Cryptography – Communication system using cryptography – Symmetric key cryptography
Reexamination Certificate
1999-06-22
2003-07-01
Peeso, Thomas R. (Department: 2132)
Cryptography
Communication system using cryptography
Symmetric key cryptography
C713S153000, C713S168000, C713S152000, C713S152000
Reexamination Certificate
active
06587562
ABSTRACT:
The invention relates to a synchronous data-stream generator for generating a stream of output data items of at least one bit in synchronisation with a clock trigger; the data-stream generator comprising:
a plurality of subgenerators M
i
, i≧1, each subgenerator M
i
comprising a respective clock input and a respective output; each subgenerator M
i
being operative to generate a data item of at least one bit on the respective output in response to a trigger received via the respective clock input;
means for combining a generated data item of each of the subgenerators M
i
, forming the output data item of the data-stream generator;
a control subgenerator C comprising an input for receiving the clock trigger and an output; the control subgenerator C being operative to generate a control data item of at least one bit on the output in response to the clock trigger; and
control means operative to provide a trigger to at least one of the clock inputs of the subgenerators M
i
in dependence on the control data item of the control subgenerator C.
The invention further relates to an encryptor and/or decryptor station comprising the synchronous data-stream generator. The invention also relates to an apparatus comprising a decryptor station comprising the synchronous data-stream generator.
The invention further relates to a method for generating an output stream of data items, each of at least one bit in synchronisation with a clock trigger; the method comprises:
generating a control stream of control data items, each of at least one bit, in synchronisation with the clock trigger; and
generating a plurality of data streams DS
i
of data items, each of at least one bit; each data stream DS
i
being generated according to a respective predetermined algorithm A
i
;
controlling the generation of data items for at least one of the data streams DS
i
in dependence on the control data items of the control stream; and
forming the output stream by combining the generated data streams DS
i
.
The invention relates also to a computer program for performing the method and a computer-readable storage medium having the program recorded thereon.
Such a synchronous data-stream generator is known as the alternating step generator from “Handbook of Applied Cryptography”, A. J. Menzes, P. C. van Oorschot, S. A. Vanstone, CRC Press, 1997, pages 209-211. In this system, two Linear Feedback Shift Registers (LFSRs) M
1
and M
2
are used to generate two data-streams. The data-streams are combined into one bit output stream of the data-stream generator via an XOR (Exclusive OR) operation. A third LFSR C is used as a control subgenerator whose output controls the clocking of the LFSRs M
1
and M
2
. The sequence of operation is as follows:
register C is clocked
if the output of C is a logic “1”, then M
1
is clocked; M
2
is not clocked but its output is repeated
if the output of C is a logic “0”, then M
2
is clocked; M
1
is not clocked but its output is repeated
the outputs of M
1
and M
2
are combined.
Non-linearity is introduced in this system by using C to irregularly clock one of the subgenerators at a time under control of the output of the subgenerator C. The data stream generator can be used in many applications. For instance, the data-stream generator may be used as a pseudo-random generator, and as such can be used for encrypting/decrypting data by adding the output bits modulo 2 (XOR) to the data bits.
It is an object of the invention to provide a synchronous data-stream generator of the kind set forth which is more resistant against known attacks. It is a further object to provide such an improved data-stream generator that is suitable for use in digital consumer electronics systems with a low gate-complexity for a hardware implementation and offering a speed suitable for encryption/decryption of digital audio/video signals.
To meet the object of the invention, the control means comprises for at least one subgenerator M
i
an associated number selector S
i
for, in dependence on the control data item of the control subgenerator C, selecting a number n
i,j
from a group H
i
of different integer numbers; at least two numbers of the group H
i
being larger than zero; the group H
i
being associated with the number selector S
i
; and in that the control means is operative to cause the associated subgenerator M
i
to provide at the output the selected n
i,j
-th data item successive to a last generated data item. In this way, the subgenerator(s) M
i
is triggered/clocked more irregularly causing a higher level of non-linear behaviour. Instead of approximately half of the time the subgenerator not being clocked (i.e. maintaining the same output), the group may be larger with at least two integers being different from zero, allowing the subgenerator to be clocked more frequently (i.e. the output is less frequently kept constant which is the result of a ‘zero’ being selected for the subgenerator and consequently the subgenerator not being clocked whereas another one is being clocked) and being provided with more than one trigger at a time (i.e. in fact skipping a number of n
i,j
−1 data items and providing the n
i,j
-th data item at the output). Such a skipping may simply be performed by in response to one clock trigger to the data-stream generator, clocking a subgenerator n
i,j
times. By not or less frequently maintaining the same output of a subgenerator, the data stream generator has more states. The hardware/software required to achieve this can be kept to a minimum, making the improvements suitable for high-speed consumer electronic applications.
A data item may simply consist of one bit (0 or 1), or be formed using more bits, representing a larger range of numbers. Combining the output of the subgenerators may be done using a simple operation, such as a bitwise XOR, or may be a more complicated (non-)linear operation.
In an embodiment according to the measure as defined in the dependent claim
2
, the balanced selection is an effective way to ensure maximum period.
According to the measure defined in the dependent claim
3
, the subgenerator is always clocked at least once, ensuring that the combined output always contains a new contribution of each of the subgenerators. Selecting from two numbers, of which one number is a ‘one’ allows for a simple and fast implementation.
According to the measure defined in the dependent claim
4
, at least two subgenerators are ‘irregularly’ and differently clocked, increasing the non-linearity further.
According to the measure defined in the dependent claim
5
, a finite state machine is used as a generator.
According to the measure defined in the dependent claim
6
, a feedback shift register is used as the finite state machine, allowing a simple and fast implementation suitable for consumer electronic applications. The feedback shift register may have a linear feedback or, alternatively, a non-linear feedback.
According to the measure defined in the dependent claim
7
, the n
i,j
-th output data item is generated in one operation (one trigger) instead of using n
i,j
successive clock triggers. In this way a high bit-rate can be achieved at an only marginal increase in gate-complexity. This makes the stream generator particularly suitable for use in digital audio/video equipment, such as a CD or DVD-like player, where a high bit-rate at low cost is required.
According to the measure defined in the dependent claim
8
, more output bits of the control data item are used, increasing the variability in the number selection process.
Preferably, the data-stream generator is used in an encryptor and/or decryptor station for generating a pseudo-random stream of data items. Symmetrical encryption/decryption can then be achieved by combining (e.g. using an XOR operation) the generated data stream with a stream to be encrypted/decrypted. Advantageously, the decryptor is incorporated in a playback device, such as a disc player, where the data stream to be encrypted is read from a record carrier, such as a tape or disc. The encryptor and decryptor may also be used
Jansen Cornelis J. A.
Roelse Petrus L. A.
Belk Michael E.
Koninklijke Philips Electronics , N.V.
Peeso Thomas R.
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