build a key from existing values

assemble_rsa_pubkey.py

#!/usr/bin/env python
from datetime import datetime
from pgpy import PGPKey, PGPUID, PGPSignature
from pgpy.constants import PubKeyAlgorithm
from pgpy.packet import PubKeyV4
from pgpy.packet.types import MPI


def assemble_rsa_pubkey(n, e, created):
    # WARNING: HERE BE DRAGONS
    # these properties are not considered part of the API and may change in the future
    # there will be actual API added to accomplish this more conveniently because it is useful, so stay tuned for that
    key = PGPKey()
    # for a privkey use PrivKeyV4
    key._key = PubKeyV4()
    # the creation date in the key packet affects the key fingerprint,
    # but this can be omitted if there's no need to maintain that,
    # e.g. if the key did not previously exist as an OpenPGP key
    key._key.created = created
    key._key.pkalg = PubKeyAlgorithm.RSAEncryptOrSign
    # the keymaterial fields differ based on what key algorithm is in use -
    # setting _key. pkalg instantiates the correct class for _key.keymaterial.
    # See pgpy/packet/fields.py (look at the __pubfields__ and __privfields__
    # properties, or the _generate method for the *Priv class that matches
    # your algorithm) for what fields your key algorithm needs
    key._key.keymaterial.n = MPI(n)
    key._key.keymaterial.e = MPI(e)
    # if we'd used PrivKeyV4 instead, for RSA, we'd also need to set
    # d, p, q, and u, where u can be derived (see pgpy.packet.fields.RSAPriv._generate)
    # for a private key, call key._key.keymaterial._compute_chksum() here
    key._key.update_hlen()
    return key


if __name__ == '__main__':
    # WARNING: these small values are here for the sake of the example being readable
    #          please don't use such small RSA keys for anything more important than example code
    n = 8503889732522138487695939124253595708816839007885044584145451609214205150733473557520173387766932445639160093079626991637560423848427124071620710948292389
    e = 65537
    uid = PGPUID.new("Pain Janksson")
    key = assemble_rsa_pubkey(n, e, datetime(2017, 11, 8, 18, 11, 42, 204921))
    key.add_uid(uid, selfsign=False)
    # confirm that it worked
    # should be: 11EF 98A4 06EB 4DCB D41A  979E 6E45 A6D2 03C1 D5A1
    print(key.fingerprint)

    sig = PGPSignature.from_blob('-----BEGIN PGP SIGNATURE-----\nVersion: PGPy v0.4.4\n\nwlwEAAELAAYFAloDSecACgkQbkWm0gPB1aGO4QH/Qks4Kfx1qhBd0Djz19PExrSM\nP8XFSyHfrZQ2qFdCU5HvF6B9NHa4lve+AFg3NnsRaxjl1eENrQCKZ+zl8xwdDw==\n=VYVv\n-----END PGP SIGNATURE-----\n')
    sv = key.verify("sign this!", sig)
    print(sv)

fwiw, here's a crossref to the issue this is for: SecurityInnovation/PGPy#220