Developer Builds Python File Encryption Tool, Documents Bugs That Shaped the Learning

A cybersecurity student built a command-line file encryption tool in Python using the Fernet library, which combines AES-128 encryption with HMAC authentication, to gain hands-on understanding rather than follow a tutorial. The project was structured into separate modules for key management, encryption, and decryption, with the encryption key kept out of version control entirely. During development, the author encountered several instructive bugs, including a circular import, an inverted conditional that would have silently overwritten the encryption key on every run, and unreachable code placed after a return statement. A particularly risky logic error meant already-encrypted files could have been rendered permanently unrecoverable without any error or crash. The project also adopted a design choice to write encrypted output as a new file rather than overwriting the original, protecting against data loss if the program crashes mid-write.