Generares a Signed JWT token based on RSA private key pulled from local file

JWTGenerator.java

package com.emoran;

import java.io.IOException;
import java.net.URISyntaxException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.Signature;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.time.Instant;
import java.util.Base64;
import com.google.gson.Gson;
import com.google.gson.JsonObject;


public class JWTGenerator {

  private String privateKey;

  public static void main(String[] args) throws Exception {
    JWTGenerator generator = new JWTGenerator();
    PrivateKey privateKey = generator.getPrivateKey();

    Instant now = Instant.now();
    long timeSecs = (System.currentTimeMillis() / 1000);

    //General Claim
    JsonObject jsonClaim = new JsonObject();
    jsonClaim.addProperty("iss", "UUID");
    jsonClaim.addProperty("aud", "URL_HERE");
    jsonClaim.addProperty("scope", "admin_read admin_write");
    jsonClaim.addProperty("exp", timeSecs +  + 60); //Expires every 60 secs
    jsonClaim.addProperty("iat", timeSecs);

    Gson gson = new Gson();
    String serviceAccountAuthPayload = gson.toJson(jsonClaim);
    System.out.println("JSON Claim"+serviceAccountAuthPayload);

    String header = java.util.Base64.getUrlEncoder().encodeToString("{\"alg\":\"RS256\"}".getBytes());
    //System.out.println("header: "+header);
    String base64Claim = java.util.Base64.getUrlEncoder().encodeToString(serviceAccountAuthPayload.getBytes());
    System.out.println("base64Claim: "+base64Claim);
    String token = generator.signJWT(header,base64Claim,privateKey);
    System.out.println("Token is: "+token);
  }


  /**
    Signs the payload witht the RSA private key
  */
  public String signJWT (String header, String payload, PrivateKey privateKey) throws Exception{
    
    String token = header + "." + payload;
    Signature sig = Signature.getInstance("SHA256WithRSA");
    sig.initSign(privateKey);
    sig.update(token.getBytes());
    byte[] signature = sig.sign();

    System.out.println("SIGN: " +Base64.getUrlEncoder().encodeToString(signature));

    return token + "." + Base64.getUrlEncoder().encodeToString(signature);
  }

  /**
    Pulls the private Key from resources folder. if "IOException : algid parse error, not a sequence" happens then you need to migrate your pem
    file using this command openssl pkcs8 -topk8 -inform PEM -outform PEM -in original.pem -out migrated.pem -nocrypt
  */
  public PrivateKey getPrivateKey() throws IOException, NoSuchAlgorithmException, InvalidKeySpecException, URISyntaxException {

    String privateKeyContent = new String(Files.readAllBytes(Paths.get(ClassLoader.getSystemResource("privateKey.pem").toURI())));
    privateKeyContent = privateKeyContent.replaceAll("\\n", "").replace("-----BEGIN PRIVATE KEY-----", "").replace("-----END PRIVATE KEY-----", "");
    PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(Base64.getDecoder().decode(privateKeyContent));
    KeyFactory kf = KeyFactory.getInstance("RSA");
    return kf.generatePrivate(spec);
  }
}