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//! Online[<sup>1</sup>] variant of the EAX mode.
//!
//! # Authentication
//! Due to *AE* (authenticated encryption) nature of EAX, it is vital to verify
//! that both public (also called *associated*) and privacy-protected
//! (encrypted) data has not been tampered with.
//!
//! Because of this, it is required for the consumers to explicitly call
//! [`finish`] after the encryption/decryption operation is complete.
//! This will either return a *tag* (when encrypting) used to authenticate data
//! or a `Result` (when decrypting) that signifies whether the data is authentic,
//! which is when the resulting tag is equal to the one created during encryption.
//!
//! ## Example
//! ```
//! use eax::{Error, online::{Eax, Decrypt, Encrypt}, cipher::generic_array::GenericArray};
//! use aes::Aes256;
//!
//! let key = GenericArray::from_slice(b"an example very very secret key.");
//! let nonce = GenericArray::from_slice(b"my unique nonces"); // 128-bits; unique per message
//! let assoc = b"my associated data";
//! let plaintext = b"plaintext message";
//! let mut buffer: [u8; 17] = *plaintext;
//!
//!// Encrypt a simple message
//! let mut cipher = Eax::<Aes256, Encrypt>::with_key_and_nonce(key, nonce);
//! cipher.update_assoc(&assoc[..]);
//! cipher.encrypt(&mut buffer[..9]);
//! cipher.encrypt(&mut buffer[9..]);
//! let tag = cipher.finish();
//!
//! assert_ne!(buffer, *plaintext);
//!
//! let mut cloned = buffer;
//!
//! // Now decrypt it, using the same key and nonce
//! let mut cipher = Eax::<Aes256, Decrypt>::with_key_and_nonce(key, nonce);
//! cipher.update_assoc(&assoc[..]);
//! cipher.decrypt_unauthenticated_hazmat(&mut buffer[..5]);
//! cipher.decrypt_unauthenticated_hazmat(&mut buffer[5..10]);
//! cipher.decrypt_unauthenticated_hazmat(&mut buffer[10..]);
//! let res = cipher.finish(&tag);
//!
//! assert_eq!(res, Ok(()));
//! assert_eq!(buffer, *plaintext);
//!
//! // Decrypting the ciphertext with tampered associated data should fail
//! let mut cipher = Eax::<Aes256, Decrypt>::with_key_and_nonce(key, nonce);
//! cipher.update_assoc(b"tampered");
//! cipher.decrypt_unauthenticated_hazmat(&mut cloned);
//! let res = cipher.finish(&tag);
//!
//! assert_eq!(res, Err(Error));
//! ```
//! [<sup>1</sup>]: https://en.wikipedia.org/wiki/Online_algorithm
//! [`Eax`]: struct.Eax.html
//! [`Decrypt`]: struct.Decrypt.html
//! [`finish`]: #method.finish
use crate::{Cmac, Error, Nonce, Tag, TagSize};
use aead::consts::U16;
use cipher::{
generic_array::functional::FunctionalSequence, BlockCipher, BlockEncrypt, Key, KeyInit,
KeyIvInit, StreamCipher,
};
use cmac::Mac;
use core::marker::PhantomData;
pub use Eax as EaxOnline;
/// Marker trait denoting whether the EAX stream is used for encryption/decryption.
pub trait CipherOp {}
/// Marker struct for EAX stream used in encryption mode.
pub struct Encrypt;
impl CipherOp for Encrypt {}
/// Marker struct for EAX stream used in decryption mode.
pub struct Decrypt;
impl CipherOp for Decrypt {}
/// Online[<sup>1</sup>] variant of the EAX mode.
///
/// This type is generic to support substituting alternative cipher
/// implementations.
///
/// In contrast to [`Eax`], can be used in an online[<sup>1</sup>] fashion and
/// operates in-place.
///
/// # Authentication
/// Due to *AE* (authenticated encryption) nature of EAX, it is vital to verify
/// that both public (also called *associated*) and privacy-protected
/// (encrypted) data has not been tampered with.
///
/// Because of this, it is required for the consumers to explicitly call
/// [`finish`] after the encryption/decryption operation is complete.
