Usage: ./prf.rb ...
                                                                                    -k, --key=KEY                    secret key
                                                                                    -r, --generate-random=NUMBER     number of bytes for generated random seed (do not use in conjunction with -s
                                                                                    -s, --seed=SEED                  random seed
                                                                                    -l, --length=NUMBER              output length
                                                                                    -h, --hash=NAME                  Name of hash function to use. The function must be supported by OpenSSL. Default is sha256.