Script for trezor agent GPG recovery based on https://github.com/romanz/trezor-crypto/blob/gpg-export/export_keys.py

requirements.txt

mnemonic
libagent
ecdsa

trezor_agent_recover.py

'''Export secret GPG key using BIP13 derivation scheme.
IMPORTANT: Never run this code with your own mnemonic on a PC
with an internet connection or with any kind of persistent storage.
It may leak your mnemonic, exposing any secret key managed by the
TREZOR - which may result in Bitcoin loss!!!
'''

from __future__ import print_function

import sys
import logging
import argparse
import getpass
import hashlib
import hmac
import struct

from mnemonic import Mnemonic  # type: ignore
from ecdsa import curves, SigningKey  # type: ignore
from libagent import util  # type: ignore
from libagent.device import interface  # type: ignore
from libagent.gpg import encode, protocol  # type: ignore

from typing import Tuple


curve_name = 'nist256p1'
test_mnemonic = "all all all all all all all all all all all all"
HARDENED_INDEX = 0x80000000
curve_name_to_curve = {"nist256p1": curves.NIST256p}
curve_name_to_seed = {"nist256p1": "Nist256p1 seed"}

logger = logging.getLogger("export_keys")


def get_curve(curve_name: str) -> curves.Curve:
    return curve_name_to_curve[curve_name]


def mnemonic_to_seed(mnemonic):
    return Mnemonic("english").to_seed(mnemonic)[:64]


def privkey_and_chaincode_from_seed(
    seed: bytes,
    curve_name: str
) -> Tuple[bytes, bytes]:
    """Return private key and chaincode from provided seed

    >>> test_seed = mnemonic_to_seed(test_mnemonic)
    >>> privkey_and_chaincode_from_seed(test_seed, "nist256p1")
    (b'\\x1e\\xa4\\\\\\x10\\xd3\\x1a\\xd4\\xb8...\\xc9#t\\xf8\\xcf\\xcb')
    """
    curve_secret = curve_name_to_seed[curve_name]
    secret = hmac.new(curve_secret.encode(), seed, hashlib.sha512).digest()

    return secret[:32], secret[32:]


def derive_private_child(
    curve_name: str,
    privkey: bytes,
    chaincode: bytes,
    index: int
) -> Tuple[bytes, bytes]:
    """Derives private child key and chaincode using parent child key,
    chaincode and index

    >>> test_seed = mnemonic_to_seed(test_mnemonic)
    >>> p, c = privkey_and_chaincode_from_seed(test_seed, "nist256p1")
    >>> derive_private_child("nist256p1", p, c, 2147483661)
    (b'(\\x9f\\xccH\\xd7\\x96yKEQ\\x83\\xde\\x11\\xfaW...\\xc9\\xc5\\x82W\\x01`')
    """
    curve = get_curve(curve_name)
    secexp = util.bytes2num(privkey)

    assert index & HARDENED_INDEX, "index not hardened"

    data = b'\x00' + privkey + index.to_bytes(4, "big")
    payload = hmac.new(chaincode, data, hashlib.sha512).digest()

    B = util.bytes2num(payload[:32])

    assert B < curve.order, "curve order too small"

    B += secexp
    B %= curve.order

    child_private = util.num2bytes(B, 32)

    return child_private, payload[32:]


def derive(seed, path, curve_name):
    """Derives gpg key from provided bip32 path

    >>> seed = mnemonic_to_seed(test_mnemonic)
    >>> sk = derive(seed,
    ...     [2147483661, 3641273873, 3222207101, 2735596413, 2741857293],
    ...     "nist256p1")
    >>> sk.verifying_key.to_string().hex()
    '32dd7bda4eb424e57ec2594bc2dad...eb1ca14a6f518c204e32b24c5f18b4'
    """
    logger.debug("seed: %s", seed.hex())

    privkey, chaincode = privkey_and_chaincode_from_seed(seed, curve_name)
    logger.debug("master privkey: %s", privkey.hex())
    logger.debug("master chaincode: %s", chaincode.hex())

    for i in path:
        privkey, chaincode = derive_private_child(
            curve_name, privkey, chaincode, i)

        logger.debug("ckd: %d -> %s %s", i, privkey.hex(), chaincode.hex())

    logger.debug("child privkey: %s", privkey.hex())

    secexp = util.bytes2num(privkey)

    curve = get_curve(curve_name)
    sk = SigningKey.from_secret_exponent(
        secexp=secexp,
        curve=curve,
        hashfunc=hashlib.sha256)

