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mgm: decrypt only after tag verification #356

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merged 4 commits into from Aug 24, 2021
Merged

mgm: decrypt only after tag verification #356

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c609888
mgm: decrypt only after tag verification
newpavlov Aug 24, 2021
6716093
simplify code
newpavlov Aug 24, 2021
eab8a94
fix
newpavlov Aug 24, 2021
991a3df
move xoring into method
newpavlov Aug 24, 2021
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2 changes: 1 addition & 1 deletion Cargo.lock
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6 changes: 6 additions & 0 deletions mgm/CHANGELOG.md
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Expand Up @@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).

## 0.4.4 (2021-08-24)
### Changed
- Decrypt ciphertext only after tag verification ([#356])

[#356]: https://github.com/RustCrypto/AEADs/pull/356

## 0.4.3 (2021-07-20)
### Changed
- Pin `subtle` dependency to v2.4 ([#349])
Expand Down
2 changes: 1 addition & 1 deletion mgm/Cargo.toml
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@@ -1,6 +1,6 @@
[package]
name = "mgm"
version = "0.4.3"
version = "0.4.4"
description = "Generic implementation of the Multilinear Galois Mode (MGM) cipher"
authors = ["RustCrypto Developers"]
edition = "2018"
Expand Down
150 changes: 70 additions & 80 deletions mgm/src/lib.rs
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Expand Up @@ -39,6 +39,7 @@ use aead::{
AeadCore, AeadInPlace, Error, Key, NewAead,
};
use cipher::{BlockCipher, BlockEncrypt, NewBlockCipher};
use subtle::ConstantTimeEq;

pub use aead;

Expand All @@ -55,6 +56,7 @@ pub type Nonce<NonceSize> = GenericArray<u8, NonceSize>;
pub type Tag<TagSize> = GenericArray<u8, TagSize>;

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

/// Trait implemented for block cipher sizes usable with MGM.
pub trait MgmBlockSize: sealed::Sealed {}
Expand All @@ -76,16 +78,20 @@ where
C: BlockEncrypt,
C::BlockSize: MgmBlockSize,
{
fn get_h(&self, counter: &Counter<C>) -> Block<C> {
fn apply_ks_block(&self, counter: &mut Counter<C>, buf: &mut [u8]) {
let mut block = C::BlockSize::ctr2block(counter);
self.cipher.encrypt_block(&mut block);
block
for i in 0..core::cmp::min(block.len(), buf.len()) {
buf[i] ^= block[i];
}
C::BlockSize::incr_r(counter);
}

fn encrypt_counter(&self, counter: &Counter<C>) -> Block<C> {
let mut block = C::BlockSize::ctr2block(counter);
self.cipher.encrypt_block(&mut block);
block
fn update_tag(&self, tag: &mut Element<C>, tag_ctr: &mut Counter<C>, block: &Block<C>) {
let mut h = C::BlockSize::ctr2block(tag_ctr);
self.cipher.encrypt_block(&mut h);
tag.mul_sum(&h, block);
C::BlockSize::incr_l(tag_ctr);
}
}

Expand Down Expand Up @@ -136,57 +142,49 @@ where
if nonce[0] >> 7 != 0 {
return Err(Error);
}
let final_block = C::BlockSize::lengths2block(adata.len(), buffer.len())?;

let mut enc_counter = nonce.clone();
let mut tag_counter = nonce.clone();
enc_counter[0] &= 0b0111_1111;
tag_counter[0] |= 0b1000_0000;

self.cipher.encrypt_block(&mut enc_counter);
self.cipher.encrypt_block(&mut tag_counter);

let mut enc_counter = C::BlockSize::block2ctr(&enc_counter);
let mut tag_counter = C::BlockSize::block2ctr(&tag_counter);
let mut tag_ctr = nonce.clone();
tag_ctr[0] |= 0b1000_0000;
self.cipher.encrypt_block(&mut tag_ctr);
let mut tag_ctr = C::BlockSize::block2ctr(&tag_ctr);

let mut tag = <C::BlockSize as Sealed>::Element::new();

// process adata
let mut iter = adata.chunks_exact(C::BlockSize::USIZE);
for chunk in (&mut iter).map(Block::<C>::from_slice) {
tag.mul_sum(&self.get_h(&tag_counter), chunk);
C::BlockSize::incr_l(&mut tag_counter);
for block in (&mut iter).map(Block::<C>::from_slice) {
self.update_tag(&mut tag, &mut tag_ctr, block);
}
let rem = iter.remainder();
if !rem.is_empty() {
let mut chunk: Block<C> = Default::default();
chunk[..rem.len()].copy_from_slice(rem);
tag.mul_sum(&self.get_h(&tag_counter), &chunk);
C::BlockSize::incr_l(&mut tag_counter);
let mut block: Block<C> = Default::default();
block[..rem.len()].copy_from_slice(rem);
self.update_tag(&mut tag, &mut tag_ctr, &block);
}

let mut enc_ctr = nonce.clone();
enc_ctr[0] &= 0b0111_1111;
self.cipher.encrypt_block(&mut enc_ctr);
let mut enc_ctr = C::BlockSize::block2ctr(&enc_ctr);

