Nodejs Crypto ECDH PublicKey Hex as X.509

Question

I am using the prime256v1 curves for generating key pairs and sign using nodejs with the default crypto module.

Using crypto

let crypto = require('crypto');
let e = crypto.createECDH('prime256v1');
e.generateKeys();
privateKey = e.getPrivateKey();
privateKeyHex = privateKey.toString('hex');
publicKey = e.getPublicKey();
publicKeyHex = publicKey.toString('hex');

I obtain a publickey which looks like the hex string below:

'049a6b0ac242afe41128cf59736412686ca83c9e902ee3fa0f13810b9d59ebfe5e49204427c23b630be12ae33815b0bda6ed8d0603386c6ea5f1906cdb0e731286'

Usign jsrsasign

let jsrsa = require('jsrsasign');
let KEYUTIL = jsrsa.KEYUTIL;
let kp = KEYUTIL.generateKeypair("EC", "prime256v1");
let pkHex = kp.pubKeyObj.pubKeyHex

which returns

'04f36e41189420db05dd8a73e3cb310b0c55809190bdedd89bf19769ac8df3cd06c1380f646e9e65e31c24affff79e43516b37e0186c3753cfdfd29894c2becc84'

Converting the PublicKey Hex to PublicKey object in Java

I want to use these publicKeys and convert it into a PublicKey object in java. Using the EC KeyFactory, I convert the hex to a byte[] and try to construct the PublicKey object in java which expects a X.509 format encoding.

public PublicKey getPublicKey(byte[] pk) throws NoSuchAlgorithmException, InvalidKeySpecException {
    EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(pk);
    KeyFactory kf = KeyFactory.getInstance("EC");
    PublicKey pub = kf.generatePublic(publicKeySpec);
    return pub;
}

To convert the hex string to a byte[] I use the following:

public byte[] hexStringToByteArray(String hexString) {
    byte[] bytes = new byte[hexString.length() / 2];

    for(int i = 0; i < hexString.length(); i += 2) {
        String sub = hexString.substring(i, i + 2);
        Integer intVal = Integer.parseInt(sub, 16);
        bytes[i / 2] = intVal.byteValue();
        String hex = "".format("0x%x", bytes[i / 2]);
    }
    return bytes;
}

Trying to do the same using the test case as follows results in an InvalidKeySpecException:

@Test
public void pkConversionTest() throws NoSuchAlgorithmException, InvalidKeySpecException {
    ECDSA.setDebug(true);
    byte[] pk = hexStringToByteArray("049a6b0ac242afe41128cf59736412686ca83c9e902ee3fa0f13810b9d59ebfe5e49204427c23b630be12ae33815b0bda6ed8d0603386c6ea5f1906cdb0e731286");
    PublicKey pub = ECDSA.getPublicKey(pk);
    System.out.println(pub);
}

returns

java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: IOException: DerInputStream.getLength(): lengthTag=26, too big.

I am however able to generate a KeyPair using java and use the publicKey hex obtained with nodejs to perform signature verify. A sample publickey hex generated from java looks as follows:

3059301306072a8648ce3d020106082a8648ce3d0301070342000425a321d5a1a74e6c04a6e3cab030401f3dbc04d5242f9bc629175c3d3988799175eb80cd96d7e76ea924630a8d86b93c54dec7cb965b58de31705eb3343846a1

How do I format the publicKey generated by nodejs in an X.509 format to be used on the java's side?

Edit:

3059301306072a8648ce3d020106082a8648ce3d030107034200 seems to be a common prefix for the publicKey hexes generated using java. By Prefixing this to the hex values of the PublicKey obtained using nodejs since the length is smaller seems to solve the problem. But can someone explain why?

Thank you.

Answer 1

But can someone explain why?

Java encodes public keys in "X.509" format or more exactly the SubjectPublicKeyInfo structure (SPKI) defined by X.509/PKIX; see rfc5280, rfc3279, and for ECC specifically rfc5480. That's why the data you pass to the key factory is in a class named X509EncodedKeySpec. This ASN.1 structure contains an AlgorithmIdentifier which identifies the algorithm used and its parameters (which for ECC is the curve/group used, in your case an OID identifying prime256 aka P-256 aka secp256r1) plus a BIT STRING type containing the actual encoded publickey value (which for ECC is the point in X9.62 format, which has several variants; here you are using uncompressed; according to the doc nodejs.crypto also supports compressed).

Your "prefix" is the DER encoding of the ASN.1 outer SEQUENCE, AlgorithmIdentifier, and tag length and padcount which begin the BIT STRING to contain the publickey point.

Basically dupe:
* How can I get a PublicKey object from EC public key bytes?
* Loading raw 64-byte long ECDSA public key in Java (Maarten's answer is effectively what you did)
* How can I generate a valid ECDSA EC key pair? (disclosure: mine)

FYI: Effectively the same issue also occurs for RSA and there are more Qs on that. And there are also similar issues with privatekeys in generic PKCS8 format compared to algorithm-specific formats, but since publickeys are usually exchanged with other systems and/or programs while privatekeys usually aren't interoperability of privatekey encoding is less often a concern.