[ excerpt from developer's web site ]

MIRACL is a Big Number Library which implements all of the primitives
necessary to design Big Number Cryptography into your real-world
application. It is primarily a tool for cryptographic system
implementors. RSA public key cryptography, Diffie-Hellman Key
exchange, DSA digital signature, they are all just a few procedure
calls away. Support is also included for even more esoteric Elliptic
Curves and Lucas function based schemes. The latest version offers
full support for Elliptic Curve Cryptography over GF(p) and GF(2m).
Less well-known techniques can also be implemented as MIRACL allows
you to work directly and efficiently with the big numbers that are
the building blocks of number-theoretic cryptography. Although
implemented as a C library, a well-thought out C++ wrapper is
provided, which greatly simplifies program development. Most example
programs (25+ of them) are provided in both C and C++ versions.

WWW: http://www.shamus.ie/