Question

Assume that you use an encryption program that uses ChaCha20 to encrypt the same message twice, using the same key during both times. Which of the following is true regarding the cipher text?
a. The cipher text from both encryptions will be different. Even though the same key was used, the PRNG nonce will increment after the first message has been encrypted so the keystream for the second message will be different than the keystream in the first message.
b. The cipher text from both encryptions will be different. Even though the same key was used, the PRNG counter will increment after the first message has been encrypted so the keystream for the second message will be different than the keystream in the first message.
c. The cipher text from both encryptions will be the same since the same key was used.
d. None of the above

Answer 1

Solution:

The answer is Option A.

Explanation:

ChaCha20 Encryption:

The ChaCha20 encryption is a stream cipher which uses the secret key, nonce and PRNG to encrypt the plain text.

There are certain guidelines that are followed by the algorithm while using these components for encryption.

The PRNG stands for pseudo random number generator.

The ChaCha20 is a stream cipher. The key stream of ChaCha20 may be used to set the key and nonce.
The same key stream must be used to encrypt and decrypt text.

These key streams are nothing but the pseudo random streams.

The secret key that is used here is generally of 256 bits long.

The cipher uses nonce. This nonce must not be reused at any case again.

The nonce that is used for encrypting must not be reused again for the encryptions that are performed with same key.