How to Call Schannel Functions from .Net/C#

I am trying to re-order/remove cipher suites due to compliance reasons (I want to use 256 bit AES and ephemeral keys) in .Net. However, using WCF TCP Transport Security, I cede all control over the security to Windows' TLS implementation and its preferred ciphers. I don't want to change the system-level ciphers.

I found this great Microsoft page that lists how to do it at the application level using BCryptEnumContextFunctions, BCryptAddContextFunction, BCryptRemoveContextFunction, but it's all C and I don't have enough P/Invoke experience to even know where to begin. I googled but didn't find anyone doing it.

The C++ code snippets on the MSDN page are below, and I need help converting them to .Net P/Invoke calls:

BCryptEnumContextFunctions

#include <stdio.h>
#include <windows.h>
#include <bcrypt.h>


void main()
{

   HRESULT Status = ERROR_SUCCESS;
   DWORD   cbBuffer = 0;
   PCRYPT_CONTEXT_FUNCTIONS pBuffer = NULL;

    Status = BCryptEnumContextFunctions(
        CRYPT_LOCAL,
        L"SSL",
        NCRYPT_SCHANNEL_INTERFACE,
        &cbBuffer,
        &pBuffer);
    if(FAILED(Status))
    {
        printf_s("\n**** Error 0x%x returned by BCryptEnumContextFunctions\n", Status);
        goto Cleanup;
    }

    if(pBuffer == NULL)
    {
        printf_s("\n**** Error pBuffer returned from BCryptEnumContextFunctions is null");
        goto Cleanup;
    }

    printf_s("\n\n Listing Cipher Suites ");
    for(UINT index = 0; index < pBuffer->cFunctions; ++index)
    {
        printf_s("\n%S", pBuffer->rgpszFunctions[index]);
    }

Cleanup:
    if (pBuffer != NULL)
    {
        BCryptFreeBuffer(pBuffer);
    }
}

BCryptAddContextFunction

#include <stdio.h>
#include <windows.h>
#include <bcrypt.h>


void main()
{

    SECURITY_STATUS Status = ERROR_SUCCESS;
    LPWSTR wszCipher = (L"RSA_EXPORT1024_DES_CBC_SHA");

    Status = BCryptAddContextFunction(
                CRYPT_LOCAL,
                L"SSL",
                NCRYPT_SCHANNEL_INTERFACE,
                wszCipher,
                CRYPT_PRIORITY_TOP);
}

BCryptRemoveContextFunction

#include <stdio.h>
#include <windows.h>
#include <bcrypt.h>


void main()
{

    SECURITY_STATUS Status = ERROR_SUCCESS;
      LPWSTR wszCipher = (L"TLS_RSA_WITH_RC4_128_SHA");

    Status = BCryptRemoveContextFunction(
                CRYPT_LOCAL,
                L"SSL",
                NCRYPT_SCHANNEL_INTERFACE,
                wszCipher);
}

Could someone please help me convert these to .Net so I can call them from managed code to adjust the ciphers? Thanks!

Edit:

Later last night, I tried the following in a test program (still have no idea what I'm doing in P/Invoke):

// I found this in bcrypt.h
const uint CRYPT_LOCAL = 0x00000001;
// I can't find this anywhere in Microsoft's headers in my SDK,
// but I found some random .c file with it in there. No idea
// what this constant actually is according to Microsoft
const uint NCRYPT_SCHANNEL_INTERFACE = 0x00010002;

public static void DoStuff()
{
    PCRYPT_CONTEXT_FUNCTIONS pBuffer = new PCRYPT_CONTEXT_FUNCTIONS();
    pBuffer.rgpszFunctions = String.Empty.PadRight(1500);
    uint cbBuffer = (uint)Marshal.SizeOf(typeof(PCRYPT_CONTEXT_FUNCTIONS));

    uint Status = BCryptEnumContextFunctions(
            CRYPT_LOCAL,
            "SSL",
            NCRYPT_SCHANNEL_INTERFACE,
            ref cbBuffer,
            ref pBuffer);

    Console.WriteLine(Status);
    Console.WriteLine(pBuffer);
    Console.WriteLine(cbBuffer);
    Console.WriteLine(pBuffer.cFunctions);
    Console.WriteLine(pBuffer.rgpszFunctions);
}
/*
    typedef struct _CRYPT_CONTEXT_FUNCTIONS {
        ULONG cFunctions;
        PWSTR rgpszFunctions;
    } CRYPT_CONTEXT_FUNCTIONS, *PCRYPT_CONTEXT_FUNCTIONS;
*/

