Repeated Words/characters in Password/Phrase

[Timothy C. May]

At 11:11 PM 11/14/95, Ted Cabeen wrote:
>Do repeated words in a PGP passphrase make the pass phrase less secure than
>a passphrase without any repeated words?  And on the same note, do repeated
>letters in a UNIX password make that password easier to break? I can't seem
>to find anything in my books on cryptography that mention this.  Thanks.

More of an information theory question than a crypto question. There are no
simple answers to this question, but some examples will help:

The password "foo" is not very good, and "foofoo" is only slightly better.
And "foofoofoo" is slightly better, and so on, to a point. But
"foofoo....foo" is not N times better than a single "foo," because the
_pattern_ is simply desribed: "repeat "foo" N times." Thus, the information
content or entropy of "foofoofoo....foo" is not N times greater than the
entropy of "foo."

A some dictionary attacks which would trivially find "foo" will not find
"foofoo," or "foofoofoo," etc., so this could be a great help. More
sophisticated dictionary attacks may of course take the 30,000 or so most
common names, words, places, and then do various permutations, reversals,
repetitions, etc.

So this is why there is not likely to be a simple answer to your question.
Repeating words in a passphrase can make the passphrase easier to remember
(such as "thequickquickbrownfox") and make certain kinds of attacks harder,
but with not as much of an increase in entropy at the increased number of
raw characters might otherwise suggest.

Other "heuristics" (simple rules of thumb) for passphrases are contained in
the PGP documents, and in numerous other places: avoid names, add
nonstandard English keyboard characters liberally (even if using real
words), etc. The "best" passphrases, it almost goes without saying, are the
longest and most "unpredictable," so that "7f#qp)djQ10hB%3t+1?U4SVp5" is
much superior to "%foo%foo".

In the real world, where passphrases must be memorized, "long and random"
is an elusive goal, which has to be weighed against the risk of other
attacks (such as capturing keystrokes with a sofware monitor, or from afar
with a van Eyk antenna, etc.).

Me, I use a nonsense phrase which has meaning to me, with a few garbage
characters added to confuse things further. I don't think my passphrase is
the weak link.

--Tim May

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