diff --git a/api/controllers/AuthenticationController.php b/api/controllers/AuthenticationController.php
index 125066b..e9d336f 100644
--- a/api/controllers/AuthenticationController.php
+++ b/api/controllers/AuthenticationController.php
@@ -5,6 +5,7 @@ use api\models\authentication\ForgotPasswordForm;
 use api\models\authentication\LoginForm;
 use api\models\authentication\RecoverPasswordForm;
 use api\models\authentication\RefreshTokenForm;
+use common\helpers\Error as E;
 use common\helpers\StringHelper;
 use Yii;
 use yii\filters\AccessControl;
@@ -48,7 +49,7 @@ class AuthenticationController extends Controller {
                 'errors' => $this->normalizeModelErrors($model->getErrors()),
             ];
 
-            if (ArrayHelper::getValue($data['errors'], 'login') === 'error.account_not_activated') {
+            if (ArrayHelper::getValue($data['errors'], 'login') === E::ACCOUNT_NOT_ACTIVATED) {
                 $data['data']['email'] = $model->getAccount()->email;
             }
 
@@ -69,7 +70,7 @@ class AuthenticationController extends Controller {
                 'errors' => $this->normalizeModelErrors($model->getErrors()),
             ];
 
-            if (ArrayHelper::getValue($data['errors'], 'login') === 'error.email_frequency') {
+            if (ArrayHelper::getValue($data['errors'], 'login') === E::RECENTLY_SENT_MESSAGE) {
                 $emailActivation = $model->getEmailActivation();
                 $data['data'] = [
                     'canRepeatIn' => $emailActivation->canRepeatIn(),
diff --git a/api/controllers/SignupController.php b/api/controllers/SignupController.php
index 996e8ef..e338f17 100644
--- a/api/controllers/SignupController.php
+++ b/api/controllers/SignupController.php
@@ -4,6 +4,7 @@ namespace api\controllers;
 use api\models\authentication\ConfirmEmailForm;
 use api\models\authentication\RepeatAccountActivationForm;
 use api\models\authentication\RegistrationForm;
+use common\helpers\Error as E;
 use Yii;
 use yii\filters\AccessControl;
 use yii\helpers\ArrayHelper;
@@ -60,7 +61,7 @@ class SignupController extends Controller {
                 'errors' => $this->normalizeModelErrors($model->getErrors()),
             ];
 
-            if ($response['errors']['email'] === 'error.recently_sent_message') {
+            if ($response['errors']['email'] === E::RECENTLY_SENT_MESSAGE) {
                 $activation = $model->getActivation();
                 $response['data'] = [
                     'canRepeatIn' => $activation->canRepeatIn(),
diff --git a/api/models/FeedbackForm.php b/api/models/FeedbackForm.php
index f9f1361..59848fa 100644
--- a/api/models/FeedbackForm.php
+++ b/api/models/FeedbackForm.php
@@ -1,6 +1,7 @@
 <?php
 namespace api\models;
 
+use common\helpers\Error as E;
 use api\models\base\ApiForm;
 use Yii;
 use yii\base\ErrorException;
@@ -18,9 +19,11 @@ class FeedbackForm extends ApiForm {
 
     public function rules() {
         return [
-            [['subject', 'email', 'message'], 'required', 'message' => 'error.{attribute}_required'],
+            ['subject', 'required', 'message' => E::SUBJECT_REQUIRED],
+            ['email', 'required', 'message' => E::EMAIL_REQUIRED],
+            ['message', 'required', 'message' => E::MESSAGE_REQUIRED],
             [['subject'], 'string', 'max' => 255],
-            [['email'], 'email'],
+            [['email'], 'email', 'message' => E::EMAIL_INVALID],
             [['message'], 'string', 'max' => 65535],
         ];
     }
diff --git a/api/models/authentication/ForgotPasswordForm.php b/api/models/authentication/ForgotPasswordForm.php
index f1102b2..61dfe5e 100644
--- a/api/models/authentication/ForgotPasswordForm.php
+++ b/api/models/authentication/ForgotPasswordForm.php
@@ -2,6 +2,7 @@
 namespace api\models\authentication;
 
 use api\models\base\ApiForm;
+use common\helpers\Error as E;
 use api\traits\AccountFinder;
 use common\components\UserFriendlyRandomKey;
 use common\models\Account;
@@ -18,7 +19,7 @@ class ForgotPasswordForm extends ApiForm {
 
