Re: [Boost-users] [iostreams] Implementing a block cipher as a filter

From: Pfligersdorffer, Christian (Christian.Pfligersdorffer_at_[hidden])
Date: 2006-09-26 06:03:10

Hello all!

Seems I am the first one to reply to my own post :-) I am not so much surprised however, since this stuff is kinda weird and complicated. 

Nevertheless I got some news for you: after replacing the gzip- with bzip2-compression filters everything worked fine!! This means there is some (very subtle) bug or incompatibility in the gzip filters, which I of course did not expect (but rather was looking in my code of course).

For the protocoll: gzip_decompressor raises a gzip_error exception with internal error_ = -3 when reaching the end of the stream. Unfortunately this behaviour could not be reproduced without actual encrypting/decrypting. Simply doing a null-transformation by replacing encrypt() and decrypt() with memcpy() does not yield the error.

I did some tests: reading a file, writing it to a filtering_ostream using filter X into some file, again reading through filtering_istream using Y and comparing with the original file.

X = block_crypter, Y = block_crypter               ok, identity

X = gzip_compressor, Y = gzip_decompressor         ok, identity

X = block_crypter|bzip2_compressor, 
Y = block_crypter|bzip2_decompressor               ok, identity

X = block_crypter|gzip_compressor, 
Y = block_crypter|gzip_decompressor                error, see above

and now get yourself a good seat:

X = gzip_compressor|block_crypter, 
Y = gzip_decompressor|block_crypter                ok, identity

Of course compressing an encrypted file is not the same as encrypting the compressed file, which is much faster and results in smaller files. So I tried this just to see what happens the other way round.

Throughout the test I used file_sink and file_source in binary-mode as devices.

Have fun fixing this ;-)

--
Christian Pfligersdorffer
Dipl.-Ing. Bakk.rer.nat.
Software Engineering
EOS GmbH - Electro Optical Systems
Robert-Stirling-Ring 1
D-82152 Krailling / Munich
Phone [phone]
Fax [phone]
 
christian.pfligersdorffer_at_[hidden]
http://www.eos.info