add greenfield authorize UI docs and fix small issues (#2156)

* add greenfield authorize UI docs and fix small issues

* Update ManageController.APIKeys.cs

* Apply suggestions from code review

Co-authored-by: Dennis Reimann <mail@dennisreimann.de>

Co-authored-by: Dennis Reimann <mail@dennisreimann.de>

BTCPayServer/Controllers/ManageController.APIKeys.cs

@@ -98,8 +98,13 @@ namespace BTCPayServer.Controllers
             permissions ??= Array.Empty<string>();
 
             var requestPermissions = Permission.ToPermissions(permissions);
+            if (redirect?.IsAbsoluteUri is false)
+            {
+                redirect = null;
+            }
             if (!string.IsNullOrEmpty(applicationIdentifier) && redirect != null)
             {
+                
                 //check if there is an app identifier that matches and belongs to the current user
                 var keys = await _apiKeyRepository.GetKeys(new APIKeyRepository.APIKeyQuery()
                 {
@@ -110,7 +115,7 @@ namespace BTCPayServer.Controllers
                     var blob = key.GetBlob();
 
                     if (blob.ApplicationIdentifier != applicationIdentifier ||
-                        blob.ApplicationAuthority != redirect.Authority)
+                        blob.ApplicationAuthority != redirect.AbsoluteUri)
                     {
                         continue;
                     }
@@ -190,7 +195,7 @@ namespace BTCPayServer.Controllers
 
         private void AdjustVMForAuthorization(AuthorizeApiKeysViewModel vm)
         {
-            var parsedPermissions = Permission.ToPermissions(vm.Permissions.Split(';')).GroupBy(permission => permission.Policy);
+            var parsedPermissions = Permission.ToPermissions(vm.Permissions?.Split(';')??Array.Empty<string>()).GroupBy(permission => permission.Policy);
 
             for (var index = vm.PermissionValues.Count - 1; index >= 0; index--)
             {
@@ -266,7 +271,7 @@ namespace BTCPayServer.Controllers
                 case "authorize":
                 case "confirm":
                     var key = command == "authorize"
-                        ? await CreateKey(viewModel, (viewModel.ApplicationIdentifier, viewModel.RedirectUrl?.Authority))
+                        ? await CreateKey(viewModel, (viewModel.ApplicationIdentifier, viewModel.RedirectUrl.AbsoluteUri))
                         : await _apiKeyRepository.GetKey(viewModel.ApiKey);
     
                     if (viewModel.RedirectUrl != null)
@@ -274,7 +279,7 @@ namespace BTCPayServer.Controllers
                         var permissions = key.GetBlob().Permissions;
                         var redirectVm = new PostRedirectViewModel()
                         {
-                            FormUrl = viewModel.RedirectUrl.ToString(),
+                            FormUrl = viewModel.RedirectUrl.AbsoluteUri,
                             Parameters =
                             {
                                 new KeyValuePair<string, string>("apiKey", key.Id),

docs/greenfield-authorization.md

@@ -0,0 +1,24 @@
+
+# GreenField API Authorization Flow
+
+The GreenField API allows two modes of authentication to its endpoints: Basic auth and API Keys. 
+
+## Basic auth
+Basic auth allows you to seamlessly integrate with BTCPay Server's user system using only a traditional user/password login form. This is however a security risk if the application is a third party as they will receive your credentials in plain text and will be able to access your full account.
+
+## API Keys
+BTCPay Server's GreenField API also allows users to generate API keys with [specific permissions](https://docs.btcpayserver.org/API/Greenfield/v1/#section/Authentication/API%20Key). **If you are integrating BTCPay Server into your third-party application, this is the recommended way.**
+
+Asking a user to generate a dedicated API key, with a specific set of permissions can be a bad UX experience. For this scenario, we have the [Authorize User UI](https://docs.btcpayserver.org/API/Greenfield/v1/#tag/Authorization). This allows external applications to request the user to generate an API key with a specific set of permissions by simply generating a URL to BTCPay Server and redirecting the user to it.
+Additionally, there are 2 optional parameters to the endpoint which allow a more seamless integration:
+* if `redirect` is specified, once the API key is created, BTCPay Server redirects the user via a POST submission to the specified `redirect` URL, with a json body containing the API key, user id, and permissions granted.
+* if `applicationIdentifier` is specified (along with `redirect`), BTCPay Server will check if there is an existing API key associated with the user that also has this application identifier, redirect host AND the permissions required match. `applicationIdentifier` is ignored if `redirect` is not specified.
+
+Some examples of a generated Authorize URL:
+* `https://mainnet.demo.btcpayserver.org/api-keys/authorize` - A simplistic request, where no permission is requested. Useful to prove that a user exists on a specific BTCPay Server instance. 
+* `https://mainnet.demo.btcpayserver.org/api-keys/authorize?applicationName=Your%20Application` - Indicates that the API key is being generated for `Your Application`
+* `https://mainnet.demo.btcpayserver.org/api-keys/authorize?applicationName=Your%20Application&redirect=http://gozo.com` - Redirects the user via a POST to `http://gozo.com` with a JSON body containing the API key and its info.
+* `https://mainnet.demo.btcpayserver.org/api-keys/authorize?applicationName=Your%20Application&redirect=http://gozo.com&applicationIdentifier=gozo` - Attempts to match a previously created API key based on the app identifier, domain and permissions and is prompted.
+* `https://mainnet.demo.btcpayserver.org/api-keys/authorize?permissions=btcpay.store.cancreateinvoice&permissions=btcpay.store.canviewinvoices` - A request asking for permissions to create and view invoices on all stores available to the user
+* `https://mainnet.demo.btcpayserver.org/api-keys/authorize?permissions=btcpay.store.cancreateinvoice&permissions=btcpay.store.canviewinvoices&selectiveStores=true` - A request asking for permissions to create and view invoices on stores but also allows the user to choose which stores the application will have the permission to.
+* `https://mainnet.demo.btcpayserver.org/api-keys/authorize?permissions=btcpay.store.cancreateinvoice&permissions=btcpay.store.canviewinvoices&strict=false` - A request asking for permissions but allows the user to remove or add to the requested permission list.