Spyros Kotoulas and Ronny Siebes

Vrije Universiteit AmsterdamThe Netherlands

{kot,ronny}@few.vu.nl

Scalable discovery of private resources

Discovery systemsPrivacy in discovery systemsDistributed obfuscated indexes

Example Design choices

EvaluationSummary

Outline

A discovery system stores content or pointers to it and retrieves

them according to a query.

Discovery systems

Centralized storage and index

Discovery systems

user 1

store object query

answer (object)

user 2

P2P storage and centralized index

Discovery systems

user 1

store description query

answer pointer

user 2get(object)

send(object)

P2P storage and P2P index

Discovery systems

user 1

stor

e de

scrip

tion

query

ask user 1

user 2get(object)

send(object)

Discovery systems

They all have one problem in common

The object or description is shown to at least one third party, which may be malicious.

Discovery systems

user 1

stor

e de

scrip

tion

query

ask user 1

user 2get(object)

send(object)

•sniff

•censor

•modify

•track

Distributed obfuscated index

[a..c] [d..f] [g..k] [l..m] [n..v] [w..z]

user 1user 1

nuditynu

usr1nu

Distributed obfuscated index

[a..c] [d..f] [g..k] [l..m] [n..v] [w..z]

user 1

user 2

usr1nu

user 2

query: nukes

nu

Distributed obfuscated index

[a..c] [d..f] [g..k] [l..m] [n..v] [w..z]

user 1

usr1nu

user 2

ask usr1

about NU

Distributed obfuscated index

[a..c] [d..f] [g..k] [l..m] [n..v] [w..z]

user 1

usr1nu

user 2

I’m usr2

anything

on NUK?

Distributed obfuscated index

[a..c] [d..f] [g..k] [l..m] [n..v] [w..z]

user 1

usr1nu

user 2

no,

nothing on

NUK

Choosing the key

The uniqueness is a trade-off between scalability and privacy.

If keys are too unique, you don’t have privacy because somebody may do a dictionary attack.If keys are too common, the system will not scale, because they map to too many peers

Choosing the key

So, all keys should map to a similar and reasonable number of terms.

Three ways to make the key Secure hash prefix (e.g. SHA-1)

For example, always take the first 25% of the characters of a hash of a word in a description:

Logica CMG, Microsoft, Xerox

x41a4c1, x7bh349, xaa4652 x41, x7b,xaa

advantage: the variance of the matching number of terms is small

disadvantage: no substring matching on published part

Three ways to make the key Fixed ratio prefix

For example, always take the first 20% of the characters of a word in a description:

Logica CMG, Microsoft, Xerox lo, mi, xe

advantage: substring matching on published part

disadvantage: the variance of the matching number of terms is big (popular prefixes vs unpopular)

Three ways to make the key Information value based prefix

Based on the frequency of the prefixes on a training set, determine a desired prefix length. For example, some prefixes are more common than others (compare “pro” with “xxf”).

Logica CMG, Microsoft, Xerox logic, mic, x

Simulationresults