如何從Java中的文件生成PublicKey對象

Question

我有一個包含公共RSA密鑰的文件（使用ssh-keygen生成）。 我想讀取該文件並生成一個PublicKey對象。

在此之前我轉換了文件，因為閱讀原始文件似乎是不可能的：

# http://unix.stackexchange.com/questions/220354/how-to-convert-public-key-from-pem-to-der-format/220356#220356
ssh-keygen -f ~/.ssh/id_rsa.pub -e -m PEM > ~/.ssh/id_rsa.pub.pem
openssl rsa -RSAPublicKey_in -in ~/.ssh/id_rsa.pub.pem -inform PEM -outform DER -out ~/.ssh/id_rsa.pub.der -RSAPublicKey_out

從Java - 使用現有公鑰文件加密String我定義了函數readFileBytes ：

public static byte[] readFileBytes(String filename) throws IOException {
    Path path = Paths.get(System.getProperty("user.home") + filename);
    return Files.readAllBytes(path);
}

現在我想讀取文件並生成PublicKey對象，但我找不到辦法做到這一點; java.security.spec.RSAPublicKeySpec不提供適合的構造函數， java.security.spec.X509EncodedKeySpec拋出錯誤java.security.spec.InvalidKeySpecException: java.security.InvalidKeyException: IOException: algid parse error, not a sequence ：

//RSAPublicKeySpec publicSpec = new RSAPublicKeySpec(readFileBytes("/.ssh/id_rsa.pub.der"));
// No fitting construktor

X509EncodedKeySpec publicSpec = new X509EncodedKeySpec(readFileBytes("/.ssh/id_rsa.pub.der"));
// Gives: "algid parse error, not a sequence"

Answer 1

我有一個項目，其中（RSA）加密是必要的，這是我如何重建publicKey給定publicKey的byte數組，只是從文件中讀取。

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;
}

然后你可以調用類似於這個調用的過程， reconstruct_public_key("RSA", readFileBytes("path/to/your/publicKey/file"));

編輯：我自己嘗試（將公鑰寫入文件，讀取該文件並重建密鑰）。 這有效：

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());
    }
}

這是generate_key過程：

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.");
        }
    }
}

如果您對此進行測試，您將看到publicKey已成功重建。

編輯：我嘗試使用ssh-keygen工具自己做。 這就是我做的：

• 首先我生成了一個RSA私鑰（ .PEM格式）
• 將公鑰部分輸出為.DER格式，因此Java可以使用它。

這就是我進行轉換的方式，與您的轉換有點不同：

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

我在這里做了文件讀取，這與你的差別不大。 我測試了這個並且Java成功地重建了公鑰。

Answer 2

---

創建RSA私鑰

openssl genrsa -out rsaprivkey.pem 1024

生成DER格式的公鑰。

openssl rsa -in rsaprivkey.pem -pubout -outform DER -out rsapubkey.der

我們使用此代碼從X.509證書中提取公鑰RSA或DSA。

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;
  }   

}