     public function rules() {
         return [
-            ['login', 'required', 'message' => 'error.login_required'],
+            ['login', 'required', 'message' => E::LOGIN_REQUIRED],
             ['login', 'validateLogin'],
             ['login', 'validateActivity'],
             ['login', 'validateFrequency'],
@@ -28,7 +29,7 @@ class ForgotPasswordForm extends ApiForm {
     public function validateLogin($attribute) {
         if (!$this->hasErrors()) {
             if ($this->getAccount() === null) {
-                $this->addError($attribute, 'error.' . $attribute . '_not_exist');
+                $this->addError($attribute, E::LOGIN_NOT_EXIST);
             }
         }
     }
@@ -37,7 +38,7 @@ class ForgotPasswordForm extends ApiForm {
         if (!$this->hasErrors()) {
             $account = $this->getAccount();
             if ($account->status !== Account::STATUS_ACTIVE) {
-                $this->addError($attribute, 'error.account_not_activated');
+                $this->addError($attribute, E::ACCOUNT_NOT_ACTIVATED);
             }
         }
     }
@@ -46,7 +47,7 @@ class ForgotPasswordForm extends ApiForm {
         if (!$this->hasErrors()) {
             $emailConfirmation = $this->getEmailActivation();
             if ($emailConfirmation !== null && !$emailConfirmation->canRepeat()) {
-                $this->addError($attribute, 'error.email_frequency');
+                $this->addError($attribute, E::RECENTLY_SENT_MESSAGE);
             }
         }
     }
diff --git a/api/models/authentication/LoginForm.php b/api/models/authentication/LoginForm.php
index 0a24652..3462000 100644
--- a/api/models/authentication/LoginForm.php
+++ b/api/models/authentication/LoginForm.php
@@ -3,6 +3,7 @@ namespace api\models\authentication;
 
 use api\models\AccountIdentity;
 use api\models\base\ApiForm;
+use common\helpers\Error as E;
 use api\traits\AccountFinder;
 use common\models\Account;
 use Yii;
@@ -19,12 +20,12 @@ class LoginForm extends ApiForm {
 
     public function rules() {
         return [
-            ['login', 'required', 'message' => 'error.login_required'],
+            ['login', 'required', 'message' => E::LOGIN_REQUIRED],
             ['login', 'validateLogin'],
 
             ['password', 'required', 'when' => function(self $model) {
                 return !$model->hasErrors();
-            }, 'message' => 'error.password_required'],
+            }, 'message' => E::PASSWORD_REQUIRED],
             ['password', 'validatePassword'],
 
             ['login', 'validateActivity'],
@@ -36,7 +37,7 @@ class LoginForm extends ApiForm {
     public function validateLogin($attribute) {
         if (!$this->hasErrors()) {
             if ($this->getAccount() === null) {
-                $this->addError($attribute, 'error.' . $attribute . '_not_exist');
+                $this->addError($attribute, E::LOGIN_NOT_EXIST);
             }
         }
     }
@@ -45,7 +46,7 @@ class LoginForm extends ApiForm {
         if (!$this->hasErrors()) {
             $account = $this->getAccount();
             if ($account === null || !$account->validatePassword($this->password)) {
-                $this->addError($attribute, 'error.' . $attribute . '_incorrect');
+                $this->addError($attribute, E::PASSWORD_INCORRECT);
             }
         }
     }
@@ -54,7 +55,7 @@ class LoginForm extends ApiForm {
         if (!$this->hasErrors()) {
             $account = $this->getAccount();
             if ($account->status !== Account::STATUS_ACTIVE) {
-                $this->addError($attribute, 'error.account_not_activated');
+                $this->addError($attribute, E::ACCOUNT_NOT_ACTIVATED);
             }
         }
     }
diff --git a/api/models/authentication/RecoverPasswordForm.php b/api/models/authentication/RecoverPasswordForm.php
index b90dd2d..31a2998 100644
--- a/api/models/authentication/RecoverPasswordForm.php
+++ b/api/models/authentication/RecoverPasswordForm.php
@@ -3,6 +3,7 @@ namespace api\models\authentication;
 
 use api\models\AccountIdentity;
 use api\models\base\KeyConfirmationForm;
+use common\helpers\Error as E;
 use common\models\EmailActivation;
 use common\validators\PasswordValidate;
 use Yii;
@@ -16,7 +17,8 @@ class RecoverPasswordForm extends KeyConfirmationForm {
 
