Google service account JSON key to Google OAuth2 access token request

main.java

package io.inbot;


import com.github.jsonj.JsonObject;
import com.github.jsonj.tools.JsonParser;

import java.io.File;
import java.io.IOException;
import java.nio.charset.Charset;
import java.security.InvalidKeyException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.Signature;
import java.security.SignatureException;
import java.security.spec.EncodedKeySpec;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.time.Instant;
import java.util.Base64;
import java.util.Scanner;

import static com.github.jsonj.tools.JsonBuilder.field;
import static com.github.jsonj.tools.JsonBuilder.object;

public class Main {

    public static void main(String[] args) throws NoSuchAlgorithmException, InvalidKeySpecException, SignatureException, InvalidKeyException, IOException {
        System.out.println("Reading JSON key from: " + args[0]);
        String content = new Scanner(new File(args[0])).useDelimiter("\\Z").next();
        JsonParser jsonParser = new JsonParser();
        JsonObject jsonKey = jsonParser.parseObject(content);

        JsonObject headers = object(
                field("alg", "RS256"),
                field("typ", "JWT")
        );

        JsonObject claims = object(
                field("iss", jsonKey.getString("client_email")),
                field("scope", "https://www.googleapis.com/auth/devstorage.full_control"),
                field("aud", "https://www.googleapis.com/oauth2/v3/token"),
                field("exp", Instant.now().plusSeconds(60 * 55).getEpochSecond()),
                field("iat", Instant.now().getEpochSecond())
        );

        String encodedHeaders = new String(Base64.getUrlEncoder().encode(headers.toString().getBytes()), Charset.defaultCharset());
        String encodedClaims = new String(Base64.getUrlEncoder().encode(claims.toString().getBytes()), Charset.defaultCharset());
        String payload = encodedHeaders + "." + encodedClaims;

        System.out.println("Encoded headers: " + encodedHeaders);
        System.out.println("Encoded claims: " + encodedClaims);

        Signature sig = Signature.getInstance("SHA256withRSA");

        KeyFactory keyFactory = KeyFactory.getInstance("RSA");

        String privKeyString = jsonKey.getString("private_key");
        String privKeyPEM = privKeyString.replace("-----BEGIN PRIVATE KEY-----\n", "");
        privKeyPEM = privKeyPEM.replace("-----END PRIVATE KEY-----", "");
        privKeyPEM = privKeyPEM.replace("\n", "");
        System.out.println(privKeyPEM);

        EncodedKeySpec privKeySpec = new PKCS8EncodedKeySpec(Base64.getDecoder().decode(privKeyPEM));

        PrivateKey privateKey = keyFactory.generatePrivate(privKeySpec);
        System.out.println(privateKey);
        sig.initSign(privateKey);
        sig.update(payload.getBytes(Charset.defaultCharset()));
        String signature = new String(Base64.getUrlEncoder().encode(sig.sign()), Charset.defaultCharset());

        System.out.println("Signature: " + signature);

        System.out.println("JWT:");
        System.out.println(payload + "." + signature);

        System.out.println("curl -XPOST -d \"grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer&assertion=" + payload + "." + signature + "\" https://www.googleapis.com/oauth2/v3/token");
    }

}