@node Hardware security modules and abstract key types
@chapter Hardware security modules and abstract key types

In several cases storing the long term cryptographic keys in a hard disk or
even in memory poses a significant risk. Once the system they are stored
is compromised the keys must be replaced as the secrecy of future sessions
is no longer guarranteed. Moreover, past sessions that were not protected by a
perfect forward secrecy offering ciphersuite are also to be assumed compromised.

If such threats need to be addressed, then it may be wise storing the keys in a security
module such as a smart card, an HSM or the TPM chip. Those modules ensure the
protection of the cryptographic keys by only allowing operations on them and
preventing their extraction.

@menu
* Abstract key types::
* Smart cards and HSMs::
* Trusted platform module::
@end menu

@node Abstract key types
@section Abstract key types
@cindex abstract types

Since there are many forms of a public or private keys supported by @acronym{GnuTLS} such as
@acronym{X.509}, @acronym{OpenPGP}, @acronym{PKCS} #11 or TPM it is desirable to allow common operations
on them. For these reasons the abstract @code{gnutls_privkey_t} and @code{gnutls_pubkey_t} were
introduced in @code{gnutls/abstract.h} header. Those types are initialized using a specific type of 
key and then can be used to perform operations in an abstract way. For example in order
to sign an X.509 certificate with a key that resides in a token the following steps must be
used.

@example
#inlude <gnutls/abstract.h>

void sign_cert( gnutls_x509_crt_t to_be_signed)
@{
gnutls_x509_crt_t ca_cert;
gnutls_privkey_t abs_key;

  /* initialize the abstract key */
  gnutls_privkey_init(&abs_key);

  /* keys stored in tokens are identified by URLs */
  gnutls_privkey_import_url(abs_key, key_url);

  gnutls_x509_crt_init(&ca_cert);
  gnutls_x509_crt_import_pkcs11_url(&ca_cert, cert_url);

  /* sign the certificate to be signed */
  gnutls_x509_crt_privkey_sign(to_be_signed, ca_cert, abs_key, 
                               GNUTLS_DIG_SHA256, 0);
@}
@end example

@menu
* Abstract public keys::
* Abstract private keys::
* Operations::
@end menu

@node Abstract public keys
@subsection Public keys
An abstract @code{gnutls_pubkey_t} can be initialized
using the functions below. It can be imported through
an existing structure like @code{gnutls_x509_crt_t},
or through an ASN.1 encoding of the X.509 @code{SubjectPublicKeyInfo}
sequence.

@showfuncC{gnutls_pubkey_import_x509,gnutls_pubkey_import_openpgp,gnutls_pubkey_import_pkcs11}

@showfuncC{gnutls_pubkey_import_url,gnutls_pubkey_import_privkey,gnutls_pubkey_import}

@showfuncdesc{gnutls_pubkey_export}

An important function is @funcref{gnutls_pubkey_import_url} which will import
public keys from URLs that identify objects stored in tokens (see @ref{Smart cards and HSMs} and @ref{Trusted platform module}).
A function to check for a supported by GnuTLS URL is @funcref{gnutls_url_is_supported}.

@showfuncdesc{gnutls_url_is_supported}

Additional functions are available that will return
information over a public key, as well as a function that given a public
key fingerprint would provide a memorable sketch.

@showfuncD{gnutls_pubkey_get_pk_algorithm,gnutls_pubkey_get_preferred_hash_algorithm,gnutls_pubkey_get_key_id,gnutls_random_art}



@node Abstract private keys
@subsection Private keys
An abstract @code{gnutls_privkey_t} can be initialized
using the functions below. It can be imported through
an existing structure like @code{gnutls_x509_privkey_t},
but unlike public keys it cannot be exported. That is
to allow abstraction over keys stored in hardware that 
makes available only operations.

@showfuncC{gnutls_privkey_import_x509,gnutls_privkey_import_openpgp,gnutls_privkey_import_pkcs11}

Other helper functions that allow directly importing from raw X.509 or
OpenPGP structures are shown below. Again, as with public keys, private keys 
can be imported from a hardware module using URLs.

@showfuncB{gnutls_privkey_import_x509_raw,gnutls_privkey_import_openpgp_raw}

@showfuncdesc{gnutls_privkey_import_url}

@showfuncB{gnutls_privkey_get_pk_algorithm,gnutls_privkey_get_type}

In order to support cryptographic operations using 
an external API, the following function is provided.
This allows for a simple extensibility API without
resorting to @acronym{PKCS} #11.

