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2. The Library

In brief GnuTLS can be described as a library which offers an API to access secure communication protocols. These protocols provide privacy over insecure lines, and were designed to prevent eavesdropping, tampering, or message forgery.

Technically GnuTLS is a portable ANSI C based library which implements the TLS 1.1 and SSL 3.0 protocols (See section Introduction to TLS, for a more detailed description of the protocols), accompanied with the required framework for authentication and public key infrastructure. Important features of the GnuTLS library include:

Additionally GnuTLS provides a limited emulation API for the widely used OpenSSL(1) library, to ease integration with existing applications.

GnuTLS consists of three independent parts, namely the "TLS protocol part", the "Certificate part", and the "Cryptographic backend" part. The `TLS protocol part' is the actual protocol implementation, and is entirely implemented within the GnuTLS library. The `Certificate part' consists of the certificate parsing, and verification functions which is partially implemented in the GnuTLS library. The Libtasn1(2), a library which offers ASN.1 parsing capabilities, is used for the X.509 certificate parsing functions. A smaller version of OpenCDK(3) is used for the OpenPGP key support in GnuTLS. The "Cryptographic backend" is provided by the Libgcrypt(4) library(5).

In order to ease integration in embedded systems, parts of the GnuTLS library can be disabled at compile time. That way a small library, with the required features, can be generated.


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2.1 General Idea

A brief description of how GnuTLS works internally is shown at the figure below. This section may be easier to understand after having seen the examples (see examples).

gnutls-internals

As shown in the figure, there is a read-only global state that is initialized once by the global initialization function. This global structure, among others, contains the memory allocation functions used, and some structures needed for the ASN.1 parser. This structure is never modified by any GnuTLS function, except for the deinitialization function which frees all memory allocated in the global structure and is called after the program has permanently finished using GnuTLS.

The credentials structure is used by some authentication methods, such as certificate authentication (see Certificate Authentication). A credentials structure may contain certificates, private keys, temporary parameters for diffie hellman or RSA key exchange, and other stuff that may be shared between several TLS sessions.

This structure should be initialized using the appropriate initialization functions. For example an application which uses certificate authentication would probably initialize the credentials, using the appropriate functions, and put its trusted certificates in this structure. The next step is to associate the credentials structure with each TLS session.

A GnuTLS session contains all the required stuff for a session to handle one secure connection. This session calls directly to the transport layer functions, in order to communicate with the peer. Every session has a unique session ID shared with the peer.

Since TLS sessions can be resumed, servers would probably need a database backend to hold the session's parameters. Every GnuTLS session after a successful handshake calls the appropriate backend function (See resume, for information on initialization) to store the newly negotiated session. The session database is examined by the server just after having received the client hello(6), and if the session ID sent by the client, matches a stored session, the stored session will be retrieved, and the new session will be a resumed one, and will share the same session ID with the previous one.


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2.2 Error Handling

In GnuTLS most functions return an integer type as a result. In almost all cases a zero or a positive number means success, and a negative number indicates failure, or a situation that some action has to be taken. Thus negative error codes may be fatal or not.

Fatal errors terminate the connection immediately and further sends and receives will be disallowed. An example of a fatal error code is GNUTLS_E_DECRYPTION_FAILED. Non-fatal errors may warn about something, i.e., a warning alert was received, or indicate the some action has to be taken. This is the case with the error code GNUTLS_E_REHANDSHAKE returned by gnutls_record_recv. This error code indicates that the server requests a re-handshake. The client may ignore this request, or may reply with an alert. You can test if an error code is a fatal one by using the gnutls_error_is_fatal.

If any non fatal errors, that require an action, are to be returned by a function, these error codes will be documented in the function's reference. See Error Codes, for all the error codes.


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2.3 Memory Handling

GnuTLS internally handles heap allocated objects differently, depending on the sensitivity of the data they contain. However for performance reasons, the default memory functions do not overwrite sensitive data from memory, nor protect such objects from being written to the swap. In order to change the default behavior the gnutls_global_set_mem_functions function is available which can be used to set other memory handlers than the defaults.

The Libgcrypt library on which GnuTLS depends, has such secure memory allocation functions available. These should be used in cases where even the system's swap memory is not considered secure. See the documentation of Libgcrypt for more information.


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2.4 Callback Functions

There are several cases where GnuTLS may need some out of band input from your program. This is now implemented using some callback functions, which your program is expected to register.

An example of this type of functions are the push and pull callbacks which are used to specify the functions that will retrieve and send data to the transport layer.

Other callback functions such as the one set by gnutls_srp_set_server_credentials_function, may require more complicated input, including data to be allocated. These callbacks should allocate and free memory using the functions shown below.


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