Which of the following describes a Substitution Cypher?

A Substitution Cipher is a method of encryption where each letter in the plaintext is replaced with a different character or symbol. This transformation creates a ciphertext that conceals the original message by altering the representation of its characters. The key aspect of this cipher is that the mapping is consistent throughout the message, meaning that a specific letter will always correspond to the same character in the ciphertext.

For instance, in a simple substitution cipher, 'A' might be replaced with 'D', 'B' with 'E', and so on, forming a systematic replacement that allows for decryption if one knows the mappings used. This type of encryption emphasizes the replacement of characters rather than complex algorithmic processes, making it foundational in the study of cryptography.

The other choices do not accurately define a Substitution Cipher. Public key methods involve asymmetric encryption where different keys are used for encrypting and decrypting data, while digital signatures relate to verifying authenticity rather than character substitution. Additionally, techniques that do not require a key typically fall under simple ciphers or unencrypted messages rather than structured substitution methods, as encryption inherently relies on key coordination to conceal information.