constructing WKD urls

[tomholub]

part of #275

We'll need a way to construct WKD URLs with an email address as an input. Signature:

class Wkd {
  static fun constructUrl(email: string, mode: ADVANCED | DIRECT): string
}

This will follow the following spec: https://tools.ietf.org/html/draft-koch-openpgp-webkey-service-11 - this uses z-base-32 which is different from base-32, and may need to be implemented by hand (unless you can find a library somewhere for ObjC, Swift or C).

A few test cases:

• recipient.hello@example.com DIRECT -> https://example.com/.well-known/openpgpkey/hu/1sbjrcaf8m3zckmmuej93nx61yn1sttg?l=recipient.hello
• recipient.hello@example.com ADVANCED -> https://openpgpkey.example.com/.well-known/openpgpkey/example.com/hu/1sbjrcaf8m3zckmmuej93nx61yn1sttg?l=recipient.hello
• UPPER@EXAMPLE.COM DIRECT -> https://example.com/.well-known/openpgpkey/hu/awhcnhf7a4ax8qha5u1rwymkfaswmjz8?l=UPPER
• UPPER@EXAMPLE.COM ADVANCED -> https://openpgpkey.example.com/.well-known/openpgpkey/example.com/hu/awhcnhf7a4ax8qha5u1rwymkfaswmjz8?l=UPPER

Existing implementation - typescript (we used a library for z-base32)

    const parts = email.split('@');
    if (parts.length !== 2) {
      return null;
    }
    const [user, recipientDomain] = parts;
    if (!user || !recipientDomain) {
      return null;
    }
    const directDomain = recipientDomain.toLowerCase();
    const advancedDomainPrefix = (directDomain === 'localhost') ? '' : 'openpgpkey.';
    const hu = opgp.util.encodeZBase32(await opgp.crypto.hash.digest(opgp.enums.hash.sha1, Buf.fromUtfStr(user.toLowerCase())));
    const directHost = (typeof this.port === 'undefined') ? directDomain : `${directDomain}:${this.port}`;
    const advancedHost = `${advancedDomainPrefix}${directHost}`;
    const userPart = `hu/${hu}?l=${encodeURIComponent(user)}`;
    const advancedUrl = `https://${advancedHost}/.well-known/openpgpkey/${directDomain}`;
    const directUrl = `https://${directHost}/.well-known/openpgpkey`;
    // ...

Existing implementation - python, where we had to implement zbase32 ourselves:

class Wkd(PubkeySource):

	# 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
	# https://metacode.biz/openpgp/web-key-directory

	__TIMEOUT = (3, 3)

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

	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, []))

This is a pre-requisite for #296

[ekievsky]

@tomholub
I see that you use port in TS implementation, but there is no usage of that in Python. Should we use it on iOS? If so, what is the value for it?

[tomholub]

That was purely for local testing - it's convenient to override the port to 8000 during tests to capture the calls on our local mock server during integration testing. On iOS, this may not be needed (and anyway shouldn't be implemented until needed).

[ekievsky]

@tomholub do we replace ?l={user} with ?l=UPPER if any letter in email address is uppercased or if the whole address is uppercased only?

[tomholub]

The case of the l parameter should remain unchanged. So hello -> hello, hELLO -> hELLO and so on.

[ekievsky]

Thanks!