Serialize Python 3 cryptography RSA public key to OpenSSH format

ssh_key_helper.py

import binascii
import struct

from cryptography import utils
from cryptography.hazmat.primitives.asymmetric import rsa


def ssh_public_key(keypair: rsa.RSAPrivateKeyWithSerialization) -> str:
    eb = utils.int_to_bytes(keypair.public_key().public_numbers().e)
    nb = utils.int_to_bytes(keypair.public_key().public_numbers().n)
    if eb[0] & 0x80: eb = bytes([0x00]) + eb
    if nb[0] & 0x80: nb = bytes([0x00]) + nb
    keyparts = [b'ssh-rsa', eb, nb]
    keystring = b''.join([struct.pack(">I", len(kp)) + kp for kp in keyparts])
    return str(b'ssh-rsa ' + binascii.b2a_base64(keystring)[:-1], encoding='utf-8')