Only for demonstration purposes.

Interactive Diffie-Hellman key exchange with GNU MP

Compile it:

g main.cpp -odh -O3 -Wall -lgmp -lgmpxx

Use it:

./dh

g prime.cpp -oprime -O3 -Wall -std=c 0x -lgmp -lgmpxx

./prime 500

... to create a prime with 500 decimal digits.