Network Defense Essentials (NDE) 2025 – 400 Free Practice Questions to Pass the Exam

The choice of a substitution cipher is pertinent because this type of cipher operates on the principle of systematically replacing letters or groups of letters with other letters, symbols, or numbers to encrypt a message. In this case, Jack's method of replacing single letters or combinations aligns perfectly with the definition of a substitution cipher.

This technique ensures that each letter or combination remains consistent throughout the message, which is crucial for deciphering it later. For instance, in a simple substitution cipher, the letter 'A' might be replaced with 'D', while 'B' could be replaced by 'E', and so forth. This method can either be monoalphabetic, with each letter consistently replaced by another, or polyalphabetic, where the substitution varies throughout the message.

Other cipher types, such as transposition ciphers, rearrange the letters instead of replacing them, making them unsuitable for the description given. Block ciphers encrypt fixed-size blocks of data and apply substitution and permutation techniques, but they do not specifically focus on replacing letters as the main function. Stream ciphers encrypt data as a continuous stream, often on a bit or byte level, rather than focusing on letter replacement, making this choice less relevant for the situation described. Thus, the substitution cipher is