Ben Laurie blathering

Unforgeable Blinded Credentials

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).


  1. IIRC, Stefan Brands’ book ( has the PKI stuff to do exactly this. PDFs are here – – for personal use.

    Comment by Superpat — 1 Apr 2006 @ 17:35

  2. Oops – that book link isn’t clickable – here’s another try:

    Comment by Superpat — 1 Apr 2006 @ 22:43

  3. What you’ve described is most of the way towards OACerts (
    If you add unlinkability, you get anonymous credentials. Both Brands and Camenisch have done a bunch of work in this area.

    Comment by Nikita — 5 Apr 2006 @ 4:34

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