The indistinguishability of two pieces of data (or two lists of pieces of data) can be represented formally in terms of a relation called static equivalence. Static equivalence depends on an underlying equational theory. The choice of an inappropriate equational theory can lead to overly pessimistic or overly optimistic notions of indistinguishability, and in turn to security criteria that require protection against impossible attacks or---worse yet---that ignore feasible ones. In this paper, we define and justify an equational theory for standard, fundamental cryptographic operations. This equational theory yields a notion of static equivalence that implies computational indistinguishability. Static equivalence remains liberal enough for use in applications. In particular, we develop and analyze a principled formal account of guessing attacks in terms of static equivalence.