verify.py

#!/usr/bin/env python3
"""
quey-verify-demo — Python verification

Verifies a Quey draw certificate locally using only Quey's public key.
No trust in Quey or any server required.

Dependencies: cryptography
    pip install cryptography
"""

import json
import sys
from urllib.request import urlopen

from cryptography.exceptions import InvalidSignature
from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey

# Quey's Ed25519 public key — constant, derived from the signing secret.
# You can fetch it yourself any time:
#   curl https://getpublickey-33ssvbagba-uc.a.run.app
QUEY_PUBLIC_KEY_HEX = "4fad999c2b586aab53155afc97f564a231350087b4c66d6a8f868963f42caf15"

# Public endpoint serving signed draw certificates.
CERT_ENDPOINT = "https://getdrawcertificate-33ssvbagba-uc.a.run.app/?id={draw_id}"

# A real, permanent demo draw to verify against.
DEMO_DRAW_ID = "89421ffc-6af3-427e-8123-fa6c799cc166"


def build_canonical(cert: dict) -> str:
    """
    Reconstruct the exact UTF-8 string Quey signs.

    Reference: buildCanonicalPayload() in Quey's Cloud Function.
    Nine fields joined by '|', in this order, with winning_indices
    joined by ','.
    """
    return "|".join([
        "QUEY-DRAW-CERT-V1",
        cert["draw_id"],
        cert["timestamp"],
        cert["input_hash"],
        str(cert["total_participants"]),
        str(cert["num_winners"]),
        ",".join(str(i) for i in cert["winning_indices"]),
        str(cert["entropy_consumed_bytes"]),
        cert["key_id"],
    ])


def fetch_certificate(draw_id: str) -> dict:
    """Fetch a draw certificate from Quey's public endpoint."""
    with urlopen(CERT_ENDPOINT.format(draw_id=draw_id), timeout=10) as resp:
        return json.loads(resp.read())


def verify(cert: dict) -> bool:
    """Verify the Ed25519 signature on a certificate."""
    public_key = Ed25519PublicKey.from_public_bytes(bytes.fromhex(QUEY_PUBLIC_KEY_HEX))
    message = build_canonical(cert).encode("utf-8")
    signature = bytes.fromhex(cert["signature"])
    try:
        public_key.verify(signature, message)
        return True
    except InvalidSignature:
        return False


def main() -> None:
    args = sys.argv[1:]
    offline = "--offline" in args
    draw_id = next((a for a in args if not a.startswith("--")), DEMO_DRAW_ID)

    if offline:
        with open("certificate.json") as f:
            cert = json.load(f)
    else:
        cert = fetch_certificate(draw_id)

    ok = verify(cert)
    if ok:
        print("✓ Signature verified")
        print(f"  Draw ID:    {cert['draw_id']}")
        print(f"  Timestamp:  {cert['timestamp']}")
        print(f"  Winners:    indices {cert['winning_indices']} "
              f"out of {cert['total_participants']} participants")
        print(f"  Public key: {QUEY_PUBLIC_KEY_HEX}")
        sys.exit(0)
    else:
        print("✗ Signature verification FAILED")
        sys.exit(1)


if __name__ == "__main__":
    main()