diff --git a/app/assets/javascripts/discourse/initializers/strip-mobile-app-url-params.js.es6 b/app/assets/javascripts/discourse/initializers/strip-mobile-app-url-params.js.es6
new file mode 100644
index 00000000000..04f78881724
--- /dev/null
+++ b/app/assets/javascripts/discourse/initializers/strip-mobile-app-url-params.js.es6
@@ -0,0 +1,30 @@
+export default {
+  name: "strip-mobile-app-url-params",
+
+  initialize() {
+    let queryStrings = window.location.search;
+
+    if (queryStrings.indexOf("user_api_public_key") !== -1) {
+      let params = queryStrings.startsWith("?")
+        ? queryStrings.substr(1).split("&")
+        : [];
+
+      params = params.filter(param => {
+        return (
+          !param.startsWith("user_api_public_key=") &&
+          !param.startsWith("auth_redirect=")
+        );
+      });
+
+      queryStrings = params.length > 0 ? `?${params.join("&")}` : "";
+
+      if (window.history && window.history.replaceState) {
+        window.history.replaceState(
+          null,
+          null,
+          `${location.pathname}${queryStrings}${location.hash}`
+        );
+      }
+    }
+  }
+};
diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 6fd9b79daf3..3f257137647 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -731,6 +731,26 @@ class ApplicationController < ActionController::Base
         redirect_to path(redirect_path)
       end
     end
+
+    # Used by clients authenticated via user API.
+    # Redirects to provided URL scheme if
+    # - request uses a valid public key and auth_redirect scheme
+    # - one_time_password scope is allowed
+    if !current_user &&
+      params.has_key?(:user_api_public_key) &&
+      params.has_key?(:auth_redirect)
+      begin
+        OpenSSL::PKey::RSA.new(params[:user_api_public_key])
+      rescue OpenSSL::PKey::RSAError
+        return render plain: I18n.t("user_api_key.invalid_public_key")
+      end
+
+      if UserApiKey.invalid_auth_redirect?(params[:auth_redirect])
+        return render plain: I18n.t("user_api_key.invalid_auth_redirect")
+      end
+      redirect_to("#{params[:auth_redirect]}?otp=true") if UserApiKey.allowed_scopes.superset?(Set.new(["one_time_password"]))
+    end
+
   end
 
   def block_if_readonly_mode
diff --git a/app/controllers/session_controller.rb b/app/controllers/session_controller.rb
index fa46aad1af6..52732b34c6f 100644
--- a/app/controllers/session_controller.rb
+++ b/app/controllers/session_controller.rb
@@ -12,7 +12,7 @@ class SessionController < ApplicationController
   before_action :check_local_login_allowed, only: %i(create forgot_password email_login)
   before_action :rate_limit_login, only: %i(create email_login)
   skip_before_action :redirect_to_login_if_required
-  skip_before_action :preload_json, :check_xhr, only: %i(sso sso_login sso_provider destroy email_login)
+  skip_before_action :preload_json, :check_xhr, only: %i(sso sso_login sso_provider destroy email_login one_time_password)
 
   ACTIVATE_USER_KEY = "activate_user"
 
@@ -321,6 +321,20 @@ class SessionController < ApplicationController
     render layout: 'no_ember'
   end
 
+  def one_time_password
+    otp_username = $redis.get "otp_#{params[:token]}"
+
+    if otp_username && user = User.find_by_username(otp_username)
+      log_on_user(user)
+      $redis.del "otp_#{params[:token]}"
+      return redirect_to path("/")
+    else
+      @error = I18n.t('user_api_key.invalid_token')
+    end
+
+    render layout: 'no_ember'
+  end
+
   def forgot_password
     params.require(:login)
 
diff --git a/app/controllers/user_api_keys_controller.rb b/app/controllers/user_api_keys_controller.rb
index 30ea1290165..ab80568668f 100644
--- a/app/controllers/user_api_keys_controller.rb
+++ b/app/controllers/user_api_keys_controller.rb
@@ -2,11 +2,11 @@ class UserApiKeysController < ApplicationController
 
   layout 'no_ember'
 
