Polybius Square cipher: Issues in current implementation

[kobby-pentangeli]

1. Input Validation: The clean_input helper function only considers alphabets and spaces as valid inputs, discarding others. This means messages containing numbers or special characters can't be encrypted or decrypted correctly, which might be used as an attack vector to feed malicious inputs.

2. Error Handling: The implementation uses Option to handle errors, returning None when encountering an error. A more detailed error handling is desired.

3. Unicode Support: The implementation currently does not handle Unicode characters well, which might be exploited in various ways, including bypassing input validation or creating unexpected outputs.