Summary

The public SenderContext Seal() API has a race condition which allows for the same AEAD nonce to be re-used for multiple Seal() calls. This can lead to complete loss of Confidentiality and Integrity of the produced messages.

Details

The SenderContext Seal() implementation allows for concurrent executions to trigger computeNonce() with the same sequence number. This results in the same nonce being used in the suite's AEAD.

PoC

This code reproduces the issue (and also checks for more things that could be wrong with the implementation).

import { CipherSuite, KdfId, AeadId, KemId } from "hpke-js";

const suite = new CipherSuite({
  kem: KemId.DhkemP256HkdfSha256,
  kdf: KdfId.HkdfSha256,
  aead: AeadId.Aes128Gcm,
});

const keypair = await suite.kem.generateKeyPair();
const skR = keypair.privateKey;
const pkR = keypair.publicKey;

const sender = await suite.createSenderContext({
  recipientPublicKey: pkR,
});

const [message0, message1] = await Promise.all([
  sender.seal(
    new TextEncoder().encode("Secret message 1: Attack at dawn").buffer
  ),
  sender.seal(
    new TextEncoder().encode("Secret message 2: Withdraw troops").buffer
  ),
]);

const recipient = await suite.createRecipientContext({
  recipientKey: skR,
  enc: sender.enc,
});

const plaintext0 = await recipient.open(message0);
console.log("✓ Decrypted message seq=0", new TextDecoder().decode(plaintext0));

try {
  console.log(
    "✓ Decrypted message seq=1",
    new TextDecoder().decode(await recipient.open(message1))
  );
  console.log("\n✓ nonce-reuse reproduction completed, code is NOT vulnerable");
} catch (error) {
  // re-sequence the recipient to verify same nonce was used for two messages
  recipient._ctx.seq = 0;
  console.log(
    "❌ Decrypted a different message with seq=0",
    new TextDecoder().decode(await recipient.open(message1))
  );

  console.log(
    "\n✓ nonce-reuse reproduction completed, code is vulnerable, nonces are reused when concurrent calls to .seal() are used"
  );
}

// Test that failed Open() doesn't increment sequence
const recipient2 = await suite.createRecipientContext({
  recipientKey: skR,
  enc: sender.enc,
});

const invalidMessage = new Uint8Array(message0.byteLength);
invalidMessage.set(new Uint8Array(message0));
invalidMessage[0] ^= 0xff; // Corrupt the first byte

try {
  await recipient2.open(invalidMessage.buffer);
} catch {}

// Now try to open the first valid message - should still work with seq=0
try {
  await recipient2.open(message0);
  console.log("✓ Successfully decrypted message with seq=0 after failed open()");
  console.log("✓ Failed open() did NOT increment sequence");
} catch (error) {
  console.log("❌ Failed to decrypt message - sequence was incorrectly incremented");
}

// Test that same message produces same ciphertext due to nonce reuse
const sender2 = await suite.createSenderContext({
  recipientPublicKey: pkR,
});

const sameMessage = new TextEncoder().encode("Identical message").buffer;
const [cipher0, cipher1] = await Promise.all([
  sender2.seal(sameMessage),
  sender2.seal(sameMessage),
]);

const cipher0Array = new Uint8Array(cipher0);
const cipher1Array = new Uint8Array(cipher1);

let identical = true;
if (cipher0Array.length !== cipher1Array.length) {
  identical = false;
} else {
  for (let i = 0; i < cipher0Array.length; i++) {
    if (cipher0Array[i] !== cipher1Array[i]) {
      identical = false;
      break;
    }
  }
}

if (identical) {
  console.log("\n❌ Same message produced IDENTICAL ciphertext (nonce reuse confirmed)");
} else {
  console.log("\n✓ Same message produced different ciphertext (nonces are unique)");
}