     public function rules() {
         return array_merge(parent::rules(), [
-            [['newPassword', 'newRePassword'], 'required', 'message' => 'error.{attribute}_required'],
+            ['newPassword', 'required', 'message' => E::NEW_PASSWORD_REQUIRED],
+            ['newRePassword', 'required', 'message' => E::NEW_RE_PASSWORD_REQUIRED],
             ['newPassword', PasswordValidate::class],
             ['newRePassword', 'validatePasswordAndRePasswordMatch'],
         ]);
@@ -25,7 +27,7 @@ class RecoverPasswordForm extends KeyConfirmationForm {
     public function validatePasswordAndRePasswordMatch($attribute) {
         if (!$this->hasErrors()) {
             if ($this->newPassword !== $this->newRePassword) {
-                $this->addError($attribute, 'error.rePassword_does_not_match');
+                $this->addError($attribute, E::NEW_RE_PASSWORD_DOES_NOT_MATCH);
             }
         }
     }
diff --git a/api/models/authentication/RefreshTokenForm.php b/api/models/authentication/RefreshTokenForm.php
index e637e98..71bc702 100644
--- a/api/models/authentication/RefreshTokenForm.php
+++ b/api/models/authentication/RefreshTokenForm.php
@@ -2,6 +2,7 @@
 namespace api\models\authentication;
 
 use api\models\base\ApiForm;
+use common\helpers\Error as E;
 use common\models\AccountSession;
 use Yii;
 
@@ -16,7 +17,7 @@ class RefreshTokenForm extends ApiForm {
 
     public function rules() {
         return [
-            ['refresh_token', 'required'],
+            ['refresh_token', 'required', 'message' => E::REFRESH_TOKEN_REQUIRED],
             ['refresh_token', 'validateRefreshToken'],
         ];
     }
@@ -25,7 +26,7 @@ class RefreshTokenForm extends ApiForm {
         if (!$this->hasErrors()) {
             /** @var AccountSession|null $token */
             if ($this->getSession() === null) {
-                $this->addError('refresh_token', 'error.refresh_token_not_exist');
+                $this->addError('refresh_token', E::REFRESH_TOKEN_NOT_EXISTS);
             }
         }
     }
diff --git a/api/models/authentication/RegistrationForm.php b/api/models/authentication/RegistrationForm.php
index 574ceb5..e01bb2c 100644
--- a/api/models/authentication/RegistrationForm.php
+++ b/api/models/authentication/RegistrationForm.php
@@ -3,6 +3,7 @@ namespace api\models\authentication;
 
 use api\components\ReCaptcha\Validator as ReCaptchaValidator;
 use api\models\base\ApiForm;
+use common\helpers\Error as E;
 use api\models\profile\ChangeUsernameForm;
 use common\components\UserFriendlyRandomKey;
 use common\models\Account;
@@ -26,14 +27,14 @@ class RegistrationForm extends ApiForm {
 
     public function rules() {
         return [
-            [[], ReCaptchaValidator::class, 'message' => 'error.captcha_invalid', 'when' => !YII_ENV_TEST],
-            ['rulesAgreement', 'required', 'message' => 'error.you_must_accept_rules'],
+            [[], ReCaptchaValidator::class, 'message' => E::CAPTCHA_INVALID, 'when' => !YII_ENV_TEST],
+            ['rulesAgreement', 'required', 'message' => E::RULES_AGREEMENT_REQUIRED],
 
             ['username', 'validateUsername', 'skipOnEmpty' => false],
             ['email', 'validateEmail', 'skipOnEmpty' => false],
 
-            ['password', 'required', 'message' => 'error.password_required'],
-            ['rePassword', 'required', 'message' => 'error.rePassword_required'],
+            ['password', 'required', 'message' => E::PASSWORD_REQUIRED],
+            ['rePassword', 'required', 'message' => E::RE_PASSWORD_REQUIRED],
             ['password', PasswordValidate::class],
             ['rePassword', 'validatePasswordAndRePasswordMatch'],
 
@@ -60,7 +61,7 @@ class RegistrationForm extends ApiForm {
     public function validatePasswordAndRePasswordMatch($attribute) {
         if (!$this->hasErrors()) {
             if ($this->password !== $this->rePassword) {
-                $this->addError($attribute, "error.rePassword_does_not_match");
+                $this->addError($attribute, E::RE_PASSWORD_DOES_NOT_MATCH);
             }
         }
     }
diff --git a/api/models/authentication/RepeatAccountActivationForm.php b/api/models/authentication/RepeatAccountActivationForm.php
index f13a824..18a7136 100644
--- a/api/models/authentication/RepeatAccountActivationForm.php
+++ b/api/models/authentication/RepeatAccountActivationForm.php
@@ -2,6 +2,7 @@
 namespace api\models\authentication;
 
