How many bits is the nonce used in ChaCha?

In the ChaCha stream cipher, the nonce is utilized to ensure uniqueness for each encryption operation, and it plays a crucial role in securing the encrypted data. The nonce in ChaCha is specifically defined as being 64 bits in length. The purpose of the nonce is to prevent replay attacks and to ensure that even if the same key is used for multiple encryptions, the outputs remain unique and secure as long as different nonces are used.

Choosing a nonce size that is both practical and secure is vital, and the 64-bit length strikes a balance between sufficient variability and manageable size in the context of stream ciphers. This allows for a large number of unique values to be available for encryption, decreasing the likelihood of nonce reuse which could compromise security. In this context, 64 bits is the correct answer as it is the established standard for the nonce in the ChaCha algorithm.