If some one is not login in  that site or page cannot able to see our post or links in signature...then if they are not visible without login . What search engine consider such links as back links or not.

If a forum website requires login to see signature links, they are no good for offpage seo backlinks

The popularity of a website is based on the links that it receives. Backlinks are the links that enter your website. Backlinks were the only source of web navigation few years back, before the emergence of search engine optimization. Now, backlinks are the backbone of search engine positioning. Backlinking is very useful for the people who cannot afford more money on advertising. These links lead to a web page and back lead to another web page. The backlink works great with the websites that is designed with high quality, well established and with good rank.