  We model (interactive) resources that provide Alice with a string $X$ and a
guarantee that any Eve interacting with her interface of the resource obtains a
(quantum) system $E$ such that the conditional (smooth) min-entropy of $X$
given $E$ is lower bounded by some $k$. This (abstract) resource specification
encompasses any setting that results in the honest players holding such a
string (or aborting). For example, it could be constructed from, e.g., noisy
channels, quantum key distribution (QKD), or a violation of Bell inequalities,
which all may be used to derive bounds on the min-entropy of $X$.
  As a first application, we use this min-entropy resource to modularize key
distribution (KD) schemes by dividing them in two parts, which may be analyzed
separately. In the first part, a KD protocol constructs a min-entropy resource
given the (physical) resources available in the specific setting considered. In
the second, it distills secret key from the min-entropy resource---i.e., it
constructs a secret key resource. We prove security for a generic key
distillation protocol that may use any min-entropy resource. Since the notion
of resource construction is composable---security of a composed protocol
follows from the security of its parts--- this reduces proving security of a KD
protocol (e.g., QKD) to proving that it constructs a min-entropy resource.
  As a second application, we provide a composable security proof for the
recent Fehr-Salvail protocol [EUROCRYPT 2017] that authenticates classical
messages with a quantum message authentication code (Q-MAC), and recycles all
the key upon successfully verifying the authenticity of the message. This
protocol uses (and recycles) a non-uniform key, which we model as consuming and
constructing a min-entropy resource.
