如何：用对称密钥对 XML 元素进行加密

2023-05-10

可以使用 System.Security.Cryptography.Xml 命名空间中的类加密 XML 文档内的元素。 XML 加密可用于存储或传输敏感 XML，而无需担心数据被轻易读取。此过程使用高级加密标准 (AES) 算法对 XML 元素进行加密。

有关如何解密使用此过程加密的 XML 元素的信息，请参阅如何：用对称密钥解密 XML 元素。

当使用对称算法（如 AES）来加密 XML 数据时，必须使用相同的密钥来加密和解密 XML 数据。此过程中的示例假定加密的 XML 将使用相同的密钥进行解密，且加密方和解密方就要使用的算法和密钥达成了一致。此示例不对加密的 XML 内的 AES 密钥进行存储或加密。

此示例适用于以下情况：单个应用程序需要根据内存中存储的会话密钥加密数据，或根据从密码派生而来的加密型强密钥进行加密。对于两个或多个应用程序需要共享加密的 XML 数据的情况，请考虑使用基于非对称算法或 X.509 证书的加密方案。

使用对称密钥加密 XML 元素

使用 Aes 类生成对称密钥。此密钥将用于加密 XML 元素。

Aes? key = null;

try
{
    // Create a new AES key.
    key = Aes.Create();

Dim key As Aes = Nothing

Try
    ' Create a new Aes key.
    key = Aes.Create()

通过从磁盘加载 XML 文件来创建 XmlDocument 对象。 XmlDocument 对象包含要加密的 XML 元素。

// Load an XML document.
XmlDocument xmlDoc = new()
{
    PreserveWhitespace = true
};
xmlDoc.Load("test.xml");

' Load an XML document.
Dim xmlDoc As New XmlDocument()
xmlDoc.PreserveWhitespace = True
xmlDoc.Load("test.xml")

在 XmlDocument 对象中查找指定元素，并创建一个新的 XmlElement 对象来表示想要加密的元素。在此示例中，加密了 "creditcard" 元素。

XmlElement? elementToEncrypt = Doc.GetElementsByTagName(ElementName)[0] as XmlElement;

Dim elementToEncrypt As XmlElement = Doc.GetElementsByTagName(ElementName)(0)

创建 EncryptedXml 类的新实例，并将其与对称密钥一起使用来加密 XmlElement。 EncryptData 方法以加密的字节数组的形式返回加密元素。

EncryptedXml eXml = new();

byte[] encryptedElement = eXml.EncryptData(elementToEncrypt, Key, false);

Dim eXml As New EncryptedXml()

Dim encryptedElement As Byte() = eXml.EncryptData(elementToEncrypt, Key, False)

构造一个 EncryptedData 对象并对其填充 XML 加密元素的 URL 标识符。此 URL 标识符可使解密方知道 XML 包含一个加密元素。可使用 XmlEncElementUrl 字段来指定 URL 标识符。
```
EncryptedData edElement = new()
{
    Type = EncryptedXml.XmlEncElementUrl
};
```
```
Dim edElement As New EncryptedData()
edElement.Type = EncryptedXml.XmlEncElementUrl
```

创建一个 EncryptionMethod 对象，将它初始化为用于生成密钥的加密算法的 URL 标识符。将 EncryptionMethod 对象传递给 EncryptionMethod 属性。

string? encryptionMethod;
if (Key is Aes)
{
    encryptionMethod = EncryptedXml.XmlEncAES256Url;
}
else
{
    // Throw an exception if the transform is not AES
    throw new CryptographicException("The specified algorithm is not supported or not recommended for XML Encryption.");
}

edElement.EncryptionMethod = new EncryptionMethod(encryptionMethod);

Dim encryptionMethod As String = Nothing

If TypeOf Key Is Aes Then
    encryptionMethod = EncryptedXml.XmlEncAES256Url
Else
    ' Throw an exception if the transform is not in the previous categories
    Throw New CryptographicException("The specified algorithm is not supported or not recommended for XML Encryption.")
End If

edElement.EncryptionMethod = New EncryptionMethod(encryptionMethod)

将加密的元素数据添加到 EncryptedData 对象。

edElement.CipherData.CipherValue = encryptedElement;

edElement.CipherData.CipherValue = encryptedElement

将原始 XmlDocument 对象中的元素替换为 EncryptedData 元素。

EncryptedXml.ReplaceElement(elementToEncrypt, edElement, false);

EncryptedXml.ReplaceElement(elementToEncrypt, edElement, False)

示例

<root>  
    <creditcard>  
        <number>19834209</number>  
        <expiry>02/02/2002</expiry>  
    </creditcard>  
</root>

using System;
using System.Xml;
using System.Security.Cryptography;
using System.Security.Cryptography.Xml;

namespace CSCrypto
{
    class Program
    {
        static void Main(string[] args)
        {
            Aes? key = null;

            try
            {
                // Create a new AES key.
                key = Aes.Create();
                // Load an XML document.
                XmlDocument xmlDoc = new()
                {
                    PreserveWhitespace = true
                };
                xmlDoc.Load("test.xml");

