!!! Overview
[{$pagename}] [HTTP Request Header] contains the [credentials] to [authenticate] a [user-agent] with a [server], usually after the [server] has responded with a [HTTP 401] [Unauthorized] and the [WWW-Authenticate] [HTTP Response Header].

In the context of an HTTP transaction, [HTTP Basic Authentication|Basic Authentication Scheme] is an [Authentication Method] for an [HTTP] [user-agent] to provide a user name and [password] when making a [HTTP Request].

[Basic Authentication Scheme] implementation is the simplest technique for enforcing [Access Controls] to [web] [resources] because it doesn't require [cookies], session identifier and login pages. Rather, [Basic Authentication Scheme] uses static, standard [HTTP Header Fields] which means that no handshakes have to be done in anticipation.

When the [user-agent] wants to send the server [authentication] [credentials] it may use the [{$pagename}] [1]

!! [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] [HTTP Header Field] for basic authentication (used most often) is constructed as following:
%%prettify 
{{{
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:

[Username] and [password] are combined into a string "username:password". Note that username cannot contain the ":" character.

The resulting string is then encoded using the [RFC 2045]-[MIME] variant of [Base64], except not limited to 76 char/line.

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:
%%prettify 
{{{
Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==
}}} /%

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [Basic Access Authentication|Wikipedia:Basic_access_authentication|target='_blank'] - based on data observed:2015-05-18