Which encryption method uses plaintext and ciphertext as integers between 0 and n-1?

• A. DES
• B. AES
• C. RSA
• D. Blowfish

Answer: (C)

The correct answer is RSA, which stands for Rivest-Shamir-Adleman, a widely used public-key cryptographic system. RSA encryption operates by treating plaintext and ciphertext as integers that exist within a specific range defined by a modulus ( n ). In this method, the original message (plaintext) is converted into an integer, which is then subjected to mathematical operations using the public and private keys to produce ciphertext. This approach allows RSA to leverage properties of number theory, particularly modular arithmetic, making it suitable for secure key exchange and digital signatures.

In contrast, the other methods listed do not function in the same manner. DES (Data Encryption Standard) and AES (Advanced Encryption Standard) are symmetric key algorithms that use block ciphers, processing data in fixed-size blocks rather than as integer representations. Blowfish, like the others, is also a symmetric block cipher that does not specifically represent plaintext and ciphertext as integers between 0 and ( n-1 ). Thus, RSA is unique in its integer-based approach, aligning with the specified characteristics of the question.