Hypertext Transfer Protocol over Secure Socket Layer or HTTPS is a URI scheme used to indicate a secure HTTP connection. It is syntactically identical to the http:// scheme normally used for accessing resources using HTTP. Using an https: URL indicates that HTTP is to be used, but with a different default TCP port (443) and an additional encryption/authentication layer between the HTTP and TCP. This system was designed by Netscape Communications Corporation to provide authentication and encrypted communication and is widely used on the World Wide Web for security-sensitive communication such as payment transactions and corporate information systems.

How it works

Strictly speaking, https is not a separate protocol, but refers to the combination of a normal HTTP interaction over an encrypted Secure Sockets Layer (SSL) or Transport Layer Security (TLS) connection. This ensures reasonable protection from eavesdroppers but is weak with man-in-the-middle attacks.

An https: URL may specify a TCP port; if it does not, the connection uses port 443 (unsecured HTTP typically uses port 80).

To prepare a web-server for accepting https connections the administrator must create a public key certificate for the web-server. These certificates can be created for Unix based servers with tools such as OpenSSL's ssl-ca or SuSE's gensslcert. This certificate must be signed by a certificate authority of one form or another, which certifies that the certificate holder is indeed the entity it claims to be. Web browsers are generally distributed with the signing certificates of major certificate authorities, so that they can verify certificates signed by them.

Organizations may also run their own certificate authority, particularly if they are responsible for setting up browsers to access their own sites (for example, sites on a company intranet), as they can trivially add their own signing certificate to those shipped with the browser.

Some sites, especially those operated by hobbyists, use self-signed certificates on public sites. Using these provides protection against simple eavesdropping, but unlike a well-known certificate, preventing a man-in-the-middle attack with a self-signed certificate requires the site to make available some other secure method of verifying the certificate.

The system can also be used for client authentication, in order to restrict access to a Web server to only authorized users. For this, typically the site administrator creates certificates for each user which are loaded into their browser. These normally contain the name and e-mail address of the authorized user, and are automatically checked by the server on each reconnect to verify the user's identity, potentially without ever entering a password.