@showfuncdesc{gnutls_privkey_import_ext2}

@node Operations
@subsection Operations
The abstract key types can be used to access signing and
signature verification operations with the underlying keys.

@showfuncdesc{gnutls_pubkey_verify_data2}
@showfuncdesc{gnutls_pubkey_verify_hash2}
@showfuncdesc{gnutls_pubkey_encrypt_data}

@showfuncdesc{gnutls_privkey_sign_data}
@showfuncdesc{gnutls_privkey_sign_hash}
@showfuncdesc{gnutls_privkey_decrypt_data}

Signing existing structures, such as certificates, CRLs,
or certificate requests, as well as associating public
keys with structures is also possible using the 
key abstractions.

@showfuncdesc{gnutls_x509_crq_set_pubkey}
@showfuncdesc{gnutls_x509_crt_set_pubkey}
@showfuncC{gnutls_x509_crt_privkey_sign,gnutls_x509_crl_privkey_sign,gnutls_x509_crq_privkey_sign}

@node Smart cards and HSMs
@section Smart cards and HSMs
@cindex PKCS #11 tokens
@cindex hardware tokens
@cindex hardware security modules
@cindex smart cards

In this section we present the smart-card and hardware security module (HSM) support 
in @acronym{GnuTLS} using @acronym{PKCS} #11 @xcite{PKCS11}. Hardware security
modules and smart cards provide a way to store private keys and perform
operations on them without exposing them. This decouples cryptographic
keys from the applications that use them and provide an additional 
security layer against cryptographic key extraction.
Since this can also be achieved in software components such as in Gnome keyring,
we will use the term security module to describe any cryptographic key 
separation subsystem.

@acronym{PKCS} #11 is plugin API allowing applications to access cryptographic
operations on a security module, as well as to objects residing on it. PKCS
#11 modules exist for hardware tokens such as smart cards@footnote{@url{http://www.opensc-project.org}},
cryptographic tokens, as well as for software modules like @acronym{Gnome Keyring}. 
The objects residing on a security module may be certificates, public keys, 
private keys or secret keys. Of those certificates and public/private key 
pairs can be used with @acronym{GnuTLS}. PKCS #11's main advantage is that 
it allows operations on private key objects such as decryption
and signing without exposing the key. In GnuTLS the PKCS #11 functionality is
available in @code{gnutls/pkcs11.h}.

Moreover @acronym{PKCS} #11 can be (ab)used to allow all applications in the same operating system to access
shared cryptographic keys and certificates in a uniform way, as in @ref{fig:pkcs11-vision}.
That way applications could load their trusted certificate list, as well as user
certificates from a common PKCS #11 module. Such a provider exists in the @acronym{Gnome} 
system, being the @acronym{Gnome Keyring}.

@float Figure,fig:pkcs11-vision
@image{pkcs11-vision,9cm}
@caption{PKCS #11 module usage.}
@end float

@menu
* PKCS11 Initialization::
* Accessing objects that require a PIN::
* Reading objects::
* Writing objects::
* Using a PKCS11 token with TLS::
* p11tool Invocation::             Invoking p11tool
@end menu

@node PKCS11 Initialization
@subsection Initialization
To allow all the  @acronym{GnuTLS} applications to access @acronym{PKCS} #11 tokens
you can use a configuration per module, stored in @code{/etc/pkcs11/modules/}. 
These are the configuration files of @acronym{p11-kit}@footnote{@url{http://p11-glue.freedesktop.org/}}.
For example a file that will load the @acronym{OpenSC} module, could be named
@code{/etc/pkcs11/modules/opensc} and contain the following:

@example
module: /usr/lib/opensc-pkcs11.so
@end example

If you use this file, then there is no need for other initialization in
@acronym{GnuTLS}, except for the PIN and token functions (see next section).  
However, you may manually initialize the PKCS #11 subsystem if the default
settings are not desirable.

@showfuncdesc{gnutls_pkcs11_init}

Note that PKCS #11 modules must be reinitialized on the child processes
after a @funcintref{fork}. @acronym{GnuTLS} provides @funcref{gnutls_pkcs11_reinit}
to be called for this purpose.

