Decode a base64 encoded JSON id.

    @param key the nacl private key to decode.
    @param caveat a base64 encoded JSON string.

def _decode_caveat_v1(key, caveat):
    '''Decode a base64 encoded JSON id.

    @param key the nacl private key to decode.
    @param caveat a base64 encoded JSON string.
    '''

    data = base64.b64decode(caveat).decode('utf-8')
    wrapper = json.loads(data)
    tp_public_key = nacl.public.PublicKey(
        base64.b64decode(wrapper['ThirdPartyPublicKey']))
    if key.public_key.key != tp_public_key:
        raise Exception('public key mismatch')  # TODO

    if wrapper.get('FirstPartyPublicKey', None) is None:
        raise Exception('target service public key not specified')

    # The encrypted string is base64 encoded in the JSON representation.
    secret = base64.b64decode(wrapper.get('Id'))
    nonce = base64.b64decode(wrapper.get('Nonce'))

    fp_public_key = nacl.public.PublicKey(base64.b64decode(
        wrapper.get('FirstPartyPublicKey')))

    box = nacl.public.Box(key.key, fp_public_key)
    c = box.decrypt(secret, nonce)
    record = json.loads(c.decode('utf-8'))
    fp_key = nacl.public.PublicKey(
        base64.b64decode(wrapper.get('FirstPartyPublicKey')))
    return ThirdPartyCaveatInfo(
        condition=record.get('Condition'),
        first_party_public_key=PublicKey(fp_key),
        third_party_key_pair=key,
        root_key=base64.b64decode(record.get('RootKey')),
        caveat=caveat,
        id=None,
        version=VERSION_1,
        namespace=legacy_namespace()
    )