升级至.NET 6后自定义字符串加解密类解密结果被截断问题求助

问题根因 #

该问题由.NET 6中CryptoStream.Read方法的行为变更导致：

• .NET 5及更早版本中，CryptoStream.Read会一次性返回所有解密完成的字节，单次调用即可读取全部明文
• .NET 6开始，CryptoStream遵循标准流的语义规范，Read方法仅保证至少返回1个字节，不保证一次返回全部可用数据。原代码仅调用了一次Read，因此只拿到了第一个AES块（128位=16字节）对应的明文，后续剩余数据未被读取，最终出现截断。

修复方案 #

修改Decrypt方法中的解密读取逻辑，确保读取完CryptoStream中的全部数据即可，推荐以下两种实现方式：

方案1：使用CopyTo简化读取（代码更简洁） #

public static string Decrypt(string cipherText, string passPhrase)
{
    var cipherTextBytesWithSaltAndIv = Convert.FromBase64String(cipherText);
    var saltStringBytes = cipherTextBytesWithSaltAndIv.Take(Keysize / 8).ToArray();
    var ivStringBytes = cipherTextBytesWithSaltAndIv.Skip(Keysize / 8).Take(Keysize / 8).ToArray();
    var cipherTextBytes = cipherTextBytesWithSaltAndIv.Skip((Keysize / 8) * 2).ToArray();

    using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations))
    {
        var keyBytes = password.GetBytes(Keysize / 8);
        using (var symmetricKey = Aes.Create())
        {
            symmetricKey.BlockSize = 128;
            symmetricKey.Mode = CipherMode.CBC;
            symmetricKey.Padding = PaddingMode.PKCS7;
            using (var decryptor = symmetricKey.CreateDecryptor(keyBytes, ivStringBytes))
            {
                using (var memoryStream = new MemoryStream(cipherTextBytes))
                {
                    using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
                    {
                        using (var resultStream = new MemoryStream())
                        {
                            cryptoStream.CopyTo(resultStream);
                            return Encoding.UTF8.GetString(resultStream.ToArray());
                        }
                    }
                }
            }
        }
    }
}

方案2：循环调用Read直到无数据返回 #

如果需要手动控制读取过程，可循环调用Read直到返回值为0：

public static string Decrypt(string cipherText, string passPhrase)
{
    var cipherTextBytesWithSaltAndIv = Convert.FromBase64String(cipherText);
    var saltStringBytes = cipherTextBytesWithSaltAndIv.Take(Keysize / 8).ToArray();
    var ivStringBytes = cipherTextBytesWithSaltAndIv.Skip(Keysize / 8).Take(Keysize / 8).ToArray();
    var cipherTextBytes = cipherTextBytesWithSaltAndIv.Skip((Keysize / 8) * 2).ToArray();

    using (var password = new Rfc2898DeriveBytes(passPhrase, saltStringBytes, DerivationIterations))
    {
        var keyBytes = password.GetBytes(Keysize / 8);
        using (var symmetricKey = Aes.Create())
        {
            symmetricKey.BlockSize = 128;
            symmetricKey.Mode = CipherMode.CBC;
            symmetricKey.Padding = PaddingMode.PKCS7;
            using (var decryptor = symmetricKey.CreateDecryptor(keyBytes, ivStringBytes))
            {
                using (var memoryStream = new MemoryStream(cipherTextBytes))
                {
                    using (var cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
                    {
                        var plainTextBytes = new byte[cipherTextBytes.Length];
                        int totalDecryptedBytes = 0;
                        int decryptedByteCount;
                        // 循环读取直到没有更多数据
                        while ((decryptedByteCount = cryptoStream.Read(plainTextBytes, totalDecryptedBytes, plainTextBytes.Length - totalDecryptedBytes)) > 0)
                        {
                            totalDecryptedBytes += decryptedByteCount;
                        }
                        return Encoding.UTF8.GetString(plainTextBytes, 0, totalDecryptedBytes);
                    }
                }
            }
        }
    }
}

两种方案修改后，.NET 5和.NET 6下的解密结果就会完全一致，不会再出现截断问题。

内容的提问来源于stack exchange，提问作者Optim