  We present criptic, the Cosmic Ray Interstellar Propagation Tool using It\^o
Calculus, a new open-source software package to simulate the propagation of
cosmic rays through the interstellar medium and to calculate the resulting
observable non-thermal emission. Criptic solves the Fokker-Planck equation
describing transport of cosmic rays on scales larger than that on which their
pitch angles become approximately isotropic, and couples this to a rich and
accurate treatment of the microphysical processes by which cosmic rays in the
energy range $\sim$MeV to $\sim$PeV lose energy and produce emission. Criptic
is deliberately agnostic as to both the cosmic ray transport model and the
state of the background plasma through which cosmic rays travel. It can solve
problems where cosmic rays stream, diffuse, or perform arbitrary combinations
of both, and the coefficients describing these transport processes can be
arbitrary functions of the background plasma state, the properties of the
cosmic rays themselves, and local integrals of the cosmic ray field itself
(e.g., the local cosmic ray pressure or pressure gradient). The code is
parallelised using a hybrid OpenMP-MPI paradigm, allowing rapid calculations
exploiting multiple cores and nodes on modern supercomputers. Here we describe
the numerical methods used in the code, our treatment of the microphysical
processes, and the set of code tests and validations we have performed.
