In XDM (following XSD) the value space of a value of type xs:base64Binary is "sequence of octets", and of course it is implementation-dependent how this is represented internally. But any processor that wants to do any computation on the value is likely to represent it internally as something like byte[]. It's then the process of validation or atomization (taking the typed value of a node) which converts from the base64 representation to the internal representation, and serialization (or casting to string) which converts back to the base64 representation.

Of course a conformant processor COULD use a base64 representation internally if it chose; or indeed a base79 representation.

Michael Kay

Saxonica