We ask for the SSN due to anti-money laundering regulations.  We understand your concern regarding the security of your SSN.  You can see our SSL certification here: 

https://seal.digicert.com/seals/popup/tag=xc1igR9i&url=dev.startengine.com&lang=en&cbr=1448144534795

You can also see the certification on the bottom center of all investment pages where it says "digicert" and has an icon of a lock.