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Path: utzoo!watmath!clyde!rutgers!sri-unix!hplabs!hao!nbires!vianet!devine
From: devine@vianet.UUCP
Newsgroups: sci.crypt
Subject: Re: bitwise exclusive-or
Message-ID: <132@vianet.UUCP>
Date: Mon, 26-Jan-87 21:34:15 EST
Article-I.D.: vianet.132
Posted: Mon Jan 26 21:34:15 1987
Date-Received: Wed, 28-Jan-87 06:17:51 EST
References: <434@ethz.UUCP>
Organization: Western Digital, Boulder Tech Ctr
Lines: 64
Keywords: xor
Summary: a bit of random thoughts (it's long, sssry)


[ ethz!wyle describes a encryption scheme based on random keys and asks:
> How is the system flawed?  Is it unbreakable?  Couldn't companies
> implement this strategy cost-effectively?

  This system is essentially a one-time pad system.  It is not breakable
in the sense of never having to re-get the random-bits.  However, that does
not mean that it is a totally secure system.  If the "pad" -- here it is the
(1) source of the random bits; (2) the storage of the random bits on a
copyable medium; (3) transmission of the the random bits by a supposedly
reliable courier and (4) both the reading and writing of the random bits
on the medium -- is kept secure, the system is a good way towards security.
And, if it is not kept secure, too bad.

  However, let me suggest some possible drawbacks:

   1. What constitutes a message?  Are there specified lengths of messages?
      Are there always headers or trailers to every message?
      What error-correction/detection features are used?  If a
      message arrives in error, what happens?  A nasty person knowing
      this information might be able to do a plain-text attack or 
      other cryptanalytic [note, this word is easy to write but very
      hard to say -Bob] attack.

   2. Can just the fact that a message is sent from A to B be used to
      suggest ssmething?  Remember, even if the messaging is secure
      an fiendish ssrt of spy can realize that since A is now sending
      a message that means A's office safe is not being watched, and
      as a result, may be open for an attack on it! Information may be
      garnered just from the fact of finding out that perssn A has
      sent a message to person Z, such as (1) they have something to
      talk about (hmm, what could it be?) or (2) that A's computer is
      on the network.

   3. A optical disk platter would be a very tempting target for a
      spy.  If one were stolen (the best theft is one were the owner
      does not realize that a theft has taken place, copying a disk
      surreptitiously fits this classification) it would likely be
      usable as the key source for months after the theft.  They need to
      balance the amount that can be stolen against the overhead of
      sending small amounts of random bits.

   4. Is a different selection of random bits always used?  How is
      the selection made?  Is the information of where to start
      kept in a secured file?  How much handshaking is done between
      the sender and recipient to ensure that both are using the
      correct selection?

   5. Because the network is not secure, an active tapper could
      interpose bad messages and delete good messages.  The messages
      might be secure by themselves but the transmission could be
      rendered useless.

   6. Systems that use the same key for encryption and decryption
      are susceptible to key distribution problems.  The random-key
      system falls into same-keyness.  Public/private keys can
      handle the key distribution problem nicely.

   7. Putting people into the encryption loop is never cheap.  Having
      a secure courier deliver the keys to the (just one?) recipient
      is expensive because the courier must be investigated and other
      security nonsense.

Bob Devine