Public key cryptography - Diffie-Hellman Key Exchange

https://www.youtube.com/watch?v=YEBfamv-_do#t=02m37s

这个video很好地解释了DH算法的原理，简单清晰。需要稍微了解些取模运算的知识：参见：https://www.khanacademy.org/computing/computer-science/cryptography/modarithmetic/a/modular-addition-and-subtraction?utm_source=pocket_saves 

so for Alice: its private number is: 15,  she compute: 3^15 % 17 = 6

for Bob, its private number is: 13, he compute: 3^13 % 17 = 12

then they exchange 6 and 12

then Alice compute:  12^15 mod 17, according to mode的指数运算，也即：(3^13)^15 mod 17

then Bob compute: 6^13 mod 17, that is :  (3^15)^13 mod 17

the resule is equal, it is 10, it can be used as the shared encrypted key, while for the eva who eavesdrop the communication cannot compute the result(eva knows: 3,17,6,12)