Update AzureOpenAIConfig validation to cater for reverse proxy endpoi…

…nt (#875)

## Motivation and Context (Why the change? What's the scenario?)

The intent of the change in this PR is to allow Azure OpenAI requests to
be made over HTTP to a reverse proxy that is running within the same
Docker Compose orchestration for local development.

We have reverse proxy in place for Azure OpenAI that routes requests to
different service instances based on available quota. When deployed to
Azure all services can use HTTPS and Kernel Memory can use the reverse
proxy for Azure OpenAI requests.

Visual Studio with Docker Compose orchestration is used for local
development. The reverse proxy project is started and exposes both HTTP
and HTTPS ports. A local dev certificate is used automatically by the
tooling and the reverse proxy can be called over HTTPS from the host
machine using `localhost`.

## High level description (Approach, Design)

Update the validation in the `AzureOpenAIConfig` class:

- Allow to define a custom list of SSL cert thumbprints to be trusted

---------

Co-authored-by: Devis Lucato <[email protected]>
Co-authored-by: Devis Lucato <[email protected]>

extensions/AzureOpenAI/AzureOpenAI/AzureOpenAIConfig.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
+using System.Collections.Generic;
 using Azure.Core;
 
 #pragma warning disable IDE0130 // reduce number of "using" statements
@@ -83,6 +84,12 @@ public enum APITypes
     /// </summary>
     public int MaxRetries { get; set; } = 10;
 
+    /// <summary>
+    /// Thumbprints of certificates that should be trusted for HTTPS requests when SSL policy errors are detected.
+    /// This should only be used for local development when using a proxy to call the OpenAI endpoints.
+    /// </summary>
+    public HashSet<string> TrustedCertificateThumbprints { get; set; } = [];
+
     /// <summary>
     /// Set credentials manually from code
     /// </summary>

extensions/AzureOpenAI/AzureOpenAI/Internals/AzureOpenAIClientBuilder.cs

@@ -1,9 +1,12 @@
-// Copyright (c) Microsoft. All rights reserved.
+// Copyright (c) Microsoft. All rights reserved.
 
 using System;
 using System.ClientModel;
 using System.ClientModel.Primitives;
+using System.Linq;
 using System.Net.Http;
+using System.Net.Security;
+using System.Security.Cryptography;
 using Azure.AI.OpenAI;
 using Azure.Identity;
 using Microsoft.Extensions.Logging;
@@ -32,6 +35,13 @@ internal static AzureOpenAIClient Build(
         // See https://github.com/Azure/azure-sdk-for-net/issues/46109
         options.AddPolicy(new SingleAuthorizationHeaderPolicy(), PipelinePosition.PerTry);
 
+        if (httpClient is null && config.TrustedCertificateThumbprints.Count > 0)
+        {
+#pragma warning disable CA2000 // False Positive: https://github.com/dotnet/roslyn-analyzers/issues/4636
+            httpClient = BuildHttpClientWithCustomCertificateValidation(config);
+#pragma warning restore CA2000
+        }
+
         if (httpClient is not null)
         {
             options.Transport = new HttpClientPipelineTransport(httpClient);
@@ -57,6 +67,47 @@ internal static AzureOpenAIClient Build(
                 throw new ConfigurationException($"Azure OpenAI: authentication type '{config.Auth:G}' is not supported");
         }
     }
+
+    private static HttpClient BuildHttpClientWithCustomCertificateValidation(AzureOpenAIConfig config)
+    {
+#pragma warning disable CA2000 // False Positive: https://github.com/dotnet/roslyn-analyzers/issues/4636
+        var handler = new HttpClientHandler();
+#pragma warning restore CA2000
+
+        handler.ClientCertificateOptions = ClientCertificateOption.Manual;
+        handler.ServerCertificateCustomValidationCallback =
+            (_, cert, _, policyErrors) =>
+            {
+                // Pass if there are no policy errors.
+                if (policyErrors == SslPolicyErrors.None) { return true; }
+
+                // Attempt to get the thumbprint of the remote certificate.
+                string? remoteCert;
+                try
+                {
+                    remoteCert = cert?.GetCertHashString();
+                }
+                catch (CryptographicException)
+                {
+                    // Fail if crypto lib is not working
+                    return false;
+                }
+                catch (ArgumentException)
+                {
+                    // Fail if thumbprint format is invalid
+                    return false;
+                }
+
+                // Fail if no thumbprint available
+                if (remoteCert == null) { return false; }
+
+                // Success if the remote cert thumbprint matches any of the trusted ones.
+                return config.TrustedCertificateThumbprints.Any(
+                    trustedCert => string.Equals(remoteCert, trustedCert, StringComparison.OrdinalIgnoreCase));
+            };
+
+        return new HttpClient(handler);
+    }
 }
 
 // Use only for local debugging - Usage:

service/Service/appsettings.json

@@ -339,7 +339,10 @@
         // See https://learn.microsoft.com/azure/ai-services/openai/reference#embeddings
         "MaxEmbeddingBatchSize": 1,
         // How many times to retry in case of throttling.
-        "MaxRetries": 10
+        "MaxRetries": 10,
+        // Thumbprints of certificates that should be trusted for HTTPS requests when SSL policy errors are detected.
+        // This should only be used for local development when using a proxy to call the OpenAI endpoints.
+        "TrustedCertificateThumbprints": []
       },
       "AzureOpenAIText": {
         // "ApiKey" or "AzureIdentity"
@@ -355,7 +358,10 @@
         // "ChatCompletion" or "TextCompletion"
         "APIType": "ChatCompletion",
         // How many times to retry in case of throttling.
-        "MaxRetries": 10
+        "MaxRetries": 10,
+        // Thumbprints of certificates that should be trusted for HTTPS requests when SSL policy errors are detected.
+        // This should only be used for local development when using a proxy to call the OpenAI endpoints.
+        "TrustedCertificateThumbprints": []
       },
       "AzureQueues": {
         // "ConnectionString" or "AzureIdentity". For other options see <AzureQueueConfig>.