                // Encrypt the "creditcard" element.
                Encrypt(xmlDoc, "creditcard", key);

                Console.WriteLine("The element was encrypted");

                Console.WriteLine(xmlDoc.InnerXml);

                Decrypt(xmlDoc, key);

                Console.WriteLine("The element was decrypted");

                Console.WriteLine(xmlDoc.InnerXml);
            }
            catch (Exception e)
            {
                Console.WriteLine(e.Message);
            }
            finally
            {
                key?.Clear();
            }
        }

        public static void Encrypt(XmlDocument Doc, string ElementName, SymmetricAlgorithm Key)
        {
            // Check the arguments.
            ArgumentNullException.ThrowIfNull(Doc);
            ArgumentNullException.ThrowIfNull(ElementName);
            ArgumentNullException.ThrowIfNull(Key);

            ////////////////////////////////////////////////
            // Find the specified element in the XmlDocument
            // object and create a new XmlElement object.
            ////////////////////////////////////////////////
            XmlElement? elementToEncrypt = Doc.GetElementsByTagName(ElementName)[0] as XmlElement;
            // Throw an XmlException if the element was not found.
            if (elementToEncrypt == null)
            {
                throw new XmlException("The specified element was not found");
            }

            //////////////////////////////////////////////////
            // Create a new instance of the EncryptedXml class
            // and use it to encrypt the XmlElement with the
            // symmetric key.
            //////////////////////////////////////////////////

            EncryptedXml eXml = new();

            byte[] encryptedElement = eXml.EncryptData(elementToEncrypt, Key, false);
            ////////////////////////////////////////////////
            // Construct an EncryptedData object and populate
            // it with the desired encryption information.
            ////////////////////////////////////////////////

            EncryptedData edElement = new()
            {
                Type = EncryptedXml.XmlEncElementUrl
            };

            // Create an EncryptionMethod element so that the
            // receiver knows which algorithm to use for decryption.
            // Determine what kind of algorithm is being used and
            // supply the appropriate URL to the EncryptionMethod element.

            string? encryptionMethod;
            if (Key is Aes)
            {
                encryptionMethod = EncryptedXml.XmlEncAES256Url;
            }
            else
            {
                // Throw an exception if the transform is not AES
                throw new CryptographicException("The specified algorithm is not supported or not recommended for XML Encryption.");
            }

            edElement.EncryptionMethod = new EncryptionMethod(encryptionMethod);

            // Add the encrypted element data to the
            // EncryptedData object.
            edElement.CipherData.CipherValue = encryptedElement;

            ////////////////////////////////////////////////////
            // Replace the element from the original XmlDocument
            // object with the EncryptedData element.
            ////////////////////////////////////////////////////
            EncryptedXml.ReplaceElement(elementToEncrypt, edElement, false);
        }

        public static void Decrypt(XmlDocument Doc, SymmetricAlgorithm Alg)
        {
            // Check the arguments.
            ArgumentNullException.ThrowIfNull(Doc);
            ArgumentNullException.ThrowIfNull(Alg);

            // Find the EncryptedData element in the XmlDocument.
            XmlElement? encryptedElement = Doc.GetElementsByTagName("EncryptedData")[0] as XmlElement;

            // If the EncryptedData element was not found, throw an exception.
            if (encryptedElement == null)
            {
                throw new XmlException("The EncryptedData element was not found.");
            }

            // Create an EncryptedData object and populate it.
            EncryptedData edElement = new();
            edElement.LoadXml(encryptedElement);

            // Create a new EncryptedXml object.
            EncryptedXml exml = new();

            // Decrypt the element using the symmetric key.
            byte[] rgbOutput = exml.DecryptData(edElement, Alg);

            // Replace the encryptedData element with the plaintext XML element.
            exml.ReplaceData(encryptedElement, rgbOutput);
        }
    }
}

Imports System.Xml
Imports System.Security.Cryptography
Imports System.Security.Cryptography.Xml

Module Program

    Sub Main(ByVal args() As String)
        Dim key As Aes = Nothing

        Try
            ' Create a new Aes key.
            key = Aes.Create()
            ' Load an XML document.
            Dim xmlDoc As New XmlDocument()
            xmlDoc.PreserveWhitespace = True
            xmlDoc.Load("test.xml")
            ' Encrypt the "creditcard" element.
            Encrypt(xmlDoc, "creditcard", key)

            Console.WriteLine("The element was encrypted")
            Console.WriteLine(xmlDoc.InnerXml)

            Decrypt(xmlDoc, key)

            Console.WriteLine("The element was decrypted")
            Console.WriteLine(xmlDoc.InnerXml)


