type VigenereCipherFunction = (Int, Int) => Int

  override def encrypt(plain: String, key: String, abc: ABC = this.alphabet) : String = {
    val func: VigenereCipherFunction = (x,y) => (x + y) % abc.length
    work(plain, key, abc, func)
  }

  override def decrypt(cipher: String, key: String, abc: ABC = this.alphabet)  : String = {
    val func: VigenereCipherFunction = (x,y) => (x - y + abc.length) % abc.length
    work(cipher, key, abc, func)
  }

  private def work(text: String, key: String, abc: ABC, func: VigenereCipherFunction): String = {

    val it = Stream.continually(key.toLowerCase).flatten.iterator

    text.toLowerCase
      .map((c) => {
        val tInd = abc.indexWhere((x) => x.equals(c))
        val next = it.next
        val kInd = abc.indexWhere((x) => x.equals(next))

        func(tInd, kInd)
      })
      .map((c) => abc(c))
      .mkString
  }