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doc: crypto docs updated to use good defaults for createDiffieHellman #5505

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doc: crypto docs updated to use good defaults for createDiffieHellman #5505

wants to merge 3 commits into from

Commits on Mar 1, 2016

  1. crypto docs updated to use good defaults 

    [Diffie-Hellman](https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange#Cryptographic_explanation) keys are composed of a `generator` a `prime` a `secret_key` and the `public_key` resulting from the math operation:

```
(generator ^ secret_key) mod prime = public_key
```

Diffie-Hellman keypairs will compute a matching shared secret if and only if the generator and prime match for both recipients.  The generator is usually **2** and the prime is what is called a [Safe Prime](https://en.wikipedia.org/wiki/Safe_prime).

Usually this matching is accomplished by using [standard published groups](http://tools.ietf.org/html/rfc3526).  We expose access those groups with the `crypto.getDiffieHellman` function.

`createDiffieHellman` is trickier to use.  The original example had the user creating 11 bit keys, and creating random groups of generators and primes.  11 bit keys are very very small, can be cracked by a single person on a single sheet of paper.  A byproduct of using such small keys were that it was a high likelihood that two calls of `createDiffieHellman(11)` would result in using the same 11 bit safe prime.  

The original example code would fail when the safe primes generated at 11 bit lengths did not match for alice and bob.

If you want to use your own generated safe `prime` then the proper use of `createDiffieHellman` is to pass the `prime` and `generator` to the recipient's constructor, so that when they compute the shared secret their `prime` and `generator` match, which is fundamental to the algorithm.
    @billautomata
    billautomata committed Mar 1, 2016
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  2. removed unnecessary encodings

    @billautomata
    billautomata committed Mar 1, 2016
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  3. removed newline

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    billautomata committed Mar 1, 2016
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