Every time you visit a website, your computer sends data to the site. Often that data is innocuous and may involve your computer telling the site what page it wants to visit next. Sometimes that data is sensitive and could be a security risk if other people see it. That's where secure websites, which most browsers indicate with a padlock symbol, come in.
Need for Secure Sites
Internet data travels from its starting point to its destination through numerous routes and relay points. At every point in the journey, it's possible that somebody could intercept and read the data. Given that users can't be certain of the route or know if every point in the chain of computers is trustworthy, some information is so sensitive it is sent in an encrypted format.
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Indicating Secure Sites
The standard method of sending encrypted data uses a secure sockets layer connection. The technical process can best be understood by thinking of it in physical terms. It is the equivalent of a website operator sending you an unlocked box that you can lock without a key. You put your message inside the box, snap the lock shut and return the box. Only the website operator -- who held on to the key all the time -- can unlock the box and read your message, making it useless to anyone who intercepts it along the way. It's appropriate, therefore, that Web browsers display a padlock symbol next to the website address to show that any data you send and receive while using the site is encrypted.
Other Security Signals
You can also confirm a website is encrypted by looking at the full Web address, also known as its URL. If the connection is secure, the URL begins HTTPS rather than HTTP. Each secure website has a security certificate that verifies the claims of the site operators. You to check the details of the certificate in your Web browser, and some browsers automatically warn you if the details do not appear correct and there is a risk of impersonation.
Alternative Padlock Meaning
In Windows 7, some files or folders in Windows Explorer may have a padlock on the relevant icon. This indicates that you have set the file to be private; no other user account on the computer or network can access the file. The padlock icon only appears when a file is set to private and is located inside a shared folder that you have otherwise made public. The private file remains private, even though other files in the folder are public. Note that you can open a file with the padlock icon with no need for a password; other users won't see the file listed at all when they access the shared folder. The icon was dropped for Windows 8.