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Neverbleed is an OpenSSL engine that runs RSA private key operations in an isolated process, thereby minimizing the risk of private key leak in case of vulnerability such as Heartbleed.

The engine is known to work together with existing versions of OpenSSL or LibreSSL, with minimal changes to the server source code.


Q. How much is the overhead?

Virtually none.

Generally speaking, private key operations are much more heavier than the overhead of inter-process communication. On my Linux VM running on Core i7 @ 2.4GHz (MacBook Pro 15" Late 2013), OpenSSL 1.0.2 without privilege separation processes 319.56 full TLS handshakes per second, whereas OpenSSL with privilege separation processes 316.72 handshakes per second (note: RSA key length: 2,048 bits, selected cipher-suite: ECDHE-RSA-AES128-GCM-SHA256).

Q. Why does the library only protect the private keys?

Because private keys are the only long-term secret being used for encrypting and/or digitally-signing the communication.

Depending on how OpenSSL is used, it might be beneficial to separate symmetric cipher operations or TLS operations as a whole. But even in such case, it would still be a good idea to isolate private key operations from them considering the impact of private key leaks. In other words, separating private key operations only to an isolated process in always a good thing to do.

Q. Is there any HTTP server that uses Neverbleed?

Neverbleed is used by H2O HTTP2 server since version 1.5.0-beta4.


The library exposes two functions: neverbleed_init and neverbleed_load_private_key_file.

The first function spawns an external process dedicated to private key operations, and the second function assigns a RSA private key stored in the specified file to an existing SSL context (SSL_CTX).


  1. adding call to neverbleed_init
  2. replacing call to SSL_CTX_use_PrivateKey_file with neverbleed_load_private_key_file

the privilege separation engine will be used for all the incoming TLS connections.

  neverbleed_t nb;

  /* initialize the OpenSSL library and the neverbleed engine */
  if (neverbleed_init(&nb, errbuf) != 0) {
    fprintf(stderr, "neverbleed_init failed: %s\n", errbuf);


  /* load certificate chain and private key */
  if (SSL_CTX_use_certificate_chain_file(ssl_ctx, certchain_fn) != 1) {
    fprintf(stderr, "failed to load certificate chain file:%s\n", certchain_fn);
  if (neverbleed_load_private_key_file(&nb, ctx, privkey_fn, errbuf) != 1) {
    fprintf(stderr, "failed to load private key from file:%s:%s\n", privkey_fn, errbuf);

Also, neverbleed_setuidgid function can be used to drop the privileges of the daemon process once it completes loading all the private keys.


privilege separation engine for OpenSSL / LibreSSL




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