The Ruby stdlib provides an entire class of secure random data generators called SecureRandom
. Whatever you want, you can probably find it there.
Stripe's keys are essentially URL-safe Base64. You can get something very similar like so:
require 'securerandom'
p "sk_test_" + SecureRandom.urlsafe_base64
(Stripe does strip out non-alphanumeric characters, but that's trivial to do with gsub if you don't want hyphens in your keys.)