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That's really neat! Seems potentially adaptable to paper currency--a verifiable QR code digital signature of the bill's serial number creates a cryptographicall

That's really neat! Seems potentially adaptable to paper currency--a verifiable QR code digital signature of the bill's serial number creates a cryptographically hard obstacle to counterfeiting!

kqp 16d ago
Copy known good serial number + signature.
- teravor 16d ago
  combine with a central authority server network and make it blind signatures and you have untraceable e-cash.
  - mitxela 16d ago
    No government wants untraceable e-cash. They want it fully traceable.
    - teravor 15d ago
      it would depend on the government and the situation. I suspect most don't realize this is possible. a small government can generate immediate demand for their currency doing this.
      - lxgr 15d ago
        The original e-cash paper is from 1983. Governments absolutely know this is possible. It's just much "too private" to get any political traction. At the very least, I suspect an acceptable modern alternative would either have caps on what can be sent/received completely anonymously (e.g. per recipient and timespan) or want non-anonymous recipients (to allow for VAT/sales tax accounting etc.)
  - lxgr 16d ago
    Blind signatures don’t work like that. Once you unblind them, they are very traceable. Chaumian e-cash can only be spent once for that reason.
    - teravor 15d ago
      when you unblind a blind signature all the signer knows is that it's a signature they signed at some point, they know nothing (true zero knowledge) about when or where they signed it among all the other signatures.
      - lxgr 15d ago
        Yes, and then you have a signed piece of data that others can verify. How does that help you with preventing duplication of signatures? E-cash depends on the secrecy of the signed data, and immediate redemption with the issuer once it's been spent/accepted. This is a terrible model for physical cash.
        teravor 15d ago
        > immediate redemption with the issuer once it's been spent/accepted. This is a terrible model for physical cash not when everything is now online. also the obvious way such cash would work is that "redemption" is just the new minting of coin. the central authority will mint a new coin by blindly signing your secret after you "destroy" the spent coin by giving them the unblinded signature.
        lxgr 15d ago
        This thread is about printing QR codes on physical cash. GGP explicitly said: > That's really neat! Seems potentially adaptable to paper currency--a verifiable QR code digital signature of the bill's serial number creates a cryptographically hard obstacle to counterfeiting! I don't see how any e-cash solutions can help here.
lxgr 16d ago
It’s a bunch of static signed data, which is hard to modify but trivial to copy. Not exactly what you’d want for bank notes. NFC and a challenge-response protocol could work, though, like e.g. the one used in biometric passports.

That's really neat! Seems potentially adaptable to paper currency--a verifiable QR code digital signature of the bill's serial number creates a cryptographically hard obstacle to counterfeiting!

kqp 16d ago

Copy known good serial number + signature.

teravor 16d ago
combine with a central authority server network and make it blind signatures and you have untraceable e-cash.
- mitxela 16d ago
  No government wants untraceable e-cash. They want it fully traceable.
  - teravor 15d ago
    it would depend on the government and the situation. I suspect most don't realize this is possible. a small government can generate immediate demand for their currency doing this.
    - lxgr 15d ago
      The original e-cash paper is from 1983. Governments absolutely know this is possible. It's just much "too private" to get any political traction. At the very least, I suspect an acceptable modern alternative would either have caps on what can be sent/received completely anonymously (e.g. per recipient and timespan) or want non-anonymous recipients (to allow for VAT/sales tax accounting etc.)
- lxgr 16d ago
  Blind signatures don’t work like that. Once you unblind them, they are very traceable. Chaumian e-cash can only be spent once for that reason.
  - teravor 15d ago
    when you unblind a blind signature all the signer knows is that it's a signature they signed at some point, they know nothing (true zero knowledge) about when or where they signed it among all the other signatures.
    - lxgr 15d ago
      Yes, and then you have a signed piece of data that others can verify. How does that help you with preventing duplication of signatures? E-cash depends on the secrecy of the signed data, and immediate redemption with the issuer once it's been spent/accepted. This is a terrible model for physical cash.
      - teravor 15d ago
        > immediate redemption with the issuer once it's been spent/accepted. This is a terrible model for physical cash not when everything is now online. also the obvious way such cash would work is that "redemption" is just the new minting of coin. the central authority will mint a new coin by blindly signing your secret after you "destroy" the spent coin by giving them the unblinded signature.
        lxgr 15d ago
        This thread is about printing QR codes on physical cash. GGP explicitly said: > That's really neat! Seems potentially adaptable to paper currency--a verifiable QR code digital signature of the bill's serial number creates a cryptographically hard obstacle to counterfeiting! I don't see how any e-cash solutions can help here.

lxgr 16d ago

It’s a bunch of static signed data, which is hard to modify but trivial to copy. Not exactly what you’d want for bank notes. NFC and a challenge-response protocol could work, though, like e.g. the one used in biometric passports.