trezor-agent/libagent/gpg/agent.py

"""GPG-agent utilities."""
import binascii
import logging

from . import client, decode, keyring, protocol
from .. import util

log = logging.getLogger(__name__)


def yield_connections(sock):
    """Run a server on the specified socket."""
    while True:
        log.debug('waiting for connection on %s', sock.getsockname())
        try:
            conn, _ = sock.accept()
        except KeyboardInterrupt:
            return
        conn.settimeout(None)
        log.debug('accepted connection on %s', sock.getsockname())
        yield conn


def serialize(data):
    """Serialize data according to ASSUAN protocol."""
    for c in [b'%', b'\n', b'\r']:
        escaped = '%{:02X}'.format(ord(c)).encode('ascii')
        data = data.replace(c, escaped)
    return data


def sig_encode(r, s):
    """Serialize ECDSA signature data into GPG S-expression."""
    r = serialize(util.num2bytes(r, 32))
    s = serialize(util.num2bytes(s, 32))
    return b'(7:sig-val(5:ecdsa(1:r32:' + r + b')(1:s32:' + s + b')))'


def open_connection(keygrip_bytes, device_type):
    """
    Connect to the device for the specified keygrip.

    Parse GPG public key to find the first user ID, which is used to
    specify the correct signature/decryption key on the device.
    """
    pubkey_dict, user_ids = decode.load_by_keygrip(
        pubkey_bytes=keyring.export_public_keys(),
        keygrip=keygrip_bytes)
    # We assume the first user ID is used to generate TREZOR-based GPG keys.
    user_id = user_ids[0]['value'].decode('ascii')
    curve_name = protocol.get_curve_name_by_oid(pubkey_dict['curve_oid'])
    ecdh = (pubkey_dict['algo'] == protocol.ECDH_ALGO_ID)

    conn = client.Client(user_id, curve_name=curve_name, device_type=device_type)
    pubkey = protocol.PublicKey(
        curve_name=curve_name, created=pubkey_dict['created'],
        verifying_key=conn.pubkey(ecdh=ecdh), ecdh=ecdh)
    assert pubkey.key_id() == pubkey_dict['key_id']
    assert pubkey.keygrip() == keygrip_bytes
    return conn


def pksign(keygrip, digest, algo, device_type):
    """Sign a message digest using a private EC key."""
    log.debug('signing %r digest (algo #%s)', digest, algo)
    keygrip_bytes = binascii.unhexlify(keygrip)
    conn = open_connection(keygrip_bytes, device_type=device_type)
    r, s = conn.sign(binascii.unhexlify(digest))
    result = sig_encode(r, s)
    log.debug('result: %r', result)
    return result


def _serialize_point(data):
    prefix = '{}:'.format(len(data)).encode('ascii')
    # https://www.gnupg.org/documentation/manuals/assuan/Server-responses.html
    return b'(5:value' + serialize(prefix + data) + b')'


def parse_ecdh(line):
    """Parse ECDH request and return remote public key."""
    prefix, line = line.split(b' ', 1)
    assert prefix == b'D'
    exp, leftover = keyring.parse(keyring.unescape(line))
    log.debug('ECDH s-exp: %r', exp)
    assert not leftover
    label, exp = exp
    assert label == b'enc-val'
    assert exp[0] == b'ecdh'
    items = exp[1:]
    log.debug('ECDH parameters: %r', items)
    return dict(items)[b'e']


def pkdecrypt(keygrip, conn, device_type):
    """Handle decryption using ECDH."""
    for msg in [b'S INQUIRE_MAXLEN 4096', b'INQUIRE CIPHERTEXT']:
        keyring.sendline(conn, msg)

    line = keyring.recvline(conn)
    assert keyring.recvline(conn) == b'END'
    remote_pubkey = parse_ecdh(line)

    keygrip_bytes = binascii.unhexlify(keygrip)
    conn = open_connection(keygrip_bytes, device_type=device_type)
    return _serialize_point(conn.ecdh(remote_pubkey))


@util.memoize
def have_key(keygrip, device_type):
    """Check if current keygrip correspond to a TREZOR-based key."""
    try:
        open_connection(keygrip_bytes=binascii.unhexlify(keygrip),
                        device_type=device_type)
        return True
    except KeyError as e:
        log.warning('HAVEKEY(%s) failed: %s', keygrip, e)
        return False


# pylint: disable=too-many-branches
def handle_connection(conn, device_type):
    """Handle connection from GPG binary using the ASSUAN protocol."""
    keygrip = None
    digest = None
    algo = None
    version = keyring.gpg_version()  # "Clone" existing GPG version

    keyring.sendline(conn, b'OK')
    for line in keyring.iterlines(conn):
        parts = line.split(b' ')
        command = parts[0]
        args = parts[1:]
        if command in {b'RESET', b'OPTION', b'SETKEYDESC'}:
            pass  # reply with OK
        elif command == b'GETINFO':
            keyring.sendline(conn, b'D ' + version)
        elif command == b'AGENT_ID':
            keyring.sendline(conn, b'D TREZOR')  # "Fake" agent ID
        elif command in {b'SIGKEY', b'SETKEY'}:
            keygrip, = args
        elif command == b'SETHASH':
            algo, digest = args
        elif command == b'PKSIGN':
            sig = pksign(keygrip, digest, algo, device_type=device_type)
            keyring.sendline(conn, b'D ' + sig)
        elif command == b'PKDECRYPT':
            sec = pkdecrypt(keygrip, conn, device_type=device_type)
            keyring.sendline(conn, b'D ' + sec)
        elif command == b'HAVEKEY':
            if not have_key(keygrip=args[0], device_type=device_type):
                keyring.sendline(conn,
                                 b'ERR 67108881 No secret key <GPG Agent>')
                return
        elif command == b'KEYINFO':
            keygrip, = args
            # Dummy reply (mainly for 'gpg --edit' to succeed).
            # For details, see GnuPG agent KEYINFO command help.
            # https://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git;a=blob;f=agent/command.c;h=c8b34e9882076b1b724346787781f657cac75499;hb=refs/heads/master#l1082
            fmt = 'S KEYINFO {0} X - - - - - - -'
            keyring.sendline(conn, fmt.format(keygrip).encode('ascii'))
        elif command == b'BYE':
            return
        elif command == b'KILLAGENT':
            keyring.sendline(conn, b'OK')
            raise StopIteration
        else:
            log.error('unknown request: %r', line)
            return

        keyring.sendline(conn, b'OK')