#!/usr/bin/env python3
"""Independent offline verifier. Reads ONLY the files on disk: the COSE bytes,
the subject document and the issuer's PEM public key. Shares no state with the
signer. This is what would run in the room with the network off."""
import cbor2, hashlib, sys, os
from cryptography.hazmat.primitives.asymmetric.utils import encode_dss_signature
from cryptography.hazmat.primitives.asymmetric import ec
from cryptography.hazmat.primitives import hashes, serialization

H = os.path.dirname(os.path.abspath(__file__))
pub = serialization.load_pem_public_key(open(os.path.join(H, "issuer-public-key.pem"), "rb").read())
subject = open(os.path.join(H, "subject-cbom.json"), "rb").read()

def check(path, external=None):
    tagged = cbor2.loads(open(os.path.join(H, path), "rb").read())
    assert tagged.tag == 18, "not a COSE_Sign1"
    protected, unprotected, body, sig = tagged.value
    hdr = cbor2.loads(protected)
    payload = body if body is not None else external
    tbs = cbor2.dumps(["Signature1", protected, b"", payload])
    pub.verify(encode_dss_signature(int.from_bytes(sig[:32], "big"),
                                    int.from_bytes(sig[32:], "big")),
               tbs, ec.ECDSA(hashes.SHA256()))
    return hdr, unprotected, payload

hdr, unp, pl = check("signed-statement.cose")
alg = {-7: "ES256"}.get(hdr.get(1), hdr.get(1))
print(f"protected header : alg={alg}  content-type={hdr.get(3)}")
print(f"kid              : {unp.get(4).decode()}")
print(f"payload sha256   : {hashlib.sha256(pl).hexdigest()}")
print(f"matches subject  : {hashlib.sha256(pl).hexdigest() == hashlib.sha256(subject).hexdigest()}")
check("signed-statement-detached.cose", external=subject)
print("detached form verifies against the subject file on disk")

import json
cb = json.loads(subject)
ca = [c for c in cb.get("components", []) if c.get("type") == "cryptographic-asset"]
print(f"subject is CycloneDX {cb.get('specVersion')} with {len(ca)} cryptographic-asset component(s):")
for c in ca:
    p = c.get("cryptoProperties", {})
    print(f"  {c.get('name')}  assetType={p.get('assetType')}  oid={p.get('oid')}")
print("VERIFIED OFFLINE")