Signing RDF Graphs

G1 (Monica ex:hasStatus Admin. 
    Monica rdf:type ex:Person. 
    G1 ex:author Andy. 
    G1 dc:date  2/10/2004 ) 
G2 (G1 ex:hasSignature  xd2shfl22k4jdsre& . 
    G1 ex:Signer Andy) 
G3 (Andy ex:publicKeyURL http://bla.bla.bla)

SELECT ?a ?b ?c ?d 
WHERE ?d (Monica ex:hasStatus Admin) 
 (?d dc:signer ?a) 
 (?a ex:publicKeyURL ?b) 
 (?d ex:hasSignature ?c)