encrypting files + filestreams?

[Larry Bates]

per9000 wrote:
> Hi python people,
> 
> I am trying to figure out the best way to encrypt files in python.
> 
> I've build a small script (see below) that encrypts the ubuntu 7.04
> iso file in 2 minutes (I like python :) ).
> 
> But I have some thoughts about it. By pure luck (?) this file happened
> to be N*512 bytes long so I do not have to add crap at the end - but
> on files of the size N*512 + M (M != 521) I will add some crap to make
> it fit in the algorithm. When I later decrypt I will have the stuff I
> do not want. How do people solve this? (By writing the number of
> relevant bytes in readable text in the beginning of the file?)
> 
> Also I wonder if this can be solved with filestreams (Are there
> streams in python? The only python file streams I found in the evil
> search engine was stuff in other forums.)
> 
> 
> Other comments are of course also welcome,
> Per
> 
> 
> # crypto_hardcoded.py starts here
> 
> from Crypto.Cipher import AES
> 
> def encrypt2(cryptor, infile, outfile):
>     """enly encrypt a few bytes at a time"""
> 
>     size = 512
>     bytes = infile.read(size)
> 
>     seek = 0
>     interval = 97
>     ctr = 0
> 
>     while len(bytes) == size:
>         seek += size
>         if ctr % interval == 0:
>             print '\r%15d bytes completed' %  (seek),
>         ctr += 1
> 
>         outfile.write(cryptor.encrypt(bytes))
>         # change to this to decrypt
>         # outfile.write(cryptor.decrypt(bytes))
>         bytes = infile.read(size)
> 
>     if len(bytes) != 0:
>         bytes += "#" * (size - len(bytes))
>         outfile.write(cryptor.encrypt(bytes))
>         seek += len(bytes)
> 
>     print '\r%15d bytes completed' %  (seek)
> 
> if __name__ == "__main__":
>     crptz = AES.new("my-secret_passwd")
>     ifile = file('/tmp/ubuntu-7.04-desktop-i386.iso','r')
>     ofile = file('/tmp/ubuntu-7.04-desktop-i386.iso.out','w')
> 
>     encrypt2(crptz, ifile, ofile)
>     ifile.close()
>     ofile.close()
> 
> # crypto_hardcoded.py ends here
> 

Padding and keeping information in a header is how I solved the problem.

-Larry