 use api\models\base\ApiForm;
+use common\helpers\Error as E;
 use common\components\UserFriendlyRandomKey;
 use common\models\Account;
 use common\models\confirmations\RegistrationConfirmation;
@@ -18,7 +19,7 @@ class RepeatAccountActivationForm extends ApiForm {
     public function rules() {
         return [
             ['email', 'filter', 'filter' => 'trim'],
-            ['email', 'required', 'message' => 'error.email_required'],
+            ['email', 'required', 'message' => E::EMAIL_REQUIRED],
             ['email', 'validateEmailForAccount'],
             ['email', 'validateExistsActivation'],
         ];
@@ -28,12 +29,12 @@ class RepeatAccountActivationForm extends ApiForm {
         if (!$this->hasErrors($attribute)) {
             $account = $this->getAccount();
             if ($account === null) {
-                $this->addError($attribute, "error.{$attribute}_not_found");
+                $this->addError($attribute, E::EMAIL_NOT_FOUND);
             } elseif ($account->status === Account::STATUS_ACTIVE) {
-                $this->addError($attribute, "error.account_already_activated");
+                $this->addError($attribute, E::ACCOUNT_ALREADY_ACTIVATED);
             } elseif ($account->status !== Account::STATUS_REGISTERED) {
                 // TODO: такие аккаунты следует логировать за попытку к саботажу
-                $this->addError($attribute, "error.account_cannot_resend_message");
+                $this->addError($attribute, E::ACCOUNT_CANNOT_RESEND_MESSAGE);
             }
         }
     }
@@ -42,7 +43,7 @@ class RepeatAccountActivationForm extends ApiForm {
         if (!$this->hasErrors($attribute)) {
             $activation = $this->getActivation();
             if ($activation !== null && !$activation->canRepeat()) {
-                $this->addError($attribute, 'error.recently_sent_message');
+                $this->addError($attribute, E::RECENTLY_SENT_MESSAGE);
             }
         }
     }
diff --git a/api/models/base/KeyConfirmationForm.php b/api/models/base/KeyConfirmationForm.php
index bc23c73..9148351 100644
--- a/api/models/base/KeyConfirmationForm.php
+++ b/api/models/base/KeyConfirmationForm.php
@@ -1,6 +1,7 @@
 <?php
 namespace api\models\base;
 
+use common\helpers\Error as E;
 use api\validators\EmailActivationKeyValidator;
 use common\models\EmailActivation;
 
@@ -13,7 +14,7 @@ class KeyConfirmationForm extends ApiForm {
     public function rules() {
         return [
             // TODO: нужно провалидировать количество попыток ввода кода для определённого IP адреса и в случае чего запросить капчу
-            ['key', 'required', 'message' => 'error.key_is_required'],
+            ['key', 'required', 'message' => E::KEY_REQUIRED],
             ['key', EmailActivationKeyValidator::class],
         ];
     }
diff --git a/api/models/base/PasswordProtectedForm.php b/api/models/base/PasswordProtectedForm.php
index 2289969..da82ba0 100644
--- a/api/models/base/PasswordProtectedForm.php
+++ b/api/models/base/PasswordProtectedForm.php
@@ -1,6 +1,7 @@
 <?php
 namespace api\models\base;
 
+use common\helpers\Error as E;
 use Yii;
 
 class PasswordProtectedForm extends ApiForm {
@@ -9,14 +10,14 @@ class PasswordProtectedForm extends ApiForm {
 
     public function rules() {
         return [
-            [['password'], 'required', 'message' => 'error.password_required'],
+            [['password'], 'required', 'message' => E::PASSWORD_REQUIRED],
             [['password'], 'validatePassword'],
         ];
     }
 
     public function validatePassword() {
         if (!$this->getAccount()->validatePassword($this->password)) {
-            $this->addError('password', 'error.password_invalid');
+            $this->addError('password', E::PASSWORD_INVALID);
         }
     }
 
diff --git a/api/models/profile/ChangeEmail/ConfirmNewEmailForm.php b/api/models/profile/ChangeEmail/ConfirmNewEmailForm.php
index efc47d1..466dc04 100644
--- a/api/models/profile/ChangeEmail/ConfirmNewEmailForm.php
+++ b/api/models/profile/ChangeEmail/ConfirmNewEmailForm.php
@@ -22,11 +22,11 @@ class ConfirmNewEmailForm extends KeyConfirmationForm {
     /**
      * @return Account
      */
-    public function getAccount() {
+    public function getAccount() : Account {
         return $this->account;
     }
 
-    public function changeEmail() {
+    public function changeEmail() : bool {
         if (!$this->validate()) {
             return false;
         }
diff --git a/api/models/profile/ChangeEmail/InitStateForm.php b/api/models/profile/ChangeEmail/InitStateForm.php
index dc96a30..378047b 100644
--- a/api/models/profile/ChangeEmail/InitStateForm.php
+++ b/api/models/profile/ChangeEmail/InitStateForm.php
@@ -21,7 +21,7 @@ class InitStateForm extends PasswordProtectedForm {
         parent::__construct($config);
     }
 
-    public function getAccount() {
+    public function getAccount() : Account {
         return $this->account;
     }
 
