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 );
             
-            
+            descriptor = new SecureBufferDesc();
-            status = NativeMethods.InitializeSecurityContext_Client(
+            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,
+                SecureBufferDataRep.Network,
-                0,
                 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
+    }
+}

Credentials/Credential.cs

@@ -78,7 +78,7 @@ namespace NSspi
                 ref this.expiry
             );
 
-            if ( status != SecurityStatus.Success )
+            if ( status != SecurityStatus.OK )
             {
                 throw new SSPIException( "Failed to call AcquireCredentialHandle", status );
             }
@@ -117,7 +117,7 @@ namespace NSspi
                     ref carrier
                 );
 
-                if ( status == SecurityStatus.Success )
+                if ( status == SecurityStatus.OK )
                 {
                     name = Marshal.PtrToStringUni( carrier.Name );
                     NativeMethods.FreeContextBuffer( carrier.Name );
@@ -155,7 +155,7 @@ namespace NSspi
 
                 this.disposed = true;
 
-                if ( disposing && result != SecurityStatus.Success )
+                if ( disposing && result != SecurityStatus.OK )
                 {
                     throw new SSPIException( "Failed to release credentials handle", result );
                 }

NSspi.csproj

@@ -20,6 +20,7 @@
     <DefineConstants>DEBUG;TRACE</DefineConstants>
     <ErrorReport>prompt</ErrorReport>
     <WarningLevel>4</WarningLevel>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
     <DebugType>pdbonly</DebugType>
@@ -47,6 +48,10 @@
     <Compile Include="Contexts\ClientContext.cs" />
     <Compile Include="Contexts\Context.cs" />
     <Compile Include="Contexts\ContextAttrib.cs" />
+    <Compile Include="Contexts\SecureBuffer.cs" />
+    <Compile Include="Contexts\SecureBufferDataRep.cs" />
+    <Compile Include="Contexts\SecureBufferDesc.cs" />
+    <Compile Include="Contexts\SecureBufferType.cs" />
     <Compile Include="Contexts\ServerContext.cs" />
     <Compile Include="Credentials\ClientCredential.cs" />
     <Compile Include="Credentials\Credential.cs" />

NativeMethods.cs

@@ -1,4 +1,5 @@
-using System;
+using NSspi.Contexts;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.InteropServices;
@@ -11,6 +12,11 @@ namespace NSspi
     {
         // http://msdn.microsoft.com/en-us/library/windows/desktop/aa374713(v=vs.85).aspx
 
+        // The REMSSPI sample:
+
+        // A C++ pure client/server example:
+        // http://msdn.microsoft.com/en-us/library/windows/desktop/aa380536(v=vs.85).aspx
+
         /*
         SECURITY_STATUS sResult = AcquireCredentialsHandle(
 		        NULL,											// [in] name of principal. NULL = principal of current security context
@@ -166,18 +172,18 @@ namespace NSspi
             CharSet = CharSet.Unicode,
             SetLastError = true
         )]
-        public static extern SecurityStatus InitializeSecurityContext_Client(
+        public static extern SecurityStatus InitializeSecurityContext_Client1(
             ref long credentialHandle,
-            ref long prevContextHandle, 
+            IntPtr zero, 
             string serverPrincipleName,
-            int requiredAttribs,
+            ContextAttrib requiredAttribs,
             int reserved1,
-            int dataRep,
+            SecureBufferDataRep dataRep,
             IntPtr inputBuffer,
             int reserved2, 
             ref long newContextHandle,
             IntPtr outputBuffer,
-            ref int contextAttribs,
+            ref ContextAttrib contextAttribs,
             ref long expiry
         );
 

Program.cs

@@ -1,4 +1,5 @@
-using System;
+using NSspi.Contexts;
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
@@ -37,13 +38,23 @@ namespace NSspi
 
         private static void CredTest()
         {
-            Credential cred = null;
+            ClientCredential cred = null;
+            ClientContext client;
             try
             {
-                cred = new Credential( SecurityPackage.Negotiate, CredentialType.Client );
+                cred = new ClientCredential( SecurityPackage.Negotiate );
+                Console.Out.WriteLine( cred.Name );
 
