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wheel / wheel / signatures / __init__.py

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"""
Create and verify jws-js format Ed25519 signatures.
"""

__all__ = [ 'sign', 'verify' ]

import json
from ..util import urlsafe_b64decode, urlsafe_b64encode, native, binary

ed25519ll = None

ALG = "Ed25519"

def get_ed25519ll():
    """Lazy import-and-test of ed25519 module"""
    global ed25519ll
    
    if not ed25519ll:
        try:
            import ed25519ll # fast (thousands / s)
        except (ImportError, OSError): # pragma nocover
            from . import ed25519py as ed25519ll # pure Python (hundreds / s)
        test()
    
    return ed25519ll

def sign(payload, keypair):
    """Return a JWS-JS format signature given a JSON-serializable payload and 
    an Ed25519 keypair."""
    get_ed25519ll()
    #
    header = {
                "alg": ALG,
                "jwk": {
                    "alg": ALG,
                    "vk": native(urlsafe_b64encode(keypair.vk))
                }
             }
    
    encoded_header = urlsafe_b64encode(binary(json.dumps(header, sort_keys=True)))
    encoded_payload = urlsafe_b64encode(binary(json.dumps(payload, sort_keys=True)))
    secured_input = b".".join((encoded_header, encoded_payload))
    sig_msg = ed25519ll.crypto_sign(secured_input, keypair.sk)
    signature = sig_msg[:ed25519ll.SIGNATUREBYTES]
    encoded_signature = urlsafe_b64encode(signature)
    
    return {"recipients": 
            [{"header":native(encoded_header),
             "signature":native(encoded_signature)}],
            "payload": native(encoded_payload)}

def assertTrue(condition, message=""):
    if not condition:
        raise ValueError(message)
    
def verify(jwsjs):
    """Return (decoded headers, payload) if all signatures in jwsjs are
    consistent, else raise ValueError.
    
    Caller must decide whether the keys are actually trusted."""
    get_ed25519ll()    
    # XXX forbid duplicate keys in JSON input using object_pairs_hook (2.7+)
    recipients = jwsjs["recipients"]
    encoded_payload = binary(jwsjs["payload"])
    headers = []
    for recipient in recipients:
        assertTrue(len(recipient) == 2, "Unknown recipient key {0}".format(recipient))
        h = binary(recipient["header"])
        s = binary(recipient["signature"])
        header = json.loads(native(urlsafe_b64decode(h)))
        assertTrue(header["alg"] == ALG, 
                "Unexpected algorithm {0}".format(header["alg"]))
        assertTrue(header["jwk"]["alg"] == ALG, 
                "Unexpected key algorithm {0}".format(header["jwk"]["alg"]))
        vk = urlsafe_b64decode(binary(header["jwk"]["vk"]))
        secured_input = b".".join((h, encoded_payload))
        sig = urlsafe_b64decode(s)
        sig_msg = sig+secured_input
        verified_input = native(ed25519ll.crypto_sign_open(sig_msg, vk))
        verified_header, verified_payload = verified_input.split('.')
        verified_header = binary(verified_header)
        decoded_header = native(urlsafe_b64decode(verified_header))
        headers.append(json.loads(decoded_header))

    verified_payload = binary(verified_payload)

    # only return header, payload that have passed through the crypto library.
    payload = json.loads(native(urlsafe_b64decode(verified_payload)))

    return headers, payload

def test():
    kp = ed25519ll.crypto_sign_keypair()
    payload = {'test': 'onstartup'}
    jwsjs = json.loads(json.dumps(sign(payload, kp)))
    verify(jwsjs)
    jwsjs['payload'] += 'x'
    try:
        verify(jwsjs)
    except ValueError:
        pass
    else: # pragma no cover
        raise RuntimeError("No error from bad wheel.signatures payload.")