A substitution cipher for the digits 0, 1, 2, 3, . . . , 9 substitutes each digit in 0, 1, 2, 3, . . . , 9 with another digit in 0, 1, 2, 3, . . . , 9. It can be represented as a 10-digit string specifying how each digit in 0, 1, 2, 3, . . . , 9 is substituted. For example, the 10-digit string '3941068257' specifies a substitution cipher in which digit 0 is substituted with digit 3, 1 with 9, 2 with 4, and so on. To encrypt a nonnegative integer, substitute each of its digits with the digit specified by the encryption key. Implement function cipher() that takes as input a 10-digit string key and a digit string (i. e., the clear text to be encrypted) and returns the encryption of the clear text.