Which of the following is a method for transforming a message into a masked form, together with a way of undoing the transformation to recover the message?

A cipher is a cryptographic algorithm that defines the steps for both encryption and decryption. Encryption is the process of transforming a plaintext message into a masked, unreadable form called ciphertext. Decryption is the corresponding process of using a key to reverse the transformation, recovering the original plaintext from the ciphertext. This two-part process of masking and unmasking is the fundamental definition of a cipher.

B. CrypTool: CrypTool is a specific educational software application that implements various cryptographic methods; it is a tool, not the fundamental method itself.
C. Steganography: Steganography's primary goal is to hide the existence of a message within other data, not just to mask its content, which is the function of a cipher.
D. MIME: MIME (Multipurpose Internet Mail Extensions) is an internet standard for formatting email messages to support various content types and encodings, not a method for cryptographic masking. ---

1. National Institute of Standards and Technology (NIST). (2021). Glossary of Key Information Security Terms (NISTIR 7298 Rev. 4). Computer Security Resource Center.

Page 40: Defines Cipher as "A cryptographic algorithm for encryption and decryption." This directly supports the correct answer by defining a cipher as the algorithm for both the masking (encryption) and unmasking (decryption) processes.

Page 188: Defines Steganography as "The art and science of communicating in a way which hides the existence of the communication." This clarifies why option C is incorrect

as its focus is on concealment

not just masking content.

2. Katz

J.

& Lindell

Y. (2014). Introduction to Modern Cryptography (2nd ed.). CRC Press. (Widely used in university cryptography courses).

Page 26

Section 2.1: "A private-key encryption scheme is defined by three algorithms... Gen (a key-generation algorithm)

Enc (an encryption algorithm)

and Dec (a decryption algorithm)." A cipher is the core component (Enc and Dec) of such a scheme

performing the transformation and its reversal.

3. Freed

N.

& Borenstein

N. (1996). Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies (RFC 2045). The Internet Engineering Task Force (IETF).

Section 1

Introduction: "MIME redefines the format of messages... This document defines the general structure of the MIME media typing system and the initial set of media types." This source confirms that MIME is a formatting/encoding standard

not a cryptographic method.