What occurs if an opponent discovers the key used in symmetric encryption?

When an opponent discovers the key used in symmetric encryption, all encrypted communication becomes readable. This is because symmetric encryption relies on a single shared secret key for both encryption and decryption. If the adversary gains access to this key, they can decrypt any message that has been encrypted using it, compromising the confidentiality of all communications.

With symmetric encryption, the security of the entire communication system hinges on the secrecy of the key. Therefore, once that key is exposed, it invalidates the protective layer provided by the encryption, granting the opponent complete access to all data that was secured with that key. This scenario highlights the critical importance of key management practices in information security, emphasizing that securing and protecting the encryption key is just as crucial as securing the encrypted data itself.