Libgcrypt

Libgcrypt is a cryptography library developed as a separated module of GnuPG.
It can also be used independently of GnuPG, but depends on its error-reporting library Libgpg-error.
It provides functions for all fundamental cryptographic building blocks:
Libgcrypt features its own "multiple precision arithmetic" implementation, with assembler implementations for a variety of processors, including Alpha, AMD64, HP PA-RISC, i386, i586, M68K, MIPS 3, PowerPC, and SPARC. It also features an "entropy gathering" utility, coming in different versions for Unix-like and Windows machines.
Usually multiple, stable branches of Libgcrypt are maintained in parallel; since 2021-01-19 this is the Libgrypt 1.9 branch, plus the old stable 1.8 branch, which will be maintained at least until 2024-12-31.