WebCryptoAPI EC key SPKI export does not always use uncompressed point format

[panva]

Our WebCryptoAPI implementation supports import of compressed point format EC keys in both raw, and spki forms. That is an optional to implement part of the API.

The specification however requires that when keys are exported they unconditionally use the uncompressed point format. This is currently not the case in Node.js when compressed point spki was used to import the CryptoKey.

The following script demonstrates the issue.

Script

import * as assert from 'assert';

const { subtle } = globalThis.crypto;

const keyData = {
  'P-256': {
    uncompressed: Buffer.from([
      48, 89, 48, 19, 6, 7, 42, 134, 72, 206, 61, 2, 1, 6, 8, 42, 134, 72, 206, 61, 3, 1, 7, 3, 66, 0, 4, 210, 16, 176, 166, 249, 217, 240, 18, 134, 128, 88, 180, 63, 164, 244, 113, 1, 133, 67, 187, 160, 12, 146, 80, 223, 146, 87, 194, 172, 174, 93, 209, 206, 3, 117, 82, 212, 129, 69, 12, 227, 155, 77, 16, 149, 112, 27, 23, 91, 250, 179, 75, 142, 108, 9, 158, 24, 241, 193, 152, 53, 131, 97, 232,
    ]),
    compressed: Buffer.from([
      48, 57, 48, 19, 6, 7, 42, 134, 72, 206, 61, 2, 1, 6, 8, 42, 134, 72, 206, 61, 3, 1, 7, 3, 34, 0, 2, 210, 16, 176, 166, 249, 217, 240, 18, 134, 128, 88, 180, 63, 164, 244, 113, 1, 133, 67, 187, 160, 12, 146, 80, 223, 146, 87, 194, 172, 174, 93, 209,
    ]),
  },

  'P-384': {
    uncompressed: Buffer.from([
      48, 118, 48, 16, 6, 7, 42, 134, 72, 206, 61, 2, 1, 6, 5, 43, 129, 4, 0, 34, 3, 98, 0, 4, 33, 156, 20, 214, 102, 23, 179, 110, 198, 216, 133, 107, 56, 91, 115, 167, 77, 52, 79, 216, 174, 117, 239, 4, 100, 53, 221, 165, 78, 59, 68, 189, 95, 189, 235, 209, 208, 141, 214, 158, 45, 125, 193, 220, 33, 140, 180, 53, 189, 40, 19, 140, 199, 120, 51, 122, 132, 47, 107, 214, 27, 36, 14, 116, 36, 159, 36, 102, 124, 42, 88, 16, 167, 107, 252, 40, 224, 51, 95, 136, 166, 80, 29, 236, 1, 151, 109, 168, 90, 251, 0, 134, 156, 182, 172, 232,
    ]),
    compressed: Buffer.from([
      48, 70, 48, 16, 6, 7, 42, 134, 72, 206, 61, 2, 1, 6, 5, 43, 129, 4, 0, 34, 3, 50, 0, 2, 33, 156, 20, 214, 102, 23, 179, 110, 198, 216, 133, 107, 56, 91, 115, 167, 77, 52, 79, 216, 174, 117, 239, 4, 100, 53, 221, 165, 78, 59, 68, 189, 95, 189, 235, 209, 208, 141, 214, 158, 45, 125, 193, 220, 33, 140, 180, 53,
    ]),
  },
  'P-521': {
    uncompressed: Buffer.from([
      48, 129, 155, 48, 16, 6, 7, 42, 134, 72, 206, 61, 2, 1, 6, 5, 43, 129, 4, 0, 35, 3, 129, 134, 0, 4, 1, 86, 244, 121, 248, 223, 30, 32, 167, 255, 192, 76, 228, 32, 195, 225, 84, 174, 37, 25, 150, 190, 228, 47, 3, 75, 132, 212, 27, 116, 63, 52, 228, 95, 49, 27, 129, 58, 156, 222, 200, 205, 165, 155, 187, 189, 49, 212, 96, 179, 41, 37, 33, 231, 193, 183, 34, 229, 102, 124, 3, 219, 47, 174, 117, 63, 1, 80, 23, 54, 207, 226, 71, 57, 67, 32, 216, 228, 175, 194, 253, 57, 181, 169, 51, 16, 97, 184, 30, 34, 65, 40, 43, 158, 23, 137, 24, 34, 181, 183, 158, 5, 47, 69, 151, 181, 150, 67, 253, 57, 55, 156, 81, 189, 81, 37, 196, 244, 139, 195, 240, 37, 206, 60, 211, 105, 83, 40, 108, 203, 56, 251,
    ]),
    compressed: Buffer.from([
      48, 88, 48, 16, 6, 7, 42, 134, 72, 206, 61, 2, 1, 6, 5, 43, 129, 4, 0, 35, 3, 68, 0, 3, 1, 86, 244, 121, 248, 223, 30, 32, 167, 255, 192, 76, 228, 32, 195, 225, 84, 174, 37, 25, 150, 190, 228, 47, 3, 75, 132, 212, 27, 116, 63, 52, 228, 95, 49, 27, 129, 58, 156, 222, 200, 205, 165, 155, 187, 189, 49, 212, 96, 179, 41, 37, 33, 231, 193, 183, 34, 229, 102, 124, 3, 219, 47, 174, 117, 63,
    ]),
  },
};

