Selenium Webdriver Ruby code for Random text and Number

I am using Selenium Webdriver with Ruby and Rspec. I have a need to Pass a Random number for SSN in one of the fields. How do I do this. Currently trying.

ssn = @driver.find_element(:id, 'ssn')
cs = rand (121111111..899999999)
ssn.send_keys(cs) 

In another field I also have a need to Pass random alpha string. I also need help for that.