-  requires_login only: [:create, :revoke, :undo_revoke]
-  skip_before_action :redirect_to_login_if_required, only: [:new]
+  requires_login only: [:create, :create_otp, :revoke, :undo_revoke]
+  skip_before_action :redirect_to_login_if_required, only: [:new, :otp]
   skip_before_action :check_xhr, :preload_json
 
-  AUTH_API_VERSION ||= 3
+  AUTH_API_VERSION ||= 4
 
   def new
 
@@ -51,17 +51,15 @@ class UserApiKeysController < ApplicationController
 
     require_params
 
-    if params.key?(:auth_redirect) && SiteSetting.allowed_user_api_auth_redirects
-        .split('|')
-        .none? { |u| WildcardUrlChecker.check_url(u, params[:auth_redirect]) }
-
-      raise Discourse::InvalidAccess
+    if params.key?(:auth_redirect)
+      raise Discourse::InvalidAccess if UserApiKey.invalid_auth_redirect?(params[:auth_redirect])
     end
 
     raise Discourse::InvalidAccess unless meets_tl?
 
     validate_params
     @application_name = params[:application_name]
+    scopes = params[:scopes].split(",")
 
     # destroy any old keys we had
     UserApiKey.where(user_id: current_user.id, client_id: params[:client_id]).destroy_all
@@ -72,7 +70,7 @@ class UserApiKeysController < ApplicationController
       user_id: current_user.id,
       push_url: params[:push_url],
       key: SecureRandom.hex,
-      scopes: params[:scopes].split(",")
+      scopes: scopes
     )
 
     # we keep the payload short so it encrypts easily with public key
@@ -87,8 +85,15 @@ class UserApiKeysController < ApplicationController
     public_key = OpenSSL::PKey::RSA.new(params[:public_key])
     @payload = Base64.encode64(public_key.public_encrypt(@payload))
 
+    if scopes.include?("one_time_password")
+      # encrypt one_time_password separately to bypass 128 chars encryption limit
+      otp_payload = one_time_password(public_key, current_user.username)
+    end
+
     if params[:auth_redirect]
-      redirect_to("#{params[:auth_redirect]}?payload=#{CGI.escape(@payload)}")
+      redirect_path = "#{params[:auth_redirect]}?payload=#{CGI.escape(@payload)}"
+      redirect_path << "&oneTimePassword=#{CGI.escape(otp_payload)}" if scopes.include?("one_time_password")
+      redirect_to(redirect_path)
     else
       respond_to do |format|
         format.html { render :show }
@@ -100,6 +105,38 @@ class UserApiKeysController < ApplicationController
     end
   end
 
+  def otp
+    require_params_otp
+
+    unless current_user
+      cookies[:destination_url] = request.fullpath
+
+      if SiteSetting.enable_sso?
+        redirect_to path('/session/sso')
+      else
+        redirect_to path('/login')
+      end
+      return
+    end
+
+    @application_name = params[:application_name]
+    @public_key = params[:public_key]
+    @auth_redirect = params[:auth_redirect]
+  end
+
+  def create_otp
+    require_params_otp
+
+    raise Discourse::InvalidAccess if UserApiKey.invalid_auth_redirect?(params[:auth_redirect])
+    raise Discourse::InvalidAccess unless meets_tl?
+
+    public_key = OpenSSL::PKey::RSA.new(params[:public_key])
+    otp_payload = one_time_password(public_key, current_user.username)
+
+    redirect_path = "#{params[:auth_redirect]}?oneTimePassword=#{CGI.escape(otp_payload)}"
+    redirect_to(redirect_path)
+  end
+
   def revoke
     revoke_key = find_key if params[:id]
 
