Add additional documentation about client authentication (#3009)

* test

* docs

* Revert "test"

This reverts commit ecd1189.

* Update Authentication.md

* Update Authentication.md

* Update index.md

* prefer async

Author: mgravell

docs/Authentication.md

@@ -0,0 +1,124 @@
+Authentication
+===
+
+There are multiple ways of connecting to a Redis server, depending on the authentication model. The simplest
+(but least secure) approach is to use the `default` user, with no authentication, and no transport security.
+This is as simple as:
+
+``` csharp
+var muxer = await ConnectionMultiplexer.ConnectAsync("myserver"); // or myserver:1241 to use a custom port
+```
+
+This approach is often used for local transient servers - it is simple, but insecure. But from there,
+we can get more complex!
+
+TLS
+===
+
+If your server has TLS enabled, SE.Redis can be instructed to use it. In some cases (AMR, etc), the
+library will recognize the endpoint address, meaning: *you do not need to do anything*. To
+*manually* enable TLS, the `ssl` token can be used:
+
+``` csharp
+var muxer = await ConnectionMultiplexer.ConnectAsync("myserver,ssl=true");
+```
+
+This will work fine if the server is using a server-certificate that is already trusted by the local
+machine. If this is *not* the case, we need to tell the library about the server. This requires
+the `ConfigurationOptions` type:
+
+``` csharp
+var options = ConfigurationOptions.Parse("myserver,ssl=true");
+// or: var options = new ConfigurationOptions { Endpoints = { "myserver" }, Ssl = true };
+// TODO configure
+var muxer = await ConnectionMultiplexer.ConnectAsync(options);
+```
+
+If we have a local *issuer* public certificate (commonly `ca.crt`), we can use:
+
+``` csharp
+options.TrustIssuer(caPath);
+```
+
+Alternatively, in advanced scenarios: to provide your own custom server validation, the `options.CertificateValidation` callback
+can be used; this uses the normal [`RemoteCertificateValidationCallback`](https://learn.microsoft.com/dotnet/api/system.net.security.remotecertificatevalidationcallback)
+API.
+
+Usernames and Passwords
+===
+
+Usernames and passwords can be specified with the `user` and `password` tokens, respectively:
+
+``` csharp
+var muxer = await ConnectionMultiplexer.ConnectAsync("myserver,ssl=true,user=myuser,password=mypassword");
+```
+
+If no `user` is provided, the `default` user is assumed. In some cases, an authentication-token can be
+used in place of a classic password.
+
+Client certificates
+===
+
+If the server is configured to require a client certificate, this can be supplied in multiple ways.
+If you have a local public / private key pair (such as `MyUser2.crt` and `MyUser2.key`), the
+`options.SetUserPemCertificate(...)` method can be used:
+
+``` csharp
+config.SetUserPemCertificate(
+    userCertificatePath: userCrtPath,
+    userKeyPath: userKeyPath
+);
+```
+
+If you have a single `pfx` file that contains the public / private pair, the `options.SetUserPfxCertificate(...)`
+method can be used:
+
+``` csharp
+config.SetUserPfxCertificate(
+    userCertificatePath: userCrtPath,
+    password: filePassword // optional
+);
+```
+
+Alternatively, in advanced scenarios: to provide your own custom client-certificate lookup, the `options.CertificateSelection` callback
+can be used; this uses the normal
+[`LocalCertificateSelectionCallback`](https://learn.microsoft.com/dotnet/api/system.net.security.remotecertificatevalidationcallback)
+API.
+
+User certificates with implicit user authentication
+===
+
+Historically, the client certificate only provided access to the server, but as the `default` user. From 8.6,
+the server can be configured to use client certificates to provide user identity. This replaces the
+usage of passwords, and requires:
+
+- An 8.6+ server, configured to use TLS with client certificates mapped - typically using the `CN` of the certificate as the user.
+- A matching `ACL` user account configured on the server, that is enabled (`on`) - i.e. the `ACL LIST` command should
+  display something like `user MyUser2 on sanitize-payload ~* &* +@all` (the details will vary depending on the user permissions).
+- At the client: access to the client certificate pair.
+
+For example:
+
+``` csharp
+string certRoot = // some path to a folder with ca.crt, MyUser2.crt and MyUser2.key
+
+var options = ConfigurationOptions.Parse("myserver:6380");
+options.SetUserPemCertificate(// automatically enables TLS
+    userCertificatePath: Path.Combine(certRoot, "MyUser2.crt"),
+    userKeyPath: Path.Combine(certRoot, "MyUser2.key"));
+options.TrustIssuer(Path.Combine(certRoot, "ca.crt"));
+await using var conn = await ConnectionMultiplexer.ConnectAsync(options);
+
+// prove we are connected as MyUser2
+var user = (string?)await conn.GetDatabase().ExecuteAsync("acl", "whoami");
+Console.WriteLine(user); // writes "MyUser2"
+```
+
+More info
+===
+
+For more information:
+
+- [Redis Security](https://redis.io/docs/latest/operate/oss_and_stack/management/security/)
+  - [ACL](https://redis.io/docs/latest/operate/oss_and_stack/management/security/acl/)
+  - [TLS](https://redis.io/docs/latest/operate/oss_and_stack/management/security/encryption/)

docs/index.md

@@ -31,6 +31,7 @@ Documentation
 ---
 
 - [Server](Server) - running a redis server
+- [Authentication](Authentication) - connecting to a Redis server with user authentication
 - [Basic Usage](Basics) - getting started and basic usage
 - [Async Timeouts](AsyncTimeouts) - async timeouts and cancellation
 - [Configuration](Configuration) - options available when connecting to redis