Chat in Ruby using a home-made RSA.

chat_client.rb

%w(socket net/http thread monitor cmath ./rsa).map(&method(:require))

from_server = RSA.new
n,e = from_server.get_params

puts "Encryption system ready!"

server  = TCPSocket.new 'localhost', 2000 # change this part

a = server.gets.to_i
b = server.gets.to_i
server.puts n, e

to_server = RSA.new(a,b)

puts "Welcome to the chat!", "What is your name?"
name = gets.strip 
server.puts to_server.encrypt(name)
puts "Welcome #{name}"


id = Thread.start(server) do |server|
	loop do
		s = server.gets.strip
		s = from_server.decrypt(s)
		puts s
	end
end

loop do
	s = gets.strip
	s = to_server.encrypt(s)
	server.puts s
end

chat_server.rb

%w(socket net/http thread cmath ./rsa).map(&method(:require))

server  = TCPServer.new 2000 # choose your port

mutex = Mutex.new
keys = []
users = []

from_client = RSA.new
n,e = from_client.get_params

id = 0

puts "Server ready!"

loop do
	Thread.start(server.accept,id) do |client,id|
		client.puts n,e
		mutex.synchronize { users << Queue.new; keys << RSA.new(client.gets.to_i,client.gets.to_i) }
		name = from_client.decrypt(client.gets.strip)
		puts name
		mutex.synchronize do
			users.each do |q|
				q << ["System",-1,"#{name} has entered the room."]
			end
		end
		writer = Thread.new do
				loop do
					user, src, message = users[id].pop
					mutex.synchronize do
						client.puts keys[id].encrypt("#{user}> #{message}") if src != id
					end
				end
			 end
		loop do
			s = client.gets.strip
			s = from_client.decrypt(s)
			puts "#{name}> #{s}"
			mutex.synchronize do
				users.each do |q|
					q << [name,id,s]
				end
			end
		 end
	end
	id += 1
end

rsa.rb

class RSA

	NUM = 100
	MAX = 10**NUM
	LEN = (NUM / CMath::log10(256)).floor

	def initialize(*args)
		if args.length == 0
			arr = []

			while arr.size < 2
				p = rand(MAX..(2*MAX))
				arr << p if prime(p)
			end

			@n = arr.inject(1,:*)

			@m = (arr[0]-1)*(arr[1]-1)

			loop do
				a = rand(2..MAX)
				if @m.gcd(a) == 1
					@e = a 
					break	
				end
			end

			@u = euclid(@e,@m).first % @m

			@mode = :decrypt
		else
			@n = args[0]; @e = args[1]; @mode = :encrypt
		end
	end

	def encrypt(s)
		raise "Wrong mode!" if @mode != :encrypt
		s.bytes.to_a.each_slice(LEN).map{|r| ex r.to_a.reverse.inject(0){|ac,x| ac*256 + x }, @e, @n }.map(&:to_s).join(" ")
	end

	def decrypt(s)
		raise "Wrong mode!" if @mode != :decrypt
		res = s.split.map(&:to_i).map do |x|
			y = ex x, @u, @n
			ans = []
			while y > 0
				ans.unshift (y % 256).chr
				y /= 256
			end
			ans.join.reverse
		end
		res.join
	end

	def get_params
		raise "Wrong mode!" if @mode != :decrypt
		[@n,@e]
	end

	private

	def ex(a,n,m)
		ans = 1
		while n > 0
			ans = (ans*a) % m if (n & 1) == 1
			a = (a*a)%m
			n >>= 1
		end
		ans
	end

	def prime(p)
		k = p - 1
		50.times do
			a = rand(2..k)
			return false if ex(a,k,p) != 1
		end
		true
	end

	def euclid(x,y)
		return euclid(y,x).reverse if y > x
		return [0,1] if (x % y) == 0
		b,a = euclid(y,x%y)
		[a,b-a*(x/y)]
	end

end