After solving Stage 9, we had all the input data that was needed to start off on Stage 10. During this time it was actually a little bit amusing to follow the discussions in the mailing lists among the ones who had not seen the plaintext of Stage 9.

Somehow, we had suspected that the final stage would be based on RSA encryption, and had been doing some experiments with the 155 digit number presented as ``the short message.'' It obviously is not an RSA modulus since it is divisible by small factors, and neither is any contiguous substring of the 155-digit string.

Since Torbjörn was known to be interested in factorization and large integer computations in general, we asked him how to best factorize the 155-digit number at hand. Torbjörn said that the only method that could be used for such a large number is the General Number Field Sieve (GNFS), but that it would require on the order of 40 CPU years. Torbjörn had previously factored several numbers using GNFS, but never anything close to a 155-digit number.

solvers@codebook.org