What does substitution refer to in cryptography?

Substitution in cryptography refers to the method where portions of plaintext are changed for matching ciphertext. This technique involves replacing elements of the plaintext, such as characters or bits, with other characters or bits, which is fundamental to many encryption algorithms. The goal is to obscure the original message so that it cannot be easily deciphered without knowledge of the substitution cipher used to encrypt it.

This method can take various forms, such as simple substitution where each letter is replaced by another letter or more complex substitutions that can involve a specific algorithm. The principle behind substitution ciphers is that even though the content of the plaintext is altered, the overall length and structure can remain the same, allowing for consistent transmission and potential recovery when the correct key or method is utilized.

In contrast, other cryptographic methods mentioned in the options involve different approaches, such as encryption streams, using multiple keys, or processing data in blocks, which do not align with the direct concept of substitution. These methods focus on varying aspects of encryption rather than the specific technique of replacing plaintext elements with their corresponding ciphertext.