Chapter 9: Authentication using PKCS #11 URIs

How to use PKCS #11 URIs in libssh?

PKCS #11 is a Cryptographic Token Interface Standard that provides an API to devices like smart cards that store cryptographic private information. Such cryptographic devices are referenced as tokens. A mechanism through which objects stored on the tokens can be uniquely identified is called PKCS #11 URI (Uniform Resource Identifier) and is defined in RFC 7512 (https://tools.ietf.org/html/rfc7512).

Pre-requisites (OpenSSL < 3.0):

OpenSSL 1.x defines an abstract layer called the "engine" to achieve cryptographic acceleration. The engine_pkcs11 module acts like an interface between the PKCS #11 modules and the OpenSSL application.

To build and use libssh with PKCS #11 support:

1. Enable the cmake option: $ cmake -DWITH_PKCS11_URI=ON
2. Build with OpenSSL.
3. Install and configure engine_pkcs11 (https://github.com/OpenSC/libp11).
4. Plug in a working smart card or configure softhsm (https://www.opendnssec.org/softhsm).

Pre-requisites (OpenSSL 3.0.8+)

The OpenSSL 3.0 is deprecating usage of low-level engines in favor of high-level "providers" to provide alternative implementation of cryptographic operations or acceleration.

To build and use libssh with PKCS #11 support using OpenSSL providers:

1. Install and configure pkcs11 provider (https://github.com/latchset/pkcs11-provider).
2. Enable the cmake options: $ cmake -DWITH_PKCS11_URI=ON -DWITH_PKCS11_PROVIDER=ON
3. Build with OpenSSL.
4. Plug in a working smart card or configure softhsm (https://www.opendnssec.org/softhsm).

New API functions

The functions ssh_pki_import_pubkey_file() and ssh_pki_import_privkey_file() that import the public and private keys from files respectively are now modified to support PKCS #11 URIs. These functions automatically detect if the provided filename is a file path or a PKCS #11 URI (when it begins with "pkcs11:"). If a PKCS #11 URI is detected, the engine is loaded and initialized. Through the engine, the private/public key corresponding to the PKCS #11 URI are loaded from the PKCS #11 device.

If you wish to authenticate using public keys on your own, follow the steps mentioned under "Authentication with public keys" in Chapter 2 - A deeper insight into authentication.

The function pki_uri_import() is used to populate the public/private ssh_key from the engine with PKCS #11 URIs as the look up.

Here is a minimalistic example of public key authentication using PKCS #11 URIs:

int authenticate_pkcs11_URI(ssh_session session)
{
  int rc;
  char priv_uri[1042] = "pkcs11:token=my-token;object=my-object;type=private?pin-value=1234";
 
  rc = ssh_options_set(session, SSH_OPTIONS_IDENTITY, priv_uri);
  assert_int_equal(rc, SSH_OK)
 
  rc = ssh_userauth_publickey_auto(session, NULL, NULL);
 
  if (rc == SSH_AUTH_ERROR)
  {
     fprintf(stderr, "Authentication with PKCS #11 URIs failed: %s\n",
       ssh_get_error(session));
     return SSH_AUTH_ERROR;
  }
 
  return rc;
}

Caveats

We recommend the users to provide a specific PKCS #11 URI so that it matches only a single slot in the engine. If the engine discovers multiple slots that could potentially contain the private keys referenced by the provided PKCS #11 URI, the engine will not try to authenticate.

For testing, the SoftHSM PKCS#11 library is used. But it has some issues with OpenSSL initialization/cleanup when used with OpenSSL 3.0 so we are using it indirectly through a p11-kit remoting as described in the following article:

https://p11-glue.github.io/p11-glue/p11-kit/manual/remoting.html