Function encrypt_into 

pub fn encrypt_into(
    input: &[u8],
    output: &mut [u8],
    key: &[u8; 32],
) -> Result<usize>

Encrypt input into output using AES-256-GCM with the provided 32-byte key. The output buffer must be at least input.len() + NONCE_LEN + TAG_LEN bytes long, returns the total number of bytes written to output (nonce + ciphertext + tag) or an error if encryption fails.