Aes Encryption… missing an important piece

Question

I have a method in my static Encryption class that looks like this:

    public static byte[] EncryptString(string toEncrypt, byte[] encryptionKey)
    {
        var toEncryptBytes = Encoding.UTF8.GetBytes(toEncrypt);
        using (var provider = new AesCryptoServiceProvider())
        {
            provider.Key = encryptionKey;
            provider.Mode = CipherMode.ECB;
            provider.Padding = PaddingMode.ISO10126;
            using (var encryptor = provider.CreateEncryptor(provider.Key, provider.IV))
            {
                using (var ms = new MemoryStream())
                {
                    using (var cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
                    {
                        cs.Write(toEncryptBytes, 0, toEncryptBytes.Length);
                        cs.FlushFinalBlock();
                    }
                    return ms.ToArray();
                }
            }
        }
    }

I have a unit test that looks like this:

    [TestMethod]
    public void EncryptStringEncryptsTest()
    {
        var toEncrypt = "My text to encrypt";
        var encryptionKey = Convert.FromBase64String("93mcgv9UBYpwgoUX0AXEaU1BqTCufPWPkFdOdoILLDA=");
        var encrypted = Encryption.EncryptString(toEncrypt, encryptionKey);

        var text = Convert.ToBase64String(encrypted);
        Assert.IsTrue(false);
    }

Every time I run this, the text value changes. I would expect it to be constant, given the same inputs. Am I wrong to expect that, or am I doing something wrong?

Answer 1

Your assumption is incorrect.

ISO10126 padding will append random data to pad your message to a multiple of the block size.

Answer 2

You are using random padding bytes, per ISO10126. So the results will not be the same each time, even with all inputs the same.