5,205 research outputs found
Somewhat Non-Committing Encryption and Efficient Adaptively Secure Oblivious Transfer
Designing efficient cryptographic protocols tolerating adaptive
adversaries, who are able to corrupt parties on the fly as the
computation proceeds, has been an elusive task. Indeed, thus far no
\emph{efficient} protocols achieve adaptive security for general
multi-party computation, or even for many specific two-party tasks
such as oblivious transfer (OT). In fact, it is difficult and
expensive to achieve adaptive security even for the task of
\emph{secure communication}, which is arguably the most basic task
in cryptography.
In this paper we make progress in this area. First, we introduce a
new notion called \emph{semi-adaptive} security which is slightly
stronger than static security but \emph{significantly weaker than
fully adaptive security}. The main difference between adaptive and
semi-adaptive security is that, for semi-adaptive security, the
simulator is not required to handle the case where \emph{both}
parties start out honest and one becomes corrupted later on during
the protocol execution. As such, semi-adaptive security is much
easier to achieve than fully adaptive security. We then give a
simple, generic protocol compiler which transforms any
semi-adaptively secure protocol into a fully adaptively secure one.
The compilation effectively decomposes the problem of adaptive
security into two (simpler) problems which can be tackled
separately: the problem of semi-adaptive security and the problem of
realizing a weaker variant of secure channels.
We solve the latter problem by means of a new primitive that we call
{\em somewhat non-committing encryption} resulting in significant
efficiency improvements over the standard method for realizing
(fully) secure channels using (fully) non-committing encryption.
Somewhat non-committing encryption has two parameters: an
equivocality parameter (measuring the number of ways that a
ciphertext can be ``opened\u27\u27) and the message sizes . Our
implementation is very efficient for small values ,
\emph{even} when is large. This translates into a very efficient
compilation of many semi-adaptively secure protocols (in particular,
for a task with small input/output domains such as bit-OT) into a
fully adaptively secure protocol.
Finally, we showcase
our methodology by applying it to the recent Oblivious Transfer
protocol by Peikert \etal\ [Crypto 2008], which is only secure
against static corruptions, to obtain the first efficient, adaptively secure and composable OT protocol.
In particular, to transfer an -bit message, we use a constant number of rounds and public key operations
Cryptographic Randomized Response Techniques
We develop cryptographically secure techniques to guarantee unconditional
privacy for respondents to polls. Our constructions are efficient and
practical, and are shown not to allow cheating respondents to affect the
``tally'' by more than their own vote -- which will be given the exact same
weight as that of other respondents. We demonstrate solutions to this problem
based on both traditional cryptographic techniques and quantum cryptography.Comment: 21 page
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