Merge pull request #53 from red0xff/merkle-hellman

Add merkle-hellman cryptosystem implementation

ciphers/merkle_hellman_cryptosystem.rb

@@ -0,0 +1,71 @@
+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}\""