Question

I was playing around with encryption and I discovered this modifies my buffer. I was NOT expecting that.

If you look at the MemoryStream with aesbuf you can clearly see i set CanWrite to false. Yet the aesbuf is modified. I know because b1 is true but b2 is false.

WTF!? Whats going on?

byte[] unencrypted_array;
var a1 = (byte[])aesbuf.Clone();
var b1 = a1.ArraysEqual(aesbuf);

using (MemoryStream ms2 = new MemoryStream(aesbuf))
using (var aes = new RijndaelManaged())
{
    using (var dec = aes.CreateDecryptor(aeskey, aesiv))
    using (MemoryStream ms = new MemoryStream(aesbuf, false))
    using (CryptoStream sr = new CryptoStream(ms, dec, CryptoStreamMode.Read))
    {
        byte[] buf = new byte[1024 * 4];
        int l;
        while ((l = sr.Read(buf, 0, buf.Length)) != 0)
        {
            ms2.Write(buf, 0, l);
        }
    }
    ms2.Flush();
    unencrypted_array = ms2.ToArray();
}
var b2 = a1.ArraysEqual(aesbuf);

Answer 1

You have:

using (MemoryStream ms2 = new MemoryStream(aesbuf))

and:

ms2.Write(buf, 0, l);

so… your own code wrote to a writeable MemoryStream that is using aesbuf as the underlying buffer. Self-inflicted.