Adds integrated auth (starter)?

- no need for #36
- maybe starting point for #43?

Sources/Mailozaurr/Authentication/SaslMechanismNtlmIntegrated.cs

@@ -0,0 +1,91 @@
+using NSspi;
+using NSspi.Contexts;
+using NSspi.Credentials;
+
+namespace Mailozaurr {
+    /// <summary>
+    /// The NTLM Integrated Auth SASL mechanism.
+    /// </summary>
+    /// <remarks>
+    /// A SASL mechanism based on NTLM using the credentials of the current user 
+    /// via Windows Integrated Authentication (SSPI).
+    /// </remarks>
+    /// <inheritdoc cref="SaslMechanism"/>
+    public class SaslMechanismNtlmIntegrated : SaslMechanism {
+        enum LoginState {
+            Initial,
+            Challenge
+        }
+
+        LoginState state;
+        ClientContext sspiContext;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="SaslMechanismNtlmIntegrated"/> class.
+        /// </summary>
+        /// <remarks>
+        /// Creates a new NTLM Integrated Auth SASL context.
+        /// </remarks>
+        public SaslMechanismNtlmIntegrated() : base(string.Empty, string.Empty) {
+        }
+
+        /// <inheritdoc cref="SaslMechanism.MechanismName"/>
+        public override string MechanismName => "NTLM";
+
+        /// <inheritdoc cref="SaslMechanism.SupportsInitialResponse"/>
+        public override bool SupportsInitialResponse => true;
+
+        /// <summary>
+        /// The authenticated user name.
+        /// </summary>
+        public virtual string AuthenticatedUserName => sspiContext.ContextUserName;
+
+        /// <inheritdoc cref="SaslMechanism.Challenge(byte[], int, int, CancellationToken)"/>
+        /// <exception cref="InvalidOperationException">
+        /// The SASL mechanism is already authenticated.
+        /// </exception>
+        protected override byte[] Challenge(byte[] token, int startIndex, int length, CancellationToken cancellationToken) {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            if (IsAuthenticated) {
+                throw new InvalidOperationException();
+            }
+
+            InitializeSSPIContext();
+
+            byte[] serverResponse;
+
+            if (state == LoginState.Initial) {
+                sspiContext.Init(null, out serverResponse);
+                state = LoginState.Challenge;
+            } else {
+                sspiContext.Init(token, out serverResponse);
+                IsAuthenticated = true;
+            }
+
+            return serverResponse;
+        }
+
+        private void InitializeSSPIContext() {
+            if (sspiContext != null) {
+                return;
+            }
+
+            var credential = new ClientCurrentCredential(PackageNames.Ntlm);
+
+            sspiContext = new ClientContext(
+                credential,
+                string.Empty,
+                ContextAttrib.InitIntegrity
+                | ContextAttrib.ReplayDetect
+                | ContextAttrib.SequenceDetect
+                | ContextAttrib.Confidentiality);
+        }
+
+
+        public override void Reset() {
+            state = LoginState.Initial;
+            base.Reset();
+        }
+    }
+}