Unforgeable Blinded Credentials

[Apu Kapadia]

I came across the same problem a couple of years ago (and indeed  
iterated through private/public key solutions with a colleague). The  
problem is that you can still give your private key to somebody else.  
There's no real deterrent unless that private key is used for many  
other purposes, thereby discouraging sharing. But if that's the case,  
there's no real anonymity anymore, since the private key is tied to  
the person's identity.

I found that Chameleon Certificates had nice properties. You have a  
"master certificate" that lists all your attributes. For  
authentication, you generate an unlinkable slave certificate with any  
subset of attributes. You have to possess the master certificate at  
time of use to generate the slave certificate, so you can't pass a  
slave certificate to a friend for later use. Then you just need to  
ensure that the master certificate includes personal details like  
credit card number, SSN, etc. to deter sharing of master  
certificates. Note that the slave certificates won't have this  
information, so this personal information is safe as long as the  
master certificate is not leaked. Since sharing an attribute amounts  
to sharing all your attributes, including personal information, this  
property serves as a good deterrent. Maybe somebody else can comment  
on the technical viability + crypto details of the paper.

P. Persiano and I. Visconti. An Anonymous Credential System and a  
Privacy-Aware PKI. In Information Security
and Privacy, 8th Australasian Conference, ACISP 2003, volume 2727 of  
Lecture Notes in Computer Science. Springer Verlag, 2003.
http://springerlink.metapress.com/openurl.asp? 
genre=article&issn=0302-9743&volume=2727&spage=27

Here's the abstract:
  In this paper we present a non-transferable anonymous credential  
system that is based on the concept of a chameleon certificate. A  
chameleon certificate is a special certificate that enjoys two  
interesting properties. Firstly, the owner can choose which  
attributes of the certificate to disclose. Moreover, a chameleon  
certificate is multi-show in the sense that several uses of the same  
chameleon certificate by the same user cannot be linked together.

We adopt the framework of Brands [2] and our construction improves  
the results of Camenisch et al. [5] and Verheul [16] since it allows  
the owner of a certificate to prove general statements on the  
attributes encoded in the certificate and our certificates enjoy the  
multi-show property.

Apu

-- 
Apu Kapadia, Ph.D.
Research Fellow, Institute for Security Technology Studies (ISTS)
Dartmouth College, Hanover NH 03755, USA
http://www.cs.dartmouth.edu/~akapadia/


On Apr 1, 2006, at 6:35 AM, Ben Laurie wrote:

> It is possible to use blind signatures to produce anonymity-preserving
> credentials. The general idea is that, say, British Airways want to
> testify that I am a silver BA Executive Club cardholder. First I  
> create
> a random number (a nonce), I blind it, then send it to BA. They  
> sign it
> with their “this guy is a silver member” signing key, I unblind the
> signature and then I can show the signed nonce to anyone who wants to
> verify that I am silver. All they need to do is check the signature
> against BA’s published silver member key. BA cannot link this nonce  
> back
> to me because they have never seen it, so they cannot distinguish me
> from any other member.
>
> However, anyone I show this proof to can then masquerade as a silver
> member, using my signed nonce. So, it occurred to me that an easy  
> way to
> prevent this is to create a private/public key pair and instead of the
> nonce use the hash of the public key. Then to prove my silver status I
> have to show that both the hash is signed by BA and that I possess the
> corresponding private key (by signing a nonce, say).
>
> It seems to me quite obvious that someone must have thought of this
> before - the question is who? Is it IP free?
>
> Obviously this kind of credential could be quite useful in identity
> management. Note, though, that this scheme doesn’t give me  
> unlinkability
> unless I only show each public/private key pair once. What I really  
> need
> is a family of unlinkable public/private key pairs that I can somehow
> get signed with a single “family” signature (obviously this would need
> to be unlinkably transformed for each member of the key family).
>
> Permalink: http://www.links.org/?p=88
>
> Cheers,
>
> Ben.


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