Private Anonymous MessagingWith Friends!

Ruchith Fernando

Purdue University

April 26, 2011

Ruchith Fernando CS 626 : Project Presentation

Problem

A user wants to send a message to all his current contacts

Even if they are offline!

Only trusts his/her immediate contacts

A contact can re-distribute messages on requests

Ruchith Fernando CS 626 : Project Presentation

Problem

Ruchith Fernando CS 626 : Project Presentation

Problem

Ruchith Fernando CS 626 : Project Presentation

Proposed Solution

Modify HIBE (Hierarchical Identity Based Encryption withConstant Size Ciphertext, Boneh et.al)

Each contact is issued a private key (only private channel forkey exchange)

Contacts generate anonymous public keys using their privatekeys

Broadcast update request to be processed by other contacts

Re-key mechanism with public data (no private channelrequirement)

Ruchith Fernando CS 626 : Project Presentation

Hierarchical Identity Based Encryption

Identities:

Alice: I1Bob: I1, I21Charlie: I1, I22

e : G×G→ G1, |G| = p

params = (g , g1, g2, g3, h1, h2), g1 = gα, α ∈ Zp

mk = g2α

Ruchith Fernando CS 626 : Project Presentation

Hierarchical Identity Based Encryption

Alice :

Kpriv alice = KeyGen(I1, params,mk)Kpubalice = I1

Bob :

Kpriv bob = KeyGen(I1, I2, params,Kpriv alice)Kpubbob = I1, I21

To encrypt for BobCT = Encrypt(msg , I1, I21 , params)

Ruchith Fernando CS 626 : Project Presentation

Changes to HIBE

Update Encrypt() to work with h1I1h2

I21 = ID

To encrypt for BobCT = Encrypt ′(msg , ID, params)

On re-key update α and only generate minimum public datafor existing contacts.

Ruchith Fernando CS 626 : Project Presentation

Usage : Encryption

Contact: First level identity (Ir1) and private key

A contact issues him/herself a second level identity with arandom Ir2Broadcsts a request for data (<user, IDr >) whereIDr = h1

Ir1h2Ir2

Any other contact of the user can respond to the request, byencrypting with paramsuser :CT = Encrypt ′(msg , IDr , paramsuser )

Ruchith Fernando CS 626 : Project Presentation

Usage : Revocation

If the user removes a contact

Re-key : Parameters

Generate a new master key : α′ ∈ Zp, g2α′

paramsuser update : Only g1 = gα′

Re-key : Contact private key

Kpriv = (g2α′ · (h1I1 · g3)

r, g r , h2

r , h3r ) = (C1,C2,C3)

Only C1 need to be published along with I1Indexed by blinded IDcontact

Ruchith Fernando CS 626 : Project Presentation

Implementation

http://code.google.com/p/anon-encrypt/

Using Java Pairing Based Cryptography Library (JPBC)1

Implemented as a library

Demo application

1http://gas.dia.unisa.it/projects/jpbc/Ruchith Fernando CS 626 : Project Presentation

Demonstration

DEMO!,

Ruchith Fernando CS 626 : Project Presentation