I can now successfully call InitializeSecurityContext and get a status of ContinueNeeded.

Contexts/ClientContext.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Runtime.InteropServices;
 using System.Text;
 using System.Threading.Tasks;
 
@@ -8,7 +9,7 @@ namespace NSspi.Contexts
 {
     public class ClientContext : Context
     {
-        public ClientContext( ClientCredential cred, string serverPrinc, ContextReqAttrib attribs )
+        public ClientContext( ClientCredential cred, string serverPrinc, ContextAttrib attribs )
             : base( cred )
         {
             long credHandle = base.Credential.CredentialHandle;
@@ -17,26 +18,58 @@ namespace NSspi.Contexts
             long newContextHandle = 0;
 
             long expiry = 0;
-            int newContextAttribs = 0;
+            ContextAttrib newContextAttribs = 0;
 
             SecurityStatus status;
+            SecureBufferDesc descriptor;
+            SecureBuffer secureBuffer;
+            byte[] tokenBuffer = new byte[100000];
+            GCHandle tokenBufferHandle;
+            GCHandle bufferArrayHandle;
+            GCHandle descriptorHandle;
+            SecureBuffer[] bufferArray;
+
+            tokenBufferHandle = GCHandle.Alloc( tokenBuffer, GCHandleType.Pinned );
+
+            secureBuffer = new SecureBuffer();
+            secureBuffer.Type = BufferType.Token;
+            secureBuffer.Count = tokenBuffer.Length;
+            secureBuffer.Buffer = tokenBufferHandle.AddrOfPinnedObject();
+
+            bufferArray = new SecureBuffer[1];
+            bufferArray[0] = secureBuffer;
+            bufferArrayHandle = GCHandle.Alloc( bufferArray, GCHandleType.Pinned );
             
-            
-            status = NativeMethods.InitializeSecurityContext_Client(
+            descriptor = new SecureBufferDesc();
+            descriptor.Version = SecureBufferDesc.ApiVersion;
+            descriptor.NumBuffers = bufferArray.Length;
+            descriptor.Buffers = bufferArrayHandle.AddrOfPinnedObject();
+
+            descriptorHandle = GCHandle.Alloc( descriptor, GCHandleType.Pinned );
+
+            status = NativeMethods.InitializeSecurityContext_Client1(
                 ref credHandle,
-                ref prevContextHandle,
+                IntPtr.Zero,
                 serverPrinc,
+                attribs,
                 0,
-                0,
-                0,
+                SecureBufferDataRep.Network,
                 IntPtr.Zero,
                 0,
                 ref newContextHandle,
-                IntPtr.Zero,
+                descriptorHandle.AddrOfPinnedObject(),
                 ref newContextAttribs,
                 ref expiry
             );
 
+            descriptorHandle.Free();
+            bufferArrayHandle.Free();
+            tokenBufferHandle.Free();
+
+            secureBuffer = bufferArray[0];
+
+            Console.Out.WriteLine( status );
+            base.ContextHandle = newContextHandle;
             
         }
     }

Contexts/ContextAttrib.cs

@@ -43,11 +43,6 @@ namespace NSspi.Contexts
         /// </summary>
         ReplayDetect = 0x00000004,
 
-        // The context must be allowed to detect out-of-order
-        // delivery of packets later through the message support
-        // functions. Use of this flag implies all of the
-        // conditions specified by the Integrity flag.
-
         /// <summary>
         /// Detect messages received out of sequence when using the message support functionality. 
         /// This flag implies all of the conditions specified by the Integrity flag - out-of-order sequence 
@@ -126,6 +121,9 @@ namespace NSspi.Contexts
         /// </summary>
         AcceptIntegrity = 0x00020000,
 
+        InitIdentify = 0x00020000,
+        AcceptIdentify = 0x00080000,
+
         /// <summary>
         /// An Schannel provider connection is instructed to not authenticate the server automatically.
         /// </summary>

Contexts/SecureBuffer.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace NSspi.Contexts
+{
+    [StructLayout( LayoutKind.Sequential )]
+    public unsafe struct SecureBuffer
+    {
+        public int Count;
+
+        public BufferType Type;
+
+        // A pointer to a byte[]
+        public IntPtr Buffer;
+    }
+}

Contexts/SecureBufferDataRep.cs

@@ -0,0 +1,18 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace NSspi.Contexts
+{
+    public enum SecureBufferDataRep : int
+    {
+        /*
+        #define SECURITY_NATIVE_DREP        0x00000010
+        #define SECURITY_NETWORK_DREP       0x00000000
+        */
+        Nativee = 0x10,
+        Network = 0x00
+    }
+}

Contexts/SecureBufferDesc.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.InteropServices;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace NSspi.Contexts
+{
+    [StructLayout( LayoutKind.Sequential)]
+    public unsafe struct SecureBufferDesc
+    {
+        public int Version;
+        public int NumBuffers;
+
+        // A pointer to a SecureBuffer[]
+        public IntPtr Buffers;
+
+        public const int ApiVersion = 0;
+    }
+
+
+}

Contexts/SecureBufferType.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace NSspi.Contexts
+{
+    public enum BufferType : int
+    {
+        Empty = 0x00,
+        Data = 0x01,
+        Token = 0x02,
+        Parameters = 0x03,
+        Missing = 0x04,
+        Extra = 0x05,
+        Trailer = 0x06,
+        Header = 0x07,
+        Padding = 0x09,
+        Stream = 0x0A,
+        ChannelBindings = 0x0E,
+        TargetHost = 0x10,
+        ReadOnlyFlag = unchecked( (int)0x80000000 ),
+        ReadOnlyWithChecksum = 0x10000000
+    }
+}