CVE-2025-27498: AEADs/ascon-aead: Plaintext exposed in decrypt_in_place_detached even on tag verification failure

Summary

In decrypt_in_place_detached, the decrypted ciphertext (which is the correct ciphertext) is exposed even if the tag is incorrect.

Details

This is because in decrypt_inplace in asconcore.rs, tag verification causes an error to be returned with the plaintext contents still in buffer. The root cause of this vulnerability is similar to https://github.com/RustCrypto/AEADs/security/advisories/GHSA-423w-p2w9-r7vq

PoC

use ascon_aead::Tag;
use ascon_aead::{Ascon128, Key, Nonce};
use ascon_aead::aead::{AeadInPlace, KeyInit};

fn main() {

    let key = Key::<Ascon128>::from_slice(b"very secret key.");
    let cipher = Ascon128::new(key);

    let nonce = Nonce::<Ascon128>::from_slice(b"unique nonce 012"); // 128-bits; unique per message

    let mut buffer: Vec<u8> = Vec::new(); // Buffer needs 16-bytes overhead for authentication tag
    buffer.extend_from_slice(b"plaintext message");

    // Encrypt `buffer` in-place detached, replacing the plaintext contents with ciphertext
    cipher.encrypt_in_place_detached(nonce, b"", &mut buffer).expect("encryption failure!");
    
    // Decrypt `buffer` in-place with the wrong tag, ignoring the decryption error
    let _ = cipher.decrypt_in_place_detached(nonce, b"", &mut buffer, Tag::<Ascon128>::from_slice(b"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"));

    assert_eq!(&buffer, b"plaintext message");
}

Impact

If a program continues to use the result of decrypt_in_place_detached after a decryption failure, the result will be unauthenticated. This may permit some forms of chosen ciphertext attacks (CCAs).