>Solitaire (...) was featured in Neal Stephenson’s book Cryptonomicon under the name “Pontifex”.

It was introduced under the name "Solitaire", "Pontifex" was used initially as a temporary code-word when characters couldn't speak freely.

I've always wondered if just using plain RC4 (adapted to a card deck) would be simpler (and probably more secure). Adding two-digit (0..52) numbers by hand is easy for almost everyone, and RC4's swap step is no more involved than the `cut` operation of Solitaire. The only downside is that it requires user to keep track of `i` and `j` - two additional two-digit numbers (in memory, because writing anything down is against the threat model of characters in the story). I agree it's error-prone. But, assuming temporarily writing things down with pen and paper is OK according to my threat model, I think RC4 would work nicely.

Of course Bruce is the world-class expert in cryptography and I'm not, so you should probably ignore my grumbling here :).