Cryptography – Communication system using cryptography – Time segment interchange
Patent
1996-04-12
1998-08-25
Cangialosi, Salvatore
Cryptography
Communication system using cryptography
Time segment interchange
380 49, H04L 900
Patent
active
057990891
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a system for encrypting and decrypting digital data.
A known system uses the so-called cipher block chaining (CBC) method. Although an encryption of digital data can be obtained by this known CBC method, which encryption can hardly be decrypted by unauthorized persons, the known system has some disadvantages. These disadvantages are present in particular in applications in the field of digital television, wherein a minor number of senders and a very high number of receivers are involved and high processing speeds are required in view of the large amount of digital data to be transmitted. In using the usual CBC method, a buffer storage is required in the receiver, in which four block lengths of digital data can be stored. Such a large buffer storage increases the costs of the receiver which is a major disadvantage in systems with a high number of receivers. Further, a header with a fixed bit pattern is generally provided at the beginning of each packet of digital data. As in the known CBC method, the first block is combined with a fixed initial vector, this could result in a recognizable bit pattern in the encrypted data. Such a recognizable bit pattern provides a potential attack for decrypting the encrypted data by unauthorized persons.
SUMMARY OF THE INVENTION
The invention aims to provide a system of the above-mentioned type wherein the disadvantages of the known system are obviated in an effective manner and which is particularly suitable for application in the field of digital television.
In this manner a system is obtained wherein at the receiver side a buffer storage is required of two times the block length in bits so that the costs are decreased. By reversing the sequence of the blocks, it is further obtained that the initial vector is combined with variable data, whereby the header part of the packet as last block is combined with a variable bit pattern, so that it is guaranteed that a fixed pattern cannot be found in the encrypted blocks. The method used in the system according to the invention can be indicated as reverse cipher block chaining or RCBC method.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be further explained by reference to the drawings in which an embodiment of the system of the invention is schematically shown.
FIG. 1 schematically shows the RCBC method used in the system of the invention.
FIG. 2 schematically shows the operation of the encrypting device of the invention by means of a block diagram.
FIG. 3 schematically shows the operation of the decryption device of the invention by means of a block diagram.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1 there is very schematically shown an embodiment of the encryption and decryption method used in the system of the invention. In the embodiment shown it is assumed that packets of data are divided in four blocks A, B, C and D each having a length of 64 bits. At the sender side the sequence of the blocks A-D is reversed in an encryption device not further shown, so that the blocks D, C, B and A are encrypted successively in time. In the first encryption step block D is subjected to an exclusive- or operation or XOR operation indicated by the symbol +. In the first step, the XOR operation is carried out with an initial vector IV also having a length of 64 bits. As shown in FIG. 1, a data block D* is obtained in this manner, which is thereafter subjected to an encryption operation by means of an encryption algorithm E which will be further explained hereafter. Thereby the encrypted data or cypher text block D' is finally obtained.
In the second step, the data block C and the encrypted data block D' are subjected to a XOR operation providing an encoded data block C* which is thereafter encrypted by means of the encryption algorithm E into an encrypted data block C'. In the next steps the encrypted data blocks B' and A' are obtained in a corresponding manner.
Before transferring the data, the sequence is again reversed, so that the encrypte
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C.E. Roberts, "System-Level Data Encryption Comes of Age with Speed and Flexibility", Electro, vol. 17, May 1992, pp. 336-339.
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Davies Donald Watts
Kuhn Gideon Jacobus
Rix Simon Paul Ashley
Cangialosi Salvatore
Irdeto B.V.
Koehler S.
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