Data encryption on the physical layer of a data transmission system

• US 20050055546A1
• Filed: 08/30/2004
• Published: 03/10/2005
• Est. Priority Date: 09/08/2003
• Status: Active Grant

• Current Assignee: ABB Research Ltd. (ABB Ltd.)
• Original Assignee: ABB Research Ltd. (ABB Ltd.)
• Inventors: Dzung, Dacfey
• Granted Patent: US 7,752,430 B2
• Time in Patent Office: Days
• US Class Current: 713/151
• CPC Class Codes: H04K 1/00 Secret communication
• CPC Class Codes: H04L 27/2602 Signal structure
• CPC Class Codes: H04L 27/2626 Arrangements specific to the transmitter only
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01
• CPC Class Codes: H04W 12/03 Protecting confidentiality, e.g. by encryption

Abstract

The present invention is concerned with a method to encrypt OFDM modulation, by multiplying its underlying QAM symbols by a complex-valued generalized key stream sequence. Performing encryption on the physical layer ensures that all services and applications running over the OFDM modem will be protected against eavesdropping. It is also proposed to include in the encryption any training symbols used for synchronization and channel estimation. Only the legitimate receiver knowing the encryption key is hence able to synchronize and correctly demodulate the received signal. Attackers will not even be able to acquire the encrypted signal.

Claims

1. Method of encrypting data on the physical layer of a data transmission system, comprising the steps of

providing a key stream sequence {k_n},

mapping each of a sequence of data blocks on a QAM symbol z_n,

altering each of the QAM symbols z_n according to an element k_n of the key stream sequence {k_n}, and creating encrypted symbols v_n, wherein the encrypted symbols v_n are assigned to N≧

2 distinct sub-carriers of an Orthogonal Frequency Division Multiplex (OFDM) transmission scheme.

2. The method according to claim 1, wherein the key stream sequence {k_n} is a K-ary sequence with K>

2, each element k_n of the key stream sequence {k_n} being chosen from K distinct values.

3. The method according to claim 2, wherein the QAM symbols z_n are multiplied with the elements k_n of the key stream sequence {k_n}.

4. The method according to claim 3, wherein the elements k_n are of the form k_n=e^jφ

n.

5. The method according to claim 4, wherein the QAM symbols z_n form a set of 2^m distinct constellation points, characterized in that K=4 and ø

_n is a multiple of π

/2.

6. The method according to claim 2, wherein K equals the number of distinct constellation points, and wherein the QAM symbols z_n are permuted among themselves.

7. The method according to claim 2, wherein training OFDM symbols are periodically inserted into the sequence of data blocks, encrypted and transmitted by a transmitter.

8. A method of decrypting data encrypted according to claim 7, wherein the training symbols are encrypted by a receiver and compared to received transmitted encrypted training symbols by a synchronizer/channel estimator, and wherein a channel transfer function h_n or synchronization information is deduced therefrom.

9. A modem for encrypting data on the physical layer of a data transmission system by a method according to claim 1, comprising

a key stream generator for providing a key stream sequence {k_n},

a QAM mapper for mapping each of a sequence of data blocks on a QAM symbol z_n,

an encryptor for altering each of the QAM symbols z_n according to an element k_n of the key stream sequence {k_n}, and for creating encrypted symbols v_n,

wherein the modem further comprises an Inverse Fast Fourier Transformer for superposing the encrypted symbols v_n according to an Orthogonal Frequency Division Multiplex (OFDM) transmission scheme.

10. A modem for decrypting data encrypted on the physical layer of a data transmission system by a method according to claim 8, comprising a synchronizer/channel estimator for comparing received transmitted encrypted training symbols with encrypted training symbols and deducing a channel transfer function h_n or synchronization information therefrom.