The invention relates to a secure data processing system including an
unscrambling module [DSC] disposed on a dedicated hardware part [HW] of
an integrated circuit and intended to unscramble a stream of data [SP]
scrambled according to a scrambling key, a module [CM] for calculating an
unscrambling key [Kp] disposed on said dedicated hardware part [HW] and
intended to manipulate data under the control of a so-called calculation
program stored on said dedicated hardware part [HW], a processor [CPU]
for in particular controlling the functioning of the unscrambling [DSC]
and calculation [CM] modules. Said system also includes a read only
memory [SME] disposed on said dedicated hardware part [HW] for storing a
secret key [Kp]. Said calculation program includes instructions for
prompting said calculation module [CM] to use said secret key [L] and at
least one data item [AC[n,p] or Kpc)] coming from outside the secure data
processing system, in order to calculate an unscrambling key [Kp]. The
unscrambling is enabled only if the unscrambling key [Kp] corresponds to
the scrambling key.