Fix lowercase support

The program expects the encrypted text to be uppercase.

- Update the cipher code to handle lowercase both in encryption and decryption.

Start by locating the cipher code responsible for encryption and decryption. Check how it currently handles uppercase encrypted text, then verify that lowercase input is supported in both directions. Done means lowercase text can be encrypted and decrypted without requiring conversion to uppercase; no test file is named in the issue.

Written by the indexing model from the issue text.