What is the purpose of a substitution cipher in cryptography?

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When evaluating the purpose of a substitution cipher in cryptography, I’ll examine each option to see what the substitution mechanism actually achieves.
Option a: 'To generate random encryption keys for symmetric encryption' — This is not the role of a substitution cipher. Generating random keys is typically done by a key schedule or a random number generator, not by constructing a fixed letter-to-letter mapping used in a substitution cipher. Therefore, this statement misattributes the function of key generation to the cipher type itself.
Option b: 'To map each letter of the plaintext to a different letter in the ciphertext' — This is the core idea of a substitution cipher: each plaintext symbol is consistently replaced by another symbol according to a fixed mapping, producing ciphertext. This captures the essential mechanism: a one-to-one (often 1-to-1) letter mapping that transforms the plaintext alphabet into a ciphertext alphabet.
Option c: 'To create digital signatures for message authentication' — Digital signatures involve asymmetric cryptography and public-key cryptography, not the simple letter-to-letter substitution of a substitution cipher. This option describes a different cryptographic primitive, not the substitution cipher’s purpose.
Option d: 'To encrypt messages using complex mathematical operations' — While some ciphers use complex mathematical transforms, a substitution cipher specifically focuses on re-labelling symbols rather than performing complex math. This description is more aligned with other cipher types (like certain block or stream ciphers) and does not precisely describe a substitution cipher's purpose.
In short, option b accurately states the fundamental purpose of a substitution cipher, distinguishing it from key generation, digital signatures, and more math-heavy encryption schemes. The incorrect options either misstate or mischaracterize the primary function of a substitution cipher, highlighting common misconceptions about cryptographic roles.

Introduction to Security and Forensic Technologies (CT046-3-1-ISFT-25-RESIT)

What is the purpose of a substitution cipher in cryptography?

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