Skip to content

eax: simplify tag computation #805

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
newpavlov merged 1 commit into from
Feb 18, 2026
+4 −27
Merged

eax: simplify tag computation #805

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
31 changes: 4 additions & 27 deletions eax/src/lib.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -133,8 +133,7 @@ pub use cipher;

use aead::{TagPosition, inout::InOutBuf};
use cipher::{
BlockCipherEncrypt, BlockSizeUser, InnerIvInit, StreamCipherCore, array::Array,
common::OutputSizeUser, consts::U16, typenum::Unsigned,
BlockCipherEncrypt, BlockSizeUser, InnerIvInit, StreamCipherCore, array::Array, consts::U16,
};
use cmac::{Cmac, Mac, digest::Output};
use core::marker::PhantomData;
Expand All @@ -159,9 +158,6 @@ pub type Nonce<NonceSize> = Array<u8, NonceSize>;
/// EAX tags
pub type Tag<TagSize> = Array<u8, TagSize>;

// TODO: Drop that once https://github.com/RustCrypto/traits/pull/1533 releases.
type OutputSize<T> = <T as OutputSizeUser>::OutputSize;

pub mod online;

/// Counter mode with a 128-bit big endian counter.
Expand Down Expand Up @@ -251,17 +247,8 @@ where
let c = Self::cmac_with_iv(&self.key, 2, buffer.get_out());

// 5. tag ← n ^ h ^ c
// (^ means xor)
let full_tag: Array<_, OutputSize<Cmac<Cipher>>> = n
.into_iter()
.zip(h)
.map(|(a, b)| a ^ b)
.zip(c)
.map(|(a, b)| a ^ b)
.take(OutputSize::<Cmac<Cipher>>::to_usize())
.collect();

let tag = Tag::<M>::try_from(&full_tag[..M::to_usize()]).expect("tag size mismatch");
let tag = Array::<u8, M>::from_fn(|i| n[i] ^ h[i] ^ c[i]);

Ok(tag)
}

Expand All @@ -286,17 +273,7 @@ where
let c = Self::cmac_with_iv(&self.key, 2, buffer.get_in());

// 5. tag ← n ^ h ^ c
// (^ means xor)
let expected_tag: Array<_, OutputSize<Cmac<Cipher>>> = n
.into_iter()
.zip(h)
.map(|(a, b)| a ^ b)
.zip(c)
.map(|(a, b)| a ^ b)
.take(OutputSize::<Cmac<Cipher>>::to_usize())
.collect();

let expected_tag = &expected_tag[..tag.len()];
let expected_tag = Array::<u8, M>::from_fn(|i| n[i] ^ h[i] ^ c[i]);

// Constant-time MAC comparison
use subtle::ConstantTimeEq;
Expand Down