store basic auth credentials as hash instead of raw value

Update how the basic auth credentials are stored in the application to use
a hashed value instead of the raw value. This prevents the raw value from
being accidentally leaked.
Add new `auth` package for functions related to authentication.

src/auth/hash.go

@@ -0,0 +1,14 @@
+package auth
+
+import (
+	"crypto/sha256"
+)
+
+func Hash(data []byte) []byte {
+	b := sha256.Sum256(data)
+	return b[:]
+}
+
+func HashString(data string) string {
+	return string(Hash([]byte(data)))
+}

src/auth/middleware.go

@@ -0,0 +1,30 @@
+package auth
+
+import (
+	"fail2ban-prometheus-exporter/cfg"
+	"net/http"
+)
+
+func BasicAuthMiddleware(handlerFunc http.HandlerFunc, appSettings *cfg.AppSettings) http.HandlerFunc {
+	authEnabled := len(appSettings.BasicAuthUsername) > 0 && len(appSettings.BasicAuthPassword) > 0
+	if authEnabled {
+		return func(w http.ResponseWriter, r *http.Request) {
+			if doesBasicAuthMatch(r, appSettings) {
+				w.WriteHeader(http.StatusUnauthorized)
+			} else {
+				handlerFunc.ServeHTTP(w, r)
+			}
+		}
+	}
+	return handlerFunc
+}
+
+func doesBasicAuthMatch(r *http.Request, appSettings *cfg.AppSettings) bool {
+	rawUsername, rawPassword, ok := r.BasicAuth()
+	if ok {
+		username := HashString(rawUsername)
+		password := HashString(rawPassword)
+		return username == appSettings.BasicAuthUsername && password == appSettings.BasicAuthPassword
+	}
+	return false
+}

src/cfg/cfg.go

@@ -1,6 +1,7 @@
 package cfg
 
 import (
+	"fail2ban-prometheus-exporter/auth"
 	"flag"
 	"fmt"
 	"os"
@@ -23,6 +24,9 @@ type AppSettings struct {
 }
 
 func Parse() *AppSettings {
+	var rawBasicAuthUsername string
+	var rawBasicAuthPassword string
+
 	appSettings := &AppSettings{}
 	flag.BoolVar(&appSettings.VersionMode, "version", false, "show version info and exit")
 	flag.StringVar(&appSettings.MetricsAddress, "web.listen-address", "0.0.0.0", "address to use for the metrics server")
@@ -30,14 +34,20 @@ func Parse() *AppSettings {
 	flag.StringVar(&appSettings.Fail2BanSocketPath, "socket", "", "path to the fail2ban server socket")
 	flag.BoolVar(&appSettings.FileCollectorEnabled, "collector.textfile", false, "enable the textfile collector")
 	flag.StringVar(&appSettings.FileCollectorPath, "collector.textfile.directory", "", "directory to read text files with metrics from")
-	flag.StringVar(&appSettings.BasicAuthUsername, "web.basic-auth.username", "", "username to use to protect endpoints with basic auth")
-	flag.StringVar(&appSettings.BasicAuthPassword, "web.basic-auth.password", "", "password to use to protect endpoints with basic auth")
+	flag.StringVar(&rawBasicAuthUsername, "web.basic-auth.username", "", "username to use to protect endpoints with basic auth")
+	flag.StringVar(&rawBasicAuthPassword, "web.basic-auth.password", "", "password to use to protect endpoints with basic auth")
 
 	flag.Parse()
+	appSettings.setBasicAuthValues(rawBasicAuthUsername, rawBasicAuthPassword)
 	appSettings.validateFlags()
 	return appSettings
 }
 
+func (settings *AppSettings) setBasicAuthValues(username, password string) {
+	settings.BasicAuthUsername = auth.HashString(username)
+	settings.BasicAuthPassword = auth.HashString(password)
+}
+
 func (settings *AppSettings) validateFlags() {
 	var flagsValid = true
 	if !settings.VersionMode {

src/exporter.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fail2ban-prometheus-exporter/auth"
 	"fail2ban-prometheus-exporter/cfg"
 	"fail2ban-prometheus-exporter/collector/f2b"
 	"fail2ban-prometheus-exporter/collector/textfile"
@@ -48,21 +49,6 @@ func metricHandler(w http.ResponseWriter, r *http.Request, collector *textfile.C
 	collector.WriteTextFileMetrics(w, r)
 }
 
-func authMiddleware(handlerFunc http.HandlerFunc, appSettings *cfg.AppSettings) http.HandlerFunc {
-	authEnabled := len(appSettings.BasicAuthUsername) > 0 && len(appSettings.BasicAuthPassword) > 0
-	if authEnabled {
-		return func(w http.ResponseWriter, r *http.Request) {
-			username, password, ok := r.BasicAuth()
-			if !ok || username != appSettings.BasicAuthUsername || password != appSettings.BasicAuthPassword {
-				w.WriteHeader(http.StatusUnauthorized)
-				return
-			}
-			handlerFunc.ServeHTTP(w, r)
-		}
-	}
-	return handlerFunc
-}
-
 func main() {
 	appSettings := cfg.Parse()
 	if appSettings.VersionMode {
@@ -78,8 +64,8 @@ func main() {
 		textFileCollector := textfile.NewCollector(appSettings)
 		prometheus.MustRegister(textFileCollector)
 
-		http.HandleFunc("/", authMiddleware(rootHtmlHandler, appSettings))
-		http.HandleFunc(metricsPath, authMiddleware(
+		http.HandleFunc("/", auth.BasicAuthMiddleware(rootHtmlHandler, appSettings))
+		http.HandleFunc(metricsPath, auth.BasicAuthMiddleware(
 			func(w http.ResponseWriter, r *http.Request) {
 				metricHandler(w, r, textFileCollector)
 			},