@node Using GnuTLS as a cryptographic library
@chapter Using GnuTLS as a cryptographic library

@acronym{GnuTLS} is not a low-level cryptographic library, i.e., 
it does not provide access to basic cryptographic primitives. However
it abstracts the internal cryptographic back-end (see @ref{Cryptographic Backend}),
providing symmetric crypto, hash and HMAC algorithms, as well access
to the random number generation. For a low-level crypto API the usage of nettle
@footnote{See @uref{https://www.lysator.liu.se/~nisse/nettle/}.} library is recommended.

@menu
* Symmetric algorithms::
* Public key algorithms::
* Cryptographic Message Syntax / PKCS7::
* Hash and MAC functions::
* Random number generation::
* Overriding algorithms::
@end menu

@node Symmetric algorithms
@section Symmetric algorithms
@cindex symmetric algorithms
@cindex symmetric cryptography

The available functions to access symmetric crypto algorithms operations
are listed in the sections below. The supported algorithms are the algorithms required by the TLS protocol.
They are listed in @ref{gnutls_cipher_algorithm_t}. Note that there two
types of ciphers, the ones providing an authenticated-encryption with
associated data (AEAD), and the legacy ciphers which provide raw access
to the ciphers. We recommend the use of the AEAD ciphers under the AEAD APIs
for new applications as they are designed to minimize the misuse of
cryptographic primitives.

@showenumdesc{gnutls_cipher_algorithm_t,The supported ciphers.}

@subheading Authenticated-encryption API

The AEAD API provides access to all ciphers supported by GnuTLS which support
authenticated encryption with associated data; these ciphers are marked with
the AEAD keyword on the table above. The AEAD cipher API is
particularly suitable for message or packet-encryption as it provides
authentication and encryption on the same API. See @code{RFC5116} for more
information on authenticated encryption.

@showfuncD{gnutls_aead_cipher_init,gnutls_aead_cipher_encrypt,gnutls_aead_cipher_decrypt,gnutls_aead_cipher_deinit}

Because the encryption function above may be difficult to use with
scattered data, we provide the following API.

@showfuncdesc{gnutls_aead_cipher_encryptv}

@subheading Legacy API

The legacy API provides low-level access to all legacy ciphers supported by GnuTLS,
and some of the AEAD ciphers (e.g., AES-GCM and CHACHA20). The restrictions
of the nettle library implementation of the ciphers apply verbatim to this
API@footnote{See the nettle manual @url{https://www.lysator.liu.se/~nisse/nettle/nettle.html}}.

@showfuncE{gnutls_cipher_init,gnutls_cipher_encrypt2,gnutls_cipher_decrypt2,gnutls_cipher_set_iv,gnutls_cipher_deinit}

@showfuncB{gnutls_cipher_add_auth,gnutls_cipher_tag}
While the latter two functions allow the same API can be used with authenticated encryption ciphers, 
it is recommended to use the following functions which are solely for AEAD ciphers. The latter
API is designed to be simple to use and also hard to misuse, by handling the tag verification
and addition in transparent way.

@node Public key algorithms
@section Public key algorithms
@cindex public key algorithms

Public key cryptography algorithms such as RSA, DSA and ECDSA, are
accessed using the abstract key API in @ref{Abstract key types}. This
is a high level API with the advantage of transparently handling keys
stored in memory and keys present in smart cards.

@showfuncF{gnutls_privkey_init,gnutls_privkey_import_url,gnutls_privkey_import_x509_raw,gnutls_privkey_sign_data,gnutls_privkey_sign_hash,gnutls_privkey_deinit}

@showfuncF{gnutls_pubkey_init,gnutls_pubkey_import_url,gnutls_pubkey_import_x509,gnutls_pubkey_verify_data2,gnutls_pubkey_verify_hash2,gnutls_pubkey_deinit}

Keys stored in memory can be imported using functions like
@funcref{gnutls_privkey_import_x509_raw}, while keys on smart cards or HSMs
should be imported using their PKCS#11 URL with
@funcref{gnutls_privkey_import_url}.

If any of the smart card operations require PIN, that should be provided
either by setting the global PIN function
(@funcref{gnutls_pkcs11_set_pin_function}), or better with the targeted to
structures functions such as @funcref{gnutls_privkey_set_pin_function}.


@subsection Key generation

All supported key types (including RSA, DSA, ECDSA, Ed25519) can be generated
with GnuTLS. They can be generated with the simpler @funcref{gnutls_privkey_generate}
or with the more advanced @funcref{gnutls_privkey_generate2}.