// process plaintext
let mut iter = buffer.chunks_exact_mut(C::BlockSize::USIZE);
for chunk in (&mut iter).map(Block::<C>::from_mut_slice) {
xor(chunk, &self.encrypt_counter(&enc_counter));
tag.mul_sum(&self.get_h(&tag_counter), chunk);
C::BlockSize::incr_r(&mut enc_counter);
C::BlockSize::incr_l(&mut tag_counter);
for block in (&mut iter).map(Block::<C>::from_mut_slice) {
self.apply_ks_block(&mut enc_ctr, block);
self.update_tag(&mut tag, &mut tag_ctr, block);
}
let rem = iter.into_remainder();
if !rem.is_empty() {
let n = rem.len();
let e = self.encrypt_counter(&enc_counter);
xor(rem, &e[..n]);

let mut ct = Block::<C>::default();
ct[..n].copy_from_slice(rem);
self.apply_ks_block(&mut enc_ctr, rem);

tag.mul_sum(&self.get_h(&tag_counter), &ct);
C::BlockSize::incr_l(&mut tag_counter);
let mut block = Block::<C>::default();
let n = rem.len();
block[..n].copy_from_slice(rem);
self.update_tag(&mut tag, &mut tag_ctr, &block);
}

tag.mul_sum(&self.get_h(&tag_counter), &final_block);
let block = C::BlockSize::lengths2block(adata.len(), buffer.len())?;
self.update_tag(&mut tag, &mut tag_ctr, &block);

let mut tag = tag.into_bytes();
self.cipher.encrypt_block(&mut tag);
Expand All @@ -205,73 +203,65 @@ where
if nonce[0] >> 7 != 0 {
return Err(Error);
}
let final_block = C::BlockSize::lengths2block(adata.len(), buffer.len())?;

let mut enc_counter = nonce.clone();
let mut tag_counter = nonce.clone();
enc_counter[0] &= 0b0111_1111;
tag_counter[0] |= 0b1000_0000;

self.cipher.encrypt_block(&mut enc_counter);
self.cipher.encrypt_block(&mut tag_counter);

let mut dec_counter = C::BlockSize::block2ctr(&enc_counter);
let mut tag_counter = C::BlockSize::block2ctr(&tag_counter);
let mut tag_ctr = nonce.clone();
tag_ctr[0] |= 0b1000_0000;
self.cipher.encrypt_block(&mut tag_ctr);

let mut tag_ctr = C::BlockSize::block2ctr(&tag_ctr);
let mut tag = <C::BlockSize as Sealed>::Element::new();

// process adata
let mut iter = adata.chunks_exact(C::BlockSize::USIZE);
for chunk in (&mut iter).map(Block::<C>::from_slice) {
tag.mul_sum(&self.get_h(&tag_counter), chunk);
C::BlockSize::incr_l(&mut tag_counter);
for block in (&mut iter).map(Block::<C>::from_slice) {
self.update_tag(&mut tag, &mut tag_ctr, block);
}
let rem = iter.remainder();
if !rem.is_empty() {
let mut chunk: Block<C> = Default::default();
chunk[..rem.len()].copy_from_slice(rem);
tag.mul_sum(&self.get_h(&tag_counter), &chunk);
C::BlockSize::incr_l(&mut tag_counter);
let mut block: Block<C> = Default::default();
block[..rem.len()].copy_from_slice(rem);
self.update_tag(&mut tag, &mut tag_ctr, &block);
}

// process ciphertext
let mut iter = buffer.chunks_exact_mut(C::BlockSize::USIZE);
for chunk in (&mut iter).map(Block::<C>::from_mut_slice) {
tag.mul_sum(&self.get_h(&tag_counter), chunk);
xor(chunk, &self.encrypt_counter(&dec_counter));
C::BlockSize::incr_r(&mut dec_counter);
C::BlockSize::incr_l(&mut tag_counter);
for block in (&mut iter).map(Block::<C>::from_mut_slice) {
self.update_tag(&mut tag, &mut tag_ctr, block);
}
let rem = iter.into_remainder();
if !rem.is_empty() {
let n = rem.len();
let e = self.encrypt_counter(&dec_counter);

let mut ct = Block::<C>::default();
ct[..n].copy_from_slice(rem);
let mut block = Block::<C>::default();
block[..n].copy_from_slice(rem);

tag.mul_sum(&self.get_h(&tag_counter), &ct);
xor(rem, &e[..n]);
C::BlockSize::incr_l(&mut tag_counter);
self.update_tag(&mut tag, &mut tag_ctr, &block);
}

tag.mul_sum(&self.get_h(&tag_counter), &final_block);
let block = C::BlockSize::lengths2block(adata.len(), buffer.len())?;
self.update_tag(&mut tag, &mut tag_ctr, &block);

let mut tag = tag.into_bytes();
self.cipher.encrypt_block(&mut tag);

use subtle::ConstantTimeEq;
if expected_tag.ct_eq(&tag).unwrap_u8() == 1 {
Ok(())
} else {
Err(Error)
if expected_tag.ct_eq(&tag).unwrap_u8() == 0 {
return Err(Error);
}

// decrypt ciphertext
let mut dec_ctr = nonce.clone();
dec_ctr[0] &= 0b0111_1111;
self.cipher.encrypt_block(&mut dec_ctr);
let mut dec_ctr = C::BlockSize::block2ctr(&dec_ctr);

let mut iter = buffer.chunks_exact_mut(C::BlockSize::USIZE);
for block in (&mut iter).map(Block::<C>::from_mut_slice) {
self.apply_ks_block(&mut dec_ctr, block);
}
let rem = iter.into_remainder();
if !rem.is_empty() {
self.apply_ks_block(&mut dec_ctr, rem);
}
}
}

fn xor(buf: &mut [u8], val: &[u8]) {
debug_assert_eq!(buf.len(), val.len());
for (a, b) in buf.iter_mut().zip(val.iter()) {
*a ^= *b;
Ok(())
}
}