/// This will either return a *tag* (when encrypting) used to authenticate data
/// or a `Result` (when decrypting) that signifies whether the data is authentic,
/// which is when the resulting tag is equal to the one created during encryption.
///
/// ## Example
/// ```
/// use eax::{Error, online::{Eax, Decrypt, Encrypt}, cipher::generic_array::GenericArray};
/// use aes::Aes256;
///
/// let key = GenericArray::from_slice(b"an example very very secret key.");
///
/// let nonce = GenericArray::from_slice(b"my unique nonces"); // 128-bits; unique per message
///
/// let assoc = b"my associated data";
/// let plaintext = b"plaintext message";
///
/// let mut buffer: [u8; 17] = *plaintext;
///
/// // Encrypt a simple message
/// let mut cipher = Eax::<Aes256, Encrypt>::with_key_and_nonce(key, nonce);
/// cipher.update_assoc(&assoc[..]);
/// cipher.encrypt(&mut buffer[..9]);
/// cipher.encrypt(&mut buffer[9..]);
/// let tag = cipher.finish();
///
/// assert_ne!(buffer, *plaintext);
///
/// let mut cloned = buffer;
///
/// // Now decrypt it, using the same key and nonce
/// let mut cipher = Eax::<Aes256, Decrypt>::with_key_and_nonce(key, nonce);
/// cipher.update_assoc(&assoc[..]);
/// cipher.decrypt_unauthenticated_hazmat(&mut buffer[..5]);
/// cipher.decrypt_unauthenticated_hazmat(&mut buffer[5..10]);
/// cipher.decrypt_unauthenticated_hazmat(&mut buffer[10..]);
/// let res = cipher.finish(&tag);
///
/// assert_eq!(res, Ok(()));
/// assert_eq!(buffer, *plaintext);
///
/// // Decrypting the ciphertext with tampered associated data should fail
/// let mut cipher = Eax::<Aes256, Decrypt>::with_key_and_nonce(key, nonce);
///
/// cipher.update_assoc(b"tampered");
/// cipher.decrypt_unauthenticated_hazmat(&mut cloned);
/// let res = cipher.finish(&tag);
///
/// assert_eq!(res, Err(Error));
/// ```
///
/// [<sup>1</sup>]: https://en.wikipedia.org/wiki/Online_algorithm
/// [`Eax`]: ../struct.Eax.html
/// [`Decrypt`]: struct.Decrypt.html
/// [`finish`]: #method.finish
pub struct Eax<Cipher, Op, M = U16>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
Op: CipherOp,
M: TagSize,
{
imp: EaxImpl<Cipher, M>,
/// Denotes whether this stream is used for encryption or decryption.
marker: PhantomData<Op>,
}
impl<Cipher, Op, M> Eax<Cipher, Op, M>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
Op: CipherOp,
M: TagSize,
{
/// Creates a stateful EAX instance that is capable of processing both
/// the associated data and the plaintext in an "on-line" fashion.
pub fn with_key_and_nonce(key: &Key<Cipher>, nonce: &Nonce<Cipher::BlockSize>) -> Self {
let imp = EaxImpl::<Cipher, M>::with_key_and_nonce(key, nonce);
Self {
imp,
marker: PhantomData,
}
}
/// Process the associated data (AD).
#[inline]
pub fn update_assoc(&mut self, aad: &[u8]) {
self.imp.update_assoc(aad);
}
/// Derives the tag from the encrypted/decrypted message so far.
///
/// If the encryption/decryption operation is finished, [`finish`] method
/// *must* be called instead.
///
///[`finish`]: #method.finish
#[inline]
pub fn tag_clone(&self) -> Tag<M> {
self.imp.tag_clone()
}
}
impl<Cipher, M> Eax<Cipher, Encrypt, M>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
M: TagSize,
{
/// Applies encryption to the plaintext.
#[inline]
pub fn encrypt(&mut self, msg: &mut [u8]) {
self.imp.encrypt(msg)
}
/// Finishes the encryption stream, returning the derived tag.
///
/// This *must* be called after the stream encryption is finished.
