feat(python): implement full certificate verification in trezorctl (fixes #3364)

python/.changelog.d/3364.added

@@ -0,0 +1 @@
+trezorctl: support for human-friendly Trezor Safe device authenticity check (requires separate installation of `cryptography` library).

python/requirements-optional.txt

@@ -3,3 +3,4 @@ web3>=5
 Pillow>=10
 stellar-sdk>=6
 rlp>=1.1.0 ; python_version<'3.7'
+cryptography>=41

python/setup.py

@@ -31,6 +31,7 @@ extras_require = {
     "qt-widgets": ["PyQt5"],
     "extra": ["Pillow>=10"],
     "stellar": ["stellar-sdk>=6"],
+    "authentication": ["cryptography>=41"],
 }
 
 extras_require["full"] = sum(extras_require.values(), [])

python/src/trezorlib/authentication.py

@@ -0,0 +1,383 @@
+from __future__ import annotations
+
+import io
+import logging
+import secrets
+import typing as t
+from importlib import metadata
+
+from . import device
+from .client import TrezorClient
+
+try:
+    cryptography_version = metadata.version("cryptography")
+    vsplit = [int(x) for x in cryptography_version.split(".")]
+    if vsplit[0] < 41:
+        raise ImportError(
+            "cryptography>=41 is required for this module, "
+            f"found cryptography=={cryptography_version}"
+        )
+except ImportError as e:
+    raise ImportError("cryptography>=41 is required for this module") from e
+
+from cryptography import exceptions, x509
+from cryptography.hazmat.primitives import hashes, serialization
+from cryptography.hazmat.primitives.asymmetric import ec, utils
+
+LOG = logging.getLogger(__name__)
+
+
+def _pk_p256(pubkey_hex: str) -> ec.EllipticCurvePublicKey:
+    return ec.EllipticCurvePublicKey.from_encoded_point(
+        ec.SECP256R1(), bytes.fromhex(pubkey_hex)
+    )
+
+
+CHALLENGE_HEADER = b"AuthenticateDevice:"
+
+
+class RootCertificate(t.NamedTuple):
+    name: str
+    device: str
+    devel: bool
+    pubkey: ec.EllipticCurvePublicKey
+
+
+ROOT_PUBLIC_KEYS = [
+    RootCertificate(
+        "Trezor Company",
+        "Trezor Safe 3",
+        False,
+        _pk_p256(
+            "04ca97480ac0d7b1e6efafe518cd433cec2bf8ab9822d76eafd34363b55d63e60"
+            "380bff20acc75cde03cffcb50ab6f8ce70c878e37ebc58ff7cca0a83b16b15fa5"
+        ),
+    ),
+    RootCertificate(
+        "Trezor Company",
+        "Trezor Safe 5",
+        False,
+        _pk_p256(
+            "041854b27fb1d9f65abb66828e78c9dc0ca301e66081ab0c6a4d104f9df1cd0ad"
+            "5a7c75f77a8c092f55cf825d2abaf734f934c9394d5e75f75a5a06a5ee9be93ae"
+        ),
+    ),
+    RootCertificate(
+        "TESTING ENVIRONMENT. DO NOT USE THIS DEVICE",
+        "Trezor Safe 3",
+        True,
+        _pk_p256(
+            "047f77368dea2d4d61e989f474a56723c3212dacf8a808d8795595ef38441427c"
+            "4389bc454f02089d7f08b873005e4c28d432468997871c0bf286fd3861e21e96a"
+        ),
+    ),
+    RootCertificate(
+        "TESTING ENVIRONMENT. DO NOT USE THIS DEVICE",
+        "Trezor Safe 5",
+        True,
+        _pk_p256(
+            "04e48b69cd7962068d3cca3bcc6b1747ef496c1e28b5529e34ad7295215ea161d"
+            "be8fb08ae0479568f9d2cb07630cb3e52f4af0692102da5873559e45e9fa72959"
+        ),
+    ),
+]
+
+
+class DeviceNotAuthentic(Exception):
+    pass
+
+
+class Certificate:
+    def __init__(self, cert_bytes: bytes) -> None:
+        self.cert_bytes = cert_bytes
+        self.cert = x509.load_der_x509_certificate(cert_bytes)
+
+    def __str__(self) -> str:
+        return self.cert.subject.rfc4514_string()
+
+    def public_key_bytes(self) -> bytes:
+        return self.cert.public_key().public_bytes(
+            serialization.Encoding.X962,
+            serialization.PublicFormat.UncompressedPoint,
+        )
+
+    def verify(self, signature: bytes, message: bytes) -> None:
+        cert_pubkey = self.cert.public_key()
+        assert isinstance(cert_pubkey, ec.EllipticCurvePublicKey)
+        cert_pubkey.verify(
+            self.fix_signature(signature),
+            message,
+            ec.ECDSA(hashes.SHA256()),
+        )
+
+    def verify_by(self, pubkey: ec.EllipticCurvePublicKey) -> None:
+        algo_params = self.cert.signature_algorithm_parameters
+        assert isinstance(algo_params, ec.ECDSA)
+        pubkey.verify(
+            self.fix_signature(self.cert.signature),
+            self.cert.tbs_certificate_bytes,
+            algo_params,
+        )
+
+    def _check_ca_extensions(self) -> bool:
+        """Check that this certificate is a valid Trezor CA.
+
+        KeyUsage must be present and allow certificate signing.
