From: "NAKAMURA, Hiroshi" <nakahiro@...>
Date: 2005-09-28T09:47:51+09:00
Subject: Re: How do I (really) encrypt a string in ruby?

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Hi,

Michal Suchanek wrote:
> However, encrypting 2.0M file gives 2.1M file.

It may be caused by PKCS#5 padding.

> Since doing encryption in C makes 2.0M ciphertext from 2.0M plaintext,
> and it is also possible to decrypt the 2.0M ciphertext to the original
> plaintext, something is rotten here.
> 
> Am I missing something?
> 
> require "openssl"
> 
> CHLEN = 256
> KBITS = 256
> KLEN = 32
> IVLEN = 16 # God knows why

AES is a 128 bits (16 bytes) block cipher.

> f=File.open("testkey");
> key=f.read(KLEN)
> f.close
> f=File.open("testiv")
> iv=f.read(IVLEN)
> f.close

> while (chunk=STDIN.read(CHLEN))
>   cipher=OpenSSL::Cipher::Cipher.new("AES-256-CBC")
>   cipher.encrypt(key,iv)
>   cipher.key=key
>   cipher.iv=iv
>   STDOUT << cipher.update(chunk)
>   STDOUT << cipher.final()
> end

This block should be:

cipher=OpenSSL::Cipher::Cipher.new("AES-256-CBC")
cipher.encrypt(key,iv)
cipher.padding=0 # avoid PKCS#5 padding
cipher.key=key
cipher.iv=iv
ciphertext = []
while (chunk=STDIN.read(CHLEN))
  ciphertext << cipher.update(chunk)
end
ciphertext << cipher.final() rescue nil # no need to call final if
padding = 0
print ciphertext.join

Regards,
// NaHi
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