#[must_use = "tag must be saved to later verify decrypted data"]
#[inline]
pub fn finish(self) -> Tag<M> {
self.imp.tag()
}
}
impl<Cipher, M> Eax<Cipher, Decrypt, M>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
M: TagSize,
{
/// Applies decryption to the ciphertext **without** verifying the
/// authenticity of decrypted message.
///
/// To correctly verify the authenticity, use the [`finish`] associated
/// function.
///
/// # ☣️ BEWARE! ☣️
/// This is a low-level operation that simultaneously decrypts the data and
/// calculates an intermediate tag used to verify the authenticity of the
/// data (used when the online decryption is finished).
///
/// Because this is exposed solely as a building block operation, an extra
/// care must be taken when using this function.
///
/// Specifically, when misused this may be vulnerable to a chosen-ciphertext
/// attack (IND-CCA). Due to online nature of this function, the decryption
/// and partial tag calculation is done simultaneously, per chunk.
/// An attacker might choose ciphertexts to be decrypted and, while the
/// final decryption will fail because the attacker can't calculate tag
/// authenticating the message, obtained decryptions may leak information
/// about the decryption scheme (e.g. leaking parts of the secret key).
///
/// [`finish`]: #method.finish
#[inline]
pub fn decrypt_unauthenticated_hazmat(&mut self, msg: &mut [u8]) {
self.imp.decrypt(msg)
}
/// Finishes the decryption stream, verifying whether the associated and
/// decrypted data stream has not been tampered with.
///
/// This *must* be called after the stream decryption is finished.
#[must_use = "decrypted data stream must be verified for authenticity"]
pub fn finish(self, expected: &Tag<M>) -> Result<(), Error> {
self.imp.verify_ct(expected)
}
}
/// Implementation of the raw EAX operations.
///
/// Main reason behind extracting the logic to a single, separate type is to
/// facilitate testing of the internal logic.
#[doc(hidden)]
struct EaxImpl<Cipher, M>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
M: TagSize,
{
nonce: Nonce<Cipher::BlockSize>,
data: Cmac<Cipher>,
message: Cmac<Cipher>,
ctr: ctr::Ctr128BE<Cipher>,
// HACK: Needed for the test harness due to AEAD trait online/offline interface mismatch
#[cfg(test)]
key: Key<Cipher>,
_tag_size: PhantomData<M>,
}
impl<Cipher, M> EaxImpl<Cipher, M>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
M: TagSize,
{
/// Creates a stateful EAX instance that is capable of processing both
/// the associated data and the plaintext in an "on-line" fashion.
fn with_key_and_nonce(key: &Key<Cipher>, nonce: &Nonce<Cipher::BlockSize>) -> Self {
let prepend_cmac = |key, init_val, data| {
let mut cmac = <Cmac<Cipher> as Mac>::new(key);
cmac.update(&[0; 15]);
cmac.update(&[init_val]);
cmac.update(data);
cmac
};
// https://crypto.stackexchange.com/questions/26948/eax-cipher-mode-with-nonce-equal-header
// has an explanation of eax.
// l = block cipher size = 128 (for AES-128) = 16 byte
// 1. n ← OMAC(0 || Nonce)
// (the 0 means the number zero in l bits)
let n = prepend_cmac(key, 0, nonce);
let n = n.finalize().into_bytes();
// NOTE: These can be updated online later
// 2. h ← OMAC(1 || associated data)
let h = prepend_cmac(key, 1, &[]);
// 3. c ← OMAC(2 || enc)
let c = prepend_cmac(key, 2, &[]);
let cipher = ctr::Ctr128BE::<Cipher>::new(key, &n);
Self {
nonce: n,
data: h,
message: c,
ctr: cipher,
#[cfg(test)]
key: key.clone(),
_tag_size: Default::default(),
}
}
/// Process the associated data (AD).
#[inline]
pub fn update_assoc(&mut self, aad: &[u8]) {
self.data.update(aad);
}
/// Applies encryption to the plaintext.
#[inline]
fn encrypt(&mut self, msg: &mut [u8]) {
self.ctr.apply_keystream(msg);
self.message.update(msg);
}
/// Applies decryption to the ciphertext.