@@ -141,15 +178,30 @@ class UserApiKeysController < ApplicationController
 
   def validate_params
     requested_scopes = Set.new(params[:scopes].split(","))
-
     raise Discourse::InvalidAccess unless UserApiKey.allowed_scopes.superset?(requested_scopes)
 
     # our pk has got to parse
     OpenSSL::PKey::RSA.new(params[:public_key])
   end
 
+  def require_params_otp
+    [
+     :public_key,
+     :auth_redirect,
+     :application_name
+    ].each { |p| params.require(p) }
+  end
+
   def meets_tl?
     current_user.staff? || current_user.trust_level >= SiteSetting.min_trust_level_for_user_api_key
   end
 
+  def one_time_password(public_key, username)
+    raise Discourse::InvalidAccess unless UserApiKey.allowed_scopes.superset?(Set.new(["one_time_password"]))
+
+    otp = SecureRandom.hex
+    $redis.setex "otp_#{otp}", 10.minutes, username
+
+    Base64.encode64(public_key.public_encrypt(otp))
+  end
 end
diff --git a/app/models/user_api_key.rb b/app/models/user_api_key.rb
index 7a3747fec98..0895d2c0b7d 100644
--- a/app/models/user_api_key.rb
+++ b/app/models/user_api_key.rb
@@ -5,6 +5,7 @@ class UserApiKey < ActiveRecord::Base
     write: [:get, :post, :patch, :put, :delete],
     message_bus: [[:post, 'message_bus']],
     push: nil,
+    one_time_password: nil,
     notifications: [[:post, 'message_bus'], [:get, 'notifications#index'], [:put, 'notifications#mark_read']],
     session_info: [
       [:get, 'session#current'],
@@ -63,6 +64,11 @@ class UserApiKey < ActiveRecord::Base
     end
   end
 
+  def self.invalid_auth_redirect?(auth_redirect)
+    return SiteSetting.allowed_user_api_auth_redirects
+        .split('|')
+        .none? { |u| WildcardUrlChecker.check_url(u, auth_redirect) }
+  end
 end
 
 # == Schema Information
diff --git a/app/views/session/one_time_password.html.erb b/app/views/session/one_time_password.html.erb
new file mode 100644
index 00000000000..a0faef79d98
--- /dev/null
+++ b/app/views/session/one_time_password.html.erb
@@ -0,0 +1,5 @@
+<%if @error%>
+  <div class='alert alert-error'>
+    <%= @error %>
+  </div>
+<%end%>
diff --git a/app/views/user_api_keys/otp.html.erb b/app/views/user_api_keys/otp.html.erb
new file mode 100644
index 00000000000..9c96d638d2d
--- /dev/null
+++ b/app/views/user_api_keys/otp.html.erb
@@ -0,0 +1,9 @@
+<h1><%= t("user_api_key.otp_description", application_name: @application_name) %></h1>
+<div class='authorize-api-key'>
+<%= form_tag(user_api_key_otp_path) do %>
+  <%= hidden_field_tag 'application_name', @application_name %>
+  <%= hidden_field_tag 'public_key', @public_key%>
+  <%= hidden_field_tag('auth_redirect', @auth_redirect) %>
+  <%= submit_tag t('user_api_key.authorize'), class: 'btn btn-danger' %>
+<% end %>
+</div>
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 34d793b808d..d3d6ba842d0 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -912,6 +912,7 @@ en:
     read_write: "read/write"
     description: '"%{application_name}" is requesting the following access to your account:'
     instructions: 'We just generated a new user API key for you to use with "%{application_name}", please paste the following key into your application:'
+    otp_description: 'Would you like to allow "%{application_name}" to access this site?'
     no_trust_level: "Sorry, you do not have the required trust level to access the user API"
     generic_error: "Sorry, we are unable to issue user API keys, this feature may be disabled by the site admin"
     scopes:
@@ -921,7 +922,10 @@ en:
       session_info: "Read user session info"
       read: "Read all"
       write: "Write all"
-
+      one_time_password: "Create a one-time login token"
+    invalid_public_key: "Sorry, the public key is invalid."
+    invalid_auth_redirect: "Sorry, this auth_redirect host is not allowed."
+    invalid_token: "Missing, invalid or expired token."
   flags:
     errors:
       already_handled: "Flag was already handled"
diff --git a/config/routes.rb b/config/routes.rb
index 0787d9eeb85..08233e7b0c6 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -331,6 +331,7 @@ Discourse::Application.routes.draw do
   get "session/csrf" => "session#csrf"
   get "session/email-login/:token" => "session#email_login"
   post "session/email-login/:token" => "session#email_login"
+  get "session/otp/:token" => "session#one_time_password", constraints: { token: /[0-9a-f]+/ }
   get "composer_messages" => "composer_messages#index"
   post "composer/parse_html" => "composer#parse_html"
 
