You need to do as below :
2.0.0-p0 :003 > require 'digest'
=> true
2.0.0-p0 :004 > Digest::SHA2.hexdigest("hello")
=> "2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824"
2.0.0-p0 :005 >
So do first require 'digest'
.
Try in tutorialspoint