mitm.py

"""
MITM certificate manager for HTTPS interception.

Generates a CA certificate (once, stored as files) and per-domain
certificates (on the fly, cached in memory) so the local proxy can
decrypt HTTPS traffic and relay it through Apps Script.

The user must install ca/ca.crt in their browser's trusted CAs once.

Requires: pip install cryptography
"""

import datetime
import logging
import os
import ssl
import tempfile

from cryptography import x509
from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.x509.oid import NameOID

log = logging.getLogger("MITM")

CA_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), "ca")
CA_KEY_FILE = os.path.join(CA_DIR, "ca.key")
CA_CERT_FILE = os.path.join(CA_DIR, "ca.crt")


class MITMCertManager:
    def __init__(self):
        self._ca_key = None
        self._ca_cert = None
        self._ctx_cache: dict[str, ssl.SSLContext] = {}
        self._cert_dir = tempfile.mkdtemp(prefix="domainfront_certs_")
        self._ensure_ca()

    def _ensure_ca(self):
        if os.path.exists(CA_KEY_FILE) and os.path.exists(CA_CERT_FILE):
            with open(CA_KEY_FILE, "rb") as f:
                self._ca_key = serialization.load_pem_private_key(
                    f.read(), password=None
                )
            with open(CA_CERT_FILE, "rb") as f:
                self._ca_cert = x509.load_pem_x509_certificate(f.read())
            log.info("Loaded CA from %s", CA_DIR)
        else:
            self._create_ca()

    def _create_ca(self):
        os.makedirs(CA_DIR, exist_ok=True)

        self._ca_key = rsa.generate_private_key(
            public_exponent=65537, key_size=2048
        )
        subject = issuer = x509.Name([
            x509.NameAttribute(NameOID.COMMON_NAME, "MasterHttpRelayVPN"),
            x509.NameAttribute(NameOID.ORGANIZATION_NAME, "MasterHttpRelayVPN"),
        ])
        now = datetime.datetime.now(datetime.timezone.utc)
        self._ca_cert = (
            x509.CertificateBuilder()
            .subject_name(subject)
            .issuer_name(issuer)
            .public_key(self._ca_key.public_key())
            .serial_number(x509.random_serial_number())
            .not_valid_before(now)
            .not_valid_after(now + datetime.timedelta(days=3650))
            .add_extension(
                x509.BasicConstraints(ca=True, path_length=0), critical=True
            )
            .add_extension(
                x509.KeyUsage(
                    digital_signature=True,
                    key_cert_sign=True,
                    crl_sign=True,
                    content_commitment=False,
                    key_encipherment=False,
                    data_encipherment=False,
                    key_agreement=False,
                    encipher_only=False,
                    decipher_only=False,
                ),
                critical=True,
            )
            .sign(self._ca_key, hashes.SHA256())
        )

        with open(CA_KEY_FILE, "wb") as f:
            f.write(
                self._ca_key.private_bytes(
                    serialization.Encoding.PEM,
                    serialization.PrivateFormat.TraditionalOpenSSL,
                    serialization.NoEncryption(),
                )
            )
        with open(CA_CERT_FILE, "wb") as f:
            f.write(self._ca_cert.public_bytes(serialization.Encoding.PEM))

        log.warning("Generated new CA certificate: %s", CA_CERT_FILE)
        log.warning(">>> Install this file in your browser's Trusted Root CAs! <<<")

    def get_server_context(self, domain: str) -> ssl.SSLContext:
        if domain not in self._ctx_cache:
            key_pem, cert_pem = self._generate_domain_cert(domain)

            cert_file = os.path.join(self._cert_dir, f"{domain}.crt")
            key_file = os.path.join(self._cert_dir, f"{domain}.key")

            ca_pem = self._ca_cert.public_bytes(serialization.Encoding.PEM)
            with open(cert_file, "wb") as f:
                f.write(cert_pem + ca_pem)
            with open(key_file, "wb") as f:
                f.write(key_pem)

            ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
            ctx.set_alpn_protocols(["http/1.1"])
            ctx.load_cert_chain(cert_file, key_file)
            self._ctx_cache[domain] = ctx

        return self._ctx_cache[domain]

    def _generate_domain_cert(self, domain: str):
        key = rsa.generate_private_key(
            public_exponent=65537, key_size=2048
        )
        subject = x509.Name([
            x509.NameAttribute(NameOID.COMMON_NAME, domain),
        ])
        now = datetime.datetime.now(datetime.timezone.utc)
        cert = (
            x509.CertificateBuilder()
            .subject_name(subject)
            .issuer_name(self._ca_cert.subject)
            .public_key(key.public_key())
            .serial_number(x509.random_serial_number())
            .not_valid_before(now)
            .not_valid_after(now + datetime.timedelta(days=365))
            .add_extension(
                x509.SubjectAlternativeName([x509.DNSName(domain)]),
                critical=False,
            )
            .sign(self._ca_key, hashes.SHA256())
        )

        key_pem = key.private_bytes(
            serialization.Encoding.PEM,
            serialization.PrivateFormat.TraditionalOpenSSL,
            serialization.NoEncryption(),
        )
        cert_pem = cert.public_bytes(serialization.Encoding.PEM)
        return key_pem, cert_pem