TheAlgorithms-Ruby/ciphers/merkle_hellman_cryptosystem.rb

require'openssl';

class MerkleHellman
  SMALLEST_KNAPSACK_ITEM = 2**32
  STEP = 2**32

  def initialize size
    @size = size;
    sum = SMALLEST_KNAPSACK_ITEM;
    @easy_knapsack = size.times.map do |k|
      x = sum + rand(STEP);
      sum += x;
      x;
    end
    @n = sum + rand(STEP);

    loop do
      @a = rand(0..@n);
      break if @a.gcd(@n) == 1;
    end

    @hard_knapsack = @easy_knapsack.map do |x|
      (@a * x) % @n;
    end
  end

  def encrypt msg
    raise ArgumentError, "max length is #{@size/8} characters" if msg.length * 8 > @size;
    c = 0;
    msg.each_codepoint.reverse_each.with_index do |ch,i|
      7.downto(0) do|j|
        wj = ch.>>(j).& 1;
        c += wj * @hard_knapsack[i*8+7-j]
      end
    end
    c;
  end

  def decrypt c
    p = @a.to_bn.mod_inverse(@n).mod_mul(c,@n).to_i;
    byte = 0;
    msg = [ ];
    @easy_knapsack.reverse_each.with_index do |x,i|
      bit = 0;
      if p >= x
        p -= x;
        bit = 1;
      end
      byte |= (bit << (i%8));
      if i % 8 == 7
        msg << byte.chr;
        byte = 0;
      end
    end
    msg.join;
  end
  attr_accessor :hard_knapsack;
end

str = "Hello there, this is my plaintext";
mh = MerkleHellman.new(str.length*8)
puts "[*] Encrypting \"#{str}\""

c = mh.encrypt(str)

puts "[*] Ciphertext : #{c}"

decrypted = mh.decrypt(c)

puts "[*] after decryption : \"#{decrypted}\""