  Interactive behaviors are ubiquitous in modern cryptography, but are also
present in $\lambda$-calculi, in the form of higher-order constructions.
Traditionally, however, typed $\lambda$-calculi simply do not fit well into
cryptography, being both deterministic and too powerful as for the complexity
of functions they can express. We study interaction in a $\lambda$-calculus for
probabilistic polynomial time computable functions. In particular, we show how
notions of context equivalence and context metric can both be characterized by
way of traces when defined on linear contexts. We then give evidence on how
this can be turned into a proof methodology for computational
indistinguishability, a key notion in modern cryptography. We also hint at what
happens if a more general notion of a context is used.