-                string name = cred.Name;
+                client = new ClientContext( 
-                Console.Out.WriteLine( name );
+                    cred, 
+                    "", 
+                    ContextAttrib.MutualAuth | 
+                    ContextAttrib.InitIdentify |
+                    ContextAttrib.Confidentiality |
+                    ContextAttrib.ReplayDetect |
+                    ContextAttrib.SequenceDetect
+                );
+                
                 Console.Out.Flush();
             }
             finally

SecurityStatus.cs

@@ -20,15 +20,49 @@ namespace NSspi
     #define SEC_E_NO_CREDENTIALS             _HRESULT_TYPEDEF_(0x8009030EL)
     */
 
-    public enum SecurityStatus : uint
+    public enum SecurityStatus : int
     {
-        Success = 0,
+        // Success / Informational
-        InsufficientMemory = 0x80090300,
+        OK                  = 0x00000000,
-        InvalidHandle = 0x80090301,
+        ContinueNeeded      = unchecked( (int)0x00090312 ),
-        InternalError = 0x80090304,
+        CompleteNeeded      = unchecked( (int)0x00090313 ),
-        SecPkgNotFound = 0x80090305,
+        CompAndContinue     = unchecked( (int)0x00090314 ),
-        NotOwner = 0x80090306,
+        ContextExpired      = unchecked( (int)0x00090317 ),
-        NoCredentials = 0x8009030E,
+        CredentialsNeeded   = unchecked( (int)0x00090320 ),
-        UnknownCredentials = 0x8009030D
+        Renegotiate         = unchecked( (int)0x00090321 ),
+
+        // Errors
+        OutOfMemory         = unchecked( (int)0x80090300 ),
+        InvalidHandle       = unchecked( (int)0x80090301 ),
+        Unsupported         = unchecked( (int)0x80090302 ),
+        TargetUnknown       = unchecked( (int)0x80090303 ),
+        InternalError       = unchecked( (int)0x80090304 ),
+        PackageNotFound     = unchecked( (int)0x80090305 ),
+        NotOwner            = unchecked( (int)0x80090306 ),
+        CannotInstall       = unchecked( (int)0x80090307 ),
+        InvalidToken        = unchecked( (int)0x80090308 ),
+        CannotPack          = unchecked( (int)0x80090309 ),
+        QopNotSupported     = unchecked( (int)0x8009030A ),
+        NoImpersonation     = unchecked( (int)0x8009030B ),
+        LogonDenied         = unchecked( (int)0x8009030C ),
+        UnknownCredentials  = unchecked( (int)0x8009030D ),
+        NoCredentials       = unchecked( (int)0x8009030E ),
+        MessageAltered      = unchecked( (int)0x8009030F ),
+        OutOfSequence       = unchecked( (int)0x80090310 ),
+        NoAuthenticatingAuthority = unchecked( (int)0x80090311 ),
+        IncompleteMessage   = unchecked( (int)0x80090318 ),
+        IncompleteCredentials = unchecked( (int)0x80090320 ),
+        BufferNotEnough     = unchecked( (int)0x80090321 ),
+        WrongPrincipal      = unchecked( (int)0x80090322 ),
+        TimeSkew            = unchecked( (int)0x80090324 ),
+        UntrustedRoot       = unchecked( (int)0x80090325 ),
+        IllegalMessage      = unchecked( (int)0x80090326 ),
+        CertUnknown         = unchecked( (int)0x80090327 ),
+        CertExpired         = unchecked( (int)0x80090328 ),
+        AlgorithmMismatch   = unchecked( (int)0x80090331 ),
+        SecurityQosFailed   = unchecked( (int)0x80090332 ),
+        SmartcardLogonRequired = unchecked( (int)0x8009033E ),
+        UnsupportedPreauth  = unchecked( (int)0x80090343 ),
+        BadBinding  = unchecked( (int)0x80090346 )
     }
 }