PKCS#7 signing & verification - Certificate extension policies

src/cryptography/hazmat/primitives/serialization/pkcs7.py

@@ -21,6 +21,13 @@
     algorithms,
 )
 from cryptography.utils import _check_byteslike
+from cryptography.x509 import Certificate
+from cryptography.x509.oid import ExtendedKeyUsageOID
+from cryptography.x509.verification import (
+    Criticality,
+    ExtensionPolicy,
+    Policy,
+)
 
 load_pem_pkcs7_certificates = rust_pkcs7.load_pem_pkcs7_certificates
 
@@ -53,6 +60,120 @@ class PKCS7Options(utils.Enum):
     NoCerts = "Don't embed signer certificate"
 
 
+def pkcs7_x509_extension_policies() -> tuple[ExtensionPolicy, ExtensionPolicy]:
+    """
+    Gets the default X.509 extension policy for S/MIME, based on RFC 8550.
+    Visit https://www.rfc-editor.org/rfc/rfc8550#section-4.4 for more info.
+    """
+    # CA policy
+    ca_policy = ExtensionPolicy.webpki_defaults_ca()
+
+    # EE policy
+    def _validate_basic_constraints(
+        policy: Policy, cert: Certificate, bc: x509.BasicConstraints | None
+    ) -> None:
+        """
+        We check that Certificates used as EE (i.e., the cert used to sign
+        a PKCS#7/SMIME message) must not have ca=true in their basic
+        constraints extension. RFC 5280 doesn't impose this requirement, but we
+        firmly agree about it being best practice.
+        """
+        if bc is not None and bc.ca:
+            raise ValueError("Basic Constraints CA must be False.")
+
+    def _validate_key_usage(
+        policy: Policy, cert: Certificate, ku: x509.KeyUsage | None
+    ) -> None:
+        """
+        Checks that the Key Usage extension, if present, has at least one of
+        the digital signature or content commitment (formerly non-repudiation)
+        bits set.
+        """
+        if (
+            ku is not None
+            and not ku.digital_signature
+            and not ku.content_commitment
+        ):
+            raise ValueError(
+                "Key Usage, if specified, must have at least one of the "
+                "digital signature or content commitment (formerly non "
+                "repudiation) bits set."
+            )
+
+    def _validate_subject_alternative_name(
+        policy: Policy,
+        cert: Certificate,
+        san: x509.SubjectAlternativeName,
+    ) -> None:
+        """
+        For each general name in the SAN, for those which are email addresses:
+        - If it is an RFC822Name, general part must be ascii.
+        - If it is an OtherName, general part must be non-ascii.
+        """
+        for general_name in san:
+            if (
+                isinstance(general_name, x509.RFC822Name)
+                and "@" in general_name.value
+                and not general_name.value.split("@")[0].isascii()
+            ):
+                raise ValueError(
+                    f"RFC822Name {general_name.value} contains non-ASCII "
+                    "characters."
+                )
+            if (
+                isinstance(general_name, x509.OtherName)
+                and "@" in general_name.value.decode()
+                and general_name.value.decode().split("@")[0].isascii()
+            ):
+                raise ValueError(
+                    f"OtherName {general_name.value.decode()} is ASCII, "
+                    "so must be stored in RFC822Name."
+                )
+
+    def _validate_extended_key_usage(
+        policy: Policy, cert: Certificate, eku: x509.ExtendedKeyUsage | None
+    ) -> None:
+        """
+        Checks that the Extended Key Usage extension, if present,
+        includes either emailProtection or anyExtendedKeyUsage bits.
+        """
+        if (
+            eku is not None
+            and ExtendedKeyUsageOID.EMAIL_PROTECTION not in eku
+            and ExtendedKeyUsageOID.ANY_EXTENDED_KEY_USAGE not in eku
+        ):
+            raise ValueError(
+                "Extended Key Usage, if specified, must include "
+                "emailProtection or anyExtendedKeyUsage."
+            )
+
+    ee_policy = (
+        ExtensionPolicy.webpki_defaults_ee()
+        .may_be_present(
+            x509.BasicConstraints,
+            Criticality.AGNOSTIC,
+            _validate_basic_constraints,
+        )
+        .may_be_present(
+            x509.KeyUsage,
+            Criticality.CRITICAL,
+            _validate_key_usage,
+        )
+        .require_present(
+            x509.SubjectAlternativeName,
+            Criticality.AGNOSTIC,
+            _validate_subject_alternative_name,
+        )
+        .may_be_present(
+            x509.ExtendedKeyUsage,
+            Criticality.AGNOSTIC,
+            _validate_extended_key_usage,
+        )
+    )
+
+    return ca_policy, ee_policy
+
+
 class PKCS7SignatureBuilder:
     def __init__(
         self,

tests/hazmat/primitives/test_pkcs7.py

@@ -18,6 +18,15 @@
 from cryptography.hazmat.primitives.asymmetric import ed25519, padding, rsa
 from cryptography.hazmat.primitives.ciphers import algorithms
 from cryptography.hazmat.primitives.serialization import pkcs7
+from cryptography.x509.oid import (
+    ExtendedKeyUsageOID,
+    ObjectIdentifier,
+)
+from cryptography.x509.verification import (
+    PolicyBuilder,
+    Store,
+    VerificationError,
+)
 from tests.x509.test_x509 import _generate_ca_and_leaf
 
 from ...hazmat.primitives.fixtures_rsa import (
@@ -139,6 +148,145 @@ def _load_cert_key():
     return cert, key
 
