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callstats java library
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package io.callstats.sdk.internal;

import java.io.UnsupportedEncodingException;
import java.util.Arrays;
import org.apache.commons.codec.binary.Base64;
import org.apache.commons.codec.binary.Hex;
import org.jose4j.jws.AlgorithmIdentifiers;
import org.jose4j.jws.JsonWebSignature;
import org.jose4j.jwt.JwtClaims;
import org.jose4j.keys.HmacKey;
import org.jose4j.lang.JoseException;
import io.callstats.sdk.ICallStatsTokenGenerator;

public class TokenGeneratorHs256 implements ICallStatsTokenGenerator {

  private HmacKey key;
  private String keyId = null;
  private String appId;
  private String userId;

  /**
   * Initialize HS256 JWT generator
   * 
   * @param appSecret Application secret. The contents will be cleared after constructor is done.
   * @param appId Application ID
   * @param userId User ID
   */
  public TokenGeneratorHs256(char[] appSecret, final int appId, final String userId) {
    this.userId = userId;
    this.appId = (new Integer(appId)).toString();

    byte[] keyIdBuilder = new byte[appSecret.length];
    boolean split = false;
    byte[] keyBytes = null;
    int keyIdx = 0;

    // Split key id and app secret
    for (int i = 0; i < appSecret.length; i++) {
      if (!split && appSecret[i] != ':') {
        keyIdBuilder[i] = (byte) appSecret[i];
      } else if (appSecret[i] == ':') {
        split = true;
        keyBytes = new byte[appSecret.length - i - 1];
      } else {
        keyBytes[keyIdx++] = (byte) appSecret[i];
      }
    }
    if (split) {
      try {
        keyId = Hex.encodeHexString(Base64.decodeBase64(new String(keyIdBuilder, "UTF-8")));
      } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
      }
      clearByteArray(keyIdBuilder);

    } else {
      // There wasn't key ID in the app secret, so keep keyid as null
      keyBytes = keyIdBuilder;
    }
    key = new HmacKey(keyBytes);
    clearByteArray(keyBytes);
    clearCharArray(appSecret);
  }

  /**
   * Clear character array
   * 
   * @param secret secret key
   */
  public static void clearCharArray(char[] secret) {
    Arrays.fill(secret, '\0');
  }

  /**
   * Clear byte array
   * 
   * @param secret secret key
   */
  public static void clearByteArray(byte[] secret) {
    Arrays.fill(secret, (byte) 0);
  }

  public String generateToken(boolean forcenew) {
    JwtClaims claims = new JwtClaims();
    claims.setClaim("appID", appId);
    claims.setClaim("userID", userId);
    if (keyId != null) {
      claims.setClaim("keyID", keyId);
    }
    claims.setExpirationTimeMinutesInTheFuture(10);
    claims.setNotBeforeMinutesInThePast(10);

    JsonWebSignature jws = new JsonWebSignature();
    jws.setPayload(claims.toJson());
    jws.setKey(key);
    jws.setAlgorithmHeaderValue(AlgorithmIdentifiers.HMAC_SHA256);
    // For backwards compatibility with old app secrets
    jws.setDoKeyValidation(false);
    try {
      String jwt = jws.getCompactSerialization();
      return jwt;
    } catch (JoseException e) {
      e.printStackTrace();
    }
    return null;
  }

}