autokey.js

function Autokey (key, alphabet) {
  this.key = []
  for (var i = 0, ii = key.length; i < ii; ++i) {
    this.key[i] = key[i].charCodeAt(0)
  }
  this.betaalph = []
  for (i = 0, ii = alphabet.length; i < ii; ++i) {
    this.betaalph[alphabet[i]] = i
  }
  this.alphabet = alphabet
  this.keylen = key.length
  this.alphalen = alphabet.length
}

Autokey.prototype.encode = function (raw) {
  var res = ''
  var cc = this.keylen
  var last = []
  var k
  for (var i = 0, ii = raw.length; i < ii; ++i) {
    if (this.betaalph[raw[i]] === undefined) {
      res += raw[i]
    } else {
      if (cc-- > 0) {
        k = this.key[cc]
      } else {
        k = this.betaalph[last.shift()]
      }

      var c = this.alphabet[(this.betaalph[raw[i]] + k) % this.alphalen]
      res += c
      last.push(c)
    }
  }
  return res
}

Autokey.prototype.decode = function (raw) {
  var res = ''
  var cc = this.keylen
  var last = []
  var k
  for (var i = 0, ii = raw.length; i < ii; ++i) {
    if (this.betaalph[raw[i]] === undefined) {
      res += raw[i]
    } else {
      if (cc-- > 0) {
        k = this.key[cc] % this.alphalen
      } else {
        k = this.betaalph[last.shift()]
      }
      res += this.alphabet[(this.betaalph[raw[i]] - k + this.alphalen) % this.alphalen]
      last.push(raw[i])
    }
  }
  return res
}

module.exports = Autokey