 
+class TestPKCS7VerifyCertificate:
+    @staticmethod
+    def build_pkcs7_certificate(
+        ca: bool = False,
+        digital_signature: bool = True,
+        usages: typing.Optional[typing.List[ObjectIdentifier]] = None,
+    ) -> x509.Certificate:
+        """
+        This static method is a helper to build certificates allowing us
+        to test all cases in PKCS#7 certificate verification.
+        """
+        # Load the standard certificate and private key
+        certificate, private_key = _load_cert_key()
+
+        # Basic certificate builder
+        certificate_builder = (
+            x509.CertificateBuilder()
+            .serial_number(certificate.serial_number)
+            .subject_name(certificate.subject)
+            .issuer_name(certificate.issuer)
+            .public_key(private_key.public_key())
+            .not_valid_before(certificate.not_valid_before)
+            .not_valid_after(certificate.not_valid_after)
+        )
+
+        # Add AuthorityKeyIdentifier extension
+        aki = x509.AuthorityKeyIdentifier(
+            b"\xfc\xeb\xb4\xd8\x12\xf2\xc9=\x99\xc3<g\xf4}7}\xe6\x13\xed\xfa",
+            None,
+            None,
+        )
+        certificate_builder = certificate_builder.add_extension(
+            aki,
+            critical=False,
+        )
+
+        # Add SubjectAlternativeName extension
+        san = x509.SubjectAlternativeName(
+            [
+                x509.RFC822Name("[email protected]"),
+            ]
+        )
+        certificate_builder = certificate_builder.add_extension(
+            san,
+            critical=True,
+        )
+
+        # Add BasicConstraints extension
+        bc_extension = x509.BasicConstraints(ca=ca, path_length=None)
+        certificate_builder = certificate_builder.add_extension(
+            bc_extension, False
+        )
+
+        # Add KeyUsage extension
+        ku_extension = x509.KeyUsage(
+            digital_signature=digital_signature,
+            content_commitment=False,
+            key_encipherment=True,
+            data_encipherment=True,
+            key_agreement=True,
+            key_cert_sign=True,
+            crl_sign=True,
+            encipher_only=False,
+            decipher_only=False,
+        )
+        certificate_builder = certificate_builder.add_extension(
+            ku_extension, True
+        )
+
+        # Add valid ExtendedKeyUsage extension
+        usages = usages or [ExtendedKeyUsageOID.EMAIL_PROTECTION]
+        certificate_builder = certificate_builder.add_extension(
+            x509.ExtendedKeyUsage(usages), True
+        )
+
+        # Build the certificate
+        return certificate_builder.sign(
+            private_key, certificate.signature_hash_algorithm, None
+        )
+
+    def test_verify_pkcs7_certificate(self):
+        # Prepare the parameters
+        certificate = self.build_pkcs7_certificate()
+        ca_policy, ee_policy = pkcs7.pkcs7_x509_extension_policies()
+
+        # Verify the certificate
+        verifier = (
+            PolicyBuilder()
+            .store(Store([certificate]))
+            .extension_policies(ca_policy=ca_policy, ee_policy=ee_policy)
+            .build_client_verifier()
+        )
+        verifier.verify(certificate, [])
+
+    @pytest.mark.parametrize(
+        "arguments",
+        [
+            {"ca": True},
+            {"digital_signature": False},
+            {"usages": [ExtendedKeyUsageOID.CLIENT_AUTH]},
+        ],
+    )
+    def test_verify_invalid_pkcs7_certificate(self, arguments: dict):
+        # Prepare the parameters
+        certificate = self.build_pkcs7_certificate(**arguments)
+
+        # Verify the certificate
+        self.verify_invalid_pkcs7_certificate(certificate)
+
+    @staticmethod
+    def verify_invalid_pkcs7_certificate(certificate: x509.Certificate):
+        ca_policy, ee_policy = pkcs7.pkcs7_x509_extension_policies()
+        verifier = (
+            PolicyBuilder()
+            .store(Store([certificate]))
+            .extension_policies(ca_policy=ca_policy, ee_policy=ee_policy)
+            .build_client_verifier()
+        )
+
+        with pytest.raises(VerificationError):
+            verifier.verify(certificate, [])
+
+    @pytest.mark.parametrize(
+        "filename", ["non-ascii-san.pem", "ascii-san.pem"]
+    )
+    def test_verify_pkcs7_certificate_wrong_san(self, filename):
+        # Read a certificate with an invalid SAN
+        pkcs7_certificate = load_vectors_from_file(
+            os.path.join("pkcs7", filename),
+            loader=lambda pemfile: x509.load_pem_x509_certificate(
+                pemfile.read()
+            ),
+            mode="rb",
+        )
+
+        # Verify the certificate
+        self.verify_invalid_pkcs7_certificate(pkcs7_certificate)
+
+
 @pytest.mark.supported(
     only_if=lambda backend: backend.pkcs7_supported(),
     skip_message="Requires OpenSSL with PKCS7 support",