Suggestion: add length check for the context of `crypto_kdf_derive_from_key`

[nikgraf]

We noticed when using a smaller context string than 8 characters sometimes (couldn't reproduce it in tests, but in two different applications) the kdf function returns a different key. When using context strings with exactly 8 characters there are no issues.

This matches the expected documentation stating: They must be crypto_kdf_CONTEXTBYTES bytes long. https://libsodium.gitbook.io/doc/key_derivation

It would be great if there was a check that throws an error in case the context is not exactly 8 characters long. Similar to invalid key length checks.

[jedisct1]

Mmmm it should already be the case:

https://github.com/jedisct1/libsodium.js/blob/e14d775b6669613906395c82293bd3955d08028c/wrapper/symbols/crypto_kdf_derive_from_key.json#L16

https://github.com/jedisct1/libsodium.js/blob/e14d775b6669613906395c82293bd3955d08028c/wrapper/macros/input_string.js#L7-L9

Maybe not in the version published on npm, but in the current code, it is.

[nikgraf]

@jedisct1 hmmm if I do git blame both code parts have been introduced more than 3 years ago, but the latest release is 5 months ago.

If you run this code you can test it yourself:

index.js

const sodium = require('libsodium-wrappers');

console.log(`Hello Node.js v${process.versions.node}!`);

async function main() {
  await sodium.ready;

  const masterKey = new Uint8Array([
    22, 83, 213, 220, 116, 80, 161, 0, 43, 65, 106, 114, 33, 69, 205, 24, 25,
    246, 101, 60, 140, 226, 94, 246, 161, 28, 253, 212, 149, 52, 225, 93,
  ]);

  const subkey1 = sodium.crypto_kdf_derive_from_key(
    sodium.crypto_secretbox_KEYBYTES,
    10,
    'locker',
    masterKey
  );
  const subkey2 = sodium.crypto_kdf_derive_from_key(
    sodium.crypto_secretbox_KEYBYTES,
    10,
    '1',
    masterKey
  );
  const subkey3 = sodium.crypto_kdf_derive_from_key(
    sodium.crypto_secretbox_KEYBYTES,
    10,
    'locker123456',
    masterKey
  );

  console.log(subkey1);
  console.log(subkey2);
  console.log(subkey3);
}

main();

package.json

{
  "name": "node-starter",
  "version": "0.0.0",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "dependencies": {
    "libsodium-wrappers": "^0.7.11"
  }
}