SignerInfos in a PKCS7 object indicate their certificates by issuer name and certificate serial number. Signature verification looks for this certificate
using PKCS7.getCertificate(serialNo, name). The latter, however, looks for a
certificate whose serial number is as given by the argument and whose *subject* name is as given by the second argument. This procedure won't find the proper certificate unless issuer and subject are the same, i.e. the certificate is self-signed.