Here's what I do:

s authorization64 = $G(pInput.Attributes("authorization"))
// ? valid
if ($PIECE(authorization64," ",1) '= "Basic"){
s Status = ##class(%SYSTEM.Status).Error(5001, "Missing basic http authentication")
Throw ##class(%Exception.StatusException).CreateFromStatus(Status)
}
//  basic http authentication looks like this: "Basic amFhcDphYXA="
s authorization = $System.Encryption.Base64Decode($PIECE(authorization64," ",2))
// decoded it looks like this: "username:password"
s username = $PIECE(authorization,":",1)
s password = $PIECE(authorization,":",2)