LCS35

LCS35 is a cryptographic challenge and a puzzle. The idea is to calculate the one-time pad

$2^\{2^t\}\; pmod\; n$

in order to decrypt the ciphertext "z", which in this case includes information to factor "n". The value of "t" was chosen to make the calculation take about 35 years since the puzzle was published in 1999.

References

http://theory.lcs.mit.edu/~rivest/lcs35-puzzle-description.txt