monkey encrypts

require "pp"

def encrypt(plaintext, key)
  pt = plaintext
  key += pt.length / 2
  ct = Array.new(pt.size)

  (0..pt.size-1).each do |i|
    ct[i] = pt[i].ord + ( pt[i].ord % key )
    key += 1
  end

  ct.map(&:chr).join
end

def decrypt(ciphertext, key)
  ct = ciphertext
  key += ct.length / 2
  pt = Array.new(ct.size)

  (0..pt.size-1).each do |i|
    possibles = []
    (0..127).each do |possible_pt|
      if possible_pt + (possible_pt % key) == ct[i].ord
        possibles << possible_pt
      end
    end
    if possibles.size == 1
      pt[i] = possibles[0].chr
    else
      pt[i] = "(" + possibles.map(&:chr).join("|") + ")"
    end
    key += 1
  end

  pt.join
 end

plaintext = "Hello, world!"
key = rand(plaintext.size / 2)
ciphertext = encrypt(plaintext, key)
replaintext = decrypt(ciphertext, key)

p key
p plaintext
p ciphertext
p replaintext