@@ -841,6 +842,8 @@ Discourse::Application.routes.draw do
   post "/user-api-key" => "user_api_keys#create"
   post "/user-api-key/revoke" => "user_api_keys#revoke"
   post "/user-api-key/undo-revoke" => "user_api_keys#undo_revoke"
+  get "/user-api-key/otp" => "user_api_keys#otp"
+  post "/user-api-key/otp" => "user_api_keys#create_otp"
 
   get "/safe-mode" => "safe_mode#index"
   post "/safe-mode" => "safe_mode#enter", as: "safe_mode_enter"
diff --git a/config/site_settings.yml b/config/site_settings.yml
index 0132538bbd2..60d4f2eff1c 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -1109,7 +1109,7 @@ files:
     regex: '^((https?:)?\/)?\/.+[^\/]'
     shadowed_by_global: true
   restrict_letter_avatar_colors:
-    default: ''
+    default: ""
     type: list
     list_type: compact
     validator: "ColorListValidator"
@@ -1231,11 +1231,11 @@ security:
   enforce_second_factor:
     client: true
     type: enum
-    default: 'no'
+    default: "no"
     choices:
-      - 'no'
-      - 'staff'
-      - 'all'
+      - "no"
+      - "staff"
+      - "all"
   force_https:
     default: false
     shadowed_by_global: true
@@ -1919,7 +1919,7 @@ user_api:
   allow_user_api_keys:
     default: true
   allow_user_api_key_scopes:
-    default: "read|write|message_bus|push|notifications|session_info"
+    default: "read|write|message_bus|push|notifications|session_info|one_time_password"
     type: list
   max_api_keys_per_user:
     default: 10
diff --git a/spec/requests/application_controller_spec.rb b/spec/requests/application_controller_spec.rb
index 06a2719519c..83327e81cae 100644
--- a/spec/requests/application_controller_spec.rb
+++ b/spec/requests/application_controller_spec.rb
@@ -345,6 +345,61 @@ RSpec.describe ApplicationController do
     end
   end
 
