.mail rivest%ai
Subject: slight bug in your encoding scheme
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I don't have your paper with me, but as I recall it, there is the following
slight bug.  If the enemy can guess the exact contents of a block,
he can confirm his guess.  In military and commercial text, exact
repetitions of substantial pieces of text are common, and long guessable
sequences that fill the blanks in standard text exist.  Much can be learned
by the confirmation of such guesses, and a computer program can check
guesses rapidly including sliding the text relative to the block
boundary.  The fix is to include enough genuinely random or well-made
pseudo-random text in each block, so that the block cannot be guessed.
I had been intending to mention this for some time after checking to
be sure that you didn't mention the problem, but I didn't get around
to checking.  Sorry if it's old hat.