@showfuncdesc{gnutls_privkey_generate2}

@node Cryptographic Message Syntax / PKCS7
@section Cryptographic Message Syntax / PKCS7
@cindex public key algorithms
@cindex cryptographic message syntax
@cindex file signing
@cindex CMS
@cindex PKCS #7

The CMS or PKCS #7 format is a commonly used format for digital signatures.
PKCS #7 is the name of the original standard when published by RSA, though
today the standard is adopted by IETF under the name CMS.

The standards include multiple ways of signing a digital document, e.g.,
by embedding the data into the signature, or creating detached signatures of the data,
including a timestamp, additional certificates etc. In certain cases the
same format is also used to transport lists of certificates and CRLs.

It is a relatively popular standard to sign structures, and is being used to
sign in PDF files, as well as for signing kernel modules and other
structures.

In GnuTLS, the basic functions to initialize, deinitialize, import, export or print information
about a PKCS #7 structure are listed below.
@showfuncE{gnutls_pkcs7_init,gnutls_pkcs7_deinit,gnutls_pkcs7_export2,gnutls_pkcs7_import,gnutls_pkcs7_print}

The following functions allow the verification of a structure using either a trust list, or
individual certificates. The @funcref{gnutls_pkcs7_sign} function is the data signing function.

@showfuncB{gnutls_pkcs7_verify_direct,gnutls_pkcs7_verify}

@showfuncdesc{gnutls_pkcs7_sign}

@showenumdesc{gnutls_pkcs7_sign_flags,Flags applicable to gnutls_pkcs7_sign()}

Other helper functions which allow to access the signatures, or certificates attached
in the structure are listed below.

@showfuncF{gnutls_pkcs7_get_signature_count,gnutls_pkcs7_get_signature_info,gnutls_pkcs7_get_crt_count,gnutls_pkcs7_get_crt_raw2,gnutls_pkcs7_get_crl_count,gnutls_pkcs7_get_crl_raw2}

To append certificates, or CRLs in the structure the following functions are provided.
@showfuncD{gnutls_pkcs7_set_crt_raw,gnutls_pkcs7_set_crt,gnutls_pkcs7_set_crl_raw,gnutls_pkcs7_set_crl}

@node Hash and MAC functions
@section Hash and MAC functions
@cindex hash functions
@cindex HMAC functions
@cindex MAC functions

The available operations to access hash functions and hash-MAC (HMAC) algorithms
are shown below. HMAC algorithms provided keyed hash functionality. The supported MAC and HMAC
algorithms are listed in @ref{gnutls_mac_algorithm_t}. Note that, despite the @code{hmac} part 
in the name of the MAC functions listed below, they can be used either for HMAC or MAC operations.

@showenumdesc{gnutls_mac_algorithm_t,The supported MAC and HMAC algorithms.}

@showfuncF{gnutls_hmac_init,gnutls_hmac,gnutls_hmac_output,gnutls_hmac_deinit,gnutls_hmac_get_len,gnutls_hmac_fast}

The available functions to access hash functions are shown below. The supported hash functions
are shown in @ref{gnutls_digest_algorithm_t}.

@showfuncF{gnutls_hash_init,gnutls_hash,gnutls_hash_output,gnutls_hash_deinit,gnutls_hash_get_len,gnutls_hash_fast}
@showfuncA{gnutls_fingerprint}

@showenumdesc{gnutls_digest_algorithm_t,The supported hash algorithms.}

@node Random number generation
@section Random number generation
@cindex random numbers

Access to the random number generator is provided using the @funcref{gnutls_rnd}
function. It allows obtaining random data of various levels.

@showenumdesc{gnutls_rnd_level_t,The random number levels.}
@showfuncdesc{gnutls_rnd}

See @ref{Random Number Generators-internals} for more information
on the random number generator operation.

@node Overriding algorithms
@section Overriding algorithms
@cindex overriding algorithms

In systems which provide a hardware accelerated cipher implementation
that is not directly supported by GnuTLS, it is possible to utilize it.
There are functions which allow overriding the default cipher, digest and MAC
implementations. Those are described below.

To override public key operations see @ref{Abstract private keys}.

@showfuncdesc{gnutls_crypto_register_cipher}
@showfuncdesc{gnutls_crypto_register_aead_cipher}
@showfuncdesc{gnutls_crypto_register_mac}
@showfuncdesc{gnutls_crypto_register_digest}