        Catch e As Exception
            Console.WriteLine(e.Message)
        Finally
            ' Clear the key.
            If Not (key Is Nothing) Then
                key.Clear()
            End If
        End Try

    End Sub


    Sub Encrypt(ByVal Doc As XmlDocument, ByVal ElementName As String, ByVal Key As SymmetricAlgorithm)
        ' Check the arguments.  
        ArgumentNullException.ThrowIfNull(Doc)
        ArgumentNullException.ThrowIfNull(ElementName)
        ArgumentNullException.ThrowIfNull(Key)

        ''''''''''''''''''''''''''''''''''''''''''''''''''
        ' Find the specified element in the XmlDocument
        ' object and create a new XmlElemnt object.
        ''''''''''''''''''''''''''''''''''''''''''''''''''
        Dim elementToEncrypt As XmlElement = Doc.GetElementsByTagName(ElementName)(0)

        ' Throw an XmlException if the element was not found.
        If elementToEncrypt Is Nothing Then
            Throw New XmlException("The specified element was not found")
        End If

        ''''''''''''''''''''''''''''''''''''''''''''''''''
        ' Create a new instance of the EncryptedXml class 
        ' and use it to encrypt the XmlElement with the 
        ' symmetric key.
        ''''''''''''''''''''''''''''''''''''''''''''''''''
        Dim eXml As New EncryptedXml()

        Dim encryptedElement As Byte() = eXml.EncryptData(elementToEncrypt, Key, False)
        ''''''''''''''''''''''''''''''''''''''''''''''''''
        ' Construct an EncryptedData object and populate
        ' it with the desired encryption information.
        ''''''''''''''''''''''''''''''''''''''''''''''''''
        Dim edElement As New EncryptedData()
        edElement.Type = EncryptedXml.XmlEncElementUrl
        ' Create an EncryptionMethod element so that the 
        ' receiver knows which algorithm to use for decryption.
        ' Determine what kind of algorithm is being used and
        ' supply the appropriate URL to the EncryptionMethod element.
        Dim encryptionMethod As String = Nothing

        If TypeOf Key Is Aes Then
            encryptionMethod = EncryptedXml.XmlEncAES256Url
        Else
            ' Throw an exception if the transform is not in the previous categories
            Throw New CryptographicException("The specified algorithm is not supported or not recommended for XML Encryption.")
        End If

        edElement.EncryptionMethod = New EncryptionMethod(encryptionMethod)
        ' Add the encrypted element data to the 
        ' EncryptedData object.
        edElement.CipherData.CipherValue = encryptedElement
        ''''''''''''''''''''''''''''''''''''''''''''''''''
        ' Replace the element from the original XmlDocument
        ' object with the EncryptedData element.
        ''''''''''''''''''''''''''''''''''''''''''''''''''
        EncryptedXml.ReplaceElement(elementToEncrypt, edElement, False)

    End Sub


    Sub Decrypt(ByVal Doc As XmlDocument, ByVal Alg As SymmetricAlgorithm)
        ' Check the arguments.  
        If Doc Is Nothing Then
            Throw New ArgumentNullException("Doc")
        End If
        If Alg Is Nothing Then
            Throw New ArgumentNullException("Alg")
        End If
        ' Find the EncryptedData element in the XmlDocument.
        Dim encryptedElement As XmlElement = Doc.GetElementsByTagName("EncryptedData")(0)

        ' If the EncryptedData element was not found, throw an exception.
        If encryptedElement Is Nothing Then
            Throw New XmlException("The EncryptedData element was not found.")
        End If


        ' Create an EncryptedData object and populate it.
        Dim edElement As New EncryptedData()
        edElement.LoadXml(encryptedElement)
        ' Create a new EncryptedXml object.
        Dim exml As New EncryptedXml()


        ' Decrypt the element using the symmetric key.
        Dim rgbOutput As Byte() = exml.DecryptData(edElement, Alg)
        ' Replace the encryptedData element with the plaintext XML element.
        exml.ReplaceData(encryptedElement, rgbOutput)
    End Sub
End Module

编译代码

在面向 .NET Framework 的项目中，包含对 System.Security.dll 的引用。
在面向 .NET Core 或 .NET 5 的项目中，安装 NuGet 包 System.Security.Cryptography.Xml。
包括以下命名空间：System.Xml、System.Security.Cryptography 和 System.Security.Cryptography.Xml。

.NET 安全性

永远不要以纯文本形式存储加密密钥，也不要以纯文本形式在计算机之间传输密钥。请转而使用安全的密钥容器来存储加密密钥。

当你使用加密密钥执行操作后，通过将每个字节设置为零或通过调用托管加密类的 Clear 方法来将它从内存中清除。

通过

如何：用对称密钥对 XML 元素进行加密

使用对称密钥加密 XML 元素

示例

编译代码

.NET 安全性

另请参阅

其他资源