C# 使用 RSA DER 格式公钥解密文件

Question

我见过很多问题，但没有一个真正提供好的答案，或者即使他们尝试了，他们也会假设提出问题的人并没有问他实际上在问什么。郑重声明，我不是在询问签名、加密或 PEM 文件!

就我而言，我将收到一个加密数据的小文件。它已使用 RSA key 对的私钥加密。

我可用的公钥是 ASN.1 格式的 DER 文件，它本身不是证书，也没有签名。

在 Java 中，这很容易，在 C# 中，据我所知这是一场噩梦。当我尝试将 X509Certificate2 解析为 key 的字节数组时，它声称“找不到对象”。

key 是使用以下 openSSL 命令生成的:

>openssl genrsa -out private_key.pem 2048

Generating RSA private key, 2048 bit long modulus
...........+++
....................................................................................................
...........................+++
e is 65537 (0x10001)

>openssl pkcs8 -topk8 -inform PEM -outform DER -in private_key.pem -out private_key.der -nocrypt

>openssl rsa -in private_key.pem -pubout -outform DER -out public_key.der
writing RSA key

编辑:我正在尝试的代码:

private byte[] ReadBytesFromStream(string streamName)
{
    using (Stream stream = this.GetType().Assembly.GetManifestResourceStream(streamName))
    {
        byte[] result = new byte[stream.Length];
        stream.Read(result, 0, (int) stream.Length);
        return result;
    }
}

public void LoadPublicKey()
{
    byte[] key = ReadBytesFromStream("my.namespace.Resources.public_key.der");
    X509Certificate2 myCertificate;
    try
    {
        myCertificate = new X509Certificate2(key);
    }
    catch (Exception e)
    {
        throw new CryptographicException(String.Format("Unable to open key file. {0}", e));
    } 
}

ReadBytesFromStream 方法确实返回了正确的字节流，public_key.der 文件是一个嵌入式资源。

Answer 1

只是为了完善这一点。虽然下面的代码仍然有点困惑，但它似乎是我可以开始工作的。

我们不能在 Windows 中使用公钥解密消息的原因解释如下: http://www.derkeiler.com/Newsgroups/microsoft.public.dotnet.security/2004-05/0270.html

接下来，您需要 C# 代码中的公钥模数和指数，在某个地方，无论是在文件中还是嵌入在此处真的不是问题。不过，我确实使用 Base64 来包装二进制 key 。我放弃了生成 Windows 实际导入的 key 。此外，对于我所需要的，公钥将是一个嵌入式资源。

我遇到的最后一个问题是从 key 生成的模数在 Windows (c#) 上不起作用。这篇 StackOverflow 帖子确实掌握了解决这个问题的关键，Windows 中的模数不能包含前导零。 Android in-app billing Verification of Receipt in Dot Net(C#)

从 Java 生成的模数在 C# 上不起作用。使用该帖子中的 stripLeadingZeros 代码创建一个有效的公钥模数:

  private static byte[] stripLeadingZeros(byte[] a) {
    int lastZero = -1;
    for (int i = 0; i < a.length; i++) {
      if (a[i] == 0) {
        lastZero = i;
      }
      else {
        break;
      }
    }
    lastZero++;
    byte[] result = new byte[a.length - lastZero];
    System.arraycopy(a, lastZero, result, 0, result.length);
    return result;
  }

对于初学者，这是我生成 key 文件的方式:

$ openssl genrsa -out private_key.pem 2048

$ openssl pkcs8 -topk8 -inform PEM -outform DER -in private_key.pem -out private_key.der -nocrypt

$ openssl rsa -in private_key.pem -pubout -outform DER -out public_key.der

Java 代码:

package com.example.signing;

import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.DataInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.IOException;
import java.security.KeyFactory;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.Signature;
import java.security.interfaces.RSAPrivateKey;
import java.security.interfaces.RSAPublicKey;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;

import jcifs.util.Base64;

public class SigningExample {
    private static String PRIVATE_KEY_FILE = "private_key.der";
    private static String PUBLIC_KEY_FILE  = "public_key.der";

    /**
     * @param args
     */
    public static void main(String[] args) {
        SigningExample.sign();
    }

    private static void sign() {
        try {
            String text = new String(SigningExample.loadFromFile("message.xml"));
            String message = Base64.encode(text.getBytes());

            RSAPrivateKey privateKey = PrivateRSAKeyReader.getKeyFile(SigningExample.PRIVATE_KEY_FILE);
            RSAPublicKey publicKey = PublicRSAKeyReader.getKeyFile(SigningExample.PUBLIC_KEY_FILE);

            byte[] modulusBytes = publicKey.getModulus().toByteArray();
            byte[] exponentBytes = publicKey.getPublicExponent().toByteArray();

            modulusBytes = SigningExample.stripLeadingZeros(modulusBytes);

            String modulusB64 = Base64.encode(modulusBytes);
            String exponentB64 = Base64.encode(exponentBytes);

            System.out.println("Copy these two into your c# code:");
            System.out.println("DotNet Modulus : " + modulusB64);
            System.out.println("DotNet Exponent: " + exponentB64);

            // Testing
            byte[] signature = SigningExample.sign(message.getBytes(), privateKey, "SHA1withRSA");

            String signedMessage = message + "\n" + Base64.encode(signature);

            SigningExample.saveToBase64File("message.signed", signedMessage.getBytes());

            System.out.println("\nMessage  :\n" + signedMessage);

