Medium-Strength Secret Key Encryption on Ubuntu -- 3

I am developing a secret key encryption system on Ubuntu. The primary goal is to safeguard sensitive data and enable secure communication. The encryption mechanism should provide a standard level of protection, not too basic yet not overly complex. The system will undergo frequency analysis against monoalphabetic substitution cipher, which requires a certain level of encryption strength to be effective.

Key Requirements:

- Implementation of a secret key encryption system on Ubuntu
- Encryption strength must be set at medium level
- Must be able to prevent unauthorized access through frequency analysis against monoalphabetic substitution cipher.
-encryption using different cipher and modes
-encryption mode - ecb vs cbc
-padding, initial vector and error propagation
-programming using the crypto library