SSL_CTX_use_PrivateKey_file
if (SSL_CTX_use_PrivateKey_file(ctx, key_file, SSL_FILETYPE_PEM) <= 0) {
int SSL_CTX_use_PrivateKey_file(SSL_CTX *ctx, const char *file, int type);
if (SSL_CTX_use_PrivateKey_file(ctx, client_key_file,
error = SSL_CTX_use_PrivateKey_file(ctx, path, SSL_FILETYPE_PEM);