@showfuncdesc{gnutls_pkcs11_reinit}

@node Accessing objects that require a PIN
@subsection Accessing objects that require a PIN

Objects stored in token such as a private keys are typically protected
from access by a PIN or password. This PIN may be required to either read
the object (if allowed) or to perform operations with it. To allow obtaining
the PIN when accessing a protected object, as well as probe
the user to insert the token the following functions allow to set a callback.

@showfuncD{gnutls_pkcs11_set_token_function,gnutls_pkcs11_set_pin_function,gnutls_pkcs11_add_provider,gnutls_pkcs11_get_pin_function}

The callback is of type @funcintref{gnutls_pin_callback_t} and will have as
input the provided userdata, the PIN attempt number, a URL describing the
token, a label describing the object and flags. The PIN must be at most 
of @code{pin_max} size and must be copied to pin variable. The function must
return 0 on success or a negative error code otherwise.

@verbatim
typedef int (*gnutls_pin_callback_t) (void *userdata, int attempt,
                                      const char *token_url,
                                      const char *token_label,
                                      unsigned int flags,
                                      char *pin, size_t pin_max);
@end verbatim

The flags are of @code{gnutls_pin_flag_t} type and are explained below.

@showenumdesc{gnutls_pin_flag_t,The @code{gnutls_pin_@-flag_t} enumeration.}

Note that due to limitations of @acronym{PKCS} #11 there are issues when multiple libraries 
are sharing a module. To avoid this problem GnuTLS uses @acronym{p11-kit}
that provides a middleware to control access to resources over the
multiple users.

To avoid conflicts with multiple registered callbacks for PIN functions,
@funcref{gnutls_pkcs11_get_pin_function} may be used to check for any previously
set functions. In addition context specific PIN functions are allowed, e.g., by
using functions below.

@showfuncE{gnutls_certificate_set_pin_function,gnutls_pubkey_set_pin_function,gnutls_privkey_set_pin_function,gnutls_pkcs11_obj_set_pin_function,gnutls_x509_crt_set_pin_function}

@node Reading objects
@subsection Reading objects

All @acronym{PKCS} #11 objects are referenced by @acronym{GnuTLS} functions by
URLs as described in @xcite{PKCS11URI}. 
This allows for a consistent naming of objects across systems and applications
in the same system. For example a public
key on a smart card may be referenced as:

@example
pkcs11:token=Nikos;serial=307521161601031;model=PKCS%2315; \
manufacturer=EnterSafe;object=test1;objecttype=public;\
id=32f153f3e37990b08624141077ca5dec2d15faed
@end example

while the smart card itself can be referenced as:
@example
pkcs11:token=Nikos;serial=307521161601031;model=PKCS%2315;manufacturer=EnterSafe
@end example

Objects stored in a @acronym{PKCS} #11 token can be extracted
if they are not marked as sensitive. Usually only private keys are marked as
sensitive and cannot be extracted, while certificates and other data can
be retrieved. The functions that can be used to access objects
are shown below.

@showfuncB{gnutls_pkcs11_obj_import_url,gnutls_pkcs11_obj_export_url}

@showfuncdesc{gnutls_pkcs11_obj_get_info}

@showfuncC{gnutls_x509_crt_import_pkcs11,gnutls_x509_crt_import_pkcs11_url,gnutls_x509_crt_list_import_pkcs11}

Properties of the physical token can also be accessed and altered with @acronym{GnuTLS}.
For example data in a token can be erased (initialized), PIN can be altered, etc.

@showfuncE{gnutls_pkcs11_token_init,gnutls_pkcs11_token_get_url,gnutls_pkcs11_token_get_info,gnutls_pkcs11_token_get_flags,gnutls_pkcs11_token_set_pin}

The following examples demonstrate the usage of the API. The first example
will list all available PKCS #11 tokens in a system and the latter will
list all certificates in a token that have a corresponding private key.

@example
int i;
char* url;

gnutls_global_init();

for (i=0;;i++) 
  @{
    ret = gnutls_pkcs11_token_get_url(i, &url);
    if (ret == GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE)
      break;

    if (ret < 0)
      exit(1);
		
    fprintf(stdout, "Token[%d]: URL: %s\n", i, url);
    gnutls_free(url);
  @}
gnutls_global_deinit();
@end example

@verbatiminclude examples/ex-pkcs11-list.c

@node Writing objects
@subsection Writing objects

With @acronym{GnuTLS} you can copy existing private keys and certificates
to a token. Note that when copying private keys it is recommended to mark
them as sensitive using the @code{GNUTLS_@-PKCS11_OBJ_@-FLAG_@-MARK_@-SENSITIVE}
to prevent its extraction. An object can be marked as private using the flag 
@code{GNUTLS_@-PKCS11_OBJ_@-FLAG_@-MARK_@-PRIVATE}, to require PIN to be
entered before accessing the object (for operations or otherwise).

