Encoding base64 with XSLT

An XML based addressbook has to be converted to LDIF (LDAP Data
Interchange Format, RFC 2849) using an XSLT-stylesheet. LDIF does not 
allow non-ASCII characters, they have to be base64-encoded.
E.g. given an element

<lastname>Müller</lastname>

has to be written like

sn:: TfxsbGVy

instead of simply:

sn: Müller

Is there any way to encode base64 with XSLT

Michael