Why Am I Getting a 'Padding is Invalid and Cannot Be Removed' Exception in My C# Rijndael Encryption?

"Padding is invalid and cannot be removed" exception in C# Rijndael encryption and decryption program

In cryptography, a "Padding is invalid and cannot be removed" exception occurs when trying to decrypt data encrypted using the Rijndael algorithm. This exception indicates that the padding used during encryption does not match what is expected by the decryption algorithm.

Understanding padding in Rijndael

Rijndael (also known as AES) operates on fixed-size blocks of data. When the size of the encrypted data is not divisible by the block size, padding is used to fill the remaining space. This padding ensures that the ciphertext is a valid, complete block.

Decryption process

During decryption, the algorithm attempts to remove padding from the ciphertext. However, if the padding does not match what is expected, a "Padding is invalid and cannot be removed" exception is thrown. This means that the encryption and decryption algorithms use different padding schemes.

Solution: Explicitly specify padding

To solve this problem, the padding scheme for encryption and decryption must be explicitly specified. This can be done using the following code:

// 加密
symmetricAlgorithm.Padding = PaddingMode.PKCS7;

// 解密
EncryptedXml exml = new EncryptedXml();
exml.Padding = PaddingMode.PKCS7;

PKCS#7 Padding

It is recommended to use PKCS#7 padding unless there is a specific reason to use a different padding scheme. PKCS#7 (also known as OAEP (Optimal Asymmetric Encryption Padding)) is a widely supported and secure padding algorithm that ensures the integrity of encrypted data.

Other notes

• Check that the key size used for encryption and decryption is the same.
• Make sure initialization vectors (IVs) are handled correctly.
• Use a library or framework that supports Rijndael encryption with PKCS#7 padding to simplify implementation.

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