            String[] newkey = new String(SigningExample.loadFromBase64File("message.signed")).split("\\n");
            System.out.println("Verified : " + SigningExample.verify(newkey[0].getBytes(), publicKey, "SHA1withRSA", Base64.decode(newkey[1])));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private static byte[] stripLeadingZeros(byte[] a) {
        int lastZero = -1;
        for (int i = 0; i < a.length; i++) {
            if (a[i] == 0) {
                lastZero = i;
            } else {
                break;
            }
        }
        lastZero++;
        byte[] result = new byte[a.length - lastZero];
        System.arraycopy(a, lastZero, result, 0, result.length);
        return result;
    }

    private static byte[] sign(byte[] data, PrivateKey prvKey,
            String sigAlg) throws Exception {
        Signature sig = Signature.getInstance(sigAlg);
        sig.initSign(prvKey);
        sig.update(data, 0, data.length);
        return sig.sign();
    }

    private static boolean verify(byte[] data, PublicKey pubKey,
            String sigAlg, byte[] sigbytes) throws Exception {
        Signature sig = Signature.getInstance(sigAlg);
        sig.initVerify(pubKey);
        sig.update(data, 0, data.length);
        return sig.verify(sigbytes);
    }

    public static void saveToBase64File(String fileName, byte[] data) throws IOException {
        BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(fileName));

        try {
            String b64 = Base64.encode(data);
            int lineLength = 64;
            int idx = 0;
            int len = b64.length();

            while (len - idx >= lineLength) {
                out.write(b64.substring(idx, idx + lineLength).getBytes());
                out.write('\n');
                idx += lineLength;
            }
            out.write(b64.substring(idx, len).getBytes());
        } catch (Exception e) {
            throw new IOException("Unexpected error", e);
        } finally {
            out.close();
        }
    }

    public static byte[] loadFromBase64File(String fileName) throws IOException {
        BufferedReader br = new BufferedReader(new FileReader(fileName));

        try {
            StringBuffer sb = new StringBuffer();

            String buffer;

            while ((buffer = br.readLine()) != null) {
                sb.append(buffer);
            }

            return Base64.decode(sb.toString());
        } catch (Exception e) {
            throw new IOException("Unexpected error", e);
        } finally {
            br.close();
        }
    }

    public static void saveToFile(String fileName, byte[] data) throws IOException {
        BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(fileName));

        try {
            out.write(data);
        } catch (Exception e) {
            throw new IOException("Unexpected error", e);
        } finally {
            out.close();
        }
    }

    public static byte[] loadFromFile(String fileName) throws IOException {
        BufferedReader br = new BufferedReader(new FileReader(fileName));

        try {
            StringBuffer sb = new StringBuffer();

            String buffer;

            while ((buffer = br.readLine()) != null) {
                sb.append(buffer);
            }

            return sb.toString().getBytes();
        } catch (Exception e) {
            throw new IOException("Unexpected error", e);
        } finally {
            br.close();
        }
    }
}

class PrivateRSAKeyReader {

    public static RSAPrivateKey getKeyFile(String filename) throws Exception {

        File f = new File(filename);
        FileInputStream fis = new FileInputStream(f);
        DataInputStream dis = new DataInputStream(fis);
        byte[] keyBytes = new byte[(int) f.length()];
        dis.readFully(keyBytes);
        dis.close();

        return PrivateRSAKeyReader.generateKey(keyBytes);
    }

    public static RSAPrivateKey getKey(String base64) throws Exception {
        byte[] keyBytes = Base64.decode(base64);

        return PrivateRSAKeyReader.generateKey(keyBytes);
    }

    private static RSAPrivateKey generateKey(byte[] keyBytes) throws Exception {
        PKCS8EncodedKeySpec spec = new PKCS8EncodedKeySpec(keyBytes);
        return (RSAPrivateKey) KeyFactory.getInstance("RSA").generatePrivate(spec);
    }
}

class PublicRSAKeyReader {

    public static RSAPublicKey getKeyFile(String filename) throws Exception {

        File f = new File(filename);
        FileInputStream fis = new FileInputStream(f);
        DataInputStream dis = new DataInputStream(fis);
        byte[] keyBytes = new byte[(int) f.length()];
        dis.readFully(keyBytes);
        dis.close();

        return PublicRSAKeyReader.generateKey(keyBytes);
    }

    public static RSAPublicKey getKey(String base64) throws Exception {
        byte[] keyBytes = Base64.decode(base64);

        return PublicRSAKeyReader.generateKey(keyBytes);
    }

    private static RSAPublicKey generateKey(byte[] keyBytes) throws Exception {
        X509EncodedKeySpec spec = new X509EncodedKeySpec(keyBytes);
        return (RSAPublicKey) KeyFactory.getInstance("RSA").generatePublic(spec);
    }
}

在 c# 中，这段代码片段应该可以解决问题:

String signedMessage = ""; // load Base64 coded the message generated in Java here.

byte[] message = Convert.FromBase64String(signedMessage);
String messageString = Encoding.ASCII.GetString(message);

String[] lines = Regex.Split(messageString, "\n");

byte[] content = Convert.FromBase64String(lines[0]); // first line of the message were the content
byte[] signature = Convert.FromBase64String(lines[1]); // second line were the signature

RSACryptoServiceProvider rsaObj = new RSACryptoServiceProvider(2048);

//Create a new instance of the RSAParameters structure.
RSAParameters rsaPars = new RSAParameters();

rsaPars.Modulus = Convert.FromBase64String("insert your modulus revealed in the Java example here");
rsaPars.Exponent = Convert.FromBase64String("AQAB"); // Exponent. Should always be this.

// Import key parameters into RSA.
rsaObj.ImportParameters(rsaPars);

// verify the message
Console.WriteLine(rsaObj.VerifyData(Encoding.ASCII.GetBytes(lines[0]), "SHA1", signature));

代码正在使用:

using System.Security;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Text;

我承认将最终消息打包到另一层 Base64 有点矫枉过正，应该改变。