4 research outputs found
Quantum Coins
One of the earliest cryptographic applications of quantum information was to
create quantum digital cash that could not be counterfeited. In this paper, we
describe a new type of quantum money: quantum coins, where all coins of the
same denomination are represented by identical quantum states. We state
desirable security properties such as anonymity and unforgeability and propose
two candidate quantum coin schemes: one using black box operations, and another
using blind quantum computation.Comment: 12 pages, 4 figure
Quantum Copy-Protection and Quantum Money
Forty years ago, Wiesner proposed using quantum states to create money that
is physically impossible to counterfeit, something that cannot be done in the
classical world. However, Wiesner's scheme required a central bank to verify
the money, and the question of whether there can be unclonable quantum money
that anyone can verify has remained open since. One can also ask a related
question, which seems to be new: can quantum states be used as copy-protected
programs, which let the user evaluate some function f, but not create more
programs for f? This paper tackles both questions using the arsenal of modern
computational complexity. Our main result is that there exist quantum oracles
relative to which publicly-verifiable quantum money is possible, and any family
of functions that cannot be efficiently learned from its input-output behavior
can be quantumly copy-protected. This provides the first formal evidence that
these tasks are achievable. The technical core of our result is a
"Complexity-Theoretic No-Cloning Theorem," which generalizes both the standard
No-Cloning Theorem and the optimality of Grover search, and might be of
independent interest. Our security argument also requires explicit
constructions of quantum t-designs. Moving beyond the oracle world, we also
present an explicit candidate scheme for publicly-verifiable quantum money,
based on random stabilizer states; as well as two explicit schemes for
copy-protecting the family of point functions. We do not know how to base the
security of these schemes on any existing cryptographic assumption. (Note that
without an oracle, we can only hope for security under some computational
assumption.)Comment: 14-page conference abstract; full version hasn't appeared and will
never appear. Being posted to arXiv mostly for archaeological purposes.
Explicit money scheme has since been broken by Lutomirski et al
(arXiv:0912.3825). Other quantum money material has been superseded by
results of Aaronson and Christiano (coming soon). Quantum copy-protection
ideas will hopefully be developed in separate wor
Classical Authenticated Key Exchange and Quantum Cryptography
the thesis, including any required final revisions, as accepted by my examiners. I understand that my thesis may be made electronically available to the public. Cryptography plays an integral role in secure communication and is usually the strongest link in the chain of security. Yet security problems abound in electronic communication: spyware, phishing, denial of service, and side-channel attacks are still major concerns. The main goal in this thesis is to consider how cryptographic techniques can be extended to offer greater defence against these non-traditional security threats. In the first part of this thesis, we consider problems in classical cryptography. We introduce multi-factor password-authenticated key exchange which allows secure authentication and key agreement based on multiple short secrets, such as a long-term password and a one-time response; it can provide an enhanced level of assurance in higher security scenarios because a multi-factor protocol is designed to remain secure even if all but one of the factors has been compromised due to attack