Which of the following best defines a substitution cipher?

A substitution cipher is a fundamental encryption technique that transforms plain text into cipher text by replacing each character or bit with another character or bit according to a specific system or key. This method retains the order of the original text but alters the actual symbols used to represent the original data.

The defining characteristic of a substitution cipher is the one-to-one correspondence between the characters or bits of the plain text and the cipher text, allowing for a direct mapping between the two. For example, in a simple substitution cipher, the letter 'A' may be replaced with 'D', 'B' with 'E', and so on, based on a consistent rule or key.

In contrast, options that suggest rearranging characters or bits refer to transposition ciphers rather than substitution, which simply changes the characters. The mention of complex algorithms is more applicable to advanced encryption methods beyond basic substitution techniques. Similarly, the use of symmetric keys pertains to more complex encryption schemes such as modern cryptographic algorithms, distinguishing them from the simpler structure of a basic substitution cipher.