Javascript - Java ECDH

Question

In my scenario there are Alice and Bob who have agreed on which curve to use.

• Alice generates the public key and the private key
• Alice sends the public key to Bob
• Bob generates his keys and generates the session key ( or secret key, or shared key ) based on the Alice public key he received.

My problem is that Alice's public key is actually a point, so it has the xy format.

I need to convert the x,y coordinates bytes into a ECPublicKey.

This is the source code I'm using

    // outerPublicKey is the raw bytes from x,y coordinates in hex format
    KeyFactory kf = KeyFactory.getInstance("EC");

    PublicKey remoteAlicePub = kf.generatePublic(new X509EncodedKeySpec(outerPublicKey));

    KeyPairGenerator bobKeyGen = KeyPairGenerator.getInstance("ECDH", "BC");
    bobKeyGen.initialize(new ECGenParameterSpec(properties.getCurveName()), new SecureRandom());

    KeyPair bobPair = bobKeyGen.generateKeyPair();
    ECPublicKey bobPub = (ECPublicKey)bobPair.getPublic();
    ECPrivateKey bobPvt = (ECPrivateKey)bobPair.getPrivate();

    byte[] bobPubEncoded = bobPub.getEncoded();
    byte[] bobPvtEncoded = bobPvt.getEncoded();

    KeyAgreement bobKeyAgree = KeyAgreement.getInstance("ECDH");
    bobKeyAgree.init(bobPvt);
    bobKeyAgree.doPhase(remoteAlicePub, true);

    return DatatypeConverter.printHexBinary(bobKeyAgree.generateSecret());

the problem is:

    new X509EncodedKeySpec(outerPublicKey);

How can I create a public key from the xy coordinates of the point? because outerPublicKey is a raw byte array of x,y coordinates, what kind of format should i use?

Answer 1

I solved in this way ( Java Server Side )

// arrive a string like this 04456cb4ba8ee9263311485baa8562c27991f7ff22d59f3d8245b9a05661d159911b632a6f8a7a080d82f4ca77e4d12bb201b89c8ec93f61d5b4dd22df42e1b482
Map<String, Object> result = new HashMap<>();
    try {

        // set provider
        Security.addProvider(new BouncyCastleProvider());

        // transform from hex to ECPublicKey
        byte[] ecRawExternalPublicKey = this.toByte(externalRawPublicKey);
        ECPublicKey ecExternalPublicKey = null;
        KeyFactory externalKeyFactor = null;

        ECNamedCurveParameterSpec ecExternalNamedCurveParameterSpec = ECNamedCurveTable.getParameterSpec("secp256r1");
        ECCurve curve = ecExternalNamedCurveParameterSpec.getCurve();
        EllipticCurve ellipticCurve = EC5Util.convertCurve(curve, ecExternalNamedCurveParameterSpec.getSeed());
        java.security.spec.ECPoint ecPoint = ECPointUtil.decodePoint(ellipticCurve, ecRawExternalPublicKey);
        java.security.spec.ECParameterSpec ecParameterSpec = EC5Util.convertSpec(ellipticCurve, ecExternalNamedCurveParameterSpec);
        java.security.spec.ECPublicKeySpec externalPublicKeySpec = new java.security.spec.ECPublicKeySpec(ecPoint, ecParameterSpec);

        externalKeyFactor = java.security.KeyFactory.getInstance("EC");
        // this is externalPubicKey
        ecExternalPublicKey = (ECPublicKey) externalKeyFactor.generatePublic(externalPublicKeySpec);

        KeyPairGenerator keyGen = KeyPairGenerator.getInstance("ECDH", "BC");
        keyGen.initialize(new ECGenParameterSpec("secp256r1"), new SecureRandom());

        KeyPair pair = keyGen.generateKeyPair();
        ECPublicKey pub = (ECPublicKey)pair.getPublic();
        ECPrivateKey pvt = (ECPrivateKey)pair.getPrivate();

        byte[] pubEncoded = pub.getEncoded();
        byte[] pvtEncoded = pvt.getEncoded();

        KeyAgreement keyAgree = KeyAgreement.getInstance("ECDH");
        keyAgree.init(pvt);
        keyAgree.doPhase(ecExternalPublicKey, true);

        System.out.println("sharedKey: "+ this.bytesToHex( keyAgree.generateSecret() ));

        // internal public key
        return "04"+ pub.getW().getAffineX().toString(16) + pub.getW().getAffineY().toString(16)

    }
    catch (Exception e ){
        e.printStackTrace();
        return null;
    }

Javascript ( Client Side )

ecdhHandShake() {
    let _this = this;

    this.keyGeneration()
    .then( k => {
        ajax({
            url: "http://localhost:5050/test/ecdh/handshake",
            headers: {
                "Content-Type": "application/json"
            },
            body: {
                publickey: this.buf2Hex(this.publicKey)
            },
            method: 'POST',
            crossDomain: true,
            responseType: 'json'
        })
        .subscribe(
            payload => {
                const publicKey = _this.hex2Arr(payload.response.publicKey);
                _this.serverPublicKey = _this.hex2Arr(payload.response.publicKey);

                _this.importServerKey()
                .then(sharedSecret => {
                    const sharedSecretHex = this.buf2Hex(sharedSecret);
                    console.log("shared key: "+ sharedSecretHex);
                })
                .catch( e => {
                    console.log(e);
                })
            },
            error => {
                console.log(error);
            },
            () => console.log('done')
        )
        ;
    })
    .catch( e => {
        console.log(e);
    })
    ;
}


keyGeneration() {       

    let _this = this;
    return window.crypto.subtle.generateKey(
        {
            name: "ECDH",
            namedCurve: "P-256", // the curve name
        },
        true, // <== Here if you want it to be exportable !!
        ["deriveKey", "deriveBits"] // usage
    )
    .then(key => {
        _this.keys = key;
        return window.crypto.subtle.exportKey(
            "raw", //can be "jwk" (public or private), "raw" (public only), "spki" (public only), or "pkcs8" (private only)
            _this.keys.publicKey
        )
        .then(rawPublicKey => {
            _this.publicKey = rawPublicKey;
            return rawPublicKey;
        })
        .catch(function(err){
            console.error(err);
        })
        ;
    })
    .catch(function(err){
        console.error(err);
    })
    ;

}

importServerKey() {

    return window.crypto.subtle.importKey(
        'raw',
        this.serverPublicKey,
        {
            name: 'ECDH',
            namedCurve: 'P-256'
        },
        true,
        []
    )
    .then(aliceKeyImported => {
        return window.crypto.subtle.deriveBits(
            {
                name: 'ECDH',
                namedCurve: 'P-256',
                public: aliceKeyImported
            },
            this.keys.privateKey,
            256)
    })
    .catch( e => {
        console.log(e);
    })
}


hex2Arr( str ) {
    if (!str) {
        return new Uint8Array();
    }
    const arr = [];
    for (let i = 0, len = str.length; i < len; i+=2) {
        arr.push(parseInt(str.substr(i, 2), 16));
    }
    return new Uint8Array(arr);
}

buf2Hex( buf ) {
    return Array.from(new Uint8Array(buf))
        .map(x => ('00' + x.toString(16)).slice(-2))
        .join('')
}