@@ -32,7 +32,7 @@ class InitStateForm extends PasswordProtectedForm {
         ]);
     }
 
-    public function sendCurrentEmailConfirmation() {
+    public function sendCurrentEmailConfirmation() : bool {
         if (!$this->validate()) {
             return false;
         }
@@ -55,7 +55,7 @@ class InitStateForm extends PasswordProtectedForm {
      * @return CurrentEmailConfirmation
      * @throws ErrorException
      */
-    public function createCode() {
+    public function createCode() : CurrentEmailConfirmation {
         $account = $this->getAccount();
         $emailActivation = new CurrentEmailConfirmation();
         $emailActivation->account_id = $account->id;
diff --git a/api/models/profile/ChangeEmail/NewEmailForm.php b/api/models/profile/ChangeEmail/NewEmailForm.php
index 07950ea..c87ba6c 100644
--- a/api/models/profile/ChangeEmail/NewEmailForm.php
+++ b/api/models/profile/ChangeEmail/NewEmailForm.php
@@ -2,6 +2,7 @@
 namespace api\models\profile\ChangeEmail;
 
 use api\models\base\KeyConfirmationForm;
+use common\helpers\Error as E;
 use common\models\Account;
 use common\models\confirmations\NewEmailConfirmation;
 use common\models\EmailActivation;
@@ -33,7 +34,7 @@ class NewEmailForm extends KeyConfirmationForm {
 
     public function rules() {
         return array_merge(parent::rules(), [
-            ['email', 'required', 'message' => 'error.email_required'],
+            ['email', 'required', 'message' => E::EMAIL_REQUIRED],
             ['email', 'validateEmail'],
         ]);
     }
diff --git a/api/models/profile/ChangePasswordForm.php b/api/models/profile/ChangePasswordForm.php
index 64bc5f2..d4d0cb6 100644
--- a/api/models/profile/ChangePasswordForm.php
+++ b/api/models/profile/ChangePasswordForm.php
@@ -2,6 +2,7 @@
 namespace api\models\profile;
 
 use api\models\base\PasswordProtectedForm;
+use common\helpers\Error as E;
 use common\models\Account;
 use common\validators\PasswordValidate;
 use Yii;
@@ -31,7 +32,8 @@ class ChangePasswordForm extends PasswordProtectedForm {
      */
     public function rules() {
         return ArrayHelper::merge(parent::rules(), [
-            [['newPassword', 'newRePassword'], 'required', 'message' => 'error.{attribute}_required'],
+            ['newPassword', 'required', 'message' => E::NEW_PASSWORD_REQUIRED],
+            ['newRePassword', 'required', 'message' => E::NEW_RE_PASSWORD_REQUIRED],
             ['newPassword', PasswordValidate::class],
             ['newRePassword', 'validatePasswordAndRePasswordMatch'],
             ['logoutAll', 'boolean'],
@@ -41,7 +43,7 @@ class ChangePasswordForm extends PasswordProtectedForm {
     public function validatePasswordAndRePasswordMatch($attribute) {
         if (!$this->hasErrors($attribute)) {
             if ($this->newPassword !== $this->newRePassword) {
-                $this->addError($attribute, 'error.newRePassword_does_not_match');
+                $this->addError($attribute, E::NEW_RE_PASSWORD_DOES_NOT_MATCH);
             }
         }
     }
@@ -49,7 +51,7 @@ class ChangePasswordForm extends PasswordProtectedForm {
     /**
      * @return boolean
      */
-    public function changePassword() {
+    public function changePassword() : bool {
         if (!$this->validate()) {
             return false;
         }
@@ -79,7 +81,7 @@ class ChangePasswordForm extends PasswordProtectedForm {
         return true;
     }
 
-    protected function getAccount() {
+    protected function getAccount() : Account {
         return $this->_account;
     }
 
diff --git a/api/models/profile/ChangeUsernameForm.php b/api/models/profile/ChangeUsernameForm.php
index a162f3b..074b48f 100644
--- a/api/models/profile/ChangeUsernameForm.php
+++ b/api/models/profile/ChangeUsernameForm.php
@@ -2,6 +2,7 @@
 namespace api\models\profile;
 
 use api\models\base\PasswordProtectedForm;
+use common\helpers\Error;
 use common\helpers\Amqp;
 use common\models\amqp\UsernameChanged;
 use common\models\UsernameHistory;
@@ -16,8 +17,8 @@ class ChangeUsernameForm extends PasswordProtectedForm {
 
     public function rules() {
         return ArrayHelper::merge(parent::rules(), [
-            [['username'], 'required', 'message' => 'error.{attribute}_required'],
-            [['username'], 'validateUsername'],
+            ['username', 'required', 'message' => Error::USERNAME_REQUIRED],
+            ['username', 'validateUsername'],
         ]);
     }
 
diff --git a/api/validators/EmailActivationKeyValidator.php b/api/validators/EmailActivationKeyValidator.php
index 3154876..9684482 100644
--- a/api/validators/EmailActivationKeyValidator.php
+++ b/api/validators/EmailActivationKeyValidator.php
@@ -1,14 +1,15 @@
 <?php
 namespace api\validators;
 
