Use crypto/rand instead of math/rand

[Gregor Riepl]

For certain use cases, it may be necessary to generate entropy using a cryptographically secure random number generator.

Would it be possible to make the randomness source customizable, so crypto/rand can be used instead of math/rand?

[Arie Roos]

@onitake I made a pull request for this, feel free to use my version of the code.