Padding is invalid and cannot be removed

Today I tried to send data encrypted between an application built in c# and an application built in Silverlight. When I encrypted the data in one application the decryption would fail with the following exception:

An exception of type ‘System.Security.Cryptography.CryptographicException’ occurred in mscorlib.dll but was not handled in user code Additional information: Padding is invalid and cannot be removed

Your Solution

To encrypt my plain text I used the following code:

public static string EncryptStringAES(string plainText, string sharedSecret) {

  string outStr = null;
  AesManaged aesAlg = null;
  try
  {
   Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedSecret, _salt); 
   aesAlg = new AesManaged(); 
   aesAlg.Key = key.GetBytes(aesAlg.KeySize / 8); 
   aesAlg.IV = key.GetBytes(aesAlg.BlockSize / 8); 
   ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV); 
   using (MemoryStream msEncrypt = new MemoryStream())
   {
      using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encryptor, CryptoStreamMode.Write))
      {
         using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
         {
            swEncrypt.Write(plainText);
         }
      } 
      outStr = Convert.ToBase64String(msEncrypt.ToArray());
   }
  } 
  finally {
    if (aesAlg != null) aesAlg.Clear();
  }
  return outStr;
}

for the decryption I used this bit of code:

public static string DecryptStringAES(string cipherText, string sharedSecret)

{
AesManaged aesAlg = null; string plaintext = null; try {
Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedSecret, _salt);
aesAlg = new AesManaged();
aesAlg.BlockSize = aesAlg.LegalBlockSizes[0].MaxSize;
aesAlg.KeySize = aesAlg.LegalKeySizes[0].MaxSize;
aesAlg.Key = key.GetBytes(aesAlg.KeySize / 8);
aesAlg.IV = key.GetBytes(aesAlg.BlockSize / 8);
ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
byte[] bytes = Convert.FromBase64String(cipherText);
using (MemoryStream msDecrypt = new MemoryStream(bytes))
{
using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
{
using (StreamReader srDecrypt = new StreamReader(csDecrypt))
plaintext = srDecrypt.ReadToEnd();
}
}
}
finally
{
if (aesAlg != null) aesAlg.Clear();
}
  return plaintext;
}

only after setting the block size and the key size the error went away.

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