SCT_CTX_set1_issuer_pubkey
int SCT_CTX_set1_issuer_pubkey(SCT_CTX *sctx, X509_PUBKEY *pubkey);
if (SCT_CTX_set1_issuer_pubkey(sctx, pub) != 1)
return SCT_CTX_set1_issuer_pubkey(sctx, X509_get_X509_PUBKEY(issuer));