+  describe 'Delegated auth' do
+    let :public_key do
+      <<~TXT
+      -----BEGIN PUBLIC KEY-----
+      MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDh7BS7Ey8hfbNhlNAW/47pqT7w
+      IhBz3UyBYzin8JurEQ2pY9jWWlY8CH147KyIZf1fpcsi7ZNxGHeDhVsbtUKZxnFV
+      p16Op3CHLJnnJKKBMNdXMy0yDfCAHZtqxeBOTcCo1Vt/bHpIgiK5kmaekyXIaD0n
+      w0z/BYpOgZ8QwnI5ZwIDAQAB
+      -----END PUBLIC KEY-----
+      TXT
+    end
+
+    let :args do
+      {
+        auth_redirect: 'http://no-good.com',
+        user_api_public_key: "not-a-valid-public-key"
+      }
+    end
+
+    it 'disallows invalid public_key param' do
+      args[:auth_redirect] = "discourse://auth_redirect"
+      get "/latest", params: args
+
+      expect(response.body).to eq(I18n.t("user_api_key.invalid_public_key"))
+    end
+
+    it 'does not allow invalid auth_redirect' do
+      args[:user_api_public_key] = public_key
+      get "/latest", params: args
+
+      expect(response.body).to eq(I18n.t("user_api_key.invalid_auth_redirect"))
+    end
+
+    it 'does not redirect if one_time_password scope is disallowed' do
+      SiteSetting.allow_user_api_key_scopes = "read|write"
+      args[:user_api_public_key] = public_key
+      args[:auth_redirect] = "discourse://auth_redirect"
+
+      get "/latest", params: args
+
+      expect(response.status).to_not eq(302)
+      expect(response).to_not redirect_to("#{args[:auth_redirect]}?otp=true")
+    end
+
+    it 'redirects correctly with valid params' do
+      args[:user_api_public_key] = public_key
+      args[:auth_redirect] = "discourse://auth_redirect"
+
+      get "/categories", params: args
+
+      expect(response.status).to eq(302)
+      expect(response).to redirect_to("#{args[:auth_redirect]}?otp=true")
+    end
+  end
+
   describe 'Content Security Policy' do
     it 'is enabled by SiteSettings' do
       SiteSetting.content_security_policy = false
diff --git a/spec/requests/session_controller_spec.rb b/spec/requests/session_controller_spec.rb
index 443c1e801bf..2795ffdb09f 100644
--- a/spec/requests/session_controller_spec.rb
+++ b/spec/requests/session_controller_spec.rb
@@ -1288,6 +1288,45 @@ RSpec.describe SessionController do
     end
   end
 
+  describe '#one_time_password' do
+    context 'missing token' do
+      it 'returns the right response' do
+        get "/session/otp"
+        expect(response.status).to eq(404)
+      end
+    end
+
+    context 'invalid token' do
+      it 'returns the right response' do
+        get "/session/otp/asd1231dasd123"
+
+        expect(response.status).to eq(404)
+      end
+
+      context 'when token is valid' do
+        it 'should authenticate user and delete token' do
+          user = Fabricate(:user)
+
+          get "/session/current.json"
+          expect(response.status).to eq(404)
+
+          token = SecureRandom.hex
+          $redis.setex "otp_#{token}", 10.minutes, user.username
+
+          get "/session/otp/#{token}"
+
+          expect(response.status).to eq(302)
+          expect(response).to redirect_to("/")
+          expect($redis.get("otp_#{token}")).to eq(nil)
+
+          get "/session/current.json"
+          expect(response.status).to eq(200)
+        end
+      end
+    end
+
+  end
+
   describe '#forgot_password' do
     it 'raises an error without a username parameter' do
       post "/session/forgot_password.json"
diff --git a/spec/requests/user_api_keys_controller_spec.rb b/spec/requests/user_api_keys_controller_spec.rb
index e2cce3d8a36..c66e5b6faee 100644
--- a/spec/requests/user_api_keys_controller_spec.rb
+++ b/spec/requests/user_api_keys_controller_spec.rb
@@ -48,7 +48,7 @@ describe UserApiKeysController do
     it "supports a head request cleanly" do
       head "/user-api-key/new"
       expect(response.status).to eq(200)
-      expect(response.headers["Auth-Api-Version"]).to eq("3")
+      expect(response.headers["Auth-Api-Version"]).to eq("4")
     end
   end
 
@@ -156,7 +156,7 @@ describe UserApiKeysController do
 
       expect(parsed["nonce"]).to eq(args[:nonce])
       expect(parsed["push"]).to eq(false)
-      expect(parsed["api"]).to eq(3)
+      expect(parsed["api"]).to eq(4)
 
       key = user.user_api_keys.first
       expect(key.scopes).to include("push")
@@ -168,7 +168,7 @@ describe UserApiKeysController do
       SiteSetting.allowed_user_api_auth_redirects = args[:auth_redirect]
       SiteSetting.allowed_user_api_push_urls = "https://push.it/here"
 
