What are the key and nonce sizes used with ChaCha?

The correct answer is that ChaCha uses a 256-bit key and a 64-bit nonce. ChaCha is a stream cipher that is designed to provide a high level of security and performance. The use of a 256-bit key enhances the cipher's resistance to brute-force attacks, making it more secure than ciphers with shorter key lengths. The nonce, which stands for "number used once," is shorter at 64 bits. This length is sufficient for ensuring that nonce reuse does not occur in practice, especially when used with a proper message counter.

The combination of a 256-bit key and a 64-bit nonce allows ChaCha to provide strong encryption while managing the risks associated with nonce reuse. This design choice leads to a practical balance between security and efficiency, which is essential in various applications such as securing internet communications and ensuring data integrity.