In a substitution cipher, what method is used to replace units of a message?

In a substitution cipher, the method of replacing units of a message typically involves a systematic approach, which aligns closely with the use of a formula. A substitution cipher works by replacing each letter or unit in the plaintext (the original message) with another letter or unit according to a defined system or rule. This systematic method can often be expressed in a formulaic manner—where specific letters are mapped to new letters based on an agreed-upon key or algorithm.

For example, in a simple Caesar cipher, each letter might be shifted a certain number of places down the alphabet. This predetermined rule ensures that the same letter in the original message will always be replaced by the same letter in the encoded message, making it a reliable and structured method of substitution.

The other methods mentioned are not applicable as direct approaches for executing a substitution within a cipher. Random selection lacks the consistency required for effective encryption, while frequency analysis is a technique used to break ciphers rather than to create them. Similarly, anagram techniques involve rearranging letters rather than consistently replacing them, which does not correspond to how substitution ciphers function.