Assalam-o-alikum brother Raphael, Congratualtions to you on becoming a Muslim. may Allah (SWT) give youi lots and lots of rewards in this life and also in the next Ameen. You are right that you can take the vow of Shahada alone with only Akllah (SWT) as the witness. One can accept Islam secretly and can practice it secretly if there is threat or any kind of danger of persecution etc. But it is recommended that one should announce his or her reversion in public as by doing that one will know other Muslims and other Muslims will then know that person. Welcome again dear brother. Wassalam.