@showfuncdesc{gnutls_pkcs11_copy_x509_privkey}

@showfuncdesc{gnutls_pkcs11_copy_x509_crt}
@showfuncdesc{gnutls_pkcs11_delete_url}


@node Using a PKCS11 token with TLS
@subsection Using a @acronym{PKCS} #11 token with TLS

It is possible to use a @acronym{PKCS} #11 token to a TLS
session, as shown in @ref{ex:pkcs11-client}. In addition
the following functions can be used to load PKCS #11 key and
certificates by specifying a PKCS #11 URL instead of a filename.

@showfuncB{gnutls_certificate_set_x509_trust_file,gnutls_certificate_set_x509_key_file}
@showfuncdesc{gnutls_certificate_set_x509_system_trust}

@include invoke-p11tool.texi

@node Trusted platform module
@section Trusted platform module
@cindex trusted platform module
@cindex TPM

In this section we present the Trusted Platform Module (TPM) support 
in @acronym{GnuTLS}. The TPM chip allows for storing and using RSA keys in a
similar way as a @acronym{PKCS} #11 module, but with slight differences
that require different handling. The basic operations supported, and used
by GnuTLS, are key generation and signing. 

In GnuTLS the TPM functionality is available in @code{gnutls/tpm.h}.

@menu
* Keys in TPM::
* Key generation::
* Using keys::
* tpmtool Invocation::            Invoking tpmtool
@end menu

@node Keys in TPM
@subsection Keys in TPM

The RSA keys in the TPM module may either be stored in a flash memory
within TPM or stored in a file in disk. In the former case the key can
provide operations as with @acronym{PKCS} #11 and is identified by
a URL. The URL is of the following form.
@verbatim
tpmkey:uuid=42309df8-d101-11e1-a89a-97bb33c23ad1;storage=user
@end verbatim

It consists from a unique identifier of the key as well as the part of the
flash memory the key is stored at. The two options for the storage field are
`user' and `system'. The user keys are typically only available to the generating
user and the system keys to all users. The stored in TPM keys are called
registered keys.

The keys that are stored in the disk are exported from the TPM but in an
encrypted form. To access them two passwords are required. The first is the TPM
Storage Root Key (SRK), and the other is a key-specific password. Also those keys are
identified by a URL of the form:
@verbatim
tpmkey:file=/path/to/file
@end verbatim

When objects require a PIN to be accessed the same callbacks as with PKCS #11
objects are expected (see @ref{Accessing objects that require a PIN}).

@node Key generation
@subsection Key generation

All keys used by the TPM must be generated by the TPM. This can be
done using @funcref{gnutls_tpm_privkey_generate}.

@showfuncdesc{gnutls_tpm_privkey_generate}

@showfuncC{gnutls_tpm_get_registered,gnutls_tpm_key_list_deinit,gnutls_tpm_key_list_get_url}

@showfuncdesc{gnutls_tpm_privkey_delete}

@node Using keys
@subsection Using keys

@subsubheading Importing keys

The TPM keys can be used directly by the abstract key types and do not require
any special structures. Moreover functions like @funcref{gnutls_certificate_set_x509_key_file}
can access TPM URLs.

@showfuncB{gnutls_privkey_import_tpm_raw,gnutls_pubkey_import_tpm_raw}

@showfuncdesc{gnutls_privkey_import_tpm_url}
@showfuncdesc{gnutls_pubkey_import_tpm_url}

@subsubheading Listing and deleting keys

The registered keys (that are stored in the TPM) can be listed using one of
the following functions. Those keys are unfortunately only identified by
their UUID and have no label or other human friendly identifier.
Keys can be deleted from permament storage using @funcref{gnutls_tpm_privkey_delete}.

@showfuncC{gnutls_tpm_get_registered,gnutls_tpm_key_list_deinit,gnutls_tpm_key_list_get_url}

@showfuncdesc{gnutls_tpm_privkey_delete}


@include invoke-tpmtool.texi