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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])
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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"
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106 changes: 57 additions & 49 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 Down Expand Up @@ -138,55 +139,55 @@ where
}
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;
let mut enc_ctr = nonce.clone();
let mut tag_ctr = nonce.clone();
enc_ctr[0] &= 0b0111_1111;
tag_ctr[0] |= 0b1000_0000;

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

let mut enc_counter = C::BlockSize::block2ctr(&enc_counter);
let mut tag_counter = C::BlockSize::block2ctr(&tag_counter);
let mut enc_ctr = C::BlockSize::block2ctr(&enc_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);
tag.mul_sum(&self.get_h(&tag_ctr), chunk);
C::BlockSize::incr_l(&mut tag_ctr);
}
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);
tag.mul_sum(&self.get_h(&tag_ctr), &chunk);
C::BlockSize::incr_l(&mut tag_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);
xor(chunk, &self.encrypt_counter(&enc_ctr));
tag.mul_sum(&self.get_h(&tag_ctr), chunk);
C::BlockSize::incr_r(&mut enc_ctr);
C::BlockSize::incr_l(&mut tag_ctr);
}
let rem = iter.into_remainder();
if !rem.is_empty() {
let n = rem.len();
let e = self.encrypt_counter(&enc_counter);
let e = self.encrypt_counter(&enc_ctr);
xor(rem, &e[..n]);

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

tag.mul_sum(&self.get_h(&tag_counter), &ct);
C::BlockSize::incr_l(&mut tag_counter);
tag.mul_sum(&self.get_h(&tag_ctr), &ct);
C::BlockSize::incr_l(&mut tag_ctr);
}

tag.mul_sum(&self.get_h(&tag_counter), &final_block);
tag.mul_sum(&self.get_h(&tag_ctr), &final_block);

let mut tag = tag.into_bytes();
self.cipher.encrypt_block(&mut tag);
Expand All @@ -205,67 +206,74 @@ 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_ctr = nonce.clone();
let mut tag_ctr = nonce.clone();
enc_ctr[0] &= 0b0111_1111;
tag_ctr[0] |= 0b1000_0000;

let mut dec_counter = C::BlockSize::block2ctr(&enc_counter);
let mut tag_counter = C::BlockSize::block2ctr(&tag_counter);
self.cipher.encrypt_block(&mut enc_ctr);
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);
tag.mul_sum(&self.get_h(&tag_ctr), chunk);
C::BlockSize::incr_l(&mut tag_ctr);
}
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);
tag.mul_sum(&self.get_h(&tag_ctr), &chunk);
C::BlockSize::incr_l(&mut tag_ctr);
}

// 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);
tag.mul_sum(&self.get_h(&tag_ctr), chunk);
C::BlockSize::incr_l(&mut tag_ctr);
}
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);

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

tag.mul_sum(&self.get_h(&tag_counter), &final_block);
let final_block = C::BlockSize::lengths2block(adata.len(), buffer.len())?;
tag.mul_sum(&self.get_h(&tag_ctr), &final_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_counter = C::BlockSize::block2ctr(&enc_ctr);
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(&dec_counter));
C::BlockSize::incr_r(&mut dec_counter);
C::BlockSize::incr_l(&mut tag_ctr);
}
let rem = iter.into_remainder();
if !rem.is_empty() {
let n = rem.len();
let e = self.encrypt_counter(&dec_counter);
xor(rem, &e[..n]);
}

Ok(())
}
}

Expand Down