5.1.4 Encryption Placement Within The Communication System

In a practical communications system, encryption is normally performed after source coding (compression) but before channel coding because:

Encryption before source coding would destroy data redundancy and prevent compression, since encrypted data resemble random symbols.
Encryption after channel coding would be ineffective because most cryptographic systems require a very low bit-error rate to maintain synchronization between transmitter and receiver.

Therefore, channel coding—which introduces controlled redundancy for error correction—must follow encryption, ensuring that ciphertext is transmitted reliably even over noisy channels