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# Diffie Hellman
Diffie-Hellman key exchange.
Alice and Bob use Diffie-Hellman key exchange to share secrets. They
start with prime numbers, pick private keys, generate and share public
keys, and then generate a shared secret key.
## Step 0
The test program supplies prime numbers p and g.
## Step 1
Alice picks a private key, a, greater than 1 and less than p. Bob does
the same to pick a private key b.
## Step 2
Alice calculates a public key A.
A = g**a mod p
Using the same p and g, Bob similarly calculates a public key B from his
private key b.
## Step 3
Alice and Bob exchange public keys. Alice calculates secret key s.
s = B**a mod p
Bob calculates
s = A**b mod p
The calculations produce the same result! Alice and Bob now share
secret s.
## Hints
This exercise requires you to perform calculations on large numbers. To correctly represent large numbers, the
[BigInteger](https://msdn.microsoft.com/en-us/library/system.numerics.biginteger(v=vs.110).aspx) struct is used.
### Submitting Exercises
Note that, when trying to submit an exercise, make sure the exercise file that you're submitting is in the `exercism/csharp/<exerciseName>` directory.
For example, if you're submitting `bob.cs` for the Bob exercise, the submit command would be something like `exercism submit <path_to_exercism_dir>/csharp/bob/bob.cs`.
## Source
Wikipedia, 1024 bit key from www.cryptopp.com/wiki. [http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange](http://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange)
## Submitting Incomplete Solutions
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
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