C# Encryption to PHP Decryption

Question

I'm trying to encrypt some (cookie) data in C# and then decrypt it in PHP. I have chosen to use Rijndael encryption. I've almost got it working, except only part of the text is decrypted! I started working from this example: Decrypt PHP encrypted string in C# Here's the text (JSON) that I am encrypting (sensitive information removed): ``` {"DisplayName":"xxx", "Username": "yyy", "EmailAddress":"zzz"} ``` So I login to the C# app which creates/encodes the cookie from stored Key and IV and then redirects to the PHP app which is supposed to decrypt/read the cookie. When I decrypt the cookie, it comes out like this: ``` {"DisplayName":"xxx","F�A ;��HP=D�������4��z����ť���k�#E���R�j�5�\�t. t�D��" ``` UPDATE: i've gotten a little bit further and this is now the result ``` string(96) "{"DisplayName":"xxx","Username":"yyy","EmailAddress"�)ق��-�J��k/VV-v� �9�B`7^" ``` As you can see, it starts decrypting it, but then gets messed up... When Decrypt the string it comes out correct (with padding,…

Accepted Answer

Since the string is partially OK, but there is gibberish at the end it would suggest a padding problem within the encryption which expects exact blocks of 256 bytes. I suggest setting the padding as PKCS7 (PaddingMode.PKCS7) instead of Zeros on the C# side which PHP will understand without issues (as it's the default mode on that parser).

Edit: Oops, I did not notice that you had the following in your PHP:

```
$enc = $_COOKIE["MyCookie"];
```

This is the caveat. PHP is likely not getting the encrypted data as-is and is running some urldecode sanitizing. You should print this variable to see that it really matches what is being sent from the C# code.

Edit2:

Convert the whitespaces to missing + characters from the cookie by adding this:

```
str_replace(' ', '+', $enc);
```

C# Encryption to PHP Decryption

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