In the series of highly boring posts ;) here’s another one; in this case, a simple explanation of two different authentication protocols available in the HTTP standard.

HTTP Basic Authentication Protocol

This is the simplest HTTP Authentication protocol available:

1. The browser sends a request to a protected resource: GET /index.html
2. The server looks for the “Authenticated” header in the request; since it does not find it, it replies back with a response with the 401 code (“Unauthorized”). The response contains a “WWW-Authenticate” header, with the value “Basic”. This response is called a “challenge”, and it also contains a “realm” text, which describes the protected resource in clear text (the “realm” is shown in the pop-up window that usually appears for you to type your password when this protocol is used).
3. The browser creates a new request GET /index.html that contains an HTTP_AUTHORIZATION header, whose value is the word “Basic” followed by the ‘username:password’ string encoded in base 64. This algorithm is a two-way algorithm: you can retrieve the username and password from the base 64-encoded string.
4. The server receives this response, and since base 64 is a two-way algorithm, it compares the MD5 (or SHA1) password hash to the one stored in the database. If they are the same, the request is processed. Otherwise, the user gets a 401 again.

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