如何从 Java 中的文件生成 PublicKey 对象

Question

我有一个包含公共 RSA 密钥的文件（使用 ssh-keygen 生成）。我想读取文件并生成一个PublicKey 对象。

在此之前我转换了文件，因为读取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

从Java - Encrypt String with existing public key file我定义了函数readFileBytes：

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

现在我想读取文件并生成 PublicKey 对象，但我找不到这样做的方法； java.security.spec.RSAPublicKeySpec 没有提供合适的构造函数，java.security.spec.X509EncodedKeySpec 抛出错误 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

---

创建 RSA 私钥

openssl genrsa -out rsaprivkey.pem 1024

生成 DER 格式的公钥。

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

我们使用此代码从 X.509 证书中提取公钥 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;
  }   

}

Answer 2

我有一个项目，其中需要 (RSA) 加密，这就是我在给定 publicKey 的 byte 数组的情况下重建 publicKey 的方式，该数组刚刚从文件中读取。

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;
}

然后你可以调用类似于这个调用的过程，reconstruct_public_key("RSA", readFileBytes("path/to/your/publicKey/file"));

编辑：我尝试自己做（将公钥写入文件，读取该文件并重建密钥）。这有效：

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());
    }
}

这是generate_key 程序：

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.");
        }
    }
}

如果您对此进行测试，您将看到publicKey 已成功重构。

编辑：我自己尝试过，使用ssh-keygen 工具。这就是我所做的：

• 首先我生成了一个 RSA 私钥（.PEM 格式）
• 将公钥部分输出为.DER格式，以便Java使用。

这就是我进行转换的方式，这与您的有点不同：

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

我的文件读起来像here，和你的差别不大。我对此进行了测试，Java 成功地重建了公钥。