+        BasicConstraints must be present, have the cA flag and a pathLenConstraint.
+
+        Any unrecognized non-critical extension is allowed. Any unrecognized critical
+        extension is disallowed.
+        """
+        missing_extension_classes = {x509.KeyUsage, x509.BasicConstraints}
+        passed = True
+
+        for ext in self.cert.extensions:
+            missing_extension_classes.discard(type(ext.value))
+
+            if isinstance(ext.value, x509.KeyUsage):
+                if not ext.value.key_cert_sign:
+                    LOG.error(
+                        "Not a valid CA certificate: %s (keyCertSign not set)", self
+                    )
+                    passed = False
+
+            elif isinstance(ext.value, x509.BasicConstraints):
+                if not ext.value.ca:
+                    LOG.error("Not a valid CA certificate: %s (cA not set)", self)
+                    passed = False
+                if ext.value.path_length is None:
+                    LOG.error(
+                        "Not a valid CA certificate: %s (pathLenConstraint missing)",
+                        self,
+                    )
+                    passed = False
+
+            elif ext.critical:
+                LOG.error(
+                    "Unknown critical extension %s in CA certificate: %s",
+                    self,
+                    type(ext.value).__name__,
+                )
+                passed = False
+
+        for ext in missing_extension_classes:
+            LOG.error("Missing extension %s in CA certificate: %s", ext.__name__, self)
+            passed = False
+
+        return passed
+
+    def is_issued_by(self, issuer: "Certificate", path_len: int) -> bool:
+        """Check if this certificate was issued by an issuer.
+
+        Returns True if:
+         * our `issuer` is the same as issuer's `subject`,
+         * the issuer is a valid CA, that is:
+           - has the cA flag set
+           - has a valid pathLenConstraint
+           - pathLenConstraint does not exceed the current path length.
+         * the issuer's public key signs this certificate.
+        """
+        if issuer.cert.subject != self.cert.issuer:
+            LOG.error("Certificate %s is not issued by %s.", self, issuer)
+            return False
+
+        if not issuer._check_ca_extensions():
+            return False
+
+        basic_constraints = issuer.cert.extensions.get_extension_for_class(
+            x509.BasicConstraints
+        ).value
+        assert basic_constraints.path_length is not None  # check_ca_extensions
+        if basic_constraints.path_length < path_len:
+            LOG.error(
+                "Issuer %s was not permitted to issue certificate %s", issuer, self
+            )
+            return False
+
+        try:
+            pubkey = issuer.cert.public_key()
+            assert isinstance(pubkey, ec.EllipticCurvePublicKey)
+            self.verify_by(pubkey)
+            return True
+        except exceptions.InvalidSignature:
+            LOG.error("Issuer %s did not sign certificate %s.", issuer, self)
+
+        return False
+
+    @staticmethod
+    def _decode_signature_permissive(sig_bytes: bytes) -> tuple[int, int]:
+        if len(sig_bytes) > 73:
+            raise ValueError("Unsupported DER signature: too long.")
+
+        reader = io.BytesIO(sig_bytes)
+        tag = reader.read(1)
+        if tag != b"\x30":
+            raise ValueError("Invalid DER signature: not a sequence.")
+        length = reader.read(1)[0]
+        if length != len(sig_bytes) - 2:
+            raise ValueError("Invalid DER signature: invalid length.")
+
+        def read_int() -> int:
+            tag = reader.read(1)
+            if tag != b"\x02":
+                raise ValueError("Invalid DER signature: not an integer.")
+            length = reader.read(1)[0]
+            if length > 33:
+                raise ValueError("Invalid DER signature: integer too long.")
+            return int.from_bytes(reader.read(length), "big")
+
+        r = read_int()
+        s = read_int()
+        if reader.tell() != len(sig_bytes):
+            raise ValueError("Invalid DER signature: trailing data.")
+        return r, s
+
+    @staticmethod
+    def fix_signature(sig_bytes: bytes) -> bytes:
+        r, s = Certificate._decode_signature_permissive(sig_bytes)
+        reencoded = utils.encode_dss_signature(r, s)
+        if reencoded != sig_bytes:
+            LOG.info(
