Add UsePrivateKey() method

Add UsePrivateKey() extension method that configures a private key file to use to auto-generate client secrets.

samples/Mvc.Client/Startup.cs

@@ -4,9 +4,6 @@
  * for more information concerning the license and the contributors participating to this project.
  */
 
-using System;
-using System.IO;
-using System.Linq;
 using Microsoft.AspNetCore.Authentication.Cookies;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Mvc;
@@ -44,30 +41,11 @@ public void ConfigureServices(IServiceCollection services)
 
             .AddApple(options =>
             {
-                options.GenerateClientSecret = true;
                 options.ClientId = Configuration["AppleClientId"];
                 options.KeyId = Configuration["AppleKeyId"];
                 options.TeamId = Configuration["AppleTeamId"];
-
-                options.PrivateKeyBytes = async (keyId) =>
-                {
-                    var privateKeyFile = HostingEnvironment.ContentRootFileProvider.GetFileInfo($"AuthKey_{keyId}.p8");
-                    string privateKey;
-
-                    using (var stream = privateKeyFile.CreateReadStream())
-                    using (var reader = new StreamReader(stream))
-                    {
-                        privateKey = await reader.ReadToEndAsync();
-                    }
-
-                    if (privateKey.StartsWith("-----BEGIN PRIVATE KEY-----", StringComparison.Ordinal))
-                    {
-                        string[] keyLines = privateKey.Split('\n');
-                        privateKey = string.Join(string.Empty, keyLines.Skip(1).Take(keyLines.Length - 2));
-                    }
-
-                    return Convert.FromBase64String(privateKey);
-                };
+                options.UsePrivateKey(
+                    (keyId) => HostingEnvironment.ContentRootFileProvider.GetFileInfo($"AuthKey_{keyId}.p8"));
             })
 
             .AddGoogle(options =>

src/AspNet.Security.OAuth.Apple/AppleAuthenticationOptionsExtensions.cs

@@ -0,0 +1,63 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (http://www.apache.org/licenses/LICENSE-2.0)
+ * See https://github.com/aspnet-contrib/AspNet.Security.OAuth.Providers
+ * for more information concerning the license and the contributors participating to this project.
+ */
+
+using System;
+using System.IO;
+using System.Linq;
+using AspNet.Security.OAuth.Apple;
+using JetBrains.Annotations;
+using Microsoft.Extensions.FileProviders;
+
+namespace Microsoft.Extensions.DependencyInjection
+{
+    /// <summary>
+    /// Extension methods to configure Sign In with Apple authentication capabilities for an HTTP application pipeline.
+    /// </summary>
+    public static class AppleAuthenticationOptionsExtensions
+    {
+        /// <summary>
+        /// Configures the application to use a specified private to generate a client secret for the provider.
+        /// </summary>
+        /// <param name="options">The Apple authentication options to configure.</param>
+        /// <param name="privateKeyFile">
+        /// A delegate to a method to return the <see cref="IFileInfo"/> for the private
+        /// key which is passed the value of <see cref="AppleAuthenticationOptions.KeyId"/>.
+        /// </param>
+        /// <param name="privateKeyPassword">The optional password for the private key.</param>
+        /// <returns>
+        /// The value of the <paramref name="options"/> argument.
+        /// </returns>
+        public static AppleAuthenticationOptions UsePrivateKey(
+            [NotNull] this AppleAuthenticationOptions options,
+            [NotNull] Func<string, IFileInfo> privateKeyFile,
+            [CanBeNull] string privateKeyPassword = null)
+        {
+            options.GenerateClientSecret = true;
+            options.PrivateKeyPassword = privateKeyPassword ?? string.Empty;
+            options.PrivateKeyBytes = async (keyId) =>
+            {
+                var fileInfo = privateKeyFile(keyId);
+                string privateKey;
+
+                using (var stream = fileInfo.CreateReadStream())
+                using (var reader = new StreamReader(stream))
+                {
+                    privateKey = await reader.ReadToEndAsync();
+                }
+
+                if (privateKey.StartsWith("-----BEGIN PRIVATE KEY-----", StringComparison.Ordinal))
+                {
+                    string[] lines = privateKey.Split('\n');
+                    privateKey = string.Join(string.Empty, lines.Skip(1).Take(lines.Length - 2));
+                }
+
+                return Convert.FromBase64String(privateKey);
+            };
+
+            return options;
+        }
+    }
+}