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}\""