We introduce the notion of Resettable Zero-Knowledge (rZK),
a new security measure for cryptographic protocols
which strengthens the classical notion of zero-knowledge.
In essence, an rZK protocol is one that remains zero knowledge
even if an adversary can interact with the prover many times, each
time resetting the prover to its initial state and forcing it to
use the same random tape.
All known examples of zero-knowledge proofs and arguments
are trivially breakable in this setting.
Moreover, by definition, all zero-knowledge proofs of knowledge
are breakable in this setting.
Under general complexity assumptions, which hold
for example if the Discrete Logarithm Problem is hard, we construct:
rZK proof-systems for NP with non-constant number of rounds,
five-round Resettable Witness-Indistinguishable proof-systems for NP,
and constant-round rZK arguments for NP in the
public key model where verifiers have
fixed, public keys associated with them.

In addition to shedding new light on what makes zero knowledge
possible (by constructing ZK protocols that
use randomness in a dramatically weaker way than before),
rZK has great relevance to applications.
Firstly, rZK protocols are closed under parallel and concurrent execution
and thus are guaranteed to be secure when implemented in fully
asynchronous networks, even if an adversary schedules the arrival
of every message sent so as to foil security.
Secondly, rZK protocols enlarge the range of physical ways
in which provers of ZK protocols can be securely implemented,
including devices which cannot reliably toss
coins on line, nor keep state between invocations.
(For instance, because ordinary smart cards
with secure hardware are resettable, they could not be used to implement
securely the provers of classical ZK protocols, but can now be used to
implement securely the provers of rZK protocols.)

We introduce the notion of Resettable Zero-Knowledge (rZK),
a new security measure for cryptographic protocols
which strengthens the classical notion of zero-knowledge.
In essence, an rZK protocol is one that remains zero knowledge
even if an adeversary can interact with the prover many times, each
time resetting the prover to its initial state and forcing him to
use the same random tape.

Under general complexity asumptions, which hold
for example if the Discrete Logarithm Problem is hard,
we construct
(1) rZK proof-systems for NP:
(2) constant-round resettable witness-indistinguishable proof-systems for NP;
and
(3) constant-round rZK arguments for NP in the
public key model where verifiers have
fixed, public keys associated with them.

In addition to shedding new light on what makes zero knowledge
possible (by constructing ZK protocols that
use randomness in a dramatically weaker way than before),
rZK has great relevance to applications.
Firstly, we show that rZK protocols are closed
under parallel and concurrent execution
and thus are guaranteed to be secure
when implemented in fully asynchronous networks,
even if an adversary schedules the arrival of every message sent.
Secondly, rZK protocols
enlarge the range of physical ways in which provers of a ZK protocols
can be securely implemented, including devices which cannot reliably toss
coins on line, nor keep state betweeen invocations.
(For instance, because ordinary smart cards
with secure hardware are resattable, they could not be used to implement
securely the provers of classical ZK protocols, but can now be used to
implement securely the provers of rZK protocols.)