Simple http://captchas.net implementation for crystal. Based on https://depth-first.com/articles/2007/09/03/fighting-spam-on-the-cheap-with-captcha-a-simple-ruby-library-for-captchas-net

captcha.cr

require "digest/md5"

module Captcha
  def get_text(secret, random, alphabet = "abcdefghijklmnopqrstuvwxyz", character_count = 6)
    if character_count < 1 || character_count > 16
      raise "Character count of #{character_count} is outside the range of 1-16"
    end

    input = "#{secret}#{random}"

    if alphabet != "abcdefghijklmnopqrstuvwxyz" || character_count != 6
      input += ":#{alphabet}:#{character_count}"
    end

    bytes = Digest::MD5.digest(input)[0..5]
    text = ""

    bytes.each do |byte|
      text += alphabet[byte % alphabet.size]
    end

    text
  end
end