How to Load an RSA Private Key from a File and Avoid the \"InvalidKeySpecException\"?

How to Load an RSA Private Key from a File

When working with RSA private keys in Java, it is often necessary to load them from a file. This can be done using the java.security package. However, sometimes errors can occur during this process.

One common error is:

java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: invalid key format

This typically indicates that the private key file is not in the correct format. The private key must be in PKCS8 format.

To convert your private key to PKCS8 format, you can use the following command:

openssl pkcs8 -topk8 -inform PEM -outform DER -in private_key_file  -nocrypt > pkcs8_key

After running this command, the converted private key will be saved in the pkcs8_key file. You can then load this file into your Java program using the following code:

<code class="java">import java.io.FileInputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.InvalidKeySpecException;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;

public class LoadRsaPrivateKeyFromFile {

    public static void main(String[] args) throws IOException, NoSuchAlgorithmException, InvalidKeySpecException {
        // Read the converted private key file
        byte[] keyBytes = Files.readAllBytes(Paths.get("pkcs8_key"));

        // Create a PKCS8EncodedKeySpec to wrap the key bytes
        PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(keyBytes);

        // Create a KeyFactory to generate the private key
        KeyFactory factory = KeyFactory.getInstance("RSA");

        // Generate the private key
        PrivateKey privateKey = factory.generatePrivate(spec);
        
        // Use the private key to sign a message
        // ...
    }
}</code>

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