Encrypt with JCE (AES/CBC/PKCS5Padding), decrypt with crypto++ v521 - doesn't work!

Mon, 30 Oct 2006 04:53:34 -0800 

Hello,
I'm new to cryptography and I'm working on my first project. I have written a Java applet to encrypt files with 128bit AES in CBC mode using PKCS5 padding. The 16 byte random IV is written to the file first, then the encrypted data. The Java app can decrypt it's own output with no problems. However, I need my C++ app to be able to decrypt it using crypto++ and it currently doesn't work. 

I get a cyrpto++ exception thrown when 'messageEnd()' is called:

StreamTransformationFilter: invalid PKCS #7 block padding found.


I think this is because the two apps are not using the same block 
padding mode but I don't know how to change crypto++ to use the same 
mode. It's possible that I've just made a newbie mistake - any help 
would be fantastic!


Here's the relevant Java source code:

------------------------------------------------
      SecretKeySpec skeySpec = new SecretKeySpec(secret_key, "AES");

      // Instantiate the cipher
      Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");
      cipher.init(Cipher.ENCRYPT_MODE, skeySpec);


      byte[] encrypted = cipher.doFinal( 
input_text_area_.getText().getBytes() );
            FileOutputStream output_stream = new 
FileOutputStream(output_file);

      output_stream.write( cipher.getIV() ); // write IV first
      output_stream.write(encrypted);
      output_stream.close();
---------------------------------------------------

Here's the C++:

---------------------------------------------------

  std::ifstream licence_file_stream( licence_file_, std::ios::in | 
std::ios::binary );


  // Key and IV setup

  const std::vector<byte> encryption_key = 
Base64::Decode(encryption_password);



  // read the first 16 byte block of data from the file - this is the 
IV...

  std::vector<byte> initialisation_vector(16);

  licence_file_stream.read( (char*) &initialisation_vector[0], 
initialisation_vector.size() );


  // read the remaining binary file content - this is the cipher text...
  std::vector<byte> cipher_text( file_size - 16 );
  licence_file_stream.read( (char*) &cipher_text[0], cipher_text.size() );
    licence_file_stream.close();

  // Decrypt

  CryptoPP::AES::Decryption aesDecryption( &encryption_key[0], 
encryption_key.size() );
  CryptoPP::CBC_Mode_ExternalCipher::Decryption cbcDecryption( 
aesDecryption, &initialisation_vector[0] );


  // init string sink...
  std::string decrypted_text;


  CryptoPP::StreamTransformationFilter stfDecryptor(cbcDecryption, new 
CryptoPP::StringSink( decrypted_text ) );

  stfDecryptor.Put( &cipher_text[0], cipher_text.size() );
  stfDecryptor.MessageEnd();
----------------------------------------------------

Thanks a lot for your help,

Ian
























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