Add system properties for persisting the 'remember me' key

This adds the 'airsonic.rememberMeKey' system property (can be set from
command-line with `-Dairsonic.rememberMeKey=<value>`) as well as a
'RememberMeKey' setting in airsonic.properties, so that the key used for
generating 'remember me' tokens can be persisted across server restarts.

It also adds a default, insecure key in case we are running in
development mode with the 'airsonic.development' property set.

airsonic-main/src/main/java/org/airsonic/player/security/GlobalSecurityConfig.java

@@ -33,6 +33,8 @@ public class GlobalSecurityConfig extends GlobalAuthenticationConfigurerAdapter
 
     static final String FAILURE_URL = "/login?error=1";
 
+    static final String DEVELOPMENT_REMEMBER_ME_KEY = "airsonic";
+
     @Autowired
     private SecurityService securityService;
 
@@ -125,6 +127,32 @@ public class GlobalSecurityConfig extends GlobalAuthenticationConfigurerAdapter
             restAuthenticationFilter.setEventPublisher(eventPublisher);
             http = http.addFilterBefore(restAuthenticationFilter, UsernamePasswordAuthenticationFilter.class);
 
+            // Try to load the 'remember me' key.
+            //
+            // Note that using a fixed key compromises security as perfect
+            // forward secrecy is not guaranteed anymore.
+            //
+            // An external entity can then re-use our authentication cookies before
+            // the expiration time, or even, given enough time, recover the password
+            // from the MD5 hash.
+            //
+            // See: https://docs.spring.io/spring-security/site/docs/3.0.x/reference/remember-me.html
+
+            String rememberMeKey = settingsService.getRememberMeKey();
+            boolean development = settingsService.isDevelopmentMode();
+            if (StringUtils.isBlank(rememberMeKey) && !development) {
+                // ...if it is empty, generate a random key on startup (default).
+                logger.debug("Generating a new ephemeral 'remember me' key in a secure way.");
+                rememberMeKey = generateRememberMeKey();
+            } else if (StringUtils.isBlank(rememberMeKey) && development) {
+                // ...if we are in development mode, we can use a fixed key.
+                logger.warn("Using a fixed 'remember me' key because we're in development mode, this is INSECURE.");
+                rememberMeKey = DEVELOPMENT_REMEMBER_ME_KEY;
+            } else {
+                // ...otherwise, use the custom key directly.
+                logger.info("Using a fixed 'remember me' key from system properties, this is insecure.");
+            }
+
             http
                     .csrf()
                     .requireCsrfProtectionMatcher(csrfSecurityRequestMatcher)
@@ -169,7 +197,7 @@ public class GlobalSecurityConfig extends GlobalAuthenticationConfigurerAdapter
                     // see http://docs.spring.io/spring-security/site/docs/3.2.4.RELEASE/reference/htmlsingle/#csrf-logout
                     .and().logout().logoutRequestMatcher(new AntPathRequestMatcher("/logout", "GET")).logoutSuccessUrl(
                     "/login?logout")
-                    .and().rememberMe().key(generateRememberMeKey());
+                    .and().rememberMe().key(rememberMeKey);
         }
 
     }

airsonic-main/src/main/java/org/airsonic/player/service/SettingsService.java

@@ -107,6 +107,7 @@ public class SettingsService {
     private static final String KEY_SONOS_SERVICE_NAME = "SonosServiceName";
     private static final String KEY_SONOS_SERVICE_ID = "SonosServiceId";
     private static final String KEY_JWT_KEY = "JWTKey";
+    private static final String KEY_REMEMBER_ME_KEY = "RememberMeKey";
 
     private static final String KEY_SMTP_SERVER = "SmtpServer";
     private static final String KEY_SMTP_ENCRYPTION = "SmtpEncryption";
@@ -794,6 +795,27 @@ public class SettingsService {
         setString(KEY_MEDIA_LIBRARY_STATISTICS, statistics.format());
     }
 
+    /**
+     * Returns whether we are running in Development mode.
+     *
+     * @return true if we are in Development mode.
+     */
+    public boolean isDevelopmentMode() {
+        return System.getProperty("airsonic.development") != null;
+    }
+
+    /**
+     * Returns the custom 'remember me' key used for generating authentication tokens.
+     *
+     * @return The 'remember me' key.
+     */
+    public String getRememberMeKey() {
+        String key = null;
+        if (StringUtils.isBlank(key)) key = getString(KEY_REMEMBER_ME_KEY, null);
+        if (StringUtils.isBlank(key)) key = System.getProperty("airsonic.rememberMeKey");
+        return key;
+    }
+
     /**
      * Returns the locale (for language, date format etc).
      *