Public key digital signatures provide a means of verifying the integrity and authenticity of a document - that it came from who it says it came from, and hasn't been altered since. These signatures work by creating a summary (hash) of the document and encrypting it with a private key. The recipient then uses a published public key to verify that the hash is correct.

This could be applied to currency as a means to prevent counterfeiting. Essentially, a bill's mint, value, and serial number would be concatenated and digitally signed, then printed on the note. To verify that it is a legitimate bill, the issuing mint's public key would be downloaded and the signature checked.

As each signature is unique, the whole bill would have to be replicated in order to counterfeit it. In this case, the serial number could be blacklisted, as they are now.

While someone with significant motivation could duplicate the design of a note, cryptographical algorithms make this virtually impossible. Massive amounts of computing power is required to perform prime factorization to produce the private key. To date, the longest number to be factorized was 576 bits (174 digits) long. According to the RSA, a 1620 bit number would require 8.0 x 10^14 GHz with 1.92 x 10^17 terabytes of RAM running for one year. In contrast, private keys are typically 4096 bytes long. And while there's nothing close to that today, the only group which might ever be able to do it is already part of the federal government.
-- rgovostes, Aug 16 2005

Digital Signature @ Wikipedia http://en.wikipedia...i/Digital_signature
A good explanation of how digital signing works [rgovostes, Aug 16 2005]

RSA Laboratories Bulletin #13 http://www.rsasecur...bs/node.asp?id=2088
AKA "A Cost-Based Security Analysis Security Analysis of Symmetric and Asymmetric Key Lengths" [rgovostes, Aug 16 2005]

Magnetic Ink on Dollar Bills http://www.gaussboy...et_applications.php
Fun and games with magnets. [Dub, Aug 16 2005]

Quantum Money http://kerneltrap.org/node/4241
The joys of cryptography [pooduck, Aug 17 2005]

Preheated. See Neal Stephenson's "Cryptonomicon".
-- normzone, Aug 16 2005


I'm not understanding how this makes the bill harder to fake - the criminal would merely copy the values over to the fake bills.
-- DrCurry, Aug 16 2005


To clarify - each bill has a unique signature, containing the information about the issuing mint and the bill's value and serial number, changing any of these details would invalidate the signature and raise a red flag.

However, I do see a point in duplicating the same bill exactly repeatedly, but its serial number could simply be blacklisted, which I believe is what they already do.
-- rgovostes, Aug 16 2005


If they already blacklist serial numbers, how is this any different?
-- Aq_Bi, Aug 17 2005


I have read about signing money using a series of photons where are polarized in a certain sequence, which has been named quantum money. However it would cost approx $1000 to print a $1 bill if I remember correctly.
-- pooduck, Aug 17 2005


[Aq_Bi] Yes, there is a blacklist of serial numbers. This means that if the bill marked with K36204536E is copied thousands of times, the dupes can be identified easily. But if the counterfeiter takes the time to alter the serial number printed on the bill, the blacklist is circumvented.

[pooduck] That is pretty cool, although what I propose would cost little more than printing the unique serial number that is already on the bill. The main difference here is that one requires a massive database of photon patterns for each bill in circulation, while my idea requires only two large numbers that could be used to verify the authenticity of any bill.
-- rgovostes, Aug 17 2005


But if the fake bill is in circulation and thought to be the REAL bill and you present the actual real bill to a cashier or a bank, will they not accept it?
-- Jscotty, Aug 17 2005



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