Why Does Rijndael Decryption Fail with 'Invalid Padding' and How Can It Be Fixed?

Invalid padding in cryptography

Question:

When using Rijndael to decrypt an encrypted XML document, I encountered an exception: "Padding is invalid and cannot be removed".

Answer:

Rijndael (also known as AES) is a block cipher that encrypts data in 16-character blocks. Padding is used to ensure that the last chunk of a message is always the appropriate size.

The default padding for Rijndael in .NET is PKCS#7 padding. However, the code provided in the question does not explicitly specify the padding mode, resulting in discrepancies between encryption and decryption operations.

To resolve this issue, the padding mode for encryption and decryption needs to be set explicitly. Specifically, the Padding property of the RijndaelManaged object should be set to PaddingMode.PKCS7.

Here is a modified code snippet that includes explicitly setting padding:

<code class="language-csharp">public void Cryptography(XmlDocument doc, bool cryptographyMode)
{
    RijndaelManaged key = null;
    try
    {
        // 创建一个新的Rijndael密钥。
        key = new RijndaelManaged()
        {
            Padding = PaddingMode.PKCS7 //显式设置PKCS7填充模式
        };
        // ... 代码其余部分保持不变 ...
    }
    catch (Exception ex)
    {
        // ... 异常处理保持不变 ...
    }
    finally
    {
        // ... 密钥清理保持不变 ...
    }
}</code>

By explicitly setting the padding mode, you can ensure that the encryption and decryption processes use the same padding mechanism, thereby resolving padding-related exceptions. This avoids decryption failures due to inconsistent padding patterns.

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