<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div><blockquote type="cite" class=""><div class=""><div bgcolor="#FFFFFF" text="#000000" class=""><blockquote class="">Coins could be made out of glass. Flaws like tiny
bubbles would be difficult to position during manufacture. As such
imperfections would make each coin unique and impossible to
replicate. Whatever process is used to draw a picture and a coin
value could be used to add a unique serial number. Authentication
would involve authenticating the physical properties like
refractive index and examining the coin's unique physical flaws
i.e. Fingerprint. Finally the central bank, for that currency, can
authenticate that 'serial number - fingerprint' combination.<br class="">
</blockquote>
</div></div></blockquote></div>There is a great deal of published work in this direction. One extreme example (first page only; the paper should be around somewhere) <a href="http://link.springer.com/chapter/10.1007%2F978-3-642-04431-1_15#page-1" class="">http://link.springer.com/chapter/10.1007%2F978-3-642-04431-1_15#page-1</a>. (A "PUF" is a Physically Uncloneable Function" - this search term will lead you to some of the published work. The particular paper - which I haven't read - uses a "PPUF" - a Public PUF).<div class=""><br class=""></div><div class="">Ideas of this sort go *way* back. Bennett et al used quantum mechanics in a (thought experiment) design for "unforgeable subway tokens" back in 1983 (<a href="https://static.aminer.org/pdf/PDF/000/120/546/quantum_cryptography_or_unforgeable_subway_tokens.pdf" class="">https://static.aminer.org/pdf/PDF/000/120/546/quantum_cryptography_or_unforgeable_subway_tokens.pdf</a>).</div><div class=""><br class=""></div><div class="">An actual fielded system - to which I don't have a reference; this is from memory - was used to make tamperproof seals: It's possible to pull a length of many fiber-optic strands. The individual strands assort and mix themselves quite randomly, unpredictably, and uncontrollably. Pick some number of them at one end and light them; record which ones are lit at the other. Thread the glass through a hasp controlling access to something. If it's cut, there's no known way to repair or replace it such that the resulting strand will reproduce the pattern.</div><div class=""><br class=""></div><div class="">There's even a science fiction book I remember reading, probably dating from the 80's, of a society in which everyone got an ID card that had a random, unpredictable value manufactured into it by some physical process that could not be reproduced afterwards. The minor but significant plot point centered around a way of hacking this "unhackable" system. As best I can recall, the attacker couldn't clone a card - but was able to break in at the point where a new card was bound to a person's identity, roll the process back, and create a second identity bound to the same brand new card.</div><div class=""><div class=""> -- Jerry</div></div><div class=""><br class=""></div></body></html>