.NET Framework中AES解密失败问题求助

问题描述 #

尝试通过HTTP头向服务端发送AES加密的用户名（已知有更优实现方式），但解密后的字符串始终为空。

加密代码（客户端.NET 8） #

using var aes = Aes.Create();
aes.Padding = PaddingMode.PKCS7;
aes.Mode = CipherMode.ECB;
aes.GenerateIV();
aes.Key = SHA256.HashData(Encoding.Unicode.GetBytes(PK));
var encryptor = aes.CreateEncryptor(aes.Key, aes.IV);

using MemoryStream memoryStream = new MemoryStream();
using CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write);
using StreamWriter streamWriter = new StreamWriter(cryptoStream, Encoding.Unicode);
string payload = UserProvider.User;

streamWriter.Write(payload);
cryptoStream.FlushFinalBlock();

byte[] array = memoryStream.ToArray();
string encryptedString = Convert.ToBase64String(array);
string ivString = Convert.ToBase64String(aes.IV);

_client.AddDefaultHeader("user", encryptedString);
_client.AddDefaultHeader("iv", ivString);

解密代码（服务端.NET Framework 4.8） #

internal static class Authentication
{
    private const string PK = "MySecretKey";
    public static string DecryptUser(string encrypted, string iv)
    {

        using (var aes = Aes.Create())
        using (var sha = SHA256.Create())
        {
            byte[] pkBytes = Encoding.Unicode.GetBytes(PK);
            byte[] pkHash = sha.ComputeHash(pkBytes);
            byte[] ivBytes = Convert.FromBase64String(iv);
            aes.Padding = PaddingMode.PKCS7;
            aes.Mode = CipherMode.ECB;
            aes.Key = pkHash;
            aes.IV = ivBytes;
            var decryptor = aes.CreateDecryptor(aes.Key, aes.IV);

            byte[] cipher = Convert.FromBase64String(encrypted);
            using (MemoryStream memoryStream = new MemoryStream(cipher))
            using (CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
            using (StreamReader streamReader = new StreamReader(cryptoStream))
            {
                string user = streamReader.ReadToEnd();

                if (user.IsNullOrWhiteSpace()) throw new Exception("Authentication failed.");

                return user;
            }
        }
    }
}

已排查项 #

• CipherMode配置正确
• PaddingMode配置正确
• 接收的Base64字符串非空且未损坏
• 尝试过将MemoryStream定位到起始位置
• 尝试发送更长内容，排除解密器最小块大小问题
• 尝试将CryptoStream复制到另一个MemoryStream，但结果缓冲区为空
• 执行ReadToEnd()后，cryptoStream.HasFlushedFinalBlock从false变为true

环境说明 #

• 服务端（解密侧）：.NET Framework 4.8
• 客户端（加密侧）：.NET 8

补充测试代码（xUnit，排除Base64/HTTP环节） #

public class UnitTest1
{
    [Fact]
    public void TestMethod1()
    {
        byte[] buffer = new byte[1024];
        using (var aes = Aes.Create())
        using (var sha = SHA256.Create())
        {
            aes.Padding = PaddingMode.PKCS7;
            aes.Mode = CipherMode.ECB;
            aes.Key = sha.ComputeHash(Encoding.Unicode.GetBytes("Password"));

            using (var encryptor = aes.CreateEncryptor(aes.Key, aes.IV))
            using (MemoryStream memoryStream = new MemoryStream())
            using (CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
            using (StreamWriter streamWriter = new StreamWriter(cryptoStream, Encoding.Unicode))
            {
                string payload = "username";

                streamWriter.Write(payload);
                cryptoStream.FlushFinalBlock();

                buffer = memoryStream.ToArray();
                Assert.NotEmpty(buffer);
            }
            using (var decryptor = aes.CreateDecryptor(aes.Key, aes.IV))
            using (MemoryStream memoryStream = new MemoryStream(buffer))
            using (CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
            using (StreamReader streamReader = new StreamReader(cryptoStream, Encoding.Unicode))
            {
                string user = streamReader.ReadToEnd();

                Assert.True(streamReader.EndOfStream);
                Assert.True(cryptoStream.HasFlushedFinalBlock);
                Assert.Equal("username", user);
            }
        }
    }
}

