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decrypt an encrypted file with 256bit block size in Java (encryption Rijndael C#)

I need to decrypt an file in Java.
The file was encrypted in C# using RijndaelManaged. This are the C# settings for the encryption (summarized and not mentioned are default value):

var crypto = new RijndaelManaged(); crypto.GenerateKey(); crypto.BlockSize = crypto.KeySize; crypto.IV = crypto.Key; crypto.Padding = PaddingMode.Zeros;

my java code so far, which produces an error:

java.security.InvalidAlgorithmParameterException: Wrong IV length: must be 16 bytes long:

Key aesKey = new SecretKeySpec(key, "AES"); Cipher cipher2 = Cipher.getInstance("AES/CBC/noPadding"); byte[] iv = key; IvParameterSpec ivspec = new IvParameterSpec(iv); cipher2.init(Cipher.DECRYPT_MODE, aesKey, new IvParameterSpec(iv)); byte[] decryptedFile = cipher2.doFinal(buildFile);

I see 2 Problems concerning the C# code:

  1. the generated Key is 256bit. Then the block size is set to to 256bit, but the standard is 128bit. As far as I know, Java is only able to use a 128bit block size, as the Error message suggests. At least I didn’t find out how to set the block size.
  2. Rijndael uses PaddingMode.Zeros, whats the equivalent Padding for Cipher?

please be aware that I can’t change the C# code

Is there even a way to decrypt this in Java?

Asked on July 16, 2020 in Java.
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