+use common\helpers\Error as E;
 use common\models\EmailActivation;
 use yii\validators\Validator;
 
 class EmailActivationKeyValidator extends Validator {
 
-    public $notExist = 'error.key_not_exists';
+    public $notExist = E::KEY_NOT_EXISTS;
 
-    public $expired = 'error.key_expire';
+    public $expired = E::KEY_EXPIRE;
 
     public function validateValue($value) {
         if (($model = $this->findEmailActivationModel($value)) === null) {
diff --git a/common/helpers/Error.php b/common/helpers/Error.php
new file mode 100644
index 0000000..fb3fb7b
--- /dev/null
+++ b/common/helpers/Error.php
@@ -0,0 +1,63 @@
+<?php
+namespace common\helpers;
+
+final class Error {
+
+    const USERNAME_REQUIRED = 'error.username_required';
+    const USERNAME_TOO_SHORT = 'error.username_too_short';
+    const USERNAME_TOO_LONG = 'error.username_too_long';
+    const USERNAME_INVALID = 'error.username_invalid';
+    const USERNAME_NOT_AVAILABLE = 'error.username_not_available';
+
+    const EMAIL_REQUIRED = 'error.email_required';
+    const EMAIL_TOO_LONG = 'error.email_too_long';
+    const EMAIL_INVALID = 'error.email_invalid';
+    const EMAIL_IS_TEMPMAIL = 'error.email_is_tempmail';
+    const EMAIL_NOT_AVAILABLE = 'error.email_not_available';
+    const EMAIL_NOT_FOUND = 'error.email_not_found';
+
+    const LOGIN_REQUIRED = 'error.login_required';
+    const LOGIN_NOT_EXIST = 'error.login_not_exist';
+
+    const PASSWORD_REQUIRED = 'error.password_required';
+    const PASSWORD_INVALID = 'error.password_invalid';
+    /**
+     * TODO: На фронте с password_incorrect и password_invalid возникла неувязочка.
+     * Один возникает у формы входа и там уместно предлагать восстановление пароля.
+     * Другой возникает у паролезащищённой формы и там уже ничего предлагать не нужно.
+     * Но по факту это ведь одна и та же ошибка.
+     * @deprecated
+     */
+    const PASSWORD_INCORRECT = 'error.password_incorrect';
+    const PASSWORD_TOO_SHORT = 'error.password_too_short';
+
+    const KEY_REQUIRED = 'error.key_required';
+    const KEY_NOT_EXISTS = 'error.key_not_exists';
+    const KEY_EXPIRE = 'error.key_expire';
+
+    const ACCOUNT_NOT_ACTIVATED = 'error.account_not_activated';
+    const ACCOUNT_ALREADY_ACTIVATED = 'error.account_already_activated';
+    const ACCOUNT_CANNOT_RESEND_MESSAGE = 'error.account_cannot_resend_message';
+
+    const RECENTLY_SENT_MESSAGE = 'error.recently_sent_message';
+
+    const NEW_PASSWORD_REQUIRED = 'error.newPassword_required';
+    const NEW_RE_PASSWORD_REQUIRED = 'error.newRePassword_required';
+    const NEW_RE_PASSWORD_DOES_NOT_MATCH = self::RE_PASSWORD_DOES_NOT_MATCH;
+
+    const REFRESH_TOKEN_REQUIRED = 'error.refresh_token_required';
+    const REFRESH_TOKEN_NOT_EXISTS = 'error.refresh_token_not_exist';
+
+    const CAPTCHA_INVALID = 'error.captcha_invalid';
+
+    const RULES_AGREEMENT_REQUIRED = 'error.rulesAgreement_required';
+
+    const RE_PASSWORD_REQUIRED = 'error.rePassword_required';
+    const RE_PASSWORD_DOES_NOT_MATCH = 'error.rePassword_does_not_match';
+
+    const UNSUPPORTED_LANGUAGE = 'error.unsupported_language';
+
+    const SUBJECT_REQUIRED = 'error.subject_required';
+    const MESSAGE_REQUIRED = 'error.message_required';
+
+}
diff --git a/common/models/Account.php b/common/models/Account.php
index 994d4d2..be4c282 100644
--- a/common/models/Account.php
+++ b/common/models/Account.php
@@ -1,6 +1,7 @@
 <?php
 namespace common\models;
 