问题根源与解决 #

核心问题1：ECB模式错误使用IV #

AES的ECB模式为无IV模式，对每个块独立加密，不需要初始化向量。但当前代码中：

• 加密侧生成并传递IV给加密器，还通过HTTP头发送IV
• 解密侧接收并设置IV到Aes实例
  虽然ECB模式会忽略IV参数，但这种冗余操作可能引发状态不一致，同时增加不必要的复杂度。

核心问题2：加密与解密的编码不匹配 #

加密侧使用Encoding.Unicode（UTF-16LE）写入数据，但服务端解密侧的StreamReader未指定编码，默认使用UTF-8读取，导致解密后的UTF-16字节被错误解析，最终得到空字符串。

核心问题3：解密时数据读取不彻底 #

直接用StreamReader读取CryptoStream可能因内部缓冲机制，无法触发填充字节的自动移除，导致数据读取不完整。

修复步骤 #

1. 移除ECB模式下的IV相关代码

• 加密侧：删除aes.GenerateIV()，将aes.CreateEncryptor(aes.Key, aes.IV)改为aes.CreateEncryptor()，不再发送iv请求头
• 解密侧：删除IV参数接收与设置逻辑，将aes.CreateDecryptor(aes.Key, aes.IV)改为aes.CreateDecryptor()

2. 统一加密解密的编码

解密侧的StreamReader必须指定Encoding.Unicode，与加密侧保持一致。

3. 优化解密数据读取方式

先将CryptoStream的内容复制到MemoryStream，再用StreamReader读取，确保所有解密数据被处理，同时触发填充字节的自动移除。

修复后的测试代码示例 #

public class UnitTest1
{
    [Fact]
    public void TestMethod1()
    {
        byte[] buffer = new byte[1024];
        using (var aes = Aes.Create())
        using (var sha = SHA256.Create())
        {
            aes.Padding = PaddingMode.PKCS7;
            aes.Mode = CipherMode.ECB;
            aes.Key = sha.ComputeHash(Encoding.Unicode.GetBytes("Password"));

            // 加密部分
            using (var encryptor = aes.CreateEncryptor())
            using (MemoryStream memoryStream = new MemoryStream())
            using (CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
            using (StreamWriter streamWriter = new StreamWriter(cryptoStream, Encoding.Unicode))
            {
                string payload = "username";
                streamWriter.Write(payload);
                cryptoStream.FlushFinalBlock();
                buffer = memoryStream.ToArray();
                Assert.NotEmpty(buffer);
            }

            // 解密部分
            using (var decryptor = aes.CreateDecryptor())
            using (MemoryStream memoryStream = new MemoryStream(buffer))
            using (CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
            using (MemoryStream decryptedStream = new MemoryStream())
            {
                cryptoStream.CopyTo(decryptedStream);
                decryptedStream.Position = 0;
                using (StreamReader streamReader = new StreamReader(decryptedStream, Encoding.Unicode))
                {
                    string user = streamReader.ReadToEnd();
                    Assert.Equal("username", user);
                }
            }
        }
    }
}

修复后的服务端解密代码示例 #

internal static class Authentication
{
    private const string PK = "MySecretKey";
    public static string DecryptUser(string encrypted)
    {
        using (var aes = Aes.Create())
        using (var sha = SHA256.Create())
        {
            byte[] pkBytes = Encoding.Unicode.GetBytes(PK);
            byte[] pkHash = sha.ComputeHash(pkBytes);
            aes.Padding = PaddingMode.PKCS7;
            aes.Mode = CipherMode.ECB;
            aes.Key = pkHash;

            var decryptor = aes.CreateDecryptor();
            byte[] cipher = Convert.FromBase64String(encrypted);

            using (MemoryStream memoryStream = new MemoryStream(cipher))
            using (CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
            using (MemoryStream decryptedStream = new MemoryStream())
            {
                cryptoStream.CopyTo(decryptedStream);
                decryptedStream.Position = 0;
                using (StreamReader streamReader = new StreamReader(decryptedStream, Encoding.Unicode))
                {
                    string user = streamReader.ReadToEnd();
                    if (string.IsNullOrWhiteSpace(user)) throw new Exception("Authentication failed.");
                    return user;
                }
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者Mateus Zampol