  The Solitaire cipher was designed by Bruce Schneier as a plot point in the
novel Cryptonomicon by Neal Stephenson. The cipher is intended to fit the
archetype of a modern stream cipher whilst being implementable by hand using a
standard deck of cards with two jokers. We find a model for repetitions in the
keystream in the stream cipher Solitaire that accounts for the large majority
of the repetition bias. Other phenomena merit further investigation. We have
proposed modifications to the cipher that would reduce the repetition bias, but
at the cost of increasing the complexity of the cipher (probably beyond the
goal of allowing manual implementation). We have argued that the state update
function is unlikely to lead to cycles significantly shorter than those of a
random bijection.
