From: smb@research.att.com
Date: Fri, 21 May 93 14:26:25 PDT
To: Stanton McCandlish <anton@hydra.unm.edu>
Subject: Re: cypto + compression
Message-ID: <9305212126.AA27376@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


	 OK well if you encrypt a compressed file, there are bound to
	 be lots more new redundencies created in the encryption
	 process (unless it is something like ROT-13).  Why not
	 compress this AGAIN, squeezing more space out o

Actually, you've got it backwards.  A decent encryption algorithm,
including DES, generates something with very little redundancy,
and hence which cannot be compress further.

$ compress </vmunix >vmunix.Z
$ des -e -k foo <vmunix.Z >vmunix.Z.des
des: WARNING: using software DES algorithm
$ compress <vmunix.Z.des >vmunix.Z.des.Z
-rwxr-xr-x  1 root      1875490 Jan  7 16:59 /vmunix
$ des -e -k foo </vmunix | compress >vmunix.des.Z
des: WARNING: using software DES algorithm
$ ls -l /vmunix vmunix*
-rwxr-xr-x  1 root      1875490 Jan  7 16:59 /vmunix
-rw-rw-r--  1 smb        794374 May 21 17:17 vmunix.Z
-rw-rw-r--  1 smb        794376 May 21 17:18 vmunix.Z.des
-rw-rw-r--  1 smb       1066555 May 21 17:18 vmunix.Z.des.Z
-rw-rw-r--  1 smb       2538235 May 21 17:21 vmunix.des.Z

As you can see, compressing after encrypting *increases* the size of
the file.