Curve25519
From Nxtwiki
Take a look at this discussion thread https://nxtforum.org/general/curve22519-in-c
It suggests that the original code can be found here https://github.com/swdunlop/WaspVM/blob/master/vm/curve25519_i64.c
In addition, user mystcoin has ported the Java code back to C here https://bitbucket.org/mystcoin/curve22519/src use your best judgement when selecting which version to use.
There is also this review of the Nxt curve25519 implementation: https://gist.github.com/doctorevil/9521116 and an alternative Python implementation for test purposes https://gist.github.com/doctorevil/9521126
