The docs at https://www.openssl.org/docs/man3.0/man3/SSL_CTX_set_tlsext_ticket_key_evp_cb.html say "returns 0 to indicate the callback function was set" The code (6d770c5ba36d43f4): int SSL_CTX_set_tlsext_ticket_key_evp_cb (SSL_CTX *ctx, int (*fp)(SSL *, unsigned char *, unsigned char *, EVP_CIPHER_CTX *, EVP_MAC_CTX *, int)) { ctx->ext.ticket_key_evp_cb = fp; return 1; } (I suspect from my application code comments that SSL_CTX_set_tlsext_ticket_key_cb() may suffer the same) -- Cheers, Jeremy