LibTomMath is a free open source portable number theoretic multiple-precision integer library written entirely in C.

LibTomCrypt is a fairly comprehensive, modular and portable cryptographic toolkit that provides developers with a vast array of well known published block ciphers, one-way hash functions, chaining modes, pseudo-random number generators, public key cryptography and a plethora of other routines.

TomsFastMath is a fast public domain, open source, large integer arithmetic library written in portable ISO C.

Script to compare LibTomCrypt RSA & ECC with different versions of LibTomMath

A library that provides multiple precision floating point arithmetic

Symmetric ciphers, hashes, microtime, random strings, big integers, and zlib compression