Add certificate signature verification

CHANGELOG.rst

@@ -51,6 +51,8 @@ Changelog
   CRLs.
 * Unrecognized non-critical X.509 extensions are now parsed into an
   :class:`~cryptography.x509.UnrecognizedExtension` object.
+* Added :class: `~cryptography.x509.CertificationVerificationContext` to allow
+  verification of certificate signatures.
 
 1.1.2 - 2015-12-10
 ~~~~~~~~~~~~~~~~~~

docs/x509/reference.rst

@@ -1116,6 +1116,34 @@ X.509 CSR (Certificate Signing Request) Builder Object
 
         The dotted string value of the OID (e.g. ``"2.5.4.3"``)
 
+X.509 Certificate Verification
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. class:: CertificateVerificationContext
+
+    .. versionadded:: 1.2
+
+    .. method:: update(certificate)
+
+       Processes the provided certificate. Raises an exception if the
+       certificate is invalid.
+
+       :param certificate: The certificate whose signature needs to be
+           verified.
+
+       :raises cryptography.x509.verification.InvalidCertificate: If the
+           certificate is an invalid type.
+
+    .. method:: verify()
+
+       Verifies the signature of the certificate provided to update against
+       the certificate associated with the context. Raises an exception if the
+       verification process fails.
+
+       :raises cryptography.x509.verification.InvalidCertificate: If the
+           certificate provided to update was not issued by the certificate
+           associated with the context.
+
 .. _general_name_classes:
 
 General Name Classes
@@ -2467,6 +2495,13 @@ Exceptions
         The integer value of the unsupported type. The complete list of
         types can be found in `RFC 5280 section 4.2.1.6`_.
 
+.. currentmodule:: cryptography.x509.verification
+
+.. class:: InvalidCertificate
+
+    This is raised by :class:`~cryptography.x509.CertificateVerificationContext`
+    when dealing with invalid certificate arguments and when certificate
+    signature verification fails.
 
 .. _`RFC 5280 section 4.2.1.1`: https://tools.ietf.org/html/rfc5280#section-4.2.1.1
 .. _`RFC 5280 section 4.2.1.6`: https://tools.ietf.org/html/rfc5280#section-4.2.1.6

src/cryptography/x509/__init__.py

@@ -34,6 +34,10 @@
     CertificatePoliciesOID, ExtendedKeyUsageOID, ExtensionOID, NameOID,
     ObjectIdentifier, SignatureAlgorithmOID, _SIG_OIDS_TO_HASH
 )
+from cryptography.x509.verification import (
+    CertificateVerificationContext, InvalidCertificate,
+    InvalidSigningCertificate
+)
 
 
 OID_AUTHORITY_INFORMATION_ACCESS = ExtensionOID.AUTHORITY_INFORMATION_ACCESS
@@ -117,6 +121,8 @@
     "UnsupportedExtension",
     "ExtensionNotFound",
     "UnsupportedGeneralNameType",
+    "InvalidCertificate",
+    "InvalidSigningCertificate",
     "NameAttribute",
     "Name",
     "ObjectIdentifier",
@@ -160,6 +166,7 @@
     "RevokedCertificateBuilder",
     "CertificateSigningRequestBuilder",
     "CertificateBuilder",
+    "CertificateVerificationContext",
     "Version",
     "_SIG_OIDS_TO_HASH",
     "OID_CA_ISSUERS",

src/cryptography/x509/verification.py

@@ -0,0 +1,93 @@
+# This file is dual licensed under the terms of the Apache License, Version
+# 2.0, and the BSD License. See the LICENSE file in the root of this repository
+# for complete details.
+
+from __future__ import absolute_import, division, print_function
+
+from cryptography.hazmat.primitives.asymmetric import ec, padding, rsa
+from cryptography.x509 import Certificate
+from cryptography.x509.oid import ExtensionOID
+
+
+class InvalidCertificate(Exception):
+    pass
+
+
+class InvalidSigningCertificate(Exception):
+    pass
+
+
+def _can_sign_certificates(certificate):
+    basic_constraints = certificate.extensions.get_extension_for_oid(
+        ExtensionOID.BASIC_CONSTRAINTS).value
+    key_usage = certificate.extensions.get_extension_for_oid(
+        ExtensionOID.KEY_USAGE).value
+
+    if not basic_constraints.ca:
+        raise InvalidSigningCertificate(
+            "The certificate is not marked as a CA in its BasicConstraints "
+            "extension."
+        )
+    elif not key_usage.key_cert_sign:
+        raise InvalidSigningCertificate(
+            "The certificate public key is not marked for verifying "
+            "certificates in its KeyUsage extension."
+        )
+    else:
+        return True
+
+
+def _is_issuing_certificate(issuing_certificate, issued_certificate):
+        return (issuing_certificate.subject == issued_certificate.issuer)
+
+
+class CertificateVerificationContext(object):
+    def __init__(self, certificate):
+        if not isinstance(certificate, Certificate):
+            raise InvalidCertificate(
+                "The signing certificate must be a Certificate."
+            )
+        _can_sign_certificates(certificate)
+
+        self._signing_cert = certificate
+
+    def update(self, certificate):
+        """
+        Processes the provided certificate. Raises an exception if the
+        certificate is invalid.
+        """
+        if not isinstance(certificate, Certificate):
+            raise InvalidCertificate(
+                "The signed certificate must be a Certificate."
+            )
+
+        self._signed_cert = certificate
+
+    def verify(self):
+        """
+        Verifies the signature of the certificate provided to update against
+        the certificate associated with the context. Raises an exception if
+        the verification process fails.
+        """
+        if not _is_issuing_certificate(self._signing_cert, self._signed_cert):
+            raise InvalidCertificate(
+                "The certificate issuer does not match the subject name of "
+                "the context certificate."
+            )
+
+        signature_hash_algorithm = self._signed_cert.signature_hash_algorithm
+        signature_bytes = self._signed_cert.signature
+        signer_public_key = self._signing_cert.public_key()
+
+        if isinstance(signer_public_key, rsa.RSAPublicKey):
+            verifier = signer_public_key.verifier(
+                signature_bytes, padding.PKCS1v15(), signature_hash_algorithm)
+        elif isinstance(signer_public_key, ec.EllipticCurvePublicKey):
+            verifier = signer_public_key.verifier(
+                signature_bytes, ec.ECDSA(signature_hash_algorithm))
+        else:
+            verifier = signer_public_key.verifier(
+                signature_bytes, signature_hash_algorithm)
+
+        verifier.update(self._signed_cert.tbs_certificate_bytes)
+        verifier.verify()