GB2381425A

GB2381425A - An encoding device which performs compression, encryption and FEC in that order, and includes synchronisation means

Info

Publication number: GB2381425A
Application number: GB0125875A
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
Anticipated expiration: 2021-10-29
Also published as: GB2381425B; GB0125875D0

Abstract

An encoding device comprises a data compression stage 5, an encryption stage 6, a forward error correction stage (FEC) stage 7 (to apply FEC to the encrypted data), and an output to output the data for onward transmission. The data is handled in frames. The frames output from the compression stage contain a variable amount of compressed data, the amount depending on how effectively the original data could be compressed. Padding bits are appended to the compressed data, some of which are then removed to ensure that each frame input to the FEC stage has a fixed length. This is achieved by applying clock signals 9, 10 to the compression and FEC stages. This synchronization process makes the sending of control information between the compression and FEC stages unnecessary and therefore reduces the vulnerability of the encryption system. A complementary decoding device is also disclosed.

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 a crypto stage, 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 because the system cannot determine which encrypted bits are data and which are redundant, 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. However, a problem with this arrangement is that it is necessary to transmit control signals across the crypto stage to provide information on the compression applied and the position in the data stream of space freed up by the compression, so that the FEC redundancy can be placed correctly. This increases the risk of unauthorised users being able to interpret the data stream, because of the risk of leakage close to the transmitter, so adds significantly to the cost to the device because of the need to undertake testing and obtain formal approvals.

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 before encryption; an encryption stage to encrypt the data; a forward error correction stage to apply forward error correction to the encrypted data and an output to output the data for onward transmission, the device further comprising synchronisation means applied to each of the compression and forward error correction stages to enable synchronisation

of plaintext and ciphertext, such that no physical connection is required across the crypto between the compression and forward error correction stages.

Preferably, the synchronisation means comprises a real time clock, although other devices for synchronisation could be used.

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 forward error correction stage to apply forward error correction to the data; a de-cryption stage to decrypt the data; a decompression stage to decompress the data; and an output to output the data; the device further comprising synchronisation means applied to each of the decompression and forward error correction stages to enable synchronisation of plaintext and ciphertext, such that no physical connection is required across the crypto between the forward error correction and decompression stages.

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

The inventors have found that the risk to security in a crypto system created by having compression and FEC on opposite sides of the crypto stage can be dealt with by applying synchronisation means, such as real time clocks, to both sides of the crypto (i. e. to the compression stage and to the FEC stage), which are synchronised at a time when no data is being transmitted.

The system of the present invention is able to operate in more noisy channel conditions than with a standard encrypted, uncompressed data transmission, but reduces the risk of compromising cryptographic security. Other advantages are that applying FEC allows the channel error rate to be higher to obtain the same quality of decrypted data, the channel signal to noise ratio can be reduced correspondingly if the decrypted data error rate is unchanged, the transmitter range can be extended or the power reduced.

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 Figure 1 in more detail; Figure 3 illustrates an example of a stream of data processed using the system of the present invention.

The communication system of Fig. 1 comprises a processor 1 and transmitter 2 for encoding data positioned remotely from a receiver 3 and processor 4 for decoding received data. The encoding side of the system is shown in more detail in Fig. 2a and the corresponding decoding side in Fig. 2b. The encoding side comprises a compression stage 5, an encryption stage 6, a forward error correction stage (FEC) 7.

The corresponding receiver processor also comprises a forward error correction stage 11 to correct errors introduced on the channel over which the data has been transmitted, de-cryption 12 and decompression 13 stages. In general, the system would be capable of both transmitting and receiving data from either location and although the same hardware could be used for both transmission and reception at the same location, it is preferred that a separate transmitter and receiver are provided.

A data stream input to the compression stage 5 is compressed using conventional data compression techniques, such as Huffman Coding, encrypted in the encryption stage 6 and then forward error correction (FEC) 7 is applied to the encrypted data. Since running an electrical signal between the unencrypted and the encrypted sides is known to give rise to security problems, the issue of synchronising frames on either side of the encryption stage needs to be dealt with in another way. In this particular example, the synchronisation is provided by real time clocks 9,10 connected to the compression and FEC stages respectively. They control the compression and FEC stages as described with respect to Fig. 3 below. The data output from the FEC 7 is combined in a multiplexer 10 for transmission from the transmitter 2. 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, although optical links tend to be significantly less error prone, so they have less need for such a system.

Fig. 3 illustrates how a data stream is processed using the present invention. The message data stream is made up of data frames which have a start and finish point defined in time. A message in plaintext having M symbols is compressed to K symbols (step A). After compression, some of the data frames have space and some have data.

A set of padding bits P are added to the compressed data to restore the length of the frame (step B). The compressed data and padding bits are encrypted separately and the encrypted padding bits discarded (step C). N-K parity bits are then added to the encrypted data to restore the frame size (step D). The real time clock present in both the compression stage and the FEC stage allows identification of encrypted message frames which have space to be made between the unencrypted and the encrypted sides without compromising the security of the data being transmitted.

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 before encryption; an encryption stage to encrypt the data; a forward error correction stage to apply forward error correction to the encrypted data and an output to output the data for onward transmission, the device further comprising synchronisation means applied to each of the compression and forward error correction stages to enable synchronisation of plaintext and ciphertext, such that no physical connection is required across the crypto between the compression and forward error correction stages.
2. An encoding device according to claim 1, wherein the synchronisation means comprises a real time clock.
3. A decoding device for a crypto system, the device comprising an input to receive transmitted data; a forward error correction stage to apply forward error correction to the data; a de-cryption stage to decrypt the data; a decompression stage to decompress the data; and an output to output the data; the device further comprising synchronisation means applied to each of the decompression and forward error correction stages, such that no physical connection is required across the crypto between the forward error correction and decompression stages.
4. A decoding device according to claim 3, wherein the synchronisation means comprises a real time clock.
5. A communication system, the system comprising an encoding device according to claim 1 and a decoding device according to claim 3.
6. An encoding device as hereinbefore described with reference to the accompanying drawings.

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7. A decoding device as hereinbefore described with reference to the accompanying drawings.
8. A communication system as hereinbefore described with reference to the accompanying drawings.