Which of the following encryption methods comes under symmetric encryption algorithm? Each correct answer represents a complete solution. Choose three.

Symmetric encryption algorithms use a single, shared secret key for both the encryption of plaintext and the decryption of ciphertext. The Data Encryption Standard (DES), Blowfish, and RC5 are all well-established symmetric-key block ciphers. DES is an early standard that operates on 64-bit blocks with a 56-bit key. Blowfish is a 64-bit block cipher with a variable-length key, designed as a faster, unpatented alternative to DES. RC5 is a parameterized block cipher notable for its variable block size, key size, and number of rounds. All three operate on the fundamental principle of a shared secret key.

D. Diffie-Hellman: This is an asymmetric key exchange protocol used to securely establish a shared secret over an insecure channel, not an algorithm for encrypting data itself.

1. Katz

J.

& Lindell

Y. (2014). Introduction to Modern Cryptography (2nd ed.). Chapman and Hall/CRC. In Chapter 3

"Private-Key (Symmetric) Cryptography

" DES

Blowfish

and RC5 are discussed as examples of symmetric ciphers. In contrast

Chapter 10

"The Diffie-Hellman Problems and Protocols

" describes Diffie-Hellman as a key-exchange protocol based on public-key principles.

2. National Institute of Standards and Technology (NIST). (1999). FIPS PUB 46-3

Data Encryption Standard (DES). This publication officially defines DES as a symmetric-key block cipher. Section 1

"Specification

" describes the cryptographic algorithm that uses a single key for both encryption and decryption.

3. Schneier

B. (1993). Description of a New Variable-Length Key

64-Bit Block Cipher (Blowfish). In Proceedings of the First International Workshop on Fast Software Encryption (FSE)

Cambridge

UK. The abstract and introduction explicitly define Blowfish as a "64-bit block cipher" and a "symmetric block cipher." (DOI: https://doi.org/10.1007/3-540-58108-124)

4. Rivest

R. L. (1994). The RC5 Encryption Algorithm. In Proceedings of the Second International Workshop on Fast Software Encryption (FSE). The abstract clearly states

"This paper introduces a new block cipher

called RC5. It is a parameterized algorithm with a variable block size

a variable key size

and a variable number of rounds." It is classified as a symmetric-key algorithm.

5. Diffie

W.

& Hellman

M. (1976). New Directions in Cryptography. IEEE Transactions on Information Theory

22(6)

644–654. The foundational paper for public-key cryptography

which introduces the Diffie-Hellman key exchange protocol as a method for key distribution

distinct from symmetric encryption algorithms. (DOI: https://doi.org/10.1109/TIT.1976.1055638)