#[inline]
fn decrypt(&mut self, msg: &mut [u8]) {
self.message.update(msg);
self.ctr.apply_keystream(msg);
}
/// Derives the tag from the encrypted/decrypted message so far.
#[inline]
fn tag(self) -> Tag<M> {
let h = self.data.finalize().into_bytes();
let c = self.message.finalize().into_bytes();
let full_tag = self.nonce.zip(h, |a, b| a ^ b).zip(c, |a, b| a ^ b);
Tag::<M>::clone_from_slice(&full_tag[..M::to_usize()])
}
/// Derives the tag from the encrypted/decrypted message so far.
#[inline]
fn tag_clone(&self) -> Tag<M> {
let h = self.data.clone().finalize().into_bytes();
let c = self.message.clone().finalize().into_bytes();
let full_tag = self.nonce.zip(h, |a, b| a ^ b).zip(c, |a, b| a ^ b);
Tag::<M>::clone_from_slice(&full_tag[..M::to_usize()])
}
/// Finishes the decryption stream, verifying whether the associated and
/// decrypted data stream has not been tampered with.
fn verify_ct(self, expected: &Tag<M>) -> Result<(), Error> {
// Check MAC using secure comparison
use subtle::ConstantTimeEq;
let resulting_tag = &self.tag()[..expected.len()];
if resulting_tag.ct_eq(expected).into() {
Ok(())
} else {
Err(Error)
}
}
}
// Because the current AEAD test harness expects the types to implement both
// `KeyInit` and `AeadMutInPlace` traits, do so here so that we can test the
// internal logic used by the public interface for the online EAX variant.
// These are not publicly implemented in general, because the traits are
// designed for offline usage and are somewhat wasteful when used in online mode.
#[cfg(test)]
mod test_impl {
use super::*;
use aead::{
consts::U0, generic_array::GenericArray, AeadCore, AeadMutInPlace, KeyInit, KeySizeUser,
};
impl<Cipher, M> KeySizeUser for EaxImpl<Cipher, M>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
M: TagSize,
{
type KeySize = Cipher::KeySize;
}
impl<Cipher, M> KeyInit for EaxImpl<Cipher, M>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
M: TagSize,
{
fn new(key: &Key<Cipher>) -> Self {
// HACK: The nonce will be initialized by the appropriate
// decrypt/encrypt functions from `AeadMutInPlace` implementation.
// This is currently done so because that trait only implements
// offline operations and thus need to re-initialize the `EaxImpl`
// instance.
let nonce = GenericArray::default();
Self::with_key_and_nonce(key, &nonce)
}
}
impl<Cipher, M> AeadCore for super::EaxImpl<Cipher, M>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
M: TagSize,
{
type NonceSize = Cipher::BlockSize;
type TagSize = M;
type CiphertextOverhead = U0;
}
impl<Cipher, M> AeadMutInPlace for super::EaxImpl<Cipher, M>
where
Cipher: BlockCipher<BlockSize = U16> + BlockEncrypt + Clone + KeyInit,
M: TagSize,
{
fn encrypt_in_place_detached(
&mut self,
nonce: &Nonce<Self::NonceSize>,
associated_data: &[u8],
buffer: &mut [u8],
) -> Result<Tag<M>, Error> {
// HACK: Reinitialize the instance
*self = Self::with_key_and_nonce(&self.key.clone(), nonce);
self.update_assoc(associated_data);
self.encrypt(buffer);
Ok(self.tag_clone())
}
fn decrypt_in_place_detached(
&mut self,
nonce: &Nonce<Self::NonceSize>,
associated_data: &[u8],
buffer: &mut [u8],
expected_tag: &Tag<M>,
) -> Result<(), Error> {
// HACK: Reinitialize the instance
*self = Self::with_key_and_nonce(&self.key.clone(), nonce);
self.update_assoc(associated_data);
self.decrypt(buffer);
let tag = self.tag_clone();
// Check mac using secure comparison
use subtle::ConstantTimeEq;
if expected_tag.ct_eq(&tag).into() {
Ok(())
} else {
Err(Error)
}
}
}
}