src/solvers/XorDecrypt.scala


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package ixee.cryptopals.solvers

import ixee.cryptopals.utils._
import ixee.cryptopals.utils.ByteUtils._
import ixee.cryptopals.utils.FunctionUtils._

object XorDecrypt {
  implicit val freq = Frequencies.cornell40kSample

  def decryptToAscii(ciphertext: Seq[Byte])(key: Byte): String = {
    new String((ciphertext xor Stream.continually(key)).toArray)
  }

  def tryBestDecrypt(ciphertext: Seq[Byte]): String = {
    (findBestSingleByteKey _ :| decryptToAscii(ciphertext) _)(ciphertext)
  }

  def findBestSingleByteKey(ciphertext: Seq[Byte]): Byte =
    candidates(ciphertext)
      .reduceOption( (a: (Double, Int), b: (Double, Int)) =>
        if (a._1 < b._1) a else b
      ).map(_._2.toByte).getOrElse(0.toByte)

  def candidates(ciphertext: Seq[Byte]) =
    (0 until 256)
      .map(_.toByte)
      .map(x => TextScorer.score(ciphertext xor Stream.continually(x)))
      .zipWithIndex
      .filter(_._1 != -1.0)

  def candidatesAsAscii(ciphertext: Seq[Byte]) =
    candidates(ciphertext).sortBy(_._1).map(x =>
      new String((ciphertext xor Stream.continually(x._2.toByte)).toArray)
    )
}