To ensure everyone has the ability to view and share files on the web painlessly, Dropbox currently does not support the creation of your own private keys. However, allowing user control over this is something we might consider adding in the future.
The security of your data is our highest priority and all files stored on Dropbox servers are encrypted. Learn more about Dropbox security.
While Dropbox does not provide for client-side encryption users are free to add their own. There are many third party applications that provide encryption at both the file and container level. Visit our forums for more information.