816,121 research outputs found
Relay-proof channels using UWB lasers
Alice is a hand-held device. Bob is a device providing a service, such as an ATM, an automatic door, or an anti-aircraft gun pointing at the gyro-copter in which Alice is travelling. Bob and Alice have never met, but share a key, which Alice uses to request a service from Bob (dispense cash, open door, don't shoot). Mort pretends to Bob that she is Alice, and her accomplice Cove pretends to Alice that he is Bob. Mort and Cove relay the appropriate challenges and responses to one another over a channel hidden from Alice and Bob. Meanwhile Alice waits impatiently in front of a different ATM, or the wrong door, or another gun. How can such an attack be prevented?Final Accepted Versio
Zero knowledge convincing protocol on quantum bit is impossible
Consider two parties: Alice and Bob and suppose that Bob is given a qubit
system in a quantum state , unknown to him. Alice knows and she is
supposed to convince Bob that she knows sending some test message. Is it
possible for her to convince Bob providing him "zero knowledge" i. e. no
information about he has? We prove that there is no "zero knowledge"
protocol of that kind. In fact it turns out that basing on Alice message, Bob
(or third party - Eve - who can intercept the message) can synthetize a copy of
the unknown qubit state with nonzero probability. This "no-go" result
puts general constrains on information processing where information {\it about}
quantum state is involved.Comment: 4 pages, RevTe
Real Estate Equity Investments and the Institutional Lender: Nothing Ventured, Nothing Gained
We consider a setup in which the channel from Alice to Bob is less noisy than the channel from Eve to Bob. We show that there exist encoding and decoding which accomplish error correction and authentication simultaneously; that is, Bob is able to correctly decode a message coming from Alice and reject a message coming from Eve with high probability. The system does not require any secret key shared between Alice and Bob, provides information theoretic security, and can safely be composed with other protocols in an arbitrary context
Parallel repetition: simplifications and the no-signaling case
Consider a game where a refereed a referee chooses (x,y) according to a
publicly known distribution P_XY, sends x to Alice, and y to Bob. Without
communicating with each other, Alice responds with a value "a" and Bob responds
with a value "b". Alice and Bob jointly win if a publicly known predicate
Q(x,y,a,b) holds.
Let such a game be given and assume that the maximum probability that Alice
and Bob can win is v<1. Raz (SIAM J. Comput. 27, 1998) shows that if the game
is repeated n times in parallel, then the probability that Alice and Bob win
all games simultaneously is at most v'^(n/log(s)), where s is the maximal
number of possible responses from Alice and Bob in the initial game, and v' is
a constant depending only on v.
In this work, we simplify Raz's proof in various ways and thus shorten it
significantly. Further we study the case where Alice and Bob are not restricted
to local computations and can use any strategy which does not imply
communication among them.Comment: 27 pages; v2:PRW97 strengthening added, references added, typos
fixed; v3: fixed error in the proof of the no-signaling theorem, minor
change
The quantum cryptographic switch
We illustrate using a quantum system the principle of a cryptographic switch,
in which a third party (Charlie) can control to a continuously varying degree
the amount of information the receiver (Bob) receives, after the sender (Alice)
has sent her information. Suppose Charlie transmits a Bell state to Alice and
Bob. Alice uses dense coding to transmit two bits to Bob. Only if the 2-bit
information corresponding to choice of Bell state is made available by Charlie
to Bob can the latter recover Alice's information. By varying the information
he gives, Charlie can continuously vary the information recovered by Bob. The
performance of the protocol subjected to the squeezed generalized amplitude
damping channel is considered. We also present a number of practical situations
where a cryptographic switch would be of use.Comment: 7 pages, 4 Figure
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