-      args[:scopes] = "push,notifications,message_bus,session_info"
+      args[:scopes] = "push,notifications,message_bus,session_info,one_time_password"
       args[:push_url] = "https://push.it/here"
 
       user = Fabricate(:user, trust_level: 0)
@@ -193,7 +193,7 @@ describe UserApiKeysController do
       api_key = UserApiKey.find_by(key: parsed["key"])
 
       expect(api_key.user_id).to eq(user.id)
-      expect(api_key.scopes.sort).to eq(["push", "message_bus", "notifications", "session_info"].sort)
+      expect(api_key.scopes.sort).to eq(["push", "message_bus", "notifications", "session_info", "one_time_password"].sort)
       expect(api_key.push_url).to eq("https://push.it/here")
 
       uri.query = ""
@@ -204,6 +204,14 @@ describe UserApiKeysController do
       post "/user-api-key.json", params: args
 
       expect(response.status).to eq(302)
+
+      one_time_password = query.split("oneTimePassword=")[1]
+      encrypted_otp = Base64.decode64(CGI.unescape(one_time_password))
+
+      parsed_otp = key.private_decrypt(encrypted_otp)
+      redis_key = "otp_#{parsed_otp}"
+
+      expect($redis.get(redis_key)).to eq(user.username)
     end
 
     it "will just show the payload if no redirect" do
@@ -251,4 +259,78 @@ describe UserApiKeysController do
       expect(response.status).to eq(302)
     end
   end
+
+  context '#create-one-time-password' do
+    let :otp_args do
+      {
+        auth_redirect: 'http://somewhere.over.the/rainbow',
+        application_name: 'foo',
+        public_key: public_key
+      }
+    end
+
+    it "does not allow anon" do
+      post "/user-api-key/otp", params: otp_args
+      expect(response.status).to eq(403)
+    end
+
+    it "refuses to redirect to disallowed place" do
+      sign_in(Fabricate(:user))
+      post "/user-api-key/otp", params: otp_args
+      expect(response.status).to eq(403)
+    end
+
+    it "will allow one-time-password for staff without TL" do
+      SiteSetting.min_trust_level_for_user_api_key = 2
+      SiteSetting.allowed_user_api_auth_redirects = otp_args[:auth_redirect]
+
+      user = Fabricate(:user, trust_level: 1, moderator: true)
+
+      sign_in(user)
+
+      post "/user-api-key/otp", params: otp_args
+      expect(response.status).to eq(302)
+    end
+
+    it "will not allow one-time-password unless TL is met" do
+      SiteSetting.min_trust_level_for_user_api_key = 2
+      SiteSetting.allowed_user_api_auth_redirects = otp_args[:auth_redirect]
+
+      user = Fabricate(:user, trust_level: 1)
+      sign_in(user)
+
+      post "/user-api-key/otp", params: otp_args
+      expect(response.status).to eq(403)
+    end
+
+    it "will not allow one-time-password if one_time_password scope is disallowed" do
+      SiteSetting.allow_user_api_key_scopes = "read|write"
+      SiteSetting.allowed_user_api_auth_redirects = otp_args[:auth_redirect]
+      user = Fabricate(:user)
+      sign_in(user)
+
+      post "/user-api-key/otp", params: otp_args
+      expect(response.status).to eq(403)
+    end
+
+    it "will return one-time-password when args are valid" do
+      SiteSetting.allowed_user_api_auth_redirects = otp_args[:auth_redirect]
+      user = Fabricate(:user)
+      sign_in(user)
+
+      post "/user-api-key/otp", params: otp_args
+      expect(response.status).to eq(302)
+
+      uri = URI.parse(response.redirect_url)
+
+      query = uri.query
+      payload = query.split("oneTimePassword=")[1]
+      encrypted = Base64.decode64(CGI.unescape(payload))
+      key = OpenSSL::PKey::RSA.new(private_key)
+
+      parsed = key.private_decrypt(encrypted)
+
+      expect($redis.get("otp_#{parsed}")).to eq(user.username)
+    end
+  end
 end