// Copyright (C) 2014 The Syncthing Authors. // // This Source Code Form is subject to the terms of the Mozilla Public // License, v. 2.0. If a copy of the MPL was not distributed with this file, // You can obtain one at https://mozilla.org/MPL/2.0/. package protocol import "fmt" var luhnBase32 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567" func codepoint32(b byte) int { switch { case 'A' <= b && b <= 'Z': return int(b - 'A') case '2' <= b && b <= '7': return int(b + 26 - '2') default: return -1 } } // luhn32 returns a check digit for the string s, which should be composed // of characters from the alphabet luhnBase32. // Doesn't follow the actual Luhn algorithm // see https://forum.syncthing.net/t/v0-9-0-new-node-id-format/478/6 for more. func luhn32(s string) (rune, error) { factor := 1 sum := 0 const n = 32 for i := range s { codepoint := codepoint32(s[i]) if codepoint == -1 { return 0, fmt.Errorf("digit %q not valid in alphabet %q", s[i], luhnBase32) } addend := factor * codepoint if factor == 2 { factor = 1 } else { factor = 2 } addend = (addend / n) + (addend % n) sum += addend } remainder := sum % n checkCodepoint := (n - remainder) % n return rune(luhnBase32[checkCodepoint]), nil }