encrypting files + filestreams?

[per9000]

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