A NEW technique for sending secret messages over the Net could undermine the
US government鈥檚 attempts to limit the spread of communication methods that can鈥檛
be deciphered by its security forces.
Ronald Rivest, a computer scientist at the Massachusetts Institute of
Technology, has posted a paper on the Internet describing a technique he calls
鈥渃haffing and winnowing鈥�. It does not encrypt messages, but instead exploits
digital authentication.
In encryption, messages are converted into a form that cannot be read without
the right mathematical key. Some techniques are for practical purposes
unbreakable. The US government, fearing spies and terrorists might use this
鈥渟trong鈥� encryption, has banned its export.
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But federal officials have endorsed authentication, in which people send a
鈥渄igital signature鈥� along with a document to prove that it is from them. To
create a signature, a numerical key known only to the sender is combined with
data in the document to produce a unique value. Anyone who has been given a
related key can use it to authenticate the signature, but not to reproduce
it.
In Rivest鈥檚 method, no part of the message is actually encrypted. Instead, it
is split into tiny pieces. Each of these is labelled with a number and digitally
signed. The message is then interspersed with nonsense data that are also
numbered and appear to be signed鈥攂ut not with the sender鈥檚 true signature.
Anyone with the correct authentication key can separate, or winnow, the 鈥渨heat鈥�
of the message from the 鈥渃haff鈥�. But no one else can put the pieces
together.
Chaffing and winnowing is less efficient than conventional encryption,
because it increases the size of the message. But experts say that it should be
very effective, and makes a nonsense of the ban on exporting strong encryption.
鈥淭his shows some of the built-in absurdities of export control,鈥� says Michael
Froomkin of the University of Miami School of Law in Coral Gables, Florida.