Need to write Irule to decode auth string to Base64 and fetch domain name from it and compare that domain name with certificate to deny/permit traffic

Something like this to get the domain value. I need to assume for the following code that the request that's sending this payload data is a POST:

when HTTP_REQUEST {
    if { [HTTP::method] eq "POST" } {
         buffer the HTTP payload and trigger the HTTP_REQUEST_DATA event
        HTTP::collect [HTTP::header Content-Length]
    }
}
when HTTP_REQUEST_DATA {
     HTTP payload data is available here
    if { [HTTP::payload] contains "" 21 "<"]
        catch { set domain [b64decode $domain] } 

        log local0. "domain = $domain"
    }
}

Another catch is we have multi SAN certificate so I want to compare this domain name with SAN's and then only permit traffic.

So do you mean you're applying this iRule to multiple HTTPS VIPs that have separate SAN certs, and you need to check the domain against each of the SAN names? Would it not be the same as the HTTP::host value?