WKD (Web Key Directory) client for fetching OpenPGP public keys in python3, including a primitive z-base-32 implementation

wkd-python-client.py

import requests
import requests.adapters
import requests.exceptions
from ssl import SSLError
import hashlib

# https://datatracker.ietf.org/doc/draft-koch-openpgp-webkey-service/?include_text=1
# https://www.sektioneins.de/en/blog/18-11-23-gnupg-wkd.html

class PubkeySource:

	_request_err = (requests.ConnectionError, requests.adapters.ReadTimeout, requests.adapters.ConnectTimeout, SSLError, requests.exceptions.Timeout)

	def _email_as_user_and_domain(self, email: str):
		user, domain = email.strip().lower().split('@')
		return user, domain


class Wkd(PubkeySource):

	__TIMEOUT = (3, 3)

	def find_pubkey(self, email:str=None):
		user, domain = self._email_as_user_and_domain(email)
		userhash = self.__sha1_zbase32(user)
		url_advanced = f'https://openpgpkey.{domain}/.well-known/openpgpkey/{domain}/hu/{userhash}?l={user}'
		url_direct = f'https://{domain}/.well-known/openpgpkey/hu/{userhash}?l={user}'
		return self.__try_url(url_advanced) or self.__try_url(url_direct)

	def __try_url(self, url: str):
		try:
			r = requests.get(url, timeout=self.__TIMEOUT)
			if r.status_code != 200:
				return None
			if r.headers['Content-Type'] == 'text/html' and ("404" in r.text or "Not Found" in r.text):
				return None  # likely a catchall 404 html page, even if responded with 200
			return r.content
		except self._request_err:
			return None
		except (ValueError, KeyError):
			return None

	def __sha1_zbase32(self, user: str):
		def chunk_string(string, length):
			return (string[0 + i:length + i] for i in range(0, len(string), length))
		def chars_from_int(int32):
			result = []
			shift = 27
			for _ in range(4):
				result.append("ybndrfg8ejkmcpqxot1uwisza345h769"[(int32 >> shift) & 31])
				shift = shift - 5
			return result
		hashed_bytes = hashlib.sha1(user.encode()).digest()
		bits_string = bin(int.from_bytes(hashed_bytes, byteorder="big"))[2:]
		chunks_of_20_bits = chunk_string(bits_string, 20)
		offset_chunks_of_20_bits = [chunk + '000000000000' for chunk in chunks_of_20_bits]
		ints = [int(bits, 2) for bits in offset_chunks_of_20_bits]
		groups_of_4_chars = [chars_from_int(int32) for int32 in ints]
		return "".join(sum(groups_of_4_chars, []))

wkd = Wkd()
print(wkd.find_pubkey("[email protected]"))