    return sk


def pack(sk):
    secexp = util.bytes2num(sk.to_string())
    mpi_bytes = protocol.mpi(secexp)
    checksum = sum(bytearray(mpi_bytes)) & 0xFFFF
    return b'\x00' + mpi_bytes + struct.pack('>H', checksum)


def sigencode(r, s, _):
    return (r, s)


def create_signer(signing_key):
    def signer(digest):
        return signing_key.sign_digest_deterministic(
            digest, hashfunc=hashlib.sha256, sigencode=sigencode)
    return signer


def export_key(
    ident, mnemonic=None, seed=None, private=False,
    user_id: str = "testing",
    curve_name: str = "nist256p1",
    timestamp: int = 1
):
    if seed is None:
        assert mnemonic is not None,  "seed or mnemonic not provided"
        seed = mnemonic_to_seed(mnemonic)

    sk = derive(
        seed,
        ident.get_bip32_address(ecdh=False), curve_name)
    signer_func = create_signer(sk)
    primary = protocol.PublicKey(
        curve_name=curve_name, created=timestamp,
        verifying_key=sk.verifying_key, ecdh=False)

    result = encode.create_primary(user_id=user_id,
                                   pubkey=primary,
                                   signer_func=signer_func,
                                   secret_bytes=(pack(sk) if private else b''))

    sk = derive(seed, ident.get_bip32_address(ecdh=True), curve_name)
    subkey = protocol.PublicKey(
        curve_name=curve_name, created=timestamp,
        verifying_key=sk.verifying_key, ecdh=True)
    result = encode.create_subkey(primary_bytes=result,
                                  subkey=subkey,
                                  signer_func=signer_func,
                                  secret_bytes=(pack(sk) if private else b''))
    return result


def main():
    print(__doc__)

    example_seed = mnemonic_to_seed(test_mnemonic).hex()
    example_identity = "First Last <first.last@example.com>"

    parser = argparse.ArgumentParser(
        description="trezor-agent gpg key recovery tool",
        formatter_class=argparse.RawTextHelpFormatter)

    parser.add_argument(
        "--mnemonic", type=str, default=None,
        help="trezor mnemonic (example: {})".format(test_mnemonic))

    parser.add_argument(
        "--seed", type=str, default=None,
        help="trezor seed (example: {})".format(example_seed[:64] + "..."))

    parser.add_argument(
        "--identity", type=str, default=None,
        help="gpg key user identity (example: '{}')".format(example_identity))

    parser.add_argument(
        "--timestamp", type=int, default=1,
        help="timestamp to use (default: 1)")

    parser.add_argument(
        "--debug", action="store_true",
        help="enable debugging")

    args = parser.parse_args()

    if not args.identity:
        user_id = input('Enter your identity (example: {}): '.format(example_identity))  # noqa
    else:
        user_id = args.identity

    if not (args.mnemonic or args.seed):
        mnemonic = getpass.getpass('Enter your mnemonic: ')
    else:
        mnemonic = args.mnemonic

    seed = bytes.fromhex(args.seed) if args.seed else None

    logging.basicConfig(
        stream=sys.stderr,
        level=logging.DEBUG if args.debug else logging.INFO)

    ident = interface.Identity('gpg://' + user_id, curve_name)

    public_key = export_key(ident, mnemonic, seed,
        private=False, user_id=args.identity, timestamp=args.timestamp)
    private_key = export_key(ident, mnemonic, seed,
        private=True, user_id=args.identity, timestamp=args.timestamp)

    print('Use "gpg2 --import" on the following GPG key blocks:\n')
    print(protocol.armor(public_key, 'PUBLIC KEY BLOCK'))
    print(protocol.armor(private_key, 'PRIVATE KEY BLOCK'))


if __name__ == '__main__':
    main()

I managed to reverse what trezor-crypto implements bip32 by using following links, if anyone is noob on bip32, bip39, bip44, like me, this can be useful:

https://github.com/trezor/python-mnemonic
https://github.com/trezor/python-shamir-mnemonic
https://github.com/warner/python-ecdsa
https://www.ntirawen.com/2019/04/hierarchical-deterministic-wallet-bip32.html
https://github.com/ofek/coincurve
https://github.com/romanz/trezor-agent
https://github.com/trezor/trezor-crypto/blob/master/bip39.c
https://github.com/trezor/trezor-crypto/blob/master/bip32.c
https://github.com/romanz/trezor-crypto/blob/gpg-export/export_keys.py