Originally Posted by

**ps1313** ...

Consider all maps of the form:

f(x) = ax + b mod 31.

a) How many of these maps are invertible?

b) Of those that are invertible, how many have no fixed points?

c) Select a cipher with no fix points and encode with it the string:

this_is,_i_believe,_a_secure_cipher.

d) Compute the inverse of the cipher selected in c) and use it to decode the string

e) Devise a method for deciphering a string encoded with this cipher. Test it with the

encoded string.

how do you find how many maps are invertible, what are maps??

I really dont understand any of these questions any help plz