Which security control involves the deliberate scrambling of a message to keep it unreadable without a key?

The concept that revolves around deliberately scrambling a message to render it unreadable without a specific key is known as encryption. Encryption is a fundamental security control used to protect sensitive data by converting it into a coded format, accessible only to those who possess the appropriate decryption key. This process ensures that even if data is intercepted, it cannot be comprehended without the key, thus safeguarding confidentiality and integrity.

In contrast, the other options do not focus on the core principle of scrambling data to maintain confidentiality. Alteration of input refers to changes made to data that can invalidate its integrity but does not involve scrambling for protection. Customer validation is about verifying the identity of users, ensuring that they are who they claim to be, typically involving authentication measures. Firewall security functions as a barrier or filter to protect networks and devices from unauthorized access and threats, rather than focusing on the encryption of individual messages or data.