Using GnuTLS as a cryptographic library
***************************************

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 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 (See http://www.lysator.liu.se/~nisse/nettle/.) library is recommended.

Symmetric algorithms
====================

The available functions to access symmetric crypto algorithms operations are shown below. The supported algorithms are the algorithms required by the TLS protocol. They are listed in gnutls_cipher_algorithm_t.

gnutls_cipher_algorithm_t,The supported ciphers.

gnutls_cipher_init,gnutls_cipher_encrypt2,gnutls_cipher_decrypt2,gnutls_cipher_set_iv,gnutls_cipher_deinit

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.

gnutls_aead_cipher_init,gnutls_aead_cipher_encrypt,gnutls_aead_cipher_decrypt,gnutls_aead_cipher_deinit

Public key algorithms
=====================

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

PKCS #7 signing
---------------

The PKCS #7 format is common format used for digital signatures. It allows to sign by embedding the data into the signature, or creating detached signatures of the data, including a timestamp, additional certificates etc. In certain cases this format is also used to transport lists of certificates and CRLs.

The basic functions to initialize, deinitialize, import, export or print information about a PKCS #7 structure are shown below. 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 gnutls_pkcs7_sign function is the data signing function.

gnutls_pkcs7_verify_direct,gnutls_pkcs7_verify gnutls_pkcs7_sign

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.

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. gnutls_pkcs7_set_crt_raw,gnutls_pkcs7_set_crt,gnutls_pkcs7_set_crl_raw,gnutls_pkcs7_set_crl

Hash and 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 gnutls_mac_algorithm_t. Note that, despite the hmac part in the name of the MAC functions listed below, they can be used either for HMAC or MAC operations.

gnutls_mac_algorithm_t,The supported MAC and HMAC algorithms.

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 gnutls_digest_algorithm_t.

gnutls_hash_init,gnutls_hash,gnutls_hash_output,gnutls_hash_deinit,gnutls_hash_get_len,gnutls_hash_fast gnutls_fingerprint

gnutls_digest_algorithm_t,The supported hash algorithms.

Random number generation
========================

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

gnutls_rnd_level_t,The random number levels. gnutls_rnd

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 Abstract private keys.

gnutls_crypto_register_cipher gnutls_crypto_register_aead_cipher gnutls_crypto_register_mac gnutls_crypto_register_digest