+                "Re-encoding malformed signature: %s -> %s",
+                sig_bytes.hex(),
+                reencoded.hex(),
+            )
+        return reencoded
+
+
+def verify_authentication_response(
+    challenge: bytes,
+    signature: bytes,
+    cert_chain: t.Iterable[bytes],
+    *,
+    whitelist: t.Collection[bytes] | None,
+    allow_development_devices: bool = False,
+    root_pubkey: bytes | ec.EllipticCurvePublicKey | None = None,
+) -> None:
+    """Evaluate the response to an AuthenticateDevice call.
+
+    Performs all steps and logs their results via the logging facility. (The log can be
+    accessed via the `LOG` object in this module.)
+
+    When done, raises DeviceNotAuthentic if the device is not authentic.
+
+    The optional argument `root_pubkey` allows you to specify a root public key either
+    as an `ec.EllipticCurvePublicKey` object or as a byte-string representing P-256
+    public key.
+    """
+    if isinstance(root_pubkey, (bytes, bytearray, memoryview)):
+        root_pubkey = ec.EllipticCurvePublicKey.from_encoded_point(
+            ec.SECP256R1(), root_pubkey
+        )
+
+    challenge_bytes = (
+        len(CHALLENGE_HEADER).to_bytes(1, "big")
+        + CHALLENGE_HEADER
+        + len(challenge).to_bytes(1, "big")
+        + challenge
+    )
+
+    cert_chain_iter = iter(cert_chain)
+
+    failed = False
+
+    try:
+        cert = Certificate(next(cert_chain_iter))
+    except Exception:
+        LOG.error("Failed to parse device certificate.")
+        raise DeviceNotAuthentic
+
+    try:
+        cert.verify(signature, challenge_bytes)
+    except exceptions.InvalidSignature:
+        LOG.error("Challenge verification failed.")
+        failed = True
+    else:
+        LOG.debug("Challenge verified successfully.")
+
+    cert_label = "Device certificate"
+    for i, issuer_bytes in enumerate(cert_chain_iter, 1):
+        try:
+            ca_cert = Certificate(issuer_bytes)
+        except Exception:
+            LOG.error(f"Failed to parse CA certificate #{i}.")
+            failed = True
+            continue
+
+        if whitelist is None:
+            LOG.warning("Skipping public key whitelist check.")
+        else:
+            if ca_cert.public_key_bytes() not in whitelist:
+                LOG.error(f"CA certificate #{i} not in whitelist: %s", ca_cert)
+                failed = True
+
+        if not cert.is_issued_by(ca_cert, i - 1):
+            failed = True
+        else:
+            LOG.debug(f"{cert_label} verified successfully: %s", cert)
+
+        cert = ca_cert
+        cert_label = f"CA #{i} certificate"
+
+    if root_pubkey is not None:
+        try:
+            cert.verify_by(root_pubkey)
+        except Exception:
+            LOG.error(f"{cert_label} was not issued by the specified root.")
+            failed = True
+        else:
+            LOG.info(f"{cert_label} was issued by the specified root.")
+
+    else:
+        for root in ROOT_PUBLIC_KEYS:
+            try:
+                cert.verify_by(root.pubkey)
+            except Exception:
+                continue
+            else:
+                LOG.debug(f"{cert_label} verified successfully: %s", cert)
+
+            if root.devel:
+                if not allow_development_devices:
+                    level = logging.ERROR
+                    failed = True
+                else:
+                    level = logging.WARNING
+            else:
+                level = logging.DEBUG
+            LOG.log(
+                level,
+                "Successfully verified a %s manufactured by %s.",
+                root.device,
+                root.name,
+            )
+            break
+        else:
+            LOG.error(f"{cert_label} was issued by an unknown root.")
+            failed = True
+
+    if failed:
+        raise DeviceNotAuthentic
+
+
+def authenticate_device(
+    client: TrezorClient,
+    challenge: bytes | None = None,
+    *,
+    whitelist: t.Collection[bytes] | None = None,
+    allow_development_devices: bool = False,
+    root_pubkey: bytes | ec.EllipticCurvePublicKey | None = None,
+) -> None:
+    if challenge is None:
+        challenge = secrets.token_bytes(16)
+
+    resp = device.authenticate(client, challenge)
+
+    return verify_authentication_response(
+        challenge,
+        resp.signature,
+        resp.certificates,
+        whitelist=whitelist,
+        allow_development_devices=allow_development_devices,
+        root_pubkey=root_pubkey,
+    )

