/// Contains utilities shared by the G and GX variants of ARX-KW
///
/// The 'G' in those two variants stands for **G**enerated -- as opposed to **E**xtended -- key, as both variants generate the
/// subkeys K1 and K2
use crate::lqb;
use crate::{ArxKwError,};
use chacha::KeyStream;

/// 384 bits worth of zeros, used in the G and GX variants of ARX-KW
///
/// This is used as the input to the ChaCha8 stream (with an all-zero LQB) used to obtain the subkeys k1 and k2 in the G and GX variants
const ZERO_384: [u8; 48] = [0_u8; 48];
const ZERO_32: [u8;8] = [0_u8;8];

/// Generates the data used as the subkeys K1 and K2 in the G and GX variants
///
/// This is done by encrypting a 384-bit all-zero message using a ChaCha8 stream initialized with
/// key `k` and an all-zero LQB
pub fn subkeys(k: &[u8;32]) -> Result<([u8;16], [u8;32]), ArxKwError> {
    let mut stream = lqb::new_chacha8_with_counter(k, ZERO_32, ZERO_32);
    let mut g: [u8;48] = ZERO_384;
    stream.xor_read(&mut g).map_err(|e| ArxKwError::ChaChaError(format!("Reached end of chacha stream: {:?} ",e)))?;
    let (k1,k2) = array_refs![&g,16,32];
    Ok((*k1,*k2))
}