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct PCRYPT_CONTEXT_FUNCTIONS
{
    public uint cFunctions;
    public string rgpszFunctions;
}

/*
    NTSTATUS WINAPI BCryptEnumContextFunctions(
    ULONG dwTable,
    LPCWSTR pszContext,
    ULONG dwInterface,
    ULONG *pcbBuffer,
    PCRYPT_CONTEXT_FUNCTIONS *ppBuffer
    );
*/
[DllImport("Bcrypt.dll", CharSet = CharSet.Unicode, SetLastError = true)]
static extern uint BCryptEnumContextFunctions(uint dwTable, string pszContext, uint dwInterface, ref uint pcbBuffer, ref PCRYPT_CONTEXT_FUNCTIONS ppBuffer);

The output right now of the only method I got even this far with is:

0
MyClass+PCRYPT_CONTEXT_FUNCTIONS
2400
8934576
[1500 spaces that I initialized rgpszFunctions with, not the cipher functions]

using System; using System.Text; using System.Runtime.InteropServices; namespace ConsoleApplication1 { class Program { [DllImport("Bcrypt.dll", CharSet = CharSet.Unicode)] static extern uint BCryptEnumContextFunctions(uint dwTable, string pszContext, uint dwInterface, ref uint pcbBuffer, ref IntPtr ppBuffer); [DllImport("Bcrypt.dll")] static extern void BCryptFreeBuffer(IntPtr pvBuffer); [DllImport("Bcrypt.dll", CharSet = CharSet.Unicode)] static extern uint BCryptAddContextFunction(uint dwTable, string pszContext, uint dwInterface, string pszFunction, uint dwPosition); [DllImport("Bcrypt.dll", CharSet = CharSet.Unicode)] static extern uint BCryptRemoveContextFunction(uint dwTable, string pszContext, uint dwInterface, string pszFunction); [StructLayout(LayoutKind.Sequential)] public struct CRYPT_CONTEXT_FUNCTIONS { public uint cFunctions; public IntPtr rgpszFunctions; } const uint CRYPT_LOCAL = 0x00000001; const uint NCRYPT_SCHANNEL_INTERFACE = 0x00010002; const uint CRYPT_PRIORITY_TOP = 0x00000000; const uint CRYPT_PRIORITY_BOTTOM = 0xFFFFFFFF; public static void DoStuff() { uint cbBuffer = 0; IntPtr ppBuffer = IntPtr.Zero; uint Status = BCryptEnumContextFunctions( CRYPT_LOCAL, "SSL", NCRYPT_SCHANNEL_INTERFACE, ref cbBuffer, ref ppBuffer); if (Status == 0) { CRYPT_CONTEXT_FUNCTIONS functions = (CRYPT_CONTEXT_FUNCTIONS)Marshal.PtrToStructure(ppBuffer, typeof(CRYPT_CONTEXT_FUNCTIONS)); Console.WriteLine(functions.cFunctions); IntPtr pStr = functions.rgpszFunctions; for (int i = 0; i < functions.cFunctions; i++) { Console.WriteLine(Marshal.PtrToStringUni(Marshal.ReadIntPtr(pStr))); pStr += IntPtr.Size; } BCryptFreeBuffer(ppBuffer); } } static void Main(string[] args) { DoStuff(); Console.ReadLine(); } } }

30 TLS_RSA_WITH_AES_128_CBC_SHA256 TLS_RSA_WITH_AES_128_CBC_SHA TLS_RSA_WITH_AES_256_CBC_SHA256 TLS_RSA_WITH_AES_256_CBC_SHA TLS_RSA_WITH_RC4_128_SHA TLS_RSA_WITH_3DES_EDE_CBC_SHA TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256 TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P384 TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA_P256 TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA_P384 TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA_P256 TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA_P384 TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256_P256 TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256_P256 TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384_P384 TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384_P384 TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA_P256 TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA_P384 TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA_P256 TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA_P384 TLS_DHE_DSS_WITH_AES_128_CBC_SHA256 TLS_DHE_DSS_WITH_AES_128_CBC_SHA TLS_DHE_DSS_WITH_AES_256_CBC_SHA256 TLS_DHE_DSS_WITH_AES_256_CBC_SHA TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA TLS_RSA_WITH_RC4_128_MD5 SSL_CK_RC4_128_WITH_MD5 SSL_CK_DES_192_EDE3_CBC_WITH_MD5 TLS_RSA_WITH_NULL_SHA256 TLS_RSA_WITH_NULL_SHA

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