Created
July 5, 2024 08:02
-
-
Save skull-squadron/8f806b28abbcaa1ba9c256391e5bd8f9 to your computer and use it in GitHub Desktop.
Find a TOTP collision
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| #!/usr/bin/env ruby | |
| # frozen_string_literal: true | |
| require 'openssl' | |
| class Base32 | |
| ALPHABET = (('A'..'Z').to_a + ('2'..'7').to_a).map(&:freeze).freeze | |
| PADDING_CHAR = '=' | |
| BITS_PER_BYTE = 8 | |
| BITS_PER_CHAR = 5 | |
| BITS_PER_CHUNK = 40 # least common mutliple | |
| CHARS_PER_CHUNK = BITS_PER_CHUNK / BITS_PER_CHAR # 8 | |
| CHUNK_LENGTH = BITS_PER_CHUNK / BITS_PER_BYTE # 5 | |
| ENCODING_MASK = ALPHABET.length - 1 # 0x1f | |
| DECODING_MASK = 0xff | |
| def self.encode(str) | |
| str.bytes | |
| .each_slice(CHUNK_LENGTH) | |
| .map { |chunk| encode_chunk(chunk) } | |
| .join | |
| end | |
| def self.decode(str) | |
| str.delete(PADDING_CHAR) | |
| .each_char | |
| .map { |char| ALPHABET.index(char) } | |
| .each_slice(CHARS_PER_CHUNK) | |
| .map { |chunk| decode_chunk(chunk) } | |
| .join | |
| end | |
| private | |
| def self.encode_chunk(chunk) | |
| bits_in_chunk = chunk.length * BITS_PER_BYTE | |
| number_of_characters = (bits_in_chunk * CHARS_PER_CHUNK + (BITS_PER_CHUNK - 1)) / BITS_PER_CHUNK | |
| if bits_in_chunk < BITS_PER_CHUNK | |
| padding = BITS_PER_CHAR - bits_in_chunk % BITS_PER_CHAR | |
| else | |
| padding = 0 | |
| end | |
| buf = chunk.reduce(0) { |buf, byte| (buf << BITS_PER_BYTE) + byte } << padding | |
| encoded = Array.new(CHARS_PER_CHUNK) | |
| j = number_of_characters - 1 | |
| number_of_characters.times do |i| | |
| encoded[j] = ALPHABET[(buf >> BITS_PER_CHAR * i) & ENCODING_MASK] | |
| j -= 1 | |
| end | |
| (CHARS_PER_CHUNK - number_of_characters).times do |i| | |
| encoded[number_of_characters + i] = PADDING_CHAR | |
| end | |
| encoded | |
| end | |
| def self.decode_chunk(chunk) | |
| number_of_original_bytes = chunk.length * BITS_PER_CHAR / BITS_PER_BYTE | |
| if chunk.length < CHARS_PER_CHUNK | |
| padding = BITS_PER_CHAR - (number_of_original_bytes * BITS_PER_BYTE) % BITS_PER_CHAR | |
| else | |
| padding = 0 | |
| end | |
| buf = chunk.reduce(0) { |buf, byte| (buf << BITS_PER_CHAR) + byte } >> padding | |
| decoded = Array.new(number_of_original_bytes) | |
| j = number_of_original_bytes - 1 | |
| number_of_original_bytes.times do |i| | |
| decoded[i] = ((buf >> BITS_PER_BYTE * j) & DECODING_MASK).chr | |
| j -= 1 | |
| end | |
| decoded | |
| end | |
| end | |
| DIGITS = 6 | |
| INTERVAL = 30 # seconds | |
| def hotp(k, c) | |
| hmac = OpenSSL::HMAC.digest('SHA1', k, [c].pack('L>')) | |
| offset = hmac[-1].ord & 0xf | |
| code = (hmac[offset].ord & 0x7f) << 24 | | |
| (hmac[offset + 1].ord & 0xff) << 16 | | |
| (hmac[offset + 2].ord & 0xff) << 8 | | |
| (hmac[offset + 3].ord & 0xff) | |
| (10 ** DIGITS + (code % 10**DIGITS)).to_s()[-DIGITS..-1] | |
| end | |
| def totp(k, ct) | |
| hotp(k, ct) | |
| end | |
| if !ARGV[1] | |
| $stderr.puts 'Usage: {otp secret in base32 format} {digits of TOTP code to match}' | |
| exit 1 | |
| end | |
| key = Base32.decode(ARGV[0]) | |
| seek = ARGV[1] | |
| c = Time.now.to_i / INTERVAL | |
| until totp(key, c) == seek do | |
| c += 1 | |
| end | |
| puts "TOTP will match #{seek} between #{Time.at(c*INTERVAL)} and #{Time.at(c*INTERVAL + INTERVAL - 1)}" |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment