how to populate a jsonwebkey from a generated ecdsa key

Question

I try to generate a public/private key pair which i will use for digital signature of a JWT with jose4j. I use Elliptic Curve Digital Signature Algorithm

My problem is that i don't know how to get the parameters representing the edcsa key meaning:

• crv
• x
• y

• d

   KeyPairGenerator g = KeyPairGenerator.getInstance("EC");
   ECGenParameterSpec kpgparams = new ECGenParameterSpec("secp256r1");
   g.initialize(kpgparams);
  
   KeyPair pair = g.generateKeyPair();
   // Instance of signature class with SHA256withECDSA algorithm
   Signature ecdsaSign = Signature.getInstance("SHA256withECDSA");
   ecdsaSign.initSign(pair.getPrivate());
  
   System.out.println("Private Keys is::" + pair.getPrivate());
   System.out.println("Public Keys is::" + pair.getPublic());
  
  JsonWebKeySet jsonWebKeySet = new JsonWebKeySet();
  PrivateKey privateKey = pair.getPrivate();
  JsonWebKey webKey = new  JsonWebKey(privateKey) {
  
      @Override
      public String getKeyType() {
          // TODO Auto-generated method stub
          return "EC";
      }
  
      @Override
      protected void fillTypeSpecificParams(Map<String, Object> params,
              OutputControlLevel outputLevel) {
          params.put("use", "sig");
          params.put("key_ops", "sign");
          params.put("alg", "ES256");
          params.put("kid", "kukuPrivateKey");
  
      }
  };
  jsonWebKeySet.addJsonWebKey(webKey);
  System.out.println("aaaa"+jsonWebKeySet.toJson());
  

Answer 1

You can create a JsonWebKey directly with the public key you generated and jose4j will take care of the parameters and encoding.

    KeyPairGenerator g = KeyPairGenerator.getInstance("EC");
    ECGenParameterSpec kpgparams = new ECGenParameterSpec("secp256r1");
    g.initialize(kpgparams);

    KeyPair keyPair = g.generateKeyPair();

    PublicJsonWebKey jwk = PublicJsonWebKey.Factory.newPublicJwk(keyPair.getPublic());
    jwk.setPrivateKey(keyPair.getPrivate());
    jwk.setUse(Use.SIGNATURE);

    System.out.println(jwk.toJson(JsonWebKey.OutputControlLevel.PUBLIC_ONLY));
    System.out.println(jwk.toJson(JsonWebKey.OutputControlLevel.INCLUDE_PRIVATE)); // to include the private key 'd'

You can also use the EcJwkGenerator utility in jose4j to generate the key pair and wrap it in a JsonWebKey,

    EllipticCurveJsonWebKey jwk = EcJwkGenerator.generateJwk(EllipticCurves.P256);
    jwk.setUse(Use.SIGNATURE);

    System.out.println(jwk.toJson(JsonWebKey.OutputControlLevel.PUBLIC_ONLY));
    System.out.println(jwk.toJson(JsonWebKey.OutputControlLevel.INCLUDE_PRIVATE)); // to include the private key 'd'

Answer 2

After struggling with it a long time i got the following

    private static String createWebKeySet() throws NoSuchAlgorithmException,

        InvalidAlgorithmParameterException, InvalidKeyException {
    KeyPairGenerator g = KeyPairGenerator.getInstance("EC");
     ECGenParameterSpec kpgparams = new ECGenParameterSpec("secp256r1");
     g.initialize(kpgparams);

     KeyPair pair = g.generateKeyPair();
     // Instance of signature class with SHA256withECDSA algorithm
     Signature ecdsaSign = Signature.getInstance("SHA256withECDSA");
     ecdsaSign.initSign(pair.getPrivate());

     System.out.println("Private Keys is::" + pair.getPrivate());
     System.out.println("Public Keys is::" + pair.getPublic());

    JsonWebKeySet jsonWebKeySet = new JsonWebKeySet();

    final ECPrivateKey privateKey = (ECPrivateKey) pair.getPrivate();
    final ECPublicKey publicKey = (ECPublicKey) pair.getPublic();
    JsonWebKey privateWebKey = new  JsonWebKey(privateKey) {


        @Override
        public String getKeyType() {
            // TODO Auto-generated method stub
            return "EC";
        }

        @Override
        protected void fillTypeSpecificParams(Map<String, Object> params,
                OutputControlLevel outputLevel) {

            params.put("use", "sig");
            params.put("key_ops", "sign");
            //params.put("alg", "ES256");
            params.put("kid", "kukuPrivateKey");
            ECParameterSpec paramSpec = privateKey.getParams();
            params.put("crv", "P-"+paramSpec.getCurve().getField().getFieldSize());

            params.put("x",  Base64.encode(publicKey.getW().getAffineX().toByteArray()));
            params.put("y",  Base64.encode(publicKey.getW().getAffineY().toByteArray()));
            params.put("d",Base64.encode(privateKey.getS().toByteArray()));

        }
    };
    jsonWebKeySet.addJsonWebKey(privateWebKey);



     JsonWebKey publicWebKey = new  JsonWebKey(publicKey) {

            @Override
            public String getKeyType() {
                // TODO Auto-generated method stub
                return "EC";
            }

            @Override
            protected void fillTypeSpecificParams(Map<String, Object> params,
                    OutputControlLevel outputLevel) {

                params.put("use", "sig");
                params.put("key_ops", "verify");
                //params.put("alg", "ES256");
                params.put("kid", "kukuPublicKey");
                ECParameterSpec paramSpec = publicKey.getParams();
                params.put("crv", "P-"+paramSpec.getCurve().getField().getFieldSize());
                params.put("x",  Base64.encode(publicKey.getW().getAffineX().toByteArray()));
                params.put("y",  Base64.encode(publicKey.getW().getAffineY().toByteArray()));

            }
        };
        jsonWebKeySet.addJsonWebKey(publicWebKey);


    return jsonWebKeySet.toJson();
}