Encoding charset of HTTP Basic Authentication

I know this is not an "xml" question but maybe someone on this list knows or can point me to the right direction ?

Is there a defined character set for the strings used in user/password in HTTP Basic Authentication ?
I can't find any reference in the W3C specs

It says its "Base64" encoded but that only makes sense on a byte array not a string.

So what encoding/charset is the string assumed to be ?
I found some apache software that lets you specify this ... but is there any 'standard' ?

Example: if someone uses a password like "$BHSED@>(B"

What charset should be used to pass that to the base64 encoding ?

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David A. Lee