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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
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.
Digital Signature @ Wikipedia
A good explanation of how digital signing works [rgovostes, Aug 16 2005]
RSA Laboratories Bulletin #13
AKA "A Cost-Based Security Analysis Security Analysis of Symmetric and Asymmetric Key Lengths" [rgovostes, Aug 16 2005]
Magnetic Ink on Dollar Bills
Fun and games with magnets. [Dub, Aug 16 2005]
The joys of cryptography [pooduck, Aug 17 2005]
||Preheated. See Neal Stephenson's "Cryptonomicon".
||I'm not understanding how this makes the bill harder to fake - the criminal would merely copy the values over to the fake bills.
||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.
||If they already blacklist serial numbers, how is this any different?
||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.
||[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
||[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.
||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?