Diffie Hellman in Python
DiffieHellman key exchange.
1  exercism fetch python diffiehellman

Diffie Hellman
DiffieHellman key exchange.
Alice and Bob use DiffieHellman 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.
1 
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.
1 
s = B**a mod p

Bob calculates
1 
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 pseudorandom, 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 pseudorandom.
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:
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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):
 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/diffiehellman
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.
Source
Wikipedia, 1024 bit key from www.cryptopp.com/wiki. 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.