diff --git a/backend/db.coffee b/backend/db.coffee
index 66b7131..bd16241 100644
--- a/backend/db.coffee
+++ b/backend/db.coffee
@@ -7,6 +7,14 @@ sqlite3 = (require 'sqlite3').verbose()
 # bruteforce countermeasure
 saltRounds = 13
 
+messages =
+  empty:    'Empty user name or password'
+  hash:     'Failed to process your data, try again later'
+  exists:   'User name is already taken'
+  noexists: 'Wrong user name or password'
+  password: 'Wrong user name or password'
+  db:       'Failed to access the database, try again later'
+
 class FFTCGDB
   constructor: (filename, truncate) ->
     @filename = filename
@@ -56,74 +64,91 @@ class FFTCGDB
       @db.close (err) ->
         if err
           logger.error "FAIL '#{err.message}'"
-          reject 'db'
+          reject null
         else
           logger.warn  "OK close '#{@filename}'"
-          resolve 'ok'
+          resolve null
+
+  validate: (login, password) ->
+    defined = (value) -> value? and value isnt ''
+
+    new Promise (resolve, reject) =>
+      if (defined login) and (defined password)
+        # both are defined
+        resolve null
+      else
+        # no user name or password given
+        logger.info "validate: FAIL empty '#{login}' or password"
+        reject null
 
   register: (login, password) ->
     new Promise (resolve, reject) =>
       # validate user input
-      if login == '' or password == ''
-        # no user name or password given
-        logger.info "reg: FAIL empty '#{login}' or password"
-        reject 'invalid'
+      @validate login, password
+      .then =>
+        # hash password
+        bcrypt.hash password, saltRounds, (err, hash) =>
+          if err
+            logger.warn "reg: FAIL hash for '#{login}'"
+            reject messages.hash
 
-      # hash password
-      bcrypt.hash password, saltRounds, (err, hash) =>
-        if err
-          logger.warn "reg: FAIL hash for '#{login}'"
-          reject 'hash'
+          else
+            # try creating row in users table
+            stmt = @db.prepare 'INSERT INTO users (login, pwdhash) VALUES (?, ?)'
+            stmt.run [login, hash], (err) ->
+              stmt.finalize()
+              if err
+                logger.warn "reg: FAIL db '#{err.code}' for '#{login}'"
+                reject messages.exists # user already exists
 
-        else
-          # try creating row in users table
-          stmt = @db.prepare 'INSERT INTO users (login, pwdhash) VALUES (?, ?)'
-          stmt.run [login, hash], (err) ->
-            stmt.finalize()
-            if err
-              logger.warn "reg: FAIL db '#{err.code}' for '#{login}'"
-              reject 'existence' # user already exists
+              else
+                logger.info "reg: OK '#{login}'"
+                # registration successful
+                resolve
+                  user: @lastID
+                  login: login
 
-            else
-              logger.info "reg: OK '#{login}'"
-              # registration successful
-              resolve
-                user: @lastID
-                login: login
+      .catch ->
+        reject messages.empty
 
   login: (login, password) ->
     new Promise (resolve, reject) =>
-      # get users table row
-      stmt = @db.prepare 'SELECT user, login, pwdhash FROM users WHERE login = ?'
-      stmt.get [login], (err, row) =>
-        stmt.finalize()
-        if err
-          logger.warn "login: FAIL db '#{err.code}' for '#{login}'"
-          reject 'db'
+      # validate user input
+      @validate login, password
+      .then =>
+        # get users table row
+        stmt = @db.prepare 'SELECT user, login, pwdhash FROM users WHERE login = ?'
+        stmt.get [login], (err, row) =>
+          stmt.finalize()
+          if err
+            logger.warn "login: FAIL db '#{err.code}' for '#{login}'"
+            reject messages.db
 
-        else if not row
-          # hash the password for timing attack reasons
-          bcrypt.hash password, saltRounds, (err, hash) ->
-            logger.debug "login: FAIL nonexistent '#{login}'"
-            reject 'existence' # user doesnt exist
+          else if not row
+            # hash the password for timing attack reasons
+            bcrypt.hash password, saltRounds, (err, hash) ->
+              logger.debug "login: FAIL nonexistent '#{login}'"
+              reject messages.noexists # user doesnt exist
 
-        else
-          bcrypt.compare password, row.pwdhash, (err, res) ->
-            if err
-              logger.warn "login: FAIL hash for '#{login}'"
-              reject 'hash'
+          else
+            bcrypt.compare password, row.pwdhash, (err, res) ->
+              if err
+                logger.warn "login: FAIL hash for '#{login}'"
+                reject messages.hash
 
-            if res == true
-              logger.debug "login: OK '#{row.login}'"
-              # login successful
-              resolve
-                user: row.user
-                login: row.login
+              if res == true
+                logger.debug "login: OK '#{row.login}'"
+                # login successful
+                resolve
+                  user: row.user
+                  login: row.login
 
-            else
-              logger.debug "login: FAIL password for '#{login}'"
-              # login failed
-              reject 'login'
+              else
+                logger.debug "login: FAIL password for '#{login}'"
+                reject messages.password # login failed
+
+      .catch ->
+        reject messages.empty
 
   addDeck: (user, deckCards) ->
     new Promise (resolve, reject) =>
@@ -133,7 +158,7 @@ class FFTCGDB
         stmt.finalize()
         if err
           logger.warn "addDeck: FAIL db '#{err.code}' for '#{user}'"
-          reject 'db'
+          reject messages.db
 
         else
           logger.debug "addDeck: OK '#{@lastID}'"
@@ -146,7 +171,7 @@ class FFTCGDB
         stmt.finalize()
         if err
           logger.warn "modDeck: FAIL db '#{err.code}' for '#{deckID}'"
-          reject 'db'
+          reject messages.db
         else
           logger.debug "modDeck: OK '#{deckID}'"
           resolve deckID
@@ -158,7 +183,7 @@ class FFTCGDB
         stmt.finalize()
         if err
           logger.warn "getDeck: FAIL db '#{err.code}' for '#{deckID}'"
-          reject 'db'
+          reject messages.db
         else
           logger.debug "getDeck: OK '#{deckID}'"
           resolve (id: row.deck, content: JSON.parse row.json for row, i in rows)
@@ -170,7 +195,7 @@ class FFTCGDB
         stmt.finalize()
         if err
           logger.warn "delDeck: FAIL db '#{err.code}' for '#{deckID}'"
-          reject 'db'
+          reject messages.db
         else
           logger.debug "delDeck: OK '#{deckID}'"
           resolve deckID