Basic access authentication

In the context of an HTTP transaction, basic access authentication is a method for an HTTP user agent to provide a user name and password when making a request.

Features

HTTP Basic authentication (BA) implementation is the simplest technique for enforcing access controls to web resources because it doesn't require cookies, session identifier and login pages. Rather, HTTP Basic authentication uses static, standard fields in the HTTP header which means that no handshakes have to be done in anticipation.

Security

The BA mechanism provides no confidentiality protection for the transmitted credentials. They are merely encoded with Base64 in transit, but not encrypted or hashed in any way. HTTPS is, therefore, typically preferred over or used in conjunction with Basic Authentication.

Because the BA field has to be sent in the header of each HTTP request, the web browser needs to cache credentials for a reasonable period of time to avoid constantly prompting the user for their username and password. Caching policy differs between browsers. Microsoft Internet Explorer by default caches them for 15 minutes.

HTTP does not provide a method for a web server to instruct the client to "log out" the user. However, there are a number of methods to clear cached credentials in certain web browsers. One of them is redirecting the user to a URL on the same domain containing credentials that are intentionally incorrect.

Unfortunately, this behavior is inconsistent between various browsers and browser versions. Microsoft Internet Explorer offers a dedicated JavaScript method to clear cached credentials:

 <script>document.execCommand('ClearAuthenticationCache', 'false');</script>

Protocol

Server side

When the server wants the user agent to authenticate itself towards the server, it must respond appropriately to unauthenticated requests.

Unauthenticated requests should return a response whose header contains a HTTP 401 Not Authorized status and a WWW-Authenticate field.

The WWW-Authenticate field for basic authentication (used most often) is constructed as following:

WWW-Authenticate: Basic realm="nmrs_m7VKmomQ2YM3:"

Client side

When the user agent wants to send the server authentication credentials it may use the Authorization field.

The Authorization field is constructed as follows:

1. Username and password are combined into a string "username:password".
2. The resulting string is then encoded using the RFC2045-MIME variant of Base64, except not limited to 76 char/line.
3. The authorization method and a space i.e. "Basic " is then put before the encoded string.

For example, if the user agent uses 'Aladdin' as the username and 'open sesame' as the password then the field is formed as follows:

Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

See also

• Digest access authentication
• HTTP header
• TLS-SRP, an alternative if one wants to avoid transmitting a password-equivalent to the server (even encrypted, like with TLS).