Java sign / verify with RSA key pair

README.md

# generate RSA key pairs
$ openssl genrsa -out private.pem 2048

# generate PKCS8 private key
$ openssl pkcs8 -topk8 -inform PEM -outform PEM -in private.pem -out private_key.pem -nocrypt

# generate public key
$ openssl rsa -in private.pem -outform PEM -pubout -out public.pem

Demo.java

public class Demo {
    public static void main(String[] args) throws Exception {
        String plainText = "Hello World";
        String signature = sign(plainText, readPrivateKey(new File("/Users/bfan01/Desktop/private_key.pem")));
        System.out.println(signature);
        System.out.println(verify(plainText, signature, readPublicKey(new File("/Users/bfan01/Desktop/public.pem"))));
    }

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

        byte[] signature = privateSignature.sign();

        return Base64.encodeBase64String(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(StandardCharsets.UTF_8));

        byte[] signatureBytes = Base64.decodeBase64(signature);

        return publicSignature.verify(signatureBytes);
    }

    public static RSAPrivateKey readPrivateKey(File file) throws Exception {
        String key = Files.readString(file.toPath(), Charset.defaultCharset());

        String privateKeyPEM = key
                .replace("-----BEGIN PRIVATE KEY-----", "")
                .replaceAll(System.lineSeparator(), "")
                .replace("-----END PRIVATE KEY-----", "");

        byte[] encoded = Base64.decodeBase64(privateKeyPEM);

        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(encoded);
        return (RSAPrivateKey) keyFactory.generatePrivate(keySpec);
    }

    public static RSAPublicKey readPublicKey(File file) throws Exception {
        String key = Files.readString(file.toPath(), Charset.defaultCharset());

        String publicKeyPEM = key
                .replace("-----BEGIN PUBLIC KEY-----", "")
                .replaceAll(System.lineSeparator(), "")
                .replace("-----END PUBLIC KEY-----", "");

        byte[] encoded = Base64.decodeBase64(publicKeyPEM);

        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        X509EncodedKeySpec keySpec = new X509EncodedKeySpec(encoded);
        return (RSAPublicKey) keyFactory.generatePublic(keySpec);
    }
}