According to Amazon,
The Fifth Edition [of An Introduction to the Theory of Numbers by Niven & Zuckerman, plus Montgomery, has] new features [that] include expanded treatment of the binomial theorem, techniques of numerical calculation and a section on public key cryptography.
A couple of years ago, I checked out an older edition from the library. I don't recall anything in it about cryptography, but given the year it was printed, I wasn't expecting that anyway.
I think that was the Fourth Edition, from 1980. In the decade to the next edition, one would think the authors would have agonized over things they could have phrased better.
And one would also think the new co-author would want to put his own stamp on the book, so that people don't think it's just a warmed over Fourth Edition with a few minor updates.
And hey, only three left in stock as of this writing. Gotta do your part to help the American economy before our idiot president takes a sledgehammer to the Dow Jones.