http://core0.staticworld.net/images/article/2013/02/encrypt_primary-100022114-gallery-100024895-orig.jpg
[Source]( http://www.pcworld.com/article/2031002/when-you-encrypt-a-file-or-a-hard-drive-is-it-really-secure-.html)
Key exchange is always a concern when security is a priority, now days thanks to asymmetric cryptography we have different solutions but back in 1970s things were a bit different.
https://i.imgsafe.org/f98410e26f.jpg
[Source](http://www.slideshare.net/lineking/key-distribution)
In symmetric type of cryptography (like 3Des) unlike asymmetric (like AES), there must be a shared key between two parties.
There is a lot of ways to do that:
* Meet in person and share the key (not a good option if you are living in different regions)
* Talking on the phone(if you are dumb enough to trust telecommunication’s infrastructure)
* Encryption(which you need another shared key to encrypt your key, so we are on the first place)
* Using different algorithms which are implemented to transfer data  on insecure networks
* …


In 1976 two cryptographer named [Whitfield Diffie](https://en.wikipedia.org/wiki/Whitfield_Diffie)-[Martin Hellman](https://en.wikipedia.org/wiki/Martin_Hellman) addressed one of the fundamental problems of cryptography and came up with an algorithm to distribute keys without sending the key over an insecure network.

https://i.imgsafe.org/f9b1417038.jpg
Ralph Merkle (worked on the Diffie-Hellman project), Martin Hellman and Whitfield Diffie in 1977 
[Source](http://www.theregister.co.uk/2016/03/01/diffie_and_hellman_scoop_turing_award_for_key_work_on_crypto/)


## How it works!
Let’s bring Alice and Bob (cryptographers’ favorite characters) to start a scenario. 
The first thing that Alice and Bob should do is to agree on a prime number (p) and a base (z). We talked and they chose p=23 and z=6(these numbers don’t need to be secure).
Now it is Alice and Bob turn to choose a number, she chooses her lucky number which is a=7, and Bob chooses b=17. 
At this time we have all the needed numbers and keys, let’s share the key
https://i.imgsafe.org/fa4732fa0b.jpg
And 16 is going to be their shared key
As you saw the key never has been transfer directly through the channel, it actually got created by both parties.
 
# The protocol is considered secure against eavesdroppers the inputs are chosen properly.

https://i.imgsafe.org/fa9b88d3cc.jpg
a very basic java code to get a better understanding of the protocol.
it can be done by more than two parties , I am not going through that because it is a bit more complex and but the basic is the same.

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