# 
# a full-ruby rsa implementation
# (c) 01/2008 Yoann Guillot
# this code is licenced under the terms of the WTFPL (sam.zoy.org/wtfpl/)
#
class RSA
	attr_accessor :privkey, :pubkey, :modulus
	# accepts bignums, hex strings or raw strings (check #bigint)
	def initialize(privkey, pubkey, modulus)
		@privkey = bigint privkey
		@pubkey  = bigint pubkey
		@modulus = bigint modulus
	end

	def decode(msg)
		msg = bigint msg
		r = RSA.modexp(msg, @privkey, @modulus)
		RSA.bigint2str(r)
	end

	def encode(msg)
		msg = bigint msg
		r = RSA.modexp(msg, @pubkey, @modulus)
		RSA.bigint2str(r)
	end

	def self.str2bigint(str)
		res = 0
		str.each_byte { |b|
			res <<= 8
			res |= b
		}
		res
	end

	def self.bigint2str(i)
		str = ''
		while i > 0
			str << (i & 0xff)
			i >>= 8
		end
		str.reverse
	end

	# full ruby modular exponentiation
	# from a source found on some ruby-lang thread
	def self.modexp(x, e, n)
		return 1%n if e == 0

		# bit size of e
		k = 0
		k += 1 while (e >> k) > 0

		y = x
		(k-2).downto(0) { |i|
			y = (y*y) % n
			y = (y*x) % n if e[i] == 1
		}
		y
	end

	def self.hex2bigint(h)
		h = '0' + h if h.length % 2 == 1
		RSA.str2bigint(h.scan(/../).map { |c| c.hex }.pack('C*'))
	end

	# calls hex2bigint or str2bigint
	def bigint(i)
		case i
		when /^[0-9a-fA-F]*$/: RSA.hex2bigint(i)
		when ::String: RSA.str2bigint(i)
		when ::Integer: i
		else raise 
		end
	end
end