LaravelのAPP_KEY

暗号化に使われるキー

Typically, this string should be 32 characters long. The key can be set in the .env environment file. If you have not renamed the .env.example file to .env, you should do that now. If the application key is not set, your user sessions and other encrypted data will not be secure!

https://laravel.com/docs/master#configuration

【Laravel5.5】APP_KEY の行方を追う - Qiita

https://github.com/laravel/framework/blob/5.7/src/Illuminate/Encryption/Encrypter.php とかで使われているっぽい

#Laravel