>Although Rivest initially released the puzzle’s code in Java, Fabrot realized it could be solved faster if he used the GNU Multiple Precision Arithmetic Library, free software written in C for doing “precise arithmetic.” So Fabrot dedicated one of the CPU cores on his home desktop computer to running squaring operations in an attempt to solve the puzzle. He says his computer was running the operation 24/7, except when he would have to leave on vacation or there was a power outage.
It seems as if your 2014 comment hinted at this project. Applause for keeping this tight lipped for those years.
2014 comment: >Java is quite fast compared to most languages but for some things it is still terribly slow compared to C / C++. I had to do some big integers crunching lately and tried: Java, Go, Go with GMP, C (GMP) and Haskell (using GMP under the hood). Eventually I went for good old C with the latest GMP version for it was the fastest for my use case (with Haskell and Go with GMP coming close but still not as fast as C). Java was, for the computation I needed to do... Eight times slower than C! I know I was hitting a edge case for a very specific kind of computation but still. [0]