pub fn rand(bytes: &mut [u8])
Reads cryptographically-secure random bytes.
This is a low-level primitive often used to construct higher-level
protocols. Unless you're sure that this is what you need, you should
probably be using something else. For example, all key types can be randomly
generated using higher-level functions (e.g.,
scrypt nonces are generated using the
scrypt_generate function, etc.