This came up last year, and I want to rephrase the question based on my understanding:
Going back over the text of the book, where they’re about to decrypt the message, the key is that it is being transmitted over and over as a number that is “tens of billions of bits long” that is the product of three prime numbers. I inferred from that that that is in binary, so. . .
With that, how long in base 10 would a number that is tens of billions of binary digits be?
(I figure with a number that big, there would be quite a few products of three primes that would provide a solution, so I’m not asking for that. )