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doc: Documents PKCS #11 URI support for libssh

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Signed-off-by: Sahana Prasad <sahana@redhat.com>
Reviewed-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
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Sahana Prasad
2020-01-21 13:52:46 +01:00
committed by Anderson Toshiyuki Sasaki
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/**
@page libssh_tutor_pkcs11 Chapter 9: Authentication using PKCS #11 URIs
@section how_to 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 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 engine.
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).
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:
@code
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;
}
@endcode
@subsection 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.
*/