Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!hao!gatech!udel!princeton!phoenix!amlovell From: amlovell@phoenix.Princeton.EDU (Anthony M Lovell) Newsgroups: sci.crypt Subject: Re: One time pads? Message-ID: <1857@phoenix.Princeton.EDU> Date: 26 Feb 88 18:14:53 GMT References: <4209@june.cs.washington.edu> <1988Feb15.151522.5094@utzoo.uucp> Organization: Princeton University, NJ Lines: 29 Summary: randomness always helps In article , jk3k+@andrew.cmu.edu (Joseph G. Keane) writes: > > > How about using the message itself, in some form or another, > > as the pad. > > comments? > > Not very secure: all you have to do is guess the first character! Even if you > have a reasonably large key (offset), a known-plaintext attack will get some > information, and it propagates forward and backward. > > --Joe As long as your method of devising a key from the plaintext is arbitrary (and remains unknown to the cracker) , he will not get his foot in the door. What if the n+5th message is the key (again adulterated in some form) for the nth message? Any scheme like this will be impregnable until guessed, and its patterns are certainly unlike those typically searched for. The arbitrary system can be changed in encrypted transmissions (with the acknowledged risk that it will not help IF the cipher is already compromised). This denies the "enemy" a large body of ciphertext to examine for these weak patterns. Not by any means the most secure or practical system (more secure than practical in my mind), but I would put my money on the cleartext remaining undiscovered for a LONG LONG time. -- amlovell@phoenix.princeton.edu