CMSAttributeTableGenerator attrGen = new DefaultSignedAttributeTableGenerator(at);
SignerInfoGeneratorBuilder genBuild =
new SignerInfoGeneratorBuilder(new BcDigestCalculatorProvider());
genBuild.setSignedAttributeGenerator(attrGen);
SignerInfoGenerator sifGen =
genBuild.build(sigGen, new X509CertificateHolder(userCertificate.getEncoded()));
gen.addSignerInfoGenerator(sifGen);
/**
* Popolo la "catena di certificazione" (certificate chain)