#!/usr/bin/env python3
# Requires the cryptography package. Verifies a detached website discovery-card JWS.
import base64,json,urllib.request
from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PublicKey

def get(url):
    with urllib.request.urlopen(url,timeout=20) as r: return json.load(r)
def decode(s): return base64.urlsafe_b64decode(s+'='*((-len(s))%4))
card=get('https://hergertsynthora.com/.well-known/agent-card.json')
jwks=get('https://hergertsynthora.com/.well-known/jwks.json')
sig=card.pop('signatures')[0]
protected=json.loads(decode(sig['protected']))
assert protected['alg']=='EdDSA'
key=next(k for k in jwks['keys'] if k['kid']==protected['kid'])
assert key['kty']=='OKP' and key['crv']=='Ed25519'
payload=json.dumps(card,sort_keys=True,separators=(',',':'),ensure_ascii=False).encode()
payload64=base64.urlsafe_b64encode(payload).rstrip(b'=').decode()
Ed25519PublicKey.from_public_bytes(decode(key['x'])).verify(decode(sig['signature']),(sig['protected']+'.'+payload64).encode())
print('Firma de tarjeta válida; la firma no certifica calidad ni frescura de productos.')