# 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.**

## Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to
indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not
every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include
a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
`raise Exception`, you should write:

```python
raise Exception("Meaningful message indicating the source of the error")
```

## Running the tests

To run the tests, run the appropriate command below ([why they are different](https://github.com/pytest-dev/pytest/issues/1629#issue-161422224)):

- Python 2.7: `py.test diffie_hellman_test.py`
- Python 3.3+: `pytest diffie_hellman_test.py`

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
`python -m pytest diffie_hellman_test.py`

### Common `pytest` options

- `-v` : enable verbose output
- `-x` : stop running tests on first failure
- `--ff` : run failures from previous test before running other test cases

For other options, see `python -m pytest -h`

## Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the `$EXERCISM_WORKSPACE/python/diffie-hellman` directory.

You can find your Exercism workspace by running `exercism debug` and looking for the line that starts with `Workspace`.

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.