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require "openid/util"
require "openid/cryptutil"
module OpenID
# Encapsulates a Diffie-Hellman key exchange. This class is used
# internally by both the consumer and server objects.
#
# Read more about Diffie-Hellman on wikipedia:
# http://en.wikipedia.org/wiki/Diffie-Hellman
class DiffieHellman
# From the OpenID specification
@@default_mod = 155172898181473697471232257763715539915724801966915404479707795314057629378541917580651227423698188993727816152646631438561595825688188889951272158842675419950341258706556549803580104870537681476726513255747040765857479291291572334510643245094715007229621094194349783925984760375594985848253359305585439638443
@@default_gen = 2
attr_reader :modulus, :generator, :public
# A new DiffieHellman object, using the modulus and generator from
# the OpenID specification
def DiffieHellman.from_defaults
DiffieHellman.new(@@default_mod, @@default_gen)
end
def initialize(modulus=nil, generator=nil, priv=nil)
@modulus = modulus.nil? ? @@default_mod : modulus
@generator = generator.nil? ? @@default_gen : generator
set_private(priv.nil? ? OpenID::CryptUtil.rand(@modulus-2) + 1 : priv)
end
def get_shared_secret(composite)
DiffieHellman.powermod(composite, @private, @modulus)
end
def xor_secret(algorithm, composite, secret)
dh_shared = get_shared_secret(composite)
packed_dh_shared = OpenID::CryptUtil.num_to_binary(dh_shared)
hashed_dh_shared = algorithm.call(packed_dh_shared)
return DiffieHellman.strxor(secret, hashed_dh_shared)
end
def using_default_values?
@generator == @@default_gen && @modulus == @@default_mod
end
private
def set_private(priv)
@private = priv
@public = DiffieHellman.powermod(@generator, @private, @modulus)
end
def DiffieHellman.strxor(s, t)
if s.length != t.length
raise ArgumentError, "strxor: lengths don't match. " +
"Inputs were #{s.inspect} and #{t.inspect}"
end
if String.method_defined? :bytes
s.bytes.zip(t.bytes).map{|sb,tb| sb^tb}.pack('C*')
else
indices = 0...(s.length)
chrs = indices.collect {|i| (s[i]^t[i]).chr}
chrs.join("")
end
end
# This code is taken from this post:
# <http://blade.nagaokaut.ac.jp/cgi-bin/scat.\rb/ruby/ruby-talk/19098>
# by Eric Lee Green.
def DiffieHellman.powermod(x, n, q)
counter=0
n_p=n
y_p=1
z_p=x
while n_p != 0
if n_p[0]==1
y_p=(y_p*z_p) % q
end
n_p = n_p >> 1
z_p = (z_p * z_p) % q
counter += 1
end
return y_p
end
end
end
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