why is it doing that will lead to a zero in the whole assignment.

Develop a program for the polyalphabetic substitution cipher. The program should ask how many
keys the user wants to use (from 2 to 5) and then ask (input) only that many keys. The keys should
be used in turn for every successive occurrence of a character. For example, suppose the plain text is
ABCABCABCABCABCABC. And the five keys are DEF…, HRY…, POI…, ATZ…, and LSY….
Then the cipher text should be DEFHRYPOIATZLSYDEF. So, the algorithm should iterate between
the keys to encrypt the entered plaintext.
Constraints:
• The length of each key should be exactly 26 characters.
• The program should check whether any character is being repeated within a key.
• The program should also check whether the entered key is different from the ones already
entered and if the size is 26 or not.
• The size of the plain text could be from 1 character to 1000 characters.
Rules:
• Submit the code with lots of comments.
• You are allowed to use pre-existing library functions of the programming language you are
using.
• Downloading and using extra/advanced math/cryptographic libraries is NOT allowed.
• Any indication of copying the code or not knowing what some part of the code is doing and
why is it doing that will lead to a zero in the whole assignment.