key)
+
+        error_msg = str(excinfo.value)
+        assert "at least 2048 bits" in error_msg
+        assert "1024 bits" in error_msg
+
+    @crypto_required
+    def test_rsa_should_accept_adequate_keys(self):
+        """Test that RSA keys at or above 2048 bits are accepted"""
+        from cryptography.hazmat.primitives.asymmetric import rsa
+
+        algo = RSAAlgorithm(RSAAlgorithm.SHA256)
+
+        # Generate a strong 2048-bit RSA key
+        strong_private_key = rsa.generate_private_key(
+            public_exponent=65537, key_size=2048
+        )
+        strong_public_key = strong_private_key.public_key()
+
+        # Should not raise exceptions
+        prepared_private = algo.prepare_key(strong_private_key)
+        prepared_public = algo.prepare_key(strong_public_key)
+
+        assert prepared_private == strong_private_key
+        assert prepared_public == strong_public_key
+
+    @crypto_required
+    def test_rsa_from_jwk_should_reject_weak_keys(self):
+        """Test that weak RSA keys are rejected when loaded from JWK (CVE-2025-45768)"""
+        from cryptography.hazmat.primitives.asymmetric import rsa
+
+        # Generate a weak 1024-bit RSA key and convert to JWK
+        weak_key = rsa.generate_private_key(public_exponent=65537, key_size=1024)
+
+        # Convert to JWK format (this will work since to_jwk doesn't validate)
+        weak_jwk = RSAAlgorithm.to_jwk(weak_key, as_dict=True)
+
+        # Now try to load it back - should fail
+        with pytest.raises(InvalidKeyError) as excinfo:
+            RSAAlgorithm.from_jwk(weak_jwk)
+
+        error_msg = str(excinfo.value)
+        assert "at least 2048 bits" in error_msg
+        assert "1024 bits" in error_msg
+
     @crypto_required
     def test_ec_jwk_public_and_private_keys_should_parse_and_verify(self):
         tests = {