解密数据
解密是加密的反向操作。 对于私钥加密，必须知道用于加密数据的密钥和 IV。 对于公钥加密，必须知道公钥（如果使用了私钥来加密数据）或私钥（如果使用了公钥来加密数据）。
对称解密
解密用对称算法加密的数据类似于用对称算法加密数据的过程。 若要对从任何托管流对象中读取的数据进行解密，应将 CryptoStream 与 .NET 提供的对称加密类一起使用。
以下示例说明了如何为 Aes 算法创建默认实现类的新实例。 该实例用于对 CryptoStream 对象执行解密。 此示例首先创建 Aes 实现类的一个新实例。 它从托管流变量 fileStream 中读取初始化向量(IV) 值。 接下来它实例化一个 CryptoStream 对象并将其初始化为 fileStream 实例的值。 Aes 实例中的 SymmetricAlgorithm.CreateDecryptor 方法传递 IV 值和用于加密的相同密钥。
Dim aes As Aes = Aes.Create()
Dim cryptStream As New CryptoStream(
fileStream, aes.CreateDecryptor(key, iv), CryptoStreamMode.Read)
Aes aes = Aes.Create();
CryptoStream cryptStream = new CryptoStream(
fileStream, aes.CreateDecryptor(key, iv), CryptoStreamMode.Read);
下面的示例显示创建流、解密流、从流中读取和关闭流的整个过程。 创建一个文件流对象，该对象读取名为 TestData.txt 的文件。 然后将使用 CryptoStream 类和 Aes 类对文件流进行解密。 此示例指定用于加密数据的对称加密示例中的密钥值。 它不会显示加密和传输这些值所需的代码。
using System.Security.Cryptography;
try
{
using (FileStream fileStream = new("TestData.txt", FileMode.Open))
{
using (Aes aes = Aes.Create())
{
byte[] iv = new byte[aes.IV.Length];
int numBytesToRead = aes.IV.Length;
int numBytesRead = 0;
while (numBytesToRead > 0)
{
int n = fileStream.Read(iv, numBytesRead, numBytesToRead);
if (n == 0) break;
numBytesRead += n;
numBytesToRead -= n;
}
byte[] key =
{
0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08,
0x09, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16
};
using (CryptoStream cryptoStream = new(
fileStream,
aes.CreateDecryptor(key, iv),
CryptoStreamMode.Read))
{
// By default, the StreamReader uses UTF-8 encoding.
// To change the text encoding, pass the desired encoding as the second parameter.
// For example, new StreamReader(cryptoStream, Encoding.Unicode).
using (StreamReader decryptReader = new(cryptoStream))
{
string decryptedMessage = await decryptReader.ReadToEndAsync();
Console.WriteLine($"The decrypted original message: {decryptedMessage}");
}
}
}
}
}
catch (Exception ex)
{
Console.WriteLine($"The decryption failed. {ex}");
}
Imports System
Imports System.IO
Imports System.Security.Cryptography
Module Module1
Sub Main()
' Decryption key must be the same value that was used
' to encrypt the stream.
Dim key As Byte() = {&H1, &H2, &H3, &H4, &H5, &H6, &H7, &H8, &H9, &H10, &H11, &H12, &H13, &H14, &H15, &H16}
Try
' Create a file stream.
Using fileStream As New FileStream("TestData.txt", FileMode.Open)
' Create a new instance of the default Aes implementation class
Using aes As Aes = Aes.Create()
' Reads IV value from beginning of the file.
Dim iv As Byte() = New Byte(aes.IV.Length - 1) {}
Dim numBytesToRead As Integer = CType(aes.IV.Length, Integer)
Dim numBytesRead As Integer = 0
While (numBytesToRead > 0)
Dim n As Integer = fileStream.Read(iv, numBytesRead, numBytesToRead)
If n = 0 Then
Exit While
End If
numBytesRead += n
numBytesToRead -= n
End While
Using cryptoStream As New CryptoStream(fileStream, aes.CreateDecryptor(key, iv), CryptoStreamMode.Read)
' By default, the StreamReader uses UTF-8 encoding.
' To change the text encoding, pass the desired encoding as the second parameter.
' For example, New StreamReader(cryptoStream, Encoding.Unicode).
Using decryptReader As New StreamReader(cryptoStream)
' Display the message.
Console.WriteLine($"The decrypted original message: {decryptReader.ReadToEnd()}")
End Using
End Using
End Using
End Using
Catch
Console.WriteLine("The decryption Failed.")
Throw
End Try
End Sub
End Module
前面的示例使用对称加密示例中用于加密数据的相同密钥和算法。 它解密由该示例创建的 TestData.txt 文件并在控制台上显示原始文本。
不对称解密
通常，一方（A 方）同时生成公钥和私钥，并将其存储在内存或加密密钥容器中。 然后 A 方将公钥发送到另一方（B 方）。 B 方使用此公钥将数据加密后发送回 A 方。接收到数据后，A 方使用对应的私钥将其解密。 A 方只有使用与 B 方用于加密数据的公钥相对应的私钥，解密才能成功。
有关如何将非对称密钥存储在安全加密密钥容器中以及随后如何获取非对称密钥的信息，请参阅 How to: Store Asymmetric Keys in a Key Container。
下面的示例阐释如何对表示一个对称密钥和 IV 的两个字节数组进行解密。 有关如何以可方便地发送到第三方的格式从 RSA 对象提取非对称公钥的信息，请参阅 RSA的托管流的值。
'Create a new instance of the RSA class.
Dim rsa As RSA = RSA.Create()
' Export the public key information and send it to a third party.
' Wait for the third party to encrypt some data and send it back.
'Decrypt the symmetric key and IV.
symmetricKey = rsa.Decrypt(encryptedSymmetricKey, RSAEncryptionPadding.Pkcs1)
symmetricIV = rsa.Decrypt(encryptedSymmetricIV, RSAEncryptionPadding.Pkcs1)
//Create a new instance of the RSA class.
RSA rsa = RSA.Create();
// Export the public key information and send it to a third party.
// Wait for the third party to encrypt some data and send it back.
//Decrypt the symmetric key and IV.
symmetricKey = rsa.Decrypt(encryptedSymmetricKey, RSAEncryptionPadding.Pkcs1);
symmetricIV = rsa.Decrypt(encryptedSymmetricIV , RSAEncryptionPadding.Pkcs1);
