protected function validateIdentity($identity)
    {
        $credentialProperty = $this->options->getCredentialProperty();
        $getter             = 'get' . ucfirst($credentialProperty);
        $documentCredential = null;

        if (method_exists($identity, $getter)) {
            $documentCredential = $identity->$getter();
        } elseif (property_exists($identity, $credentialProperty)) {
            $documentCredential = $identity->{$credentialProperty};
        } else {
            throw new Exception\UnexpectedValueException(
                sprintf(
                    'Property (%s) in (%s) is not accessible. You should implement %s::%s()',
                    $credentialProperty,
                    get_class($identity),
                    get_class($identity),
                    $getter
                )
            );
        }

        $credentialValue = $this->credential;
        $callable        = $this->options->getCredentialCallable();

        if ($callable) {
            $credentialValue = call_user_func($callable, $identity, $credentialValue);
        }

        if ($credentialValue !== true && $credentialValue !== $documentCredential) {
            $this->authenticationResultInfo['code']       = AuthenticationResult::FAILURE_CREDENTIAL_INVALID;
            $this->authenticationResultInfo['messages'][] = 'Supplied credential is invalid.';

            return $this->createAuthenticationResult();
        }

        $this->authenticationResultInfo['code']       = AuthenticationResult::SUCCESS;
        $this->authenticationResultInfo['identity']   = $identity;
        $this->authenticationResultInfo['messages'][] = 'Authentication successful.';

        return $this->createAuthenticationResult();
    }