NTL is a high-performance, portable C++ library providing data structures and algorithms for manipulating signed, arbitrary length integers, and for vectors, matrices, and polynomials over the integers and over finite fields.
On modern platforms supporting C++11, NTL can be compiled in thread safe and exception safe modes.
If you are interested in contributing to the development of NTL, please contact me (see contact info below). I have a few projects in mind, and would be happy to discuss other ideas as well.
Pay what you like: NTL is freely available under a GPL license. If you find the software useful, please feel free to donate to the author whatever amount you would like by clicking on the paypal button below. And if you don't feel like donating, that's OK too.
Thanks for your help!
For those keeping score: I did a quick comparison of v9.2 against v5.5 (which was when development kind of lapsed between 2009 and 2013). For one of my basic benchmarks, namely, multiplication of degree 1023 polynomials modulo a 1024-bit prime, I measure a 1.8x speedup. I'm pretty sure a number of other things have improved, too.
[More detailed information about recent changes]