From: "Martin Boßlet" <martin.bosslet@...>
Date: 2012-05-17T10:18:56+09:00
Subject: Re: ruby openssl bug, reset cipher fails

2012/5/17 roob noob <lists@ruby-forum.com>:
> This is mentioned in this thread
> http://www.ruby-forum.com/topic/4293246#1061067  but I thought it was a
> different problem then so the thread isn't named correctly. Hopefully
> this will get the attention of people interested more in OpenSSL than in
> helping noobs understand initialize.
>
>
> ruby 1.9.3p125 [x86_64-linux]
>
>
> irb(main):001:0> require 'openssl'
> => true
> irb(main):002:0> message = "whatever"
> => "whatever"
> irb(main):003:0> @sha256 = OpenSSL::Digest::SHA256.new
> => #<OpenSSL::Digest::SHA256:
> e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855>
> irb(main):004:0> @cipher = OpenSSL::Cipher::Cipher.new("AES-256-CTR")
> => #<OpenSSL::Cipher::Cipher:0x00000002467830>
> irb(main):005:0> 2.times do
> irb(main):006:1* key = @sha256.digest("whatever")
> irb(main):007:1> @sha256.reset
> irb(main):008:1> @cipher.encrypt
> irb(main):009:1> @cipher.key = key
> irb(main):010:1> ciphertext = @cipher.update(message)
> irb(main):011:1> ciphertext << @cipher.final
> irb(main):012:1> @cipher.reset
> irb(main):013:1> puts ciphertext
> irb(main):014:1> end
> �a�xtT��
> 9]K���/
> => 2
>
> --
> Posted via http://www.ruby-forum.com/.

Hi,

this is not a bug and Cipher#reset is working fine. What's "going
wrong" here is that you also have to take the IV into account when
trying to reproduce a certain ciphertext. In fact, you don't even need
to call #reset explicitly, #encrypt implies the same functionality
already. Let me explain:

require 'openssl'

message = "whatever"
sha256 = OpenSSL::Digest::SHA256.new
cipher = OpenSSL::Cipher::Cipher.new("AES-256-CTR")
iv = "0" * 32 # you shouldn't do this of course, see my remarks below

2.times do
  key = sha256.digest("whatever")
  sha256.reset
  cipher.encrypt
  cipher.key = key
  cipher.iv = iv
  ciphertext = cipher.update(message)
  ciphertext << cipher.final
  #cipher.reset
  puts ciphertext #will reproduce the same ciphertext twice
end

Of course, it's bad practice to choose a deterministic IV like that,
it was just for demonstration. Generally, you should prefer to use
#random_iv and #random_key in production code as outlined in
http://www.ruby-doc.org/stdlib-1.9.3/libdoc/openssl/rdoc/OpenSSL/Cipher.html.
Hope that clarifies the issue?

-Martin