+use common\helpers\Error as E;
 use common\components\UserPass;
 use common\validators\LanguageValidator;
 use Ely\Yii2\TempmailValidator;
@@ -57,22 +58,22 @@ class Account extends ActiveRecord {
     public function rules() {
         return [
             [['username'], 'filter', 'filter' => 'trim'],
-            [['username'], 'required', 'message' => 'error.username_required'],
+            [['username'], 'required', 'message' => E::USERNAME_REQUIRED],
             [['username'], 'string', 'min' => 3, 'max' => 21,
-                'tooShort' => 'error.username_too_short',
-                'tooLong' => 'error.username_too_long',
+                'tooShort' => E::USERNAME_TOO_SHORT,
+                'tooLong' => E::USERNAME_TOO_LONG,
             ],
             [['username'], 'match', 'pattern' => '/^[\p{L}\d-_\.!?#$%^&*()\[\]:;]+$/u',
-                'message' => 'error.username_invalid',
+                'message' => E::USERNAME_INVALID,
             ],
-            [['username'], 'unique', 'message' => 'error.username_not_available'],
+            [['username'], 'unique', 'message' => E::USERNAME_NOT_AVAILABLE],
 
             [['email'], 'filter', 'filter' => 'trim'],
-            [['email'], 'required', 'message' => 'error.email_required'],
-            [['email'], 'string', 'max' => 255, 'tooLong' => 'error.email_too_long'],
-            [['email'], 'email', 'checkDNS' => true, 'enableIDN' => true, 'message' => 'error.email_invalid'],
-            [['email'], TempmailValidator::class, 'message' => 'error.email_is_tempmail'],
-            [['email'], 'unique', 'message' => 'error.email_not_available'],
+            [['email'], 'required', 'message' => E::EMAIL_REQUIRED],
+            [['email'], 'string', 'max' => 255, 'tooLong' => E::EMAIL_TOO_LONG],
+            [['email'], 'email', 'checkDNS' => true, 'enableIDN' => true, 'message' => E::EMAIL_INVALID],
+            [['email'], TempmailValidator::class, 'message' => E::EMAIL_IS_TEMPMAIL],
+            [['email'], 'unique', 'message' => E::EMAIL_NOT_AVAILABLE],
 
             [['lang'], LanguageValidator::class],
             [['lang'], 'default', 'value' => 'en'],
diff --git a/common/validators/LanguageValidator.php b/common/validators/LanguageValidator.php
index a553bac..409e4a4 100644
--- a/common/validators/LanguageValidator.php
+++ b/common/validators/LanguageValidator.php
@@ -1,12 +1,13 @@
 <?php
 namespace common\validators;
 
+use common\helpers\Error as E;
 use Yii;
 use yii\validators\Validator;
 
 class LanguageValidator extends Validator {
 
-    public $message = 'error.unsupported_language';
+    public $message = E::UNSUPPORTED_LANGUAGE;
 
     protected function validateValue($value) {
         if (empty($value)) {
diff --git a/common/validators/PasswordValidate.php b/common/validators/PasswordValidate.php
index 24ac883..b4391f7 100644
--- a/common/validators/PasswordValidate.php
+++ b/common/validators/PasswordValidate.php
@@ -1,6 +1,7 @@
 <?php
 namespace common\validators;
 
+use common\helpers\Error as E;
 use yii\validators\StringValidator;
 
 /**
@@ -10,6 +11,6 @@ class PasswordValidate extends StringValidator {
 
     public $min = 8;
 
-    public $tooShort = 'error.password_too_short';
+    public $tooShort = E::PASSWORD_TOO_SHORT;
 
 }
diff --git a/tests/codeception/api/functional/EmailConfirmationCest.php b/tests/codeception/api/functional/EmailConfirmationCest.php
index 1fe0dc0..2f4e8da 100644
--- a/tests/codeception/api/functional/EmailConfirmationCest.php
+++ b/tests/codeception/api/functional/EmailConfirmationCest.php
@@ -13,7 +13,7 @@ class EmailConfirmationCest {
         $I->canSeeResponseContainsJson([
             'success' => false,
             'errors' => [
-                'key' => 'error.key_is_required',
+                'key' => 'error.key_required',
             ],
         ]);
 
diff --git a/tests/codeception/api/functional/ForgotPasswordCest.php b/tests/codeception/api/functional/ForgotPasswordCest.php
index c39662d..5f36a93 100644
--- a/tests/codeception/api/functional/ForgotPasswordCest.php
+++ b/tests/codeception/api/functional/ForgotPasswordCest.php
@@ -39,7 +39,7 @@ class ForgotPasswordCest {
         $I->canSeeResponseContainsJson([
             'success' => false,
             'errors' => [
-                'login' => 'error.email_frequency',
+                'login' => 'error.recently_sent_message',
             ],
         ]);
         $I->canSeeResponseJsonMatchesJsonPath('$.data.canRepeatIn');
diff --git a/tests/codeception/api/functional/RegisterCest.php b/tests/codeception/api/functional/RegisterCest.php
index 608bb7c..210afb3 100644
--- a/tests/codeception/api/functional/RegisterCest.php
+++ b/tests/codeception/api/functional/RegisterCest.php
@@ -18,7 +18,7 @@ class RegisterCest {
     public function testIncorrectRegistration(FunctionalTester $I) {
         $route = new SignupRoute($I);
 
