I have a spring Boot application for users who want to send a message with spring Mail Sender to someone, for this the application needs username, application, access password and host. How and where should this password be stored. I use keycloak server for user authorisation. I should use a symmetric algorithm, which needs a secretkey for decryption, where should I store this secret key and how?

i am introducing the keycloak token mechanism in my application and after adding a custom JwtTokenConverter and the securityConfig my app won’t start with this error. Before that the app worked. I read that it could be a problem related to a dependancies conflict, but i didin’t add any dependancies to the project before and after adding the changes above.