KBS: fix JWE format

Fixes confidential-containers#583.

Due to RFC 7516, the JWE AEAD Auth Tag should be expcilitly be included
inside the `tag` part. Before this commit, the tag is actually included
as the suffix of the `ciphertest`. Although this is also secure, it's
not standard.

We fix this by expcilitly extract the tag and include it into the jwe
body. Also, we use an AAD `CoCo` to do AEAD. This should be align with
the guest-components side.

Signed-off-by: Xynnn007 <[email protected]>

kbs/src/jwe.rs

@@ -2,7 +2,7 @@
 // Licensed under the Apache License, Version 2.0, see LICENSE for details.
 // SPDX-License-Identifier: Apache-2.0
 
-use aes_gcm::{aead::Aead, Aes256Gcm, KeyInit, Nonce};
+use aes_gcm::{aead::AeadMutInPlace, Aes256Gcm, KeyInit, Nonce};
 use anyhow::{anyhow, bail, Context, Result};
 use base64::{engine::general_purpose::URL_SAFE_NO_PAD, Engine};
 use kbs_types::{Response, TeePubKey};
@@ -12,8 +12,9 @@ use serde_json::json;
 
 const RSA_ALGORITHM: &str = "RSA1_5";
 const AES_GCM_256_ALGORITHM: &str = "A256GCM";
+const AEAD_AAD: &[u8] = b"CoCo";
 
-pub fn jwe(tee_pub_key: TeePubKey, payload_data: Vec<u8>) -> Result<Response> {
+pub fn jwe(tee_pub_key: TeePubKey, mut payload_data: Vec<u8>) -> Result<Response> {
     let TeePubKey::RSA { alg, k_mod, k_exp } = tee_pub_key else {
         bail!("Only RSA key is support for TEE pub key")
     };
@@ -25,11 +26,11 @@ pub fn jwe(tee_pub_key: TeePubKey, payload_data: Vec<u8>) -> Result<Response> {
     let mut rng = rand::thread_rng();
 
     let aes_sym_key = Aes256Gcm::generate_key(&mut OsRng);
-    let cipher = Aes256Gcm::new(&aes_sym_key);
+    let mut cipher = Aes256Gcm::new(&aes_sym_key);
     let iv = rng.gen::<[u8; 12]>();
     let nonce = Nonce::from_slice(&iv);
-    let encrypted_payload_data = cipher
-        .encrypt(nonce, payload_data.as_slice())
+    let tag = cipher
+        .encrypt_in_place_detached(nonce, AEAD_AAD, &mut payload_data)
         .map_err(|e| anyhow!("AES encrypt Resource payload failed: {e}"))?;
 
     let k_mod = URL_SAFE_NO_PAD
@@ -59,7 +60,7 @@ pub fn jwe(tee_pub_key: TeePubKey, payload_data: Vec<u8>) -> Result<Response> {
             .context("serde protected_header failed")?,
         encrypted_key: URL_SAFE_NO_PAD.encode(wrapped_sym_key),
         iv: URL_SAFE_NO_PAD.encode(iv),
-        ciphertext: URL_SAFE_NO_PAD.encode(encrypted_payload_data),
-        tag: "".to_string(),
+        ciphertext: URL_SAFE_NO_PAD.encode(payload_data),
+        tag: URL_SAFE_NO_PAD.encode(tag),
     })
 }