Otto Migrator stores credentials for the files that you plan to migrate. Therefore, it is highly recommended that you use FileMaker's Encryption at Rest for this file. Otto Migrator is shipped with a default Encryption Key "Ottom@tic" that you should change using FileMaker Pro Advanced Developer Utilities.
This is the default full access account for the Otto Migrator file. There is no password. You'll be prompted to change the credentials for this account.