How to sign string with private key

public static String sign(String plainText, PrivateKey privateKey) throws Exception {
    Signature privateSignature = Signature.getInstance("SHA256withRSA");
    privateSignature.initSign(privateKey);
    privateSignature.update(plainText.getBytes(UTF_8));

    byte[] signature = privateSignature.sign();

    return Base64.getEncoder().encodeToString(signature);
}

public static boolean verify(String plainText, String signature, PublicKey publicKey) throws Exception {
    Signature publicSignature = Signature.getInstance("SHA256withRSA");
    publicSignature.initVerify(publicKey);
    publicSignature.update(plainText.getBytes(UTF_8));

    byte[] signatureBytes = Base64.getDecoder().decode(signature);

    return publicSignature.verify(signatureBytes);
}

I guess what you say is you know the key pair before hand and want to sign/verify with that.

Please see the following code.

import java.security.KeyPair;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.Signature;

import sun.misc.BASE64Encoder;

public class MainClass {
    public static void main(String[] args) throws Exception {

        KeyPair keyPair = getKeyPair();

        byte[] data = "test".getBytes("UTF8");

        Signature sig = Signature.getInstance("SHA1WithRSA");
        sig.initSign(keyPair.getPrivate());
        sig.update(data);
        byte[] signatureBytes = sig.sign();
        System.out.println("Signature:" + new BASE64Encoder().encode(signatureBytes));

        sig.initVerify(keyPair.getPublic());
        sig.update(data);

        System.out.println(sig.verify(signatureBytes));
    }

    private static KeyPair getKeyPair() throws NoSuchAlgorithmException {
        KeyPairGenerator kpg = KeyPairGenerator.getInstance("RSA");
        kpg.initialize(1024);
        return kpg.genKeyPair();
    }
}

Here you need to change the method getKeyPair() to supply your known key pair. You may load it from a java key store [JKS].

You can't just have an arbitrary byte array either as your public key or private key. They should be generated in relation.

I use bouncy-castle to sign data and verify it.

you should add maven dependency:

<dependency>
    <groupId>org.bouncycastle</groupId>
    <artifactId>bcprov-jdk15on</artifactId>
    <version>1.56</version>
</dependency>
<dependency>
    <groupId>org.bouncycastle</groupId>
    <artifactId>bcpkix-jdk15on</artifactId>
    <version>1.56</version>
</dependency>

Load RSA private or public key from a disk file into a Java object

First, we need to be able to load RSA private or public key from a disk file into a Java object of a proper class from Bouncy Castle

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import org.apache.commons.lang3.Validate;
import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.asn1.x509.SubjectPublicKeyInfo;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.crypto.util.PrivateKeyFactory;
import org.bouncycastle.crypto.util.PublicKeyFactory;
import org.bouncycastle.openssl.PEMKeyPair;
import org.bouncycastle.openssl.PEMParser;

public class KeyUtil {

    public static AsymmetricKeyParameter loadPublicKey(InputStream is) {
        SubjectPublicKeyInfo spki = (SubjectPublicKeyInfo) readPemObject(is);
        try {
            return PublicKeyFactory.createKey(spki);
        } catch (IOException ex) {
            throw new RuntimeException("Cannot create public key object based on input data", ex);
        }
    }

    public static AsymmetricKeyParameter loadPrivateKey(InputStream is) {
        PEMKeyPair keyPair = (PEMKeyPair) readPemObject(is);
        PrivateKeyInfo pki = keyPair.getPrivateKeyInfo();
        try {
            return PrivateKeyFactory.createKey(pki);
        } catch (IOException ex) {
            throw new RuntimeException("Cannot create private key object based on input data", ex);
        }
    }

    private static Object readPemObject(InputStream is) {
        try {
            Validate.notNull(is, "Input data stream cannot be null");
            InputStreamReader isr = new InputStreamReader(is, "UTF-8");
            PEMParser pemParser = new PEMParser(isr);

            Object obj = pemParser.readObject();
            if (obj == null) {
                throw new Exception("No PEM object found");
            }
            return obj;
        } catch (Throwable ex) {
            throw new RuntimeException("Cannot read PEM object from input data", ex);
        }
    }
}

---

Creation of an RSA digital signature

        // GIVEN: InputStream prvKeyInpStream
    AsymmetricKeyParameter privKey = KeyUtil.loadPrivateKey(prvKeyInpStream);

    // GIVEN: byte[] messageBytes = ...
    RSADigestSigner signer = new RSADigestSigner(new SHA512Digest());
    signer.init(true, privKey);
    signer.update(messageBytes, 0, messageBytes.length);

    try {
        byte[] signature = signer.generateSignature();
    } catch (Exception ex) {
        throw new RuntimeException("Cannot generate RSA signature. " + ex.getMessage(), ex);
    }

---

Verification of an RSA digital signature

// GIVEN: InputStream pubKeyInpStream
AsymmetricKeyParameter publKey = KeyUtil.loadPublicKey(pubKeyInpStream);

// GIVEN: byte[] messageBytes
RSADigestSigner signer = new RSADigestSigner(new SHA512Digest());
signer.init(false, publKey);
signer.update(messageBytes, 0, messageBytes.length);

// GIVEN: byte[] signature - see code sample above
boolean isValidSignature = signer.verifySignature(signature);