GB2381426A

GB2381426A - An encoding device which performs compression, FEC and encryption, in that order

Info

Publication number: GB2381426A
Application number: GB0125879A
Authority: GB
Inventors: John Joseph Spicer
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-10-29
Filing date: 2001-10-29
Publication date: 2003-04-30
Also published as: GB0125879D0

Abstract

An encoding device for a crypto system comprises a data compression stage (7) to compress data; a forward error coding stage (8) to apply forward error correction to the data; an encryption stage (9) to encrypt the data and an output to output the data for onward transmission, wherein the compression and forward error coding stages are combined and applied to plaintext data. A corresponding decoding device is also claimed.

Description

COMMUNICATION SYSTEM This invention relates to an encoding and decoding device for a communication system.

There are many systems where it is desirable that communication between two parties can only be understood by the parties themselves. For example, mobile phone users wishing to retain privacy or companies protecting commercially sensitive data transmitted between their offices around the world. Whatever method or equipment is used in the communication, the data can be encrypted so that only authorised users have access. One way of doing this is to pass the input data through an encryption process, before transmitting it, which converts plaintext to ciphertext. A similar arrangement in reverse at the receiver decrypts the data to enable the authorised party to read it. As the amount of data to be transmitted can be very large, or in some cases the data links are very slow, it is desirable to be able to compress the data. It has been found that applying data compression to the data after it is encrypted does not work, so the data compression is always applied to the plaintext. By contrast it is well understood that forward error correction must be applied to the ciphertext, because to apply it to the plaintext would introduce additional redundancy, thereby potentially compromising cryptographic security.

In accordance with a first aspect of the present invention an encoding device for a crypto system comprises a data compression stage to compress data; a forward error coding stage to apply forward error correction to the data; an encryption stage to encrypt the data and an output to output the data for onward transmission, wherein the compression and forward error coding stages are combined and applied to plaintext data.

In accordance with a second aspect of the present invention a decoding device for a crypto system comprises an input to receive transmitted data; a decryption stage to decrypt the data; a forward error coding stage to apply forward error correction to the data; a de-compression stage to de-compress the data; and an output to output the data, wherein the forward error coding stage and de-compression stage are combined and applied to plaintext data.

In accordance with a third aspect of the present invention, a communication system comprises and encoding device according to the first aspect and a decoding device according to the second aspect.

The inventors have found that the performance of a crypto system can be improved by combining the data compression and forward error coding stages and applying both of these to the plaintext. The redundancy removed by data compression is reintroduced by forward error correction (FEC), so that the data input to the crypto is equivalent to uncompressed data and does not compromise cryptographic security unduly.

A further benefit of the system of the present invention is that it is able to operate in more noisy conditions than with a standard uncompressed data input. Other advantages are that applying FEC allows the communication channel error rate to be higher to obtain the same quality of output data, the signal to noise ratio can be reduced correspondingly for an unchanged output data error rate, the transmitter range can be extended or the power reduced. Combining the data compression stage and the forward error correction on the same side of the crypto, reduces manufacturing costs and by not crossing the interface between plaintext and ciphertext, cryptographic security is maintained.

An example of a communication system according to the present invention will now be described with reference to the accompanying drawings in which :- Figure 1 is a block diagram of a communication system according to the present invention; Figure 2 illustrates encoding and decoding stages for the system of Fig. 1 in more detail; Figure 3 illustrates how an example of a data stream is processed using the system of the present invention.

The communication system of Fig. 1 comprises an input stage 1, an encryption stage 2 and a transmitter 3 at one location, with a corresponding receiver 4, decryption stage 5 and output stage 6 provided at another remote location. The encoding side of the system is shown in more detail in Fig. 2a. The structure of the receive side is similar, but reversed as shown in Fig. 2b. In general, the system would be capable of both transmitting and receiving data from either location.

Data input to the input stage 1 is compressed 7 using one of many conventional data compression techniques, such as Huffman Coding and then forward error correction (FEC) 8, such as Reed Solomon, is applied to the compressed data. The data output from the FEC 8 is encrypted in the crypto 9 and the encrypted data is combined in a multiplexer 10 for transmission from the transmitter 11 via channel 15. The medium for transmission is not constrained to any particular type, and may be one of, for example, radio, acoustic, optical or wire links depending on the application. At the receive side, the data is decrypted 12, forward error corrected 13 and decompressed 14.

Fig. 3 illustrates a data stream at each stage in processing (using a single data source for simplicity) and how the data stream is affected by the compression, encryption and correction stages before transmission. In Fig. 3a, each point is given a reference and the appearance of a section of the data stream at the corresponding point is shown in Fig. 3b.

The unprocessed data stream at point X has M symbols which have been compressed at point Y to only K symbols, where the K symbols are all information bits, without redundancy. FEC is applied, so that at point Z, there are K information bits, plus N-K parity bits, where N is the total length of the data stream. This brings the number of symbols back to its original level before the redundancy was removed. The data stream is then encrypted, so that at point W, there are the same number of symbols as at point Z, but their order has been scrambled, so that the information and parity bits are not obviously separated. Decoding on receipt is similar, but in reverse.

There are various ways of applying the encryption. One of these is to multiply the cipher stream bit by bit with a 1 or 0 using an XOR gate. This gives the encrypted text in the same order as the plaintext. Another method takes blocks of data and scrambles these within the blocks. However, this is less desirable for the system of the present invention as channel errors can be introduced which overwhelm the FEC.

Although the symbol by symbol method can also introduce errors, these tend to be limited in their effect.

A particular example of an application of the present invention is a radio telemetry system, which carries commercially sensitive information from an outstation into a company's management and control system. A sensor may be installed to measure particular parameters in a production process at a remote site, for example the rate and quality of oil flow from an oil extraction plant. This data would need to be

protected from observation by competitors. The invention described above could be used to provide this protection, whilst at the same time allow a greater range for the radio telemetry link than a scheme that used only encryption/decryption to protect the data.

Claims

CLAIMS 1. An encoding device for a crypto system, the device comprising a data compression stage to compress data; a forward error coding stage to apply forward error correction to the data; an encryption stage to encrypt the data and an output to output the data for onward transmission, wherein the compression and forward error coding stages are combined and applied to plaintext data.
2. A decoding device for a crypto system, the device comprising an input to receive transmitted data; a decryption stage to decrypt the data; a forward error coding stage to apply forward error correction to the data; a decompression stage to decompress the data; and an output to output the data, wherein the forward error coding stage and decompression stage are combined and applied to plaintext data.
3. A communication system, the system comprising an encoding device according to claim 1 and a decoding device according to claim 2.
4. An encoding device as hereinbefore described with reference to the accompanying drawings.
5. A decoding device as hereinbefore described with reference to the accompanying drawings.
6. A communication system as hereinbefore described with reference to the accompanying drawings.