Chennai Mathematical Institute


3.30 pm, Seminar Hall
Almost Public Quantum Coins

Amit Behra
Ben-Gurion University, Israel.


In a quantum money scheme, a bank can issue money that users cannot counterfeit. Similar to bills of paper money, most quantum money schemes assign a unique serial number to each money state. Though the use of serial numbers can thwart counterfeiters, they can also be tracked by third parties, thus potentially compromising the privacy of the users of quantum money. However, in a quantum coins scheme, there are no serial numbers and all the money states are exact copies of each other, therefore providing a better level of privacy for the users. In a private quantum money scheme, only the bank can verify the money states, whereas, in a public scheme, all users can verify the money using the bank’s public key. In this work, we propose a way to lift any private quantum coin scheme (which is known to exist based on the existence of one-way functions) to an almost public quantum coin scheme. It satisfies all the properties of a public quantum money scheme but in a limited way. Verification of a new coin is done by comparing it to the coins the user already possesses by using a projector to the symmetric subspace. This is the first construction that is provably secure based on standard assumptions and has almost all the properties of a public coin scheme and a public quantum money scheme. The lifting technique can also be used on a known private quantum money scheme to obtain an inefficient (almost) public quantum money scheme that is secure against nonadaptive computationally unbounded adversaries. This is joint work with Or Sattath.