An integer composition is a sequence of positive integers (parts) that sum to $s$, and a $t$-composition of $s$ is a composition with $t$ parts. If the parts are allowed to be equal to zero, the $t$-compositions are called weak, and if the parts are bounded by some integer $n$, they are called $n$-restricted. We study the use of $n$-restricted weak $t$-compositions of a number $s$ for cryptographic algorithms called digital signature schemes. We focus on the Winternitz signature scheme. We modify Winternitz to create signatures that map messages to $n$-restricted $t$-compositions of some fixed $s$, which makes the cost of signature generation and verification constant for any message. To do this mapping efficiently, we propose and analyse several unranking algorithms for these combinatorial objects. We also do an experimental comparison of their performance for parameter sets relevant to the cryptographic applications. This includes joint work with Lucas Perin, Ricardo Custodio, Lucia Moura and Daniel Panario.