如何将私钥从十六进制转换为PrivateKey对象？

Question

I generated the following key hexstring for my private key:

""

How can I convert this back to a private key object in Java? I have tried converting this string back to a byte array but I'm clueless as to how I should continue.

This is how I converted my private key to a hex string:

public static String getHexString(byte[] b){
  String result = "";
  for (int i = 0; i < b.length; i++){
    result += Integer.toString((b[i] & 0xff) + 0x100, 16).substring(1);
   }
   return result;
 }

I then used the getEncoded() method on my private key as input for this method.

Answer 1

When you call getEncoded() on an RSA private key, most providers will return the private key in PKCS #8 format. This is the case with your example.

To reverse the process, use a KeyFactory with a PKCS8EncodedKeySpec as follows:

String key = "30820278020...1608"; // omitted for brevity

byte[] keyData = DatatypeConverter.parseHexBinary(key);
KeyFactory factory = KeyFactory.getInstance("RSA");
RSAPrivateKey privateKey = (RSAPrivateKey) factory
    .generatePrivate(new PKCS8EncodedKeySpec(keyData));

Signature signature = Signature.getInstance("SHA512withRSA");
signature.initSign(privateKey);
signature.update("Hello, World".getBytes());
System.out.println(DatatypeConverter.printHexBinary(signature.sign()));