const curves = Object.keys(keyData);
const testVectors = [
  {
    name: 'ECDSA',
    privateUsages: ['sign'],
    publicUsages: ['verify'],
  },
  {
    name: 'ECDH',
    privateUsages: ['deriveKey', 'deriveBits'],
    publicUsages: [],
  },
];

async function testRoundTripSpki({ name, publicUsages }, namedCurve) {
  const { compressed, uncompressed } = keyData[namedCurve];

  const key = await subtle.importKey(
    'spki',
    compressed,
    { name, namedCurve },
    true,
    publicUsages
  );

  const spki = await subtle.exportKey('spki', key);
  assert.deepStrictEqual(
    Buffer.from(spki).toString('hex'),
    uncompressed.toString('hex')
  );
}

async function testRoundTripRaw({ name, publicUsages }, namedCurve) {
  let { compressed, uncompressed } = keyData[namedCurve];
  let i = uncompressed.indexOf(Buffer.from([0x00, 0x04])) + 1;
  uncompressed = uncompressed.subarray(i);

  i =
    compressed.indexOf(Buffer.from([0x00, 0x02])) + 1 ||
    compressed.indexOf(Buffer.from([0x00, 0x03])) + 1;

  compressed = compressed.subarray(i);

  const key = await subtle.importKey(
    'raw',
    compressed,
    { name, namedCurve },
    true,
    publicUsages
  );

  const raw = await subtle.exportKey('raw', key);

  assert.strictEqual(
    Buffer.from(raw).toString('hex'),
    uncompressed.toString('hex')
  );
}

for (const namedCurve of curves) {
  for (const vector of testVectors) {
    await testRoundTripSpki(vector, namedCurve).then(
      () =>
        console.log(namedCurve, 'spki'.padEnd(4), vector.name.padEnd(5), '✅'),
      () =>
        console.log(namedCurve, 'spki'.padEnd(4), vector.name.padEnd(5), '❌')
    );
    await testRoundTripRaw(vector, namedCurve).then(
      () =>
        console.log(namedCurve, 'raw'.padEnd(4), vector.name.padEnd(5), '✅'),
      () =>
        console.log(namedCurve, 'raw'.padEnd(4), vector.name.padEnd(5), '❌')
    );
    console.log('\n');
  }
}

P-256 spki ECDSA ❌
P-256 raw  ECDSA ✅


P-256 spki ECDH  ❌
P-256 raw  ECDH  ✅


P-384 spki ECDSA ❌
P-384 raw  ECDSA ✅


P-384 spki ECDH  ❌
P-384 raw  ECDH  ✅


P-521 spki ECDSA ❌
P-521 raw  ECDSA ✅


P-521 spki ECDH  ❌
P-521 raw  ECDH  ✅

I'm opening this issue so that I have an issue tracker link to add to the WPT status file once it gets updated.

[panva]

@tniessen and I tried to find a way of ensuring EC keys use uncompressed point format upon export but short of some very brute and inefficient JS hacks using the Web Crypto API itself we didn't find a way.

[panva]

@jasnell do you have any ideas on how to resolve this issue in either the spki import invocation or the spki export invocation?

[panva]

cc @nodejs/crypto

[tniessen]

@tniessen and I tried to find a way of ensuring EC keys use uncompressed point format upon export but short of some very brute and inefficient JS hacks using the Web Crypto API itself we didn't find a way.

To clarify, it's certainly possible in C++ but it requires some refactoring because the internal code passes KeyObjectData objects around, which we cannot modify, so we either need to create a temporary copy or pass around lower-level OpenSSL types.

[panva]

I think that if we can identify the KeyObjectData is using compressed point then creating a temporary copy for the spki export in this edge-case would be okay?

[bnoordhuis]

This is about crypto.subtle.exportKey('spki', key) not writing uncompressed ECDH and ECDSA keys, correct?

Seems like that should be a trivial fix in PKEY_SPKI_Export() from src/crypto/crypto_keys.cc; the logic to go from compressed to uncompressed already exists in ECDH::ConvertKey(), it's basically a wrapper around openssl's EC_POINT_point2oct(). Is there some subtlety that I'm missing?

[panva]

It's just that the existing functionality works with ec point and the key representation we have is ec key.

If you see an obvious way to do this, great!

[bnoordhuis]

#46021. It's admittedly not very elegant but oh well.

[tniessen]

Is there some subtlety that I'm missing?
It's admittedly not very elegant but oh well.

Only that one :) Thanks for working on this.