AES decrypt fails with “Padding is invalid and cannot be removed”

Question

I am making cross-platform application with server being written in Java, so I ended up with C# for Windows. I finally got through all obstacles like different endianities for BigIntegers in these two languages (Java: big endian, C#: little), so I successfuly made key-exchange as well. Now this is where problem comes, in Java I use AES/CBC/PKCS5Padding for encryption, but in C# there was no PKCS5 available, but as I read in other posts to this topic here on SO, PKCS7 is told to be same as PKCS5 or that Java really uses PKCS7 instead of PKCS5. I don't know what's correct.

So this is the code:

using (System.Security.Cryptography.RijndaelManaged rijndael = new System.Security.Cryptography.RijndaelManaged())
{
    byte[] iv = new byte[16];
    for (int i = 0; i < 16; i++)
        iv[i] = 0;
    rijndael.Padding = PaddingMode.PKCS7;
    rijndael.Mode = CipherMode.CBC;
    rijndael.Key = Sys.PrivateKey;
    rijndael.KeySize = 128;
    rijndael.BlockSize = 128;
    rijndael.IV = iv;
    Sys.LogWrite("Decrypt input bytes length: " + buff.Length + ", keyLength: " + Sys.PrivateKey.Length);
    Sys.LogWriteBuffer("Input bytes", buff);
    Sys.LogWriteBuffer("Input key", Sys.PrivateKey);
    ICryptoTransform decryptor = rijndael.CreateDecryptor(rijndael.Key, rijndael.IV);
    buff=rijndael.CreateDecryptor().TransformFinalBlock(buff, 0, buff.Length);
    Sys.LogWriteBuffer("buffer: ", buff);
}

Please ignore that IV is always 00 00 00 00, this is only for testing purposes. Data come from server always and key is generated in key-exchange:

Decrypt input bytes length: 32, keyLength: 16
Input bytes: 7C 25 3F 49 9E D5 51 67 E2 86 F9 86 2E C1 8F 22 70 51 65 74 FC 39 2C 52 A6 83 36 B5 9A C7 27 B9 
Input key: 1C 13 C2 33 50 57 00 DB FD 60 E2 1C 27 2A A5 00 

If I put it to any online AES decryptor like this http://aes.online-domain-tools.com/ I always get correct result: [[27,{"Established":true}]] But in C# I only get this:

System.Security.Cryptography.CryptographicException: Padding is invalid and cannot be removed.
   at System.Security.Cryptography.RijndaelManagedTransform.DecryptData(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount, Byte[]& outputBuffer, Int32 outputOffset, PaddingMode paddingMode, Boolean fLast)
   at System.Security.Cryptography.RijndaelManagedTransform.TransformFinalBlock(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)

I even tried PaddingMode.None, that didn't throw any exception, but output was just random bytes. What am I doing wrong when even online decryptor can solve it correctly?

Other example input data which should produce same result [[27,{"Established":true}]] :

Decrypt input bytes length: 32, keyLength: 16
Input bytes: 85 5C 55 24 44 B8 77 A5 EF CE E7 A1 45 EC F3 84 2F 8B 74 1F AB D9 BE D0 82 64 BC 0D B0 50 73 63 
Input key: B2 21 FA 17 63 E6 4C 25 48 03 84 64 8B 70 05 00 

Decrypt input bytes length: 32, keyLength: 16
Input bytes: 8B FC 47 B4 91 05 B7 E1 6C 0E 61 78 D2 51 6B 77 EF 80 30 49 37 05 DA 79 47 52 D1 24 B9 DE A7 F3 
Input key: E7 BF E0 AA AC F1 26 42 06 D6 59 44 F9 33 74 63 

Notice that IV is always 16 null bytes.

Answer 1

I finally found the issue and it's unbelievable! Instead of creating decryptor this way:

ICryptoTransform decryptor = rijndael.CreateDecryptor(rijndael.Key, rijndael.IV);

I should have it created this way:

ICryptoTransform decryptor = rijndael.CreateDecryptor(Sys.PrivateKey, iv);

So if anyone in future had similar problem, here is already generalized function:

        public static String Decrypt(byte[] buff, byte[] key, byte[] iv)
        {
            using (System.Security.Cryptography.RijndaelManaged rijndael = new System.Security.Cryptography.RijndaelManaged())
            {
                rijndael.Padding = PaddingMode.PKCS7;
                rijndael.Mode = CipherMode.CBC;
                rijndael.KeySize = 128;
                rijndael.BlockSize = 128;
                ICryptoTransform decryptor = rijndael.CreateDecryptor(key, iv);
                System.IO.MemoryStream memoryStream = new System.IO.MemoryStream(buff);
                CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read);
                byte[] output = new byte[buff.Length];
                int readBytes = cryptoStream.Read(output, 0, output.Length);
                return System.Text.Encoding.UTF8.GetString(output, 0, readBytes);
            }
        }

Hope it helps.