问如何从Java中的文件生成PublicKey对象

EN

public PublicKey reconstruct_public_key(String algorithm, byte[] pub_key) {
    PublicKey public_key = null;

    try {
        KeyFactory kf = KeyFactory.getInstance(algorithm);
        EncodedKeySpec pub_key_spec = new X509EncodedKeySpec(pub_key);
        public_key = kf.generatePublic(pub_key_spec);
    } catch(NoSuchAlgorithmException e) {
        System.out.println("Could not reconstruct the public key, the given algorithm oculd not be found.");
    } catch(InvalidKeySpecException e) {
        System.out.println("Could not reconstruct the public key");
    }

    return public_key;
}

public static void main(String args[]) {
    String path = "./pub_key_test.txt";

    // Generate a keypair to write to file
    KeyPair kp = generate_key();
    PublicKey pub_key = kp.getPublic();
    File file = new File(path);

    try {
        // Write to file
        file.createNewFile();
        FileOutputStream out = new FileOutputStream(path);

        out.write(pub_key.getEncoded()); // Write public key to the file
        out.close();

        // Read from file
        FileInputStream in = new FileInputStream(path);
        byte[] pub_key_arr = new byte[in.available()];
        in.read(pub_key_arr, 0, in.available());
        in.close();

        // Reconstruct public key
        PublicKey reconstructed_pub_key = reconstruct_public_key("RSA", pub_key_arr);
    } catch(IOException e) {
        System.out.println("Could not open the file : " + e.getStackTrace());
    }
}

public KeyPair generate_key() {
    while(true) { // Else the compiler will complain that this procedure does not always return a "KeyPair"
        try {
            final KeyPairGenerator key_generator = KeyPairGenerator.getInstance("RSA");
            key_generator.initialize(2048); // Keys of 2048 bits (minimum key length for RSA keys) are safe enough (according to the slides 128bit keys > 16 years to brute force it)

            final KeyPair keys = key_generator.generateKeyPair();
            return keys;
        } catch(NoSuchAlgorithmException e) {
            System.out.println("The given encryption algorithm (RSA) does not exist. -- generate_key() - Cryptography.");
        }
    }
}

openssl rsa -in private_key_file.pem -pubout -outform DER -out java_readable_file.der

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.PublicKey;
import java.security.cert.CertificateException;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.X509EncodedKeySpec;

/**
 * This class is capable of extracting a public key from a X.509 certficate 
 * and returning the PublicKey representation from a referenced byte array.
 * 
 */
public class ExtractPublicKey {

  // Certificate Filename (Including Path Info)
  private static final String certFilename = "cacert.pem";

  // Public Key Filename (Including Path Info)
  private static final String pubKeyFilename = "rsapublic.key";

  public static PublicKey generatePublicKey(byte[] encodedKey)
      throws NoSuchAlgorithmException, InvalidKeySpecException {

    X509EncodedKeySpec pubSpec = new X509EncodedKeySpec(encodedKey);
    boolean isSupportedKey = false;
    KeyFactory factory;
    PublicKey retKey = null;

    //first try the DSA alg
    try {
      factory = KeyFactory.getInstance("DSA");
      retKey = factory.generatePublic(pubSpec);
      isSupportedKey = true;
    } catch (InvalidKeySpecException e) {
      System.out.println("Could not create DSA Public Key: " + e.toString());      
    }

    //if DSA didnt work, then try RSA    
    if (!isSupportedKey) {
      try {
        factory = KeyFactory.getInstance("RSA");
        retKey = factory.generatePublic(pubSpec);
        isSupportedKey = true;
      } catch (InvalidKeySpecException e) {
        System.out.println("Could not create RSA Public Key: " + e.toString());
      }      
    }

    // if not DSA or RSA
    if (!isSupportedKey) {
      throw new InvalidKeySpecException("Unsupported key spec: Not RSA or DSA");
    }

    return retKey;
  }   

}