I was introduced to this idea on the comp.risks forum, where it was presented as an attack on an answering machine with a 5-digit passcode, which, naively, would require an average of 50,000 guesses of 5 digits each, or 250,000 key presses. But this particular device not only didn't lock out after some number of incorrect guesses, it didn't require a separator character between guesses. It just listened for the correct five digits consecutively, regardless of what you had entered before. So a superpermutation just 153 digits long was sufficient to crack every device.
Possibly. It was 20 years ago. I'm reconstructing via my (possibly flawed) understanding of the linked article. (My vague recollection was something like 250 digits for a 4-digit passcode.)
And I just realized I mistakenly conflated 5-digit permutations of 5 digits with 5-digit passcodes of 10 digits. Whoops.
You’re thinking of De Brujin sequences, not superpermutations, unless there’s an additional restriction that a digit cannot be used twice in a password.
Yes, and in this case the sequence is specifically the "classic" De Bruijn sequence [1]; however, there are other sequences that bear De Bruin's name, such as the Moser de Bruijn sequence, which is the one I reference most due to its relation to Z-order curves and pairing functions.
