How to write a memcpy method in C#

Question

Hi all I'm trying to expand some cryptographic functionality in the .Net 3.5 framework which hopefully will enable CMS functions to run with CNG providers under CAPI. I've reproduced a lot of code using Reflector but one method will not compile a memory copy using two pointers and a size. It looks like the following: internal static void memcpy ( IntPtr source IntPtr dest uint size) { for ( uint i = 0 ; i size ; i ++) { (( long ) dest )[ i ] = Marshal . ReadByte ( new IntPtr ((( long ) source ) + i )); } } I've been doing some Googling and some playing around but can't get it right. I know it's not the kind of thing you want to be 'playing around with' that's why I'm going to cross my fingers and hope that someone knows how this code snippet should look in C#.

Barry Carnahan · Answer

I thought I was clear in my original post but I'll repeat what I'm trying to do. I am using Red Gate's .Net Reflector to extract code from the .Net Framework System.Security dll's in order to rebuild the CMS (Cryptographic Message Syntax) functions SignedCms and EnvelopedCms. Why am I doing this? Because Microsoft has coded these classes to work only with legacy (CAPI) CSP's (Cryptographic Service Provider) and I need them to use handles to CNG (Crypotgraphic New Generation) keys if that's even possible. Why do I want to use a handle to a CNG key? Because the private keys I need to use reside in a HSM (Hardware security module) and the algorithm's we need to use are not supported by third party legacy CSP's in the .Net Framework. The code I posted in my original post gives an error when trying to build the project. The code was generated by Reflector. My only question forget everything about cryptography that I said before is how this code should be in order to copy one memory block to another using two pointers and a size in C# (as opposed to C++). In my second post I give an example of other code generated by Reflector that uses the memcpy method that won't compile.

Sam Hobbs · Answer

I know how frustrating it is to use API style functions in C#. There is an API for Outlook Express that is difficult to use in C#. In VB 6 I tried to use HTML Help and gave up. I did manage to use the Cryptography API in VB 6; see my CryptEncrypt and CryptDecrypt  but that does not help you. Do you know C++? The Cryptography API is best used using C++. Using C++ you could do parts in managed code and parts in unmanaged code. If you are not familiar with C++ then that explains why you are having difficulties; pointers can be confusing and since C# uses pointers indirectly pointers in C# can be really confusing in C#. It is not clear to me what it is that you need to do. If you had been more clear then you probably would have gotten clearer answers. Please be specific about what you need to do; that is do you need to copy a long data type or some other data type or do you need to copy an arbitrary number of bytes. Also please read the relevant articles in this web site. If you have questions about the material in the articles then you are much more likely to get help with that.

Barry Carnahan · Answer

If I try to change the long to a byte array (byte[]) then I get the error message Cannot convert type 'System.IntPtr' to 'byte[]'. I have a feeling that Reflector totally messed up the reconstruction of this method. Since I'm using CAPI the copied data will probably be referred to by it's pointer so it's important that this works properly. Here's an example of how it's used in the code: CAPIBase . CERT_CHAIN_ELEMENT structure = new CAPIBase . CERT_CHAIN_ELEMENT ( Marshal .SizeOf( typeof ( CAPIBase . CERT_CHAIN_ELEMENT ))); uint size = ( uint ) Marshal .ReadInt32(pChainElement); if (size > Marshal .SizeOf(structure)) { size = ( uint ) Marshal .SizeOf(structure); } X509Utils .memcpy(pChainElement new IntPtr (( void *)&structure) size); Where pChainElement is an IntPtr passed into the method. Anyone have an idea how I can fix that memcpy method?

Sam Hobbs · Answer

Use of the long data type for this seems incorrect to me especially since the size varies. Use a byte array.