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libdvbcsa

Free implementation of the DVB Common Scrambling Algorithm


Introduction
============

libdvbcsa is a free and portable implementation of the DVB Common
Scrambling algorithm with decryption and encryption capabilities.

It comes in two flavors: a classical single packet implementation and
a faster parallel bitslice implementation.


Installation
============

Some configuration options are available to tune performance of the
parallel implementation. See INSTALL.


Algorithm overview
==================

The DVB CSA is composed of a two distinct ciphers which are applied to
scrambled content data packets. The block cipher and the stream cipher
both use the same 64 bits key. This key is called a control word.


Classical implementation API
============================

The classical implementation can process a single packet on each
function call. It is the slowest implementation and must be used when
data packets are not available as a large batch at the same time.

This implementation average processing bitrate is between 20 Mbits/s
and 50 Mbits/s on modern PCs.

    #include <dvbcsa/dvbcsa.h>

Two functions are available to allocate and free expanded key context:

    struct dvbcsa_key_s * dvbcsa_key_alloc();
    void dvbcsa_key_free(struct dvbcsa_key_s *key);

The control word can be changed as needed using this function:

    void dvbcsa_key_set (const dvbcsa_cw_t cw,
                         struct dvbcsa_key_s *key);

Data encryption and decryption is done with these functions:

     void dvbcsa_decrypt (const struct dvbcsa_key_s *key,
     	  	          unsigned char *data, unsigned int len);

     void dvbcsa_encrypt (const struct dvbcsa_key_s *key,
                          unsigned char *data, unsigned int len);


Parallel implementation API
===========================

The parallel implementation is faster but data packets need to be
batched together.

This implementation average processing bitrate is between 80 Mbits/s
and 200 Mbits/s on modern PCs. Performance heavily depends on bitslice
word width used, see install section.

    #include <dvbcsa/dvbcsa.h>

Two functions are available to allocate and free expanded key context:

    struct dvbcsa_bs_key_s * dvbcsa_bs_key_alloc();

    void dvbcsa_bs_key_free(struct dvbcsa_bs_key_s *key);

The control word can be changed as needed using this function:

    void dvbcsa_bs_key_set(const dvbcsa_cw_t cw,
                           struct dvbcsa_bs_key_s *key);

Packet batch must be available as an array of struct dvbcsa_bs_batch_s
to invoke encryption or decryption functions.

    struct dvbcsa_bs_batch_s
    {
      unsigned char		*data;	/* pointer to payload */
      unsigned int		len;	/* payload bytes lenght */
    };

The array must not be greater than the maximum batch size returned by:

    unsigned int dvbcsa_bs_batch_size(void);

An extra entry  with NULL data pointer must be  added to terminate the
array. Arrays with less entries  than the maximum batch size will take
the _same_ time to process as a full batch array.

An additional maximum packet lenght  parameter must be provided to the
processing  functions. Packet  greater than  this limit  will  only be
partially  processed. It  must  be  a multiple  of  8. This  parameter
directly  control algorithm  cycles  count (and  processing time)  and
should be  kept as low as  possible. When processing  Mpeg TS packets,
it should be 184.

Encryption and decryption batch processing functions are:

    void dvbcsa_bs_decrypt(const struct dvbcsa_bs_key_s *key,
		           const struct dvbcsa_bs_batch_s *pcks,
		           unsigned int maxlen);

    void dvbcsa_bs_encrypt(const struct dvbcsa_bs_key_s *key,
		           const struct dvbcsa_bs_batch_s *pcks,
		           unsigned int maxlen);

Example:

    int i, s = dvbcsa_bs_batch_size();
    struct dvbcsa_bs_batch_s b[s + 1];

    struct dvbcsa_bs_key_s *key = dvbcsa_bs_key_alloc();
    unsigned char cw[8] = "testtest";

    dvbcsa_bs_key_set(cw, key);

    for (i = 0; i < s; i++)
      {
        b[i].data = ... ;
        b[i].len = ... ;
      }

    b[i].data = NULL;

    dvbcsa_bs_encrypt(key, b, 184);


Portability
===========

This library has been successfully tested on different platforms with
32 bits and 64 bits word width, little-endian and big-endian bytes
ordering.