In cryptography, which cipher type maps each letter to a fixed different letter?

In cryptography, which cipher type maps each letter to a fixed different letter?

• A. Substitution Cipher
• B. Transposition Cipher
• C. Cryptography
• D. APT

Answer: (A)

The main idea here is substitution: replacing each plaintext letter with a fixed ciphertext letter according to a key, and keeping that mapping constant throughout the message. Because the mapping is fixed, the same letter always turns into the same other letter, making it possible to reverse the process and recover the original text by applying the inverse mapping. This differentiates substitution from a transposition cipher, which simply reorders the letters without changing them, scrambling the message rather than substituting. Cryptography is the broader field that studies these techniques, while APT refers to a type of cyber threat, not a cipher.

The main idea here is substitution: replacing each plaintext letter with a fixed ciphertext letter according to a key, and keeping that mapping constant throughout the message. Because the mapping is fixed, the same letter always turns into the same other letter, making it possible to reverse the process and recover the original text by applying the inverse mapping. This differentiates substitution from a transposition cipher, which simply reorders the letters without changing them, scrambling the message rather than substituting. Cryptography is the broader field that studies these techniques, while APT refers to a type of cyber threat, not a cipher.