# US 2021/0345102 A1 · Physical layer key generation

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- Number: US 2021/0345102 A1
- Document ID: US20210345102A1
- Status: Published application
- Kind: A1
- Application number: 17/014,611
- Filed: 2020-09-08
- Published: 2021-11-04
- Earliest claimed: 2019-11-08
- Applicants: Massachusetts Institute of Technology
- Inventors: Moe Z. Win; Tianyi Peng; Wenhan Dai; Zehao Yu
- Web page: https://patentgrep.com/patent/US20210345102A1

## CPC

- H04W 12/03: Protecting confidentiality, e.g. by encryption
- H04L 5/0007: {the frequencies being orthogonal, e.g. OFDM(A) or DMT}
- H04L 9/085: {Secret sharing or secret splitting, e.g. threshold schemes}
- H04L 9/0875: {based on channel impulse response [CIR]}
- H04L 25/0204: {of multiple channels}
- H04W: WIRELESS COMMUNICATION NETWORKS (broadcast communication H04H; communication systems using wireless links for non-selective communication, e.g. wireless extensions H04M1/72)
- H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M)

## Abstract

Physical layer key generation provides privacy protection technique suitable for devices with limited computational ability. A key generation algorithm is based on OFDM waveforms. By exploiting the holistic CSI, key generation rate (KGR) is improved significantly. A cross-layer encryption protocol is based on the key generation algorithm and the AES. The secrecy of the encryption is enhanced compared to traditional encryption schemes with one pre-shared key (e.g., WPA2-PSK), even when some generated keys are leaked to the eavesdropper. The results lead to practical and robust applications of physical layer key generation.

## Claims

### Claim 1

A method of sending an electronic communication from a sender to a receiver through a reciprocal communication channel, the method comprising:
correlating phase-related channel state information between the sender and the receiver, thereby producing correlated phase-related channel state information that is substantially equal at the sender and receiver;
generating an encryption key using the correlated phase-related channel state information; and
encrypting the electronic communication using the encryption key and an encryption protocol.

### Claim 2 (depends on 1)

The method of claim 1 wherein the correlated phase-related channel information is substantially different from the phase-related channel information.

### Claim 3 (depends on 2)

The method of claim 2 wherein the correlated phase-related channel information includes a relative phase of a signal used to transmit the electronic communication.

### Claim 4 (depends on 1)

The method of claim 1 wherein the communication channel includes a central frequency, and correlated phase-related channel state information is independent of:
a phase offset λb in a signal carrying the electronic message, the offset associated with a delay of detecting the signal;
a sampling frequency offset λo; and
a central frequency offset β.

### Claim 5 (depends on 4)

The method of claim 4 wherein producing correlated phase-related channel state information further comprises:
identifying uncorrelated channel state information;
identifying a linear term in the uncorrelated channel state information with slope λb+λo and intercept β+∠h;
subtracting the linear term from the uncorrelated channel state information, thereby producing correlated phase-related channel state information.

### Claim 6 (depends on 1)

The method of claim 1, wherein generating the encryption key further comprises:
identifying a previous encryption key;
generating an update term using the correlated phase-related channel state information; and
combining the previous encryption key with the update term, thereby generating the encryption key.

### Claim 7 (depends on 6)

The method of claim 6, wherein combining the previous encryption key with the update term includes computing the XOR sum of the previous encryption key with the update term.

### Claim 8 (depends on 6)

The method of claim 6, wherein combining the previous encryption key with the update term accomplished by a mathematically unstable operation on the previous encryption key and the update term.

### Claim 9 (depends on 6)

The method of claim 6, wherein sequentially combining the previous encryption key with the update term results in an outage probability that is an exponentially decreasing function of a number of compromised transmissions.

### Claim 10 (depends on 5)

The method of claim 5, in which the uncorrelated phase-related channel state information h associated with a frequency fk is given by
h(fk)=Σl=1αle−j2πfkτl
where αl and τl denote the amplitude and delay of an lth path, respectively, and j2=−1.

### Claim 11 (depends on 1)

The method of claim 1, wherein correlating the phase-related channel state information includes converting an analog signal to a digital signal having n possible values, where n is selected based on a performance metric.

### Claim 12 (depends on 11)

The method of claim 11, wherein the performance metric includes a minimum key generation rate.

### Claim 13 (depends on 11)

The method of claim 11, wherein the performance metric includes a maximum key disagreement rate.

### Claim 14

A method for generating successive share secret values at a first communication station in communication with a second communication station over a reciprocal communication channel, the method comprising:
measuring reciprocal channel to yield first characteristics of said channel;
computing a second secret value using (a) a first secret value that depends prior characteristics of the channel and (b) the first characteristics;
using the second secret value for a cryptographic function involving the second communication station.

### Claim 15 (depends on 14)

The method of claim 14 wherein computing the second secret value further uses (c) information received from the second station.

### Claim 16

A non-transitory machine-readable medium comprising instructions stored thereon when executed on a processor cause the processor to generating successive share secret values at a first communication station in communication with a second communication station over a reciprocal communication channel by:
measuring reciprocal channel to yield first characteristics of said channel;
computing a second secret value using (a) a first secret value that depends prior characteristics of the channel and (b) the first characteristics;
using the second secret value for a cryptographic function involving the second communication station.