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// Copyright 2022 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//! Traits for AES-CTR.
use super::{AesBlock, AesKey, BLOCK_SIZE};
/// The number of bytes used for the nonce, with the remaining bytes in a block used as the counter.
///
/// Other lengths may be used, but 12 is a good general purpose choice.
pub const AES_CTR_NONCE_LEN: usize = 12;
/// The nonce portion of the nonce+counter block used by CTR mode.
pub type AesCtrNonce = [u8; AES_CTR_NONCE_LEN];
/// An implementation of AES-CTR.
///
/// An AesCtr impl must only be used for encryption _or_ decryption, not both. Since CTR mode
/// is stateful, mixing encrypts and decrypts may advance the internal state in unexpected ways.
/// Create separate encrypt/decrypt instances if both operations are needed.
pub trait AesCtr {
/// The [AesKey] this cipher uses. See [super::Aes128Key] and [super::Aes256Key] for the common AES-128 and
/// AES-256 cases.
type Key: AesKey;
/// Build a `Self` from key material.
fn new(key: &Self::Key, nonce_and_counter: NonceAndCounter) -> Self;
/// Encrypt the data in place, advancing the counter state appropriately.
fn encrypt(&mut self, data: &mut [u8]);
/// Decrypt the data in place, advancing the counter state appropriately.
fn decrypt(&mut self, data: &mut [u8]);
}
/// The combined nonce and counter that CTR increments and encrypts to form the keystream.
pub struct NonceAndCounter {
block: AesBlock,
}
impl NonceAndCounter {
/// Appends 4 zero bytes of counter to the nonce
pub fn from_nonce(nonce: AesCtrNonce) -> Self {
let mut block = [0; BLOCK_SIZE];
block[..12].copy_from_slice(nonce.as_slice());
NonceAndCounter { block }
}
/// Initialize from an already concatenated nonce and counter
// Not recommended for general use, so restricted so only test vectors can use it
#[cfg(feature = "test_vectors")]
pub fn from_block(block: AesBlock) -> Self {
Self { block }
}
/// Nonce and counter as an AES block-sized byte array
pub fn as_block_array(&self) -> AesBlock {
self.block
}
}
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