python/src/trezorlib/cli/device.py

@@ -14,9 +14,10 @@
 # You should have received a copy of the License along with this library.
 # If not, see <https://www.gnu.org/licenses/lgpl-3.0.html>.
 
+import logging
 import secrets
 import sys
-from typing import TYPE_CHECKING, Optional, Sequence, Tuple
+from typing import TYPE_CHECKING, BinaryIO, Optional, Sequence, Tuple
 
 import click
 
@@ -46,6 +47,8 @@ SD_PROTECT_OPERATIONS = {
     "refresh": messages.SdProtectOperationType.REFRESH,
 }
 
+LOG = logging.getLogger(__name__)
+
 
 @click.group(name="device")
 def cli() -> None:
@@ -347,15 +350,88 @@ def set_busy(
 
 @cli.command()
 @click.argument("hex_challenge", required=False)
+@click.option("-R", "--root", type=click.File("rb"), help="Custom root certificate.")
+@click.option(
+    "-r", "--raw", is_flag=True, help="Print raw cryptographic data and exit."
+)
+@click.option(
+    "-s",
+    "--skip-whitelist",
+    is_flag=True,
+    help="Do not check intermediate certificates against the whitelist.",
+)
 @with_client
-def authenticate(client: "TrezorClient", hex_challenge: Optional[str]) -> None:
-    """Get information to verify the authenticity of the device."""
+def authenticate(
+    client: "TrezorClient",
+    hex_challenge: Optional[str],
+    root: Optional[BinaryIO],
+    raw: Optional[bool],
+    skip_whitelist: Optional[bool],
+) -> None:
+    """Verify the authenticity of the device.
+
+    Use the --raw option to get the raw challenge, signature, and certificate data.
+
+    Otherwise, trezorctl will attempt to decode the signatures and check their
+    authenticity. By default, it will also check the public keys against a built-in
+    whitelist, and in the future also against a whitelist downloaded from Trezor
+    servers. You can skip this check with the --skip-whitelist option.
+
+    \b
+    When not using --raw, 'cryptography' library is required. You can install it via:
+      pip3 install trezor[authentication]
+    """
     if hex_challenge is None:
         hex_challenge = secrets.token_hex(32)
-    click.echo(f"Challenge: {hex_challenge}")
+
     challenge = bytes.fromhex(hex_challenge)
+
+    if raw:
         msg = device.authenticate(client, challenge)
+
+        click.echo(f"Challenge: {hex_challenge}")
         click.echo(f"Signature of challenge: {msg.signature.hex()}")
         click.echo(f"Device certificate: {msg.certificates[0].hex()}")
         for cert in msg.certificates[1:]:
             click.echo(f"CA certificate: {cert.hex()}")
+        return
+
+    try:
+        from .. import authentication
+    except ImportError as e:
+        click.echo("Failed to import the authentication module.")
+        click.echo(f"Error: {e}")
+        click.echo("Make sure you have the required dependencies:")
+        click.echo("  pip3 install trezor[authentication]")
+        sys.exit(4)
+
+    if root is not None:
+        root_bytes = root.read()
+    else:
+        root_bytes = None
+
+    class ColoredFormatter(logging.Formatter):
+        LEVELS = {
+            logging.ERROR: click.style("ERROR", fg="red"),
+            logging.WARNING: click.style("WARNING", fg="yellow"),
+            logging.INFO: click.style("INFO", fg="blue"),
+            logging.DEBUG: click.style("OK", fg="green"),
+        }
+
+        def format(self, record: logging.LogRecord) -> str:
+            prefix = self.LEVELS[record.levelno]
+            bold_args = tuple(
+                click.style(str(arg), bold=True) for arg in record.args or ()
+            )
+            return f"[{prefix}] {record.msg}" % bold_args
+
+    handler = logging.StreamHandler()
+    handler.setFormatter(ColoredFormatter())
+    authentication.LOG.addHandler(handler)
+    authentication.LOG.setLevel(logging.DEBUG)
+
+    try:
+        authentication.authenticate_device(client, challenge, root_pubkey=root_bytes)
+    except authentication.DeviceNotAuthentic:
+        click.echo("Device is not authentic.")
+        sys.exit(5)