1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177

#![warn(
  clippy::cargo,
  clippy::nursery,
  clippy::pedantic,
  missing_debug_implementations,
  missing_docs,
  rust_2018_idioms
)]
#![deny(unsafe_code)]
#![no_std]

//! [ECB-Mix-ECB][1] (EME) block cipher mode implementation.
//!
//! # Usage example
//! ```
//! use aes::Aes128;
//! use eme_mode::{block_modes::BlockMode, block_padding::Pkcs7, Eme};
//!
//! type Aes128Eme = Eme<Aes128, Pkcs7>;
//!
//! let key = [0; 16];
//! let iv = [1; 16];
//! let plaintext = b"Hello world!";
//! let cipher = Aes128Eme::new_var(&key, &iv).unwrap();
//!
//! // buffer must have enough space for message+padding
//! let mut buffer = [0u8; 16];
//! // copy message to the buffer
//! let pos = plaintext.len();
//! buffer[..pos].copy_from_slice(plaintext);
//! let ciphertext = cipher.encrypt(&mut buffer, pos).unwrap();
//!
//! assert_eq!(
//!   ciphertext,
//!   [147, 227, 119, 228, 187, 150, 249, 88, 176, 145, 53, 209, 217, 99, 70, 245]
//! );
//!
//! // re-create cipher mode instance
//! let cipher = Aes128Eme::new_var(&key, &iv).unwrap();
//! let mut buf = ciphertext.to_vec();
//! let decrypted_ciphertext = cipher.decrypt(&mut buf).unwrap();
//!
//! assert_eq!(decrypted_ciphertext, plaintext);
//! ```
//!
//! [1]: https://eprint.iacr.org/2003/147.pdf

#[cfg(feature = "std")]
extern crate std;

use block_cipher::{
  generic_array::{typenum::U16, ArrayLength, GenericArray},
  BlockCipher, NewBlockCipher,
};
use block_modes::BlockMode;
use block_padding::Padding;
use core::marker::PhantomData;

pub use block_cipher;
pub use block_modes;
pub use block_padding;

type Block<C> = GenericArray<u8, <C as BlockCipher>::BlockSize>;

#[inline]
fn xor(buf: &mut GenericArray<u8, U16>, key: &GenericArray<u8, U16>) {
  for (a, b) in buf.iter_mut().zip(key) {
    *a ^= *b;
  }
}

/// Multiply by 2 in GF(2**128)
///
/// Based on the IEEE P1619/D11 draft
#[inline]
fn multiply_by_2(out: &mut GenericArray<u8, U16>, input: &GenericArray<u8, U16>) {
  out.iter_mut().zip(input).fold(false, |carry, (o, i)| {
    let (n, overflow) = i.overflowing_mul(2);
    *o = n + carry as u8;
    overflow
  });
  if input[15] >= 128 {
    out[0] ^= 135;
  }
}

#[inline]
fn multiply_by_2_ip(out: &mut GenericArray<u8, U16>) {
  let tmp = *out;
  multiply_by_2(out, &tmp);
}

/// [ECB-Mix-ECB][1] (EME) block cipher mode instance.
///
/// [1]: https://eprint.iacr.org/2003/147.pdf
#[derive(Debug, Clone)]
pub struct Eme<C: BlockCipher + BlockCipher, P: Padding> {
  cipher: C,
  iv: Block<C>,
  _p: PhantomData<P>,
}

impl<C, P> Eme<C, P>
where
  C: BlockCipher<BlockSize = U16>,
  <C as BlockCipher>::ParBlocks: ArrayLength<GenericArray<u8, <C as BlockCipher>::BlockSize>>,
  P: Padding,
{
  fn process_blocks(&self, blocks: &mut [Block<C>], mode: impl Fn(&C, &mut Block<C>)) {
    let l_0 = {
      let mut buf = GenericArray::clone_from_slice(&[0; 16][..]);
      self.cipher.encrypt_block(&mut buf);
      buf
    };
    let mut l = GenericArray::clone_from_slice(&[0; 16][..]);
    multiply_by_2(&mut l, &l_0);

    for block in blocks.iter_mut() {
      xor(block, &l);
      mode(&self.cipher, block); // PPPj = AES-enc(k; PPj)
      multiply_by_2_ip(&mut l);
    }

    let mut mp: Block<C> = GenericArray::clone_from_slice(&self.iv);
    for block in blocks.iter_mut() {
      xor(&mut mp, block);
    }

    let mut m: Block<C> = GenericArray::clone_from_slice(&mp);
    blocks[0].copy_from_slice(&mp); // Store mc in blocks[0]
    mode(&self.cipher, &mut blocks[0]); // mc = AES-enc(k; mp)
    xor(&mut m, &blocks[0]); // m = mp xor mc
    for block in blocks.iter_mut().skip(1) {
      multiply_by_2_ip(&mut m);
      xor(block, &m); // CCCj = 2**(j-1)*m xor PPPj
    }
    xor(&mut blocks[0], &self.iv); // CCC1 = (xorSum CCCj) xor t xor mc

    {
      let (first, rest) = blocks.split_first_mut().unwrap();
      for block in rest.iter_mut() {
        xor(first, block);
      }
    }
    multiply_by_2(&mut l, &l_0); // reset l = 2*AES-enc(k; 0)
    for block in blocks.iter_mut() {
      mode(&self.cipher, block); // CCj = AES-enc(k; CCCj)
      xor(block, &l); // Cj = 2**(j-1)*l xor CCj
      multiply_by_2_ip(&mut l);
    }
  }
}

impl<C, P> BlockMode<C, P> for Eme<C, P>
where
  C: BlockCipher<BlockSize = U16> + NewBlockCipher,
  <C as BlockCipher>::ParBlocks: ArrayLength<GenericArray<u8, <C as BlockCipher>::BlockSize>>,
  P: Padding,
{
  type IvSize = C::BlockSize;

  fn new(cipher: C, iv: &GenericArray<u8, C::BlockSize>) -> Self {
    Self {
      cipher,
      iv: *iv,
      _p: PhantomData::default(),
    }
  }

  fn encrypt_blocks(&mut self, blocks: &mut [Block<C>]) {
    self.process_blocks(blocks, C::encrypt_block);
  }

  fn decrypt_blocks(&mut self, blocks: &mut [Block<C>]) {
    self.process_blocks(blocks, C::decrypt_block);
  }
}