use super::{
iv::{Iv, IV_LEN},
Nonce,
};
use crate::endian::*;
use core::convert::TryInto;
#[repr(C)]
pub struct Counter<U32> {
u32s: [U32; COUNTER_LEN],
}
const COUNTER_LEN: usize = 4;
impl<U32> Counter<U32>
where
U32: Copy,
U32: Encoding<u32>,
U32: From<[u8; 4]>,
U32: Layout,
[U32; 4]: ArrayEncoding<[u8; IV_LEN]>,
{
pub fn zero(nonce: Nonce) -> Self {
Self::new(nonce, 0)
}
pub fn one(nonce: Nonce) -> Self {
Self::new(nonce, 1)
}
#[cfg(test)]
pub fn from_test_vector(nonce: &[u8], initial_counter: u32) -> Self {
Self::new(
Nonce::try_assume_unique_for_key(nonce).unwrap(),
initial_counter,
)
}
fn new(nonce: Nonce, initial_counter: u32) -> Self {
let mut r = Self {
u32s: [U32::ZERO; COUNTER_LEN],
};
let nonce_index = (U32::COUNTER_INDEX + 1) % COUNTER_LEN;
(&mut r.u32s[nonce_index..][..3])
.iter_mut()
.zip(nonce.as_ref().chunks_exact(4))
.for_each(|(initial, nonce)| {
let nonce: &[u8; 4] = nonce.try_into().unwrap();
*initial = U32::from(*nonce);
});
r.u32s[U32::COUNTER_INDEX] = U32::from(initial_counter);
r
}
#[inline]
pub fn increment(&mut self) -> Iv {
let current = Self { u32s: self.u32s };
self.increment_by_less_safe(1);
current.into()
}
#[inline]
pub fn increment_by_less_safe(&mut self, increment_by: u32) {
let counter = &mut self.u32s[U32::COUNTER_INDEX];
let old_value: u32 = (*counter).into();
*counter = U32::from(old_value + increment_by);
}
}
pub trait Layout {
const COUNTER_INDEX: usize;
}
impl Layout for BigEndian<u32> {
const COUNTER_INDEX: usize = 3;
}
impl Layout for LittleEndian<u32> {
const COUNTER_INDEX: usize = 0;
}
impl<U32> Into<Iv> for Counter<U32>
where
[U32; 4]: ArrayEncoding<[u8; IV_LEN]>,
{
fn into(self) -> Iv {
Iv::assume_unique_for_key(*self.u32s.as_byte_array())
}
}