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BDRC authorization library based on auth0
There is a newer version: 3.0.0
package io.bdrc.auth;

import java.io.IOException;
import java.math.BigInteger;
import java.net.URL;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.cert.CertificateException;
import java.security.interfaces.RSAPublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.RSAPublicKeySpec;
import java.util.Base64;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.auth0.jwt.JWT;
import com.auth0.jwt.JWTVerifier;
import com.auth0.jwt.algorithms.Algorithm;
import com.auth0.jwt.impl.PublicClaims;
import com.auth0.jwt.interfaces.Verification;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;

/*******************************************************************************
 * Copyright (c) 2018 Buddhist Digital Resource Center (BDRC)
 *
 * If this file is a derivation of another work the license header will appear
 * below; otherwise, this work is licensed under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with the
 * License.
 *
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *
 * See the License for the specific language governing permissions and
 * limitations under the License.
 ******************************************************************************/

public class BdrcJwks {

    static JsonNode node;
    static RSAPublicKey publicKey;
    static Algorithm algo;
    static Verification verification;
    static JWTVerifier verifier;

    public final static Logger log = LoggerFactory.getLogger(BdrcJwks.class.getName());

    public static final String ALG = PublicClaims.ALGORITHM;
    public static final String KID = PublicClaims.KEY_ID;
    public static final String KTY = "kty";
    public static final String USE = "use";
    public static final String X5C = "x5c";
    public static final String N = "n";
    public static final String E = "e";
    public static final String X5T = "x5t";

    static {

        try {
            final ObjectMapper mapper = new ObjectMapper();
            final URL url = new URL(AuthProps.getProperty("jwksUrl"));
            node = mapper.readTree(url);
            publicKey = buildPublicKey();
            algo = Algorithm.RSA256(publicKey, null);
            verifier=JWT.require(algo).build();

        } catch (IOException | CertificateException | InvalidKeySpecException | NoSuchAlgorithmException e) {
            log.error("initialization error", e);
        }
    }

    public static String getValue(String key) {
        if (key.equals(X5C)) {
            return node.findValue(X5C).get(0).asText();
        }
        return node.findValue(key).asText();
    }

    static RSAPublicKey buildPublicKey()
            throws CertificateException, IOException, InvalidKeySpecException, NoSuchAlgorithmException {
        BigInteger modulus = new BigInteger(1, Base64.getUrlDecoder().decode(getValue(N)));
        BigInteger exponent = new BigInteger(1, Base64.getUrlDecoder().decode(getValue(E)));
        return (RSAPublicKey) KeyFactory.getInstance("RSA").generatePublic(new RSAPublicKeySpec(modulus, exponent));
    }

    public static RSAPublicKey getPublicKey() {
        return publicKey;
    }
}