irb(main):001:0> user_given_password = "asdf"
=> "asdf"

irb(main):002:0> some_random_number = rand.to_s[3..10]
=> "39190336"

irb(main):006:0> passwd = user_given_password.crypt("$1$#{some_random_number}")
=> "$1$39190336$Noa/hIYTzqFLPOCFJP5nw0"

#notice that:
irb(main):010:0> passwd[3..10] == some_random_number
=> true

#so:

irb(main):021:0> user_given_password.crypt("$1$#{passwd[3..10]}") == passwd
=> true