-        $I->wantTo('get error.you_must_accept_rules if we don\'t accept rules');
+        $I->wantTo('get error.rulesAgreement_required if we don\'t accept rules');
         $route->register([
             'username' => 'ErickSkrauch',
             'email' => 'erickskrauch@ely.by',
@@ -28,17 +28,17 @@ class RegisterCest {
         $I->canSeeResponseContainsJson([
             'success' => false,
             'errors' => [
-                'rulesAgreement' => 'error.you_must_accept_rules',
+                'rulesAgreement' => 'error.rulesAgreement_required',
             ],
         ]);
 
-        $I->wantTo('don\'t see error.you_must_accept_rules if we accept rules');
+        $I->wantTo('don\'t see error.rulesAgreement_requireds if we accept rules');
         $route->register([
             'rulesAgreement' => true,
         ]);
         $I->cantSeeResponseContainsJson([
             'errors' => [
-                'rulesAgreement' => 'error.you_must_accept_rules',
+                'rulesAgreement' => 'error.rulesAgreement_required',
             ],
         ]);
 
diff --git a/tests/codeception/api/unit/models/authentication/ForgotPasswordFormTest.php b/tests/codeception/api/unit/models/authentication/ForgotPasswordFormTest.php
index a5edf8b..cd9f568 100644
--- a/tests/codeception/api/unit/models/authentication/ForgotPasswordFormTest.php
+++ b/tests/codeception/api/unit/models/authentication/ForgotPasswordFormTest.php
@@ -79,7 +79,7 @@ class ForgotPasswordFormTest extends DbTestCase {
             ]);
 
             $model->validateFrequency('login');
-            expect($model->getErrors('login'))->equals(['error.email_frequency']);
+            expect($model->getErrors('login'))->equals(['error.recently_sent_message']);
         });
 
         $this->specify('empty errors if email was sent a long time ago', function() {
diff --git a/tests/codeception/api/unit/models/profile/ChangePasswordFormTest.php b/tests/codeception/api/unit/models/profile/ChangePasswordFormTest.php
index bf75d54..a0a197d 100644
--- a/tests/codeception/api/unit/models/profile/ChangePasswordFormTest.php
+++ b/tests/codeception/api/unit/models/profile/ChangePasswordFormTest.php
@@ -27,7 +27,7 @@ class ChangePasswordFormTest extends DbTestCase {
     }
 
     public function testValidatePasswordAndRePasswordMatch() {
-        $this->specify('error.newRePassword_does_not_match expected if passwords not match', function() {
+        $this->specify('error.rePassword_does_not_match expected if passwords not match', function() {
             $account = new Account();
             $account->setPassword('12345678');
             $model = new ChangePasswordForm($account, [
@@ -36,7 +36,7 @@ class ChangePasswordFormTest extends DbTestCase {
                 'newRePassword' => 'another-password',
             ]);
             $model->validatePasswordAndRePasswordMatch('newRePassword');
-            expect($model->getErrors('newRePassword'))->equals(['error.newRePassword_does_not_match']);
+            expect($model->getErrors('newRePassword'))->equals(['error.rePassword_does_not_match']);
         });
 
         $this->specify('no errors expected if passwords are valid', function() {
@@ -51,7 +51,7 @@ class ChangePasswordFormTest extends DbTestCase {
             expect($model->getErrors('newRePassword'))->isEmpty();
         });
 
-        $this->specify('error.newRePassword_does_not_match expected even if there are errors on other attributes', function() {
+        $this->specify('error.rePassword_does_not_match expected even if there are errors on other attributes', function() {
             // this is very important, because password change flow may be combined of two steps
             // therefore we need to validate password sameness before we will validate current account password
             $account = new Account();
@@ -61,7 +61,7 @@ class ChangePasswordFormTest extends DbTestCase {
                 'newRePassword' => 'another-password',
             ]);
             $model->validate();
-            expect($model->getErrors('newRePassword'))->equals(['error.newRePassword_does_not_match']);
+            expect($model->getErrors('newRePassword'))->equals(['error.rePassword_does_not_match']);
         });
     }