# PSKA: Usable and Secure Key

AI and Robotics

Nov 21, 2013 (4 years and 7 months ago)

101 views

PSKA: Usable and Secure Key
Agreement Scheme for Body Area
Networks

Authors:

Krishna K.
Venkatasubramanian
,
Ayan

Banerjee,
Sandeep

K.S. Gupta

Presenter:Francis

Usher

Problem

Domain: Body Area Networks (BANs)

Sensors in BANs collect and disseminate
sensitive health

Security via cryptography requires key
distribution

Cryptography (overview)

Problem of sharing data securely

Symmetric
-
key cryptography

Secret key
k

is used to obscure message
m

into
cyphertext

c

Given
c
, only
k

can be used to reveal
m

break cryptosystem with negligible probability

Problem: how do we communicate keys?

Key sharing (generic approaches)

Pre
-
sharing

Manufacturer embeds long
-
term keys in device

Problem: not dynamic enough to handle key
compromise situations

Asymmetric crypto handshake

Asymmetric crypto doesn’t require shared secret

Problem: Usually requires contacting trusted
identity authority

Physiological Signal
-
based key sharing
(for BANs)

Design goals:

Length & randomness

Low latency

Distinctiveness

Temporal variance

Previous work: Inter
-
pulse
-
interval (IPI)
-
based,
independent, mutual key generation

Physiological Signal based Key Agreement (PSKA)

Use shared physiological signals to build & access
“fuzzy vault” containing session key

IPI
-
based key generation

Synchronized sensors measure IPI (EKG/PPG)

Encode measurements as key

4 observations:

Meets randomness goal, however:

High
-
latency

Two keys generated tend to differ in half of bits

This distance tends not to vary much in time between
generation or across different patients

No good tradeoff threshold between false
positive/negative rates

Digression: Shamir’s Secret
Sharing
(
p
recursor)

Secret value v to be shared among k people

Should take at least n people to determine secret

Degree
-
n
-
1

polynomial, random coefficients

P(x) =
𝑃
𝑥
=

𝑣
+
𝑐
1
𝑥
+
𝑐
2
𝑥
2
+

+
𝑐


1
𝑥


1

Evaluate at
k >= n

random points

Any n of these k points uniquely determines P

Otherwise even dist.
o
f choices for v

PSKA: Sharing keys using fuzzy vaults

Different
sensors
measure phys. signals

“Loosely synchronized”

Transform signals to create “features”

Generate random polynomial representing key

Map features under polynomial

Obscure feature maps using “chaff” points

Only similar feature set can infer polynomial from
vault (features + chaff)

Use MACs to affirm that key was shared correctly

Fuzzy Vault Security

Perfect match will always unlock vault

Close match corrected by oversampling

Hard to pick right set of points if lots of chaff

Picking the correct
s

elements of
m
(brute force)

𝑚
𝑠
=

!

𝑠
!

𝑠
!

Analysis of technique

Long & random keys

Low latency (only ~ 5
-
10 seconds of data)

Distinctiveness (across subjects)

Temporal variance (across time)

Feature generation

Use peaks from frequency domain (FFTs)

Concatenate indexes & values across windows

Features should demonstrate distinctiveness
and temporal
variance

Feasability
: implementation

Power, resource constraints

Can technique be implemented as efficiently
as competitors?

Classical, elliptic
-
curve
Diffie

Hellman

Evaluation in VHDL (formal hardware
specification language)

Metrics: clock cycles, memory footprint

Possible attacks

Fuzzy vault attacks

Some based on application to biometrics

One attack based on vault
-
construction artifact

Early points (features) have more “free area”

Dismissed but not thoroughly argued against

Idea for future work

Use fuzzy vaults to communicate public keys

Use asymmetric crypto handshakes to establish
session keys

Frequent update of public keys

Eliminates problem of contacting trusted authority
since physiological signals good for authentication
of body
-
area presence