Which statement accurately defines a cipher?

A cipher is fundamentally defined as an algorithm that performs encryption or decryption. It is a systematic method used to convert plaintext (readable data) into ciphertext (encrypted data) and vice versa. By employing mathematical functions and operations, ciphers ensure that data remains secure and confidential by making it unreadable to unauthorized parties. When data is encrypted, it becomes indecipherable without the appropriate key or algorithm to decrypt it back into its original form.

In this context, other options present concepts that do not align with the specific definition of a cipher. An algorithm for data compression pertains to reducing the size of data without changing its meaning, which is unrelated to encryption or decryption processes. A protocol for digital communications outlines rules for transferring data between devices but does not directly define the transformation of data itself. Lastly, a type of computer virus refers to malicious software designed to harm or exploit devices, which is entirely different from the purpose of ciphers in securing information. Thus, the correct understanding of a cipher centers around its role in encryption and decryption, as captured by the chosen option.