# 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.

## Should I use random or secrets?

Python, as of version 3.6, includes two different random modules.

The module called `random` is pseudo-random, meaning it does not generate
true randomness, but follows an algorithm that simulates randomness.
Since random numbers are generated through a known algorithm, they are not truly random.

The `random` module is not correctly suited for cryptography and should not be used,
precisely because it is pseudo-random.

For this reason, in version 3.6, Python introduced the `secrets` module, which generates
cryptographically strong random numbers that provide the greater security required for cryptography.

Since this is only an exercise, `random` is fine to use, but note that **it would be
very insecure if actually used for cryptography.**

### Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the `exercism/python/<exerciseName>` directory.

For example, if you're submitting `bob.py` for the Bob exercise, the submit command would be something like `exercism submit <path_to_exercism_dir>/python/bob/bob.py`.


For more detailed information about running tests, code style and linting,
please see the [help page](http://exercism.io/languages/python).
## 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.