c# - 解密 mime 内容时出错 - 遇到 ASN1 错误标签值

Question

我编写了一个函数来创建 mime 消息并使用存储在证书存储中的用户公共证书加密内容，而私钥存储在我的智能卡上。我的函数只是将 pkcs7mime 内容保存到文件中。我有另一个测试功能，它只是读取文件并尝试解密内容。但是，当我尝试解密时，出现错误：ASN1 bad tag value met。

我的加密功能代码：

private static void EncryptMime()
    {
        MimeMessage mm = new MimeMessage();
        var ctx = new WindowsSecureMimeContext();
        mm.From.Add(new MailboxAddress("Bob", "[e-mail]"));
        mm.To.Add(new MailboxAddress("Alice", "[e-mail]"));

        mm.Subject = "Smime Test";

        mm.Body = new TextPart("plain")
        {
            Text = @"This is a test."
        };

        var signer = mm.From.Mailboxes.FirstOrDefault();
        var recipients = mm.To.Mailboxes.AsEnumerable();
        ApplicationPkcs7Mime p7m = ApplicationPkcs7Mime.SignAndEncrypt(ctx, signer, DigestAlgorithm.Sha1, recipients, mm.Body);

        MemoryStream ms = new MemoryStream();
        p7m.WriteTo(ms, true);
        byte[] bytes = ms.GetBuffer();
        File.WriteAllBytes("smime.p7m", bytes);
    }

这是我的解密功能：

private static void DecryptMime()
    {

        CryptographyContext.Register(typeof(WindowsSecureMimeContext));

        string messagetext = "";
        // Read the p7m file           
        byte[] bytes = 
        File.ReadAllBytes("smime.p7m");
        MemoryStream ms = new MemoryStream(bytes);
        ApplicationPkcs7Mime p7m = new ApplicationPkcs7Mime(SecureMimeType.EnvelopedData, ms);
        // WindowsSecureMimeContext ctx = new WindowsSecureMimeContext(StoreLocation.CurrentUser);

        if (p7m != null && p7m.SecureMimeType == SecureMimeType.EnvelopedData)
        {
            Console.WriteLine("Decrypting message...");
            try
            {
                p7m = p7m.Decrypt() as ApplicationPkcs7Mime;
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error decrypting: " + ex.Message);
            }
        }

        if (p7m != null && p7m.SecureMimeType == SecureMimeType.CompressedData)
        {
            Console.WriteLine("Decompressing message...");
            p7m = p7m.Decompress() as ApplicationPkcs7Mime;
        }

        if (p7m != null && p7m.SecureMimeType == SecureMimeType.SignedData)
        {
            Console.WriteLine("Verifying message...");
            p7m.Verify(out MimeEntity entity);
            MimeMessage mm = new MimeMessage(entity);
            messagetext = mm.GetTextBody(MimeKit.Text.TextFormat.Text);
            Console.WriteLine("Decrypted Message: " + messagetext);
        }           
    }

编辑：我尝试使用以下代码隔离问题：

MemoryStream ms = new MemoryStream();
        p7m.WriteTo(ms);    
ApplicationPkcs7Mime new_p7m = new ApplicationPkcs7Mime(SecureMimeType.EnvelopedData, ms);  

        if (new_p7m != null && new_p7m.SecureMimeType == SecureMimeType.EnvelopedData)
        {
            Console.WriteLine("Decrypting message...");
            try
            {
                new_p7m = new_p7m.Decrypt() as ApplicationPkcs7Mime;
            }
            catch (Exception ex)
            {
                Console.WriteLine("Error decrypting: " + ex.Message);
            }
        }

所以问题似乎早在 WriteTo() 函数就开始了......

更新#2：好的，所以我在 p7m 和 new_p7m 对象上都做了一个 toString。貌似内容不一样。。。

p7m: Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
    name=smime.p7m
Content-Disposition: attachment; filename=smime.p7m
Content-Transfer-Encoding: base64

MIIOOwYJKoZIhvcNAQcDoIIOLDCCDigCAQIxggEwMIIBLAIBAoAU2NQqDvYHJuMeC27IpyiV
....

New p7m: Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
    name=smime.p7m
Content-Disposition: attachment; filename=smime.p7m
Content-Transfer-Encoding: base64

Q29udGVudC1UeXBlOiBhcHBsaWNhdGlvbi9wa2NzNy1taW1lOyBzbWltZS10eXBlPWVudmVs
....

到，不知何故，实际内容正在改变……很奇怪……

标签： c#mimekit

Answer 1

我打赌您将内容保存到磁盘的代码正在写入损坏的文件：

MemoryStream ms = new MemoryStream();
p7m.WriteTo(ms, true);
byte[] bytes = ms.GetBuffer();
File.WriteAllBytes("smime.p7m", bytes);

上述代码的问题在于，GetBuffer()在大多数情况下，返回的字节数比内存流实际使用的字节数多。上述逻辑还将内容保存在 base64 中，这可能不是您想要的。以下代码片段会将 base64 内容解码为 smime.p7m 文件：

using (var stream = File.Create ("smime.p7m"))
    p7m.Content.DecodeTo (stream);

然后，要重建 MIME 部分以进行解密，请执行以下操作：

var bytes = File.ReadAllBytes ("smime.p7m");
var ms = new MemoryStream (bytes);
var p7m = new ApplicationPkcs7Mime (SecureMimeType.EnvelopedData, ms);

您面临的问题是内容之前是 base64 编码的，而不应该是。

当然，除非真的需要只保存原始内容，然后重新构建 application/pkcs7-mime 部分，否则为什么不像这样保存整个 MIME 部分呢？

using (var stream = File.Create ("smime.p7m"))
    p7m.WriteTo (stream);

然后像这样重新加载它：

using (var stream = File.OpenRead ("smime.p7m"))
    p7m = (ApplicationPkcs7Mime) MimeEntity.Load (stream);

这样你就不必担心会出错。