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Java implementation of JSON Web Token (JWT)
There is a newer version: 4.4.0
package com.auth0.jwt;

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.apache.commons.codec.binary.Base64;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.io.IOException;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.security.SignatureException;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;

/**
 * JWT Java Implementation
 * 
 * Adapted from https://bitbucket.org/lluisfaja/javajwt/wiki/Home
 * See JWTVerifier.java
 */
public class JWTVerifier {

    private final String secret;
    private final String audience;
    private final String issuer;
    private final Base64 decoder;

    private final ObjectMapper mapper;

    private Map algorithms;

    public JWTVerifier(String secret, String audience, String issuer) {
        if (secret == null || "".equals(secret)) {
            throw new IllegalArgumentException("Secret cannot be null or empty");
        }

        decoder = new Base64(true);
        mapper = new ObjectMapper();

        algorithms = new HashMap();
        algorithms.put("HS256", "HmacSHA256");
        algorithms.put("HS384", "HmacSHA384");
        algorithms.put("HS512", "HmacSHA512");

        this.secret = secret;
        this.audience = audience;
        this.issuer = issuer;
    }

    public JWTVerifier(String secret, String audience) {
        this(secret, audience, null);
    }

    public JWTVerifier(String secret) {
        this(secret, null, null);
    }

    /**
     * Performs JWT validation
     *
     * @param token token to verify
     * @throws SignatureException    when signature is invalid
     * @throws IllegalStateException when token's structure, expiration, issuer or audience are invalid
     */
    public Map verify(String token)
            throws NoSuchAlgorithmException, InvalidKeyException, IllegalStateException,
            IOException, SignatureException {
        if (token == null || "".equals(token)) {
            throw new IllegalStateException("token not set");
        }

        String[] pieces = token.split("\\.");

        // check number of segments
        if (pieces.length != 3) {
            throw new IllegalStateException("Wrong number of segments: " + pieces.length);
        }

        // get JWTHeader JSON object. Extract algorithm
        JsonNode jwtHeader = decodeAndParse(pieces[0]);

        String algorithm = getAlgorithm(jwtHeader);

        // get JWTClaims JSON object
        JsonNode jwtPayload = decodeAndParse(pieces[1]);

        // check signature
        verifySignature(pieces, algorithm);

        // additional JWTClaims checks
        verifyExpiration(jwtPayload);
        verifyIssuer(jwtPayload);
        verifyAudience(jwtPayload);

        return mapper.treeToValue(jwtPayload, Map.class);
    }

    void verifySignature(String[] pieces, String algorithm) throws NoSuchAlgorithmException, InvalidKeyException, SignatureException {
        Mac hmac = Mac.getInstance(algorithm);
        hmac.init(new SecretKeySpec(decoder.decodeBase64(secret), algorithm));
        byte[] sig = hmac.doFinal(new StringBuilder(pieces[0]).append(".").append(pieces[1]).toString().getBytes());

        if (!Arrays.equals(sig, decoder.decodeBase64(pieces[2]))) {
            throw new SignatureException("signature verification failed");
        }
    }

    void verifyExpiration(JsonNode jwtClaims) {
        final long expiration = jwtClaims.has("exp") ? jwtClaims.get("exp").asLong(0) : 0;

        if (expiration != 0 && System.currentTimeMillis() / 1000L >= expiration) {
            throw new IllegalStateException("jwt expired");
        }
    }

    void verifyIssuer(JsonNode jwtClaims) {
        final String issuerFromToken = jwtClaims.has("iss") ? jwtClaims.get("iss").asText() : null;

        if (issuerFromToken != null && issuer != null && !issuer.equals(issuerFromToken)) {
            throw new IllegalStateException("jwt issuer invalid");
        }
    }

    void verifyAudience(JsonNode jwtClaims) {
        if (audience == null)
            return;
        JsonNode audNode = jwtClaims.get("aud");
        if (audNode == null)
            return;
        if (audNode.isArray()) {
            for (JsonNode jsonNode : audNode) {
                if (audience.equals(jsonNode.textValue()))
                    return;
            }
        } else if (audNode.isTextual()) {
            if (audience.equals(audNode.textValue()))
                return;
        }
        throw new IllegalStateException("jwt audience invalid");
    }

    String getAlgorithm(JsonNode jwtHeader) {
        final String algorithmName = jwtHeader.has("alg") ? jwtHeader.get("alg").asText() : null;

        if (jwtHeader.get("alg") == null) {
            throw new IllegalStateException("algorithm not set");
        }

        if (algorithms.get(algorithmName) == null) {
            throw new IllegalStateException("unsupported algorithm");
        }

        return algorithms.get(algorithmName);
    }

    JsonNode decodeAndParse(String b64String) throws IOException {
        String jsonString = new String(decoder.decodeBase64(b64String), "UTF-8");
        JsonNode jwtHeader = mapper.readValue(jsonString, JsonNode.class);
        return jwtHeader;
    }
}