New pySim.esim.x509_cert module for X.509 certificate handling

Change-Id: Ia8cc2dac02fcd96624dc6d9348f103373eeeb614
---
A pySim/esim/x509_cert.py
1 file changed, 146 insertions(+), 0 deletions(-)

diff --git a/pySim/esim/x509_cert.py b/pySim/esim/x509_cert.py
new file mode 100644
index 0000000..f6c6d43
--- /dev/null
+++ b/pySim/esim/x509_cert.py
@@ -0,0 +1,137 @@
+# Implementation of X.509 certificate handling in GSMA eSIM
+# as per SGP22 v3.0
+#
+# (C) 2024 by Harald Welte <laforge@osmocom.org>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+#
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+import requests
+from typing import Optional, List
+
+from cryptography.hazmat.primitives.asymmetric import ec, padding
+from cryptography.hazmat.primitives import hashes
+from cryptography.exceptions import InvalidSignature
+from cryptography import x509
+
+from pySim.utils import b2h
+
+def check_signed(signed: x509.Certificate, signer: x509.Certificate) -> bool:
+    """Verify if 'signed' certificate was signed using 'signer'."""
+    # this code only works for ECDSA, but this is all we need for GSMA eSIM
+    pkey = signer.public_key()
+    # this 'signed.signature_algorithm_parameters' below requires cryptopgraphy 41.0.0 :(
+    pkey.verify(signed.signature, signed.tbs_certificate_bytes, signed.signature_algorithm_parameters)
+
+def cert_get_subject_key_id(cert: x509.Certificate) -> bytes:
+    """Obtain the subject key identifier of the given cert object (as raw bytes)."""
+    ski_ext = cert.extensions.get_extension_for_class(x509.SubjectKeyIdentifier).value
+    return ski_ext.key_identifier
+
+def cert_get_auth_key_id(cert: x509.Certificate) -> bytes:
+    """Obtain the authority key identifier of the given cert object (as raw bytes)."""
+    aki_ext = cert.extensions.get_extension_for_class(x509.AuthorityKeyIdentifier).value
+    return aki_ext.key_identifier
+
+class VerifyError(Exception):
+    """An error during certificate verification,"""
+    pass
+
+class CertificateSet:
+    """A set of certificates consisting of a trusted [self-signed] CA root certificate,
+    and an optional number of intermediate certificates.  Can be used to verify the certificate chain
+    of any given other certificate."""
+    def __init__(self, root_cert: x509.Certificate):
+        check_signed(root_cert, root_cert)
+        # TODO: check other mandatory attributes for CA Cert
+        usage_ext = root_cert.extensions.get_extension_for_class(x509.KeyUsage).value
+        if not usage_ext.key_cert_sign:
+            raise ValueError('Given root certificate key usage does not permit signing of certificates')
+        if not usage_ext.crl_sign:
+            raise ValueError('Given root certificate key usage does not permit signing of CRLs')
+        self.root_cert = root_cert
+        self.intermediate_certs = {}
+        self.crl = None
+
+    def load_crl(self, urls: Optional[List[str]] = None):
+        if urls and type(urls) is str:
+            urls = [urls]
+        if not urls:
+            # generate list of CRL URLs from root CA certificate
+            crl_ext = self.root_cert.extensions.get_extension_for_class(x509.CRLDistributionPoints).value
+            name_list = [x.full_name for x in crl_ext]
+            merged_list = []
+            for n in name_list:
+                merged_list += n
+            uri_list = filter(lambda x: isinstance(x, x509.UniformResourceIdentifier), merged_list)
+            urls = [x.value for x in uri_list]
+
+        for url in urls:
+            try:
+                crl_bytes = requests.get(url)
+            except requests.exceptions.ConnectionError:
+                continue
+            crl = x509.load_der_x509_crl(crl_bytes)
+            if not crl.is_signature_valid(self.root_cert.public_key()):
+                raise ValueError('Given CRL has incorrect signature and cannot be trusted')
+            # FIXME: various other checks
+            self.crl = crl
+            # FIXME: should we support multiple CRLs? we only support a single CRL right now
+            return
+        # FIXME: report on success/failure
+
+    @property
+    def root_cert_id(self) -> bytes:
+        return cert_get_subject_key_id(self.root_cert)
+
+    def add_intermediate_cert(self, cert: x509.Certificate):
+        """Add a potential intermediate certificate to the CertificateSet."""
+        # TODO: check mandatory attributes for intermediate cert
+        usage_ext = cert.extensions.get_extension_for_class(x509.KeyUsage).value
+        if not usage_ext.key_cert_sign:
+            raise ValueError('Given intermediate certificate key usage does not permit signing of certificates')
+        aki = cert_get_auth_key_id(cert)
+        ski = cert_get_subject_key_id(cert)
+        if aki == ski:
+            raise ValueError('Cannot add self-signed cert as intermediate cert')
+        self.intermediate_certs[ski] = cert
+        # TODO: we could test if this cert verifies against the root, and mark it as pre-verified
+        # so we don't need to verify again and again the chain of intermediate certificates
+
+    def verify_cert_crl(self, cert: x509.Certificate):
+        if not self.crl:
+            # we cannot check if there's no CRL
+            return
+        if self.crl.get_revoked_certificate_by_serial_number(cert.serial_nr):
+            raise VerifyError('Certificate is present in CRL, verification failed')
+
+    def verify_cert_chain(self, cert: x509.Certificate, max_depth: int = 100):
+        """Verify if a given certificate's signature chain can be traced back to the root CA of this
+        CertificateSet."""
+        depth = 1
+        c = cert
+        while True:
+            aki = cert_get_auth_key_id(c)
+            if aki == self.root_cert_id:
+                # last step:
+                check_signed(c, self.root_cert)
+                return
+            parent_cert = self.intermediate_certs.get(aki, None)
+            if not aki:
+                raise VerifyError('Could not find intermediate certificate for AuthKeyId %s' % b2h(aki))
+            check_signed(c, parent_cert)
+            # if we reach here, we passed (no exception raised)
+            c = parent_cert
+            depth += 1
+            if depth > max_depth:
+                raise VerifyError('Maximum depth %u exceeded while verifying certificate chain' % max_depth)

