我有一个.pem格式的私钥。内容以“begin rsa private key”（开始rsa私钥）开头，并且是ascii铠装的。
我需要在签名对象中使用此密钥，但它会抛出一个错误，说明密钥格式无效。请帮忙！

package com.scb;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.Signature;
import java.security.spec.PKCS8EncodedKeySpec;
import java.util.Base64;

public class Dec {

public static void main(String[] args) {

    String input = "hello";
    try {

     //Capturing the key file
     byte[] keyBytes = Files.readAllBytes(Paths.get("C:\\Users\\note3\\Desktop\\Key pair\\0x719D855F-sec.asc"));

        PKCS8EncodedKeySpec spec =
          new PKCS8EncodedKeySpec(keyBytes);

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

        Signature privateSignature = Signature.getInstance("SHA256withRSA");
        privateSignature.initSign(kf.generatePrivate(spec));
        privateSignature.update(input.getBytes("UTF-8"));
        byte[] s = privateSignature.sign();
        System.out.println(Base64.getEncoder().encodeToString(s));
    }
    catch(Exception e) {
        e.printStackTrace();
    }
}}

错误截图