Cryptographic Spread Spectrum Relay Communication

Zahoor AhmedJ.P.CancesV.Meghdadi

Cryptographic Spread Spectrum Relay Communication

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Break up

Spread Spectrum Cryptography Relay Network (Orthogonal) Conclusions

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Introduction

Spread Spectrum Cryptography

to hide the Signal below noise level Message Encryption

Doubly secured information

Transmission through Relay Channel

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Spread Spectrum

Spread spectrum is an RF communications system in which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher frequency signalReason ?secure communicationsMakes jamming and interception harderprevent detection.

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+1

+1

+1

-1

-1

-1

t

t

t

Data Signal

PN Signal

Data x PN

Tb=8Tc Tb=8Tc Tb=8Tc

Tb

Tc

1 -1 - 1

1,-1,- 1,1,-1,1,1,-1

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Example

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The data transmitted with bit duration Tb d(t)= dn { 1,-1}

PN Sequence N=Tb/Tc

transmitted with chip duration Tc

The Data in channel

Spread Spectrum

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Cryptography

Message

Secret key

Cryptogram

To recover the original message, the receiver computes

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SS and Cryptography

To encrypt the message data or PN sequenceSuppose we want to transmit the Message with SF N=Tb/Tc PN Sequence to synchronize bit and chip time, add S in M

where is N time repetition of M

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Synchronization Problem

Cryptogram

Since the energy of SS is lower than ambient noise, so it is not possible to recover the message without the knowledge of S

Problem ?Good cryptographic PN Sequences confronts theproblem of synchronization.

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Suggestion

To counter the problem of synchronization, we use a technique very like that of steganography We can spread N over a more larger bandwidth to mask the message inside ambient noise. We mask the secret SS signal with a classical dummy communication. This method is more robust than classical stegano.

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SS using BPSK Channel

Let Be the non-secret message

Then the transmitted signal will be let Be the hidden message

Then the secret communication is

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SS using BPSK Channel

Then the transmitted signal is

The receiver uses non-secret communication to

synchronization, recover the signalThe secret message is smaller than the noise, so

is not detectable if the PN sequence is not known to compute correlation

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SS and Orthogonal transmission

If an error occurs on symbol M, this error induces very high noise on secret message mTo avoid this problem we use a relay channel in which two signals are sent orthogonally, so now it is unlikely that that the non secret message can cause an error in the secret message. So now the transmitted signal will be

Where

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Relay Network

DestinationSource

Relay

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Relay Network(Orthogonal)

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Relay network Capacity

Achievability is proved by cut-set bound

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Conclusion

SS Communications are difficult to detect. Encrypted PN sequences makes the communication

more secure. The selection of good key make the communication

absolutely secure Synchronization problem is addressed by Splitting the

signal into phase and quadrature.

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