# 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
from cryptography.hazmat.primitives import hashes
from cryptography import x509
from cryptography.hazmat.primitives.serialization import load_pem_private_key, Encoding
from cryptography.hazmat.primitives.asymmetric.utils import decode_dss_signature
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 cryptography 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
def cert_policy_has_oid(cert: x509.Certificate, match_oid: x509.ObjectIdentifier) -> bool:
"""Determine if given certificate has a certificatePolicy extension of matching OID."""
for policy_ext in filter(lambda x: isinstance(x.value, x509.CertificatePolicies), cert.extensions):
if any(policy.policy_identifier == match_oid for policy in policy_ext.value._policies):
return True
return False
ID_RSP = "2.23.146.1"
ID_RSP_CERT_OBJECTS = '.'.join([ID_RSP, '2'])
ID_RSP_ROLE = '.'.join([ID_RSP_CERT_OBJECTS, '1'])
class oid:
id_rspRole_ci = x509.ObjectIdentifier(ID_RSP_ROLE + '.0')
id_rspRole_euicc_v2 = x509.ObjectIdentifier(ID_RSP_ROLE + '.1')
id_rspRole_eum_v2 = x509.ObjectIdentifier(ID_RSP_ROLE + '.2')
id_rspRole_dp_tls_v2 = x509.ObjectIdentifier(ID_RSP_ROLE + '.3')
id_rspRole_dp_auth_v2 = x509.ObjectIdentifier(ID_RSP_ROLE + '.4')
id_rspRole_dp_pb_v2 = x509.ObjectIdentifier(ID_RSP_ROLE + '.5')
id_rspRole_ds_tls_v2 = x509.ObjectIdentifier(ID_RSP_ROLE + '.6')
id_rspRole_ds_auth_v2 = x509.ObjectIdentifier(ID_RSP_ROLE + '.7')
class VerifyError(Exception):
"""An error during certificate verification,"""
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
if not cert_policy_has_oid(root_cert, oid.id_rspRole_ci):
raise ValueError("Given root certificate doesn't have rspRole_ci OID")
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 isinstance(urls, 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, timeout=10)
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)
def ecdsa_dss_to_tr03111(sig: bytes) -> bytes:
"""convert from DER format to BSI TR-03111; first get long integers; then convert those to bytes."""
r, s = decode_dss_signature(sig)
return r.to_bytes(32, 'big') + s.to_bytes(32, 'big')
class CertAndPrivkey:
"""A pair of certificate and private key, as used for ECDSA signing."""
def __init__(self, required_policy_oid: Optional[x509.ObjectIdentifier] = None,
cert: Optional[x509.Certificate] = None, priv_key = None):
self.required_policy_oid = required_policy_oid
self.cert = cert
self.priv_key = priv_key
def cert_from_der_file(self, path: str):
with open(path, 'rb') as f:
cert = x509.load_der_x509_certificate(f.read())
if self.required_policy_oid:
# verify it is the right type of certificate (id-rspRole-dp-auth, id-rspRole-dp-auth-v2, etc.)
assert cert_policy_has_oid(cert, self.required_policy_oid)
self.cert = cert
def privkey_from_pem_file(self, path: str, password: Optional[str] = None):
with open(path, 'rb') as f:
self.priv_key = load_pem_private_key(f.read(), password)
def ecdsa_sign(self, plaintext: bytes) -> bytes:
"""Sign some input-data using an ECDSA signature compliant with SGP.22,
which internally refers to Global Platform 2.2 Annex E, which in turn points
to BSI TS-03111 which states "concatenated raw R + S values". """
sig = self.priv_key.sign(plaintext, ec.ECDSA(hashes.SHA256()))
# convert from DER format to BSI TR-03111; first get long integers; then convert those to bytes
return ecdsa_dss_to_tr03111(sig)
def get_authority_key_identifier(self) -> x509.AuthorityKeyIdentifier:
"""Return the AuthorityKeyIdentifier X.509 extension of the certificate."""
return list(filter(lambda x: isinstance(x.value, x509.AuthorityKeyIdentifier), self.cert.extensions))[0].value
def get_subject_alt_name(self) -> x509.SubjectAlternativeName:
"""Return the SubjectAlternativeName X.509 extension of the certificate."""
return list(filter(lambda x: isinstance(x.value, x509.SubjectAlternativeName), self.cert.extensions))[0].value
def get_cert_as_der(self) -> bytes:
"""Return certificate encoded as DER."""
return self.cert.public_bytes(Encoding.DER)
def get_curve(self) -> ec.EllipticCurve:
return self.cert.public_key().public_numbers().curve