Google Plus sms security pin will be useful to update your Google+ stream via SMS safely if you are more concerned about SMS spoofing. Read the post if you don’t know How to Update Google Plus Stream via SMS. Google Plus gives you two option while updating your stream. First one, without Security Pin & Second one, with Security pin.
You can setup SMS security pin for your Google plus stream update by visiting Google+ Setting Page. It must have 4-8 alphanumeric characters. Once you have set SMS Security Pin then you have to use that pin in the beginning of each message which you are trying to post on your Google+ stream via SMS.
SMS Spoofing is a new SMS technology which is used to set a number by replacing its originating mobile number i.e. sender ID with alphanumeric text and that’s the main reason your Google Plus security pin must contain alphanumeric text. To know more about SMS Spoofing, read this Wikipedia Article.