*21:17* [Pub][ePrint]
Computationally-Fair Group and Identity-Based Key-Exchange, by Andrew C. Yao and Yunlei Zhao
In this work, we re-examine some fundamental group key-exchange and identity-based key-exchange protocols, specifically the Burmester-Desmedet group key-exchange protocol [7] (re-ferred to as the BD-protocol) and the Chen-Kudla identity-based key-exchange protocol [9](referred to as the CK-protocol). We identify some new attacks on these protocols, showing in particular that these protocols are not computationally fair. Specifically, with our attacks, an

adversary can do the following damages:

(1) It can compute the session-key output with much lesser computational complexity than that of the victim honest player, and can maliciously nullify the contributions from the victim honest players.

(2) It can set the session-key output to be some pre-determined value, which can be efficiently and publicly computed without knowing any secrecy supposed to be held by the attacker.

We remark these attacks are beyond the traditional security models for group key-exchange and identity-based key-exchange.

Then, based on the computationally fair Diffie-Hellman key-

exchange in [21], we present some fixing approaches, and prove that the fixed protocols are computationally fair.

*21:17* [Pub][ePrint]
Fair Exchange of Short Signatures Without Trusted Third Party, by Philippe Camacho
We propose a protocol to exchange Boneh-Boyen short signatures in a fair way, without relying on a trusted third party. Our protocol is quite practical and is the first of the sort to the bestof our knowledge.Our construction uses a new non-interactive zero-knowledge (NIZK) argument to prove that a commitment is the encryption of a bit vector.

We also design a NIZK argument to prove that a commitment to a bit vector $v=(b_1,b_2,...,b_\\secparam)$ is such that $\\sum_{i \\in [\\secparam]}b_i2^{i-1}=\\Blinding$ where $\\Blinding$

is the discrete logarithm of some public value $\\BasicCommitment=g^\\Blinding$.These arguments may be of independent interest.

*21:17* [Pub][ePrint]
Ring Group Signatures, by Liqun Chen
In many applications of group signatures, not only a signer\'sidentity but also which group the signer belongs to is sensitive

information regarding signer privacy. In this paper, we study these

applications and combine a group signature with a ring signature to

create a ring group signature, which specifies a set of possible

groups without revealing which member of which group produced the

signature. The main contributions of this paper are a formal

definition of a ring group signature scheme and its security model,

a generic construction and a concrete example of such a scheme. Both

the construction and concrete scheme are provably secure if the

underlying group signature and ring signature schemes are

secure.

*21:17* [Pub][ePrint]
Efficient Dynamic Provable Possession of Remote Data via Update Trees, by Yihua Zhang and Marina Blanton
The emergence and wide availability of remote storage service providers prompted work inthe security community that allows a client to verify integrity and availability of the data that

she outsourced to an untrusted remove storage server at a relatively low cost. Most recent

solutions to this problem allow the client to read and update (i.e., insert, modify, or delete)

stored data blocks while trying to lower the overhead associated with verifying the integrity

of the stored data. In this work we develop a novel scheme, performance of which favorably

compares with the existing solutions. Our solution enjoys a number of new features such as a

natural support for operations on ranges of blocks, revision control, and support for multiple

user access to shared content. The performance guarantees that we achieve stem from a novel

data structure termed a balanced update tree and removing the need to verify update operations.