Utility to generate RSA key pair in JWKS format. The program takes in two arguments - the key identifier and the modulus for the key and generates RSA key pair which is converted to JWKS format and then written to signing-keystone.jwks file.

RsaJwkGenerator.java

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.interfaces.RSAPrivateCrtKey;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;
import java.util.Base64;

/**
 * Utility to generate RSA JWKS key pair.
 */
public class RsaJwkGenerator {

    public static void main(String[] args) throws IOException {
        generateJwk(args[0], args[1]);
    }

    private static void generateJwk(String kid, String modulus) throws IOException {
        KeyPair keyPair;
        try {
            KeyPairGenerator keyPairGenerator = KeyPairGenerator.getInstance("RSA");
            int keySize = Integer.parseInt(modulus);
            keyPairGenerator.initialize(keySize);
            keyPair = keyPairGenerator.generateKeyPair();
        } catch (Exception ex) {
            throw new IllegalStateException(ex);
        }
        final RSAPublicKey publicKey = (RSAPublicKey) keyPair.getPublic();
        final RSAPrivateKey privateKey = (RSAPrivateKey) keyPair.getPrivate();

        String kty = "RSA";

        // process RSA public key
        String n = Base64.getUrlEncoder().encodeToString(publicKey.getModulus().toByteArray());
        String e = Base64.getUrlEncoder().encodeToString(publicKey.getPublicExponent().toByteArray());

        // process RSA private key
        String d = null;
        String p = null;
        String q = null;
        String dp = null;
        String dq = null;
        String qi = null;
        if (privateKey instanceof RSAPrivateCrtKey) {
            RSAPrivateCrtKey privateCrtKey = (RSAPrivateCrtKey) privateKey;
            d = Base64.getUrlEncoder().encodeToString(privateCrtKey.getPrivateExponent().toByteArray());
            p = Base64.getUrlEncoder().encodeToString(privateCrtKey.getPrimeP().toByteArray());
            q = Base64.getUrlEncoder().encodeToString(privateCrtKey.getPrimeQ().toByteArray());
            dp = Base64.getUrlEncoder().encodeToString(privateCrtKey.getPrimeExponentP().toByteArray());
            dq = Base64.getUrlEncoder().encodeToString(privateCrtKey.getPrimeExponentQ().toByteArray());
            qi = Base64.getUrlEncoder().encodeToString(privateCrtKey.getCrtCoefficient().toByteArray());
        } else {
            throw new IllegalStateException("Unable to generate RSA key");
        }

        // prepare JWKS
        String jwks = "{\"keys\":[{" + "\"e\":\"" + e + "\"," +
                "\"kty\":\"" + kty + "\"," +
                "\"n\":\"" + n + "\"," +
                "\"d\":\"" + d + "\"," +
                "\"p\":\"" + p + "\"," +
                "\"q\":\"" + q + "\"," +
                "\"dp\":\"" + dp + "\"," +
                "\"dq\":\"" + dq + "\"," +
                "\"qi\":\"" + qi + "\"," +
                "\"kid\":\"" + kid + "\"}]}";

        // write JWKS to file
        BufferedWriter writer = new BufferedWriter(new FileWriter("signing-keystore.jwks"));
        writer.write(jwks);
        writer.close();
    }
}