Basic Cryptography

Question 1

Briefly describe additive cipher.

Answer 1

An additive cipher is a type of substitution cipher that replaces a plaintext letter with another letter that is offset a number of positions down the alphabet. The key determines how many positions to offset.

Question 2

How to encrypt and decrypt using additive cipher?

Answer 2

Encrypt: Ciphertext = (26 + Plaintext + Key) MOD 26
Decrypt: Plaintext = (26 + Ciphertext - Key) MOD 26

Question 3

What is the role of key length in cryptography?

Answer 3

The key length determines the number of combinations required to break an encryption algorithm. The longer the key length, the longer the time it takes to brute force the encryption, therefore the more secure the encryption.

Question 4

What is asymmetric encryption and give some examples?

Answer 4

An encryption algorithm that uses two keys: a public key and a private key. One is for encryption, and one is for decryption. Example: RSA

Question 5

What is symmetric encryption and give some examples?

Answer 5

An encryption algorithm that encrypts a plaintext message and decrypts a ciphertext using the same key.

Examples:
Stream Ciphers: RC4
Block Ciphers: DES, 3DES, AES