Solitaire is a "by-hand" encryption algorithm developed by Bruce Schneier, using a deck of cards. It is strongly biased and not too strong when used to encrypt a lot of data. Paul Crowley exposed some of the weaknesses and developed Mirdek as an alternative, but it turned out to be even weaker under some circumstances.

Schneier developed the cipher for Neal Stephenson's novel Cryptonomicon, in which the cipher is known as Pontifex.