PREFIX  rdf:    <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX  foaf:   <http://xmlns.com/foaf/0.1/>
PREFIX  jfn:    <http://jena.hpl.hp.com/ARQ/function#>

SELECT ?name
{ ?x a foaf:Person ;
     foaf:mbox ?mbox .
  FILTER ( jfn:sha1sum(?mbox) = jfn:sha1sum("mailto:person@example.org") )
  ?x foaf:name ?name .
}