*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.

*17:37* [PhD][New]
Jens Zumbrägel: Public-key cryptography based on simple semirings
Name: Jens Zumbrägel

Topic: Public-key cryptography based on simple semirings

Category: public-key cryptography

Description: The discrete logarithm problem is the basic ingredient of many\r\n public-key cryptosystems. It can be stated as follows: Given a\r\n cyclic group (*G*,?) of order *n*, a\r\n generator *g* of *G*, and another\r\n element *h*?*G*, find the unique\r\n integer *a*?[0,*n*) such that\r\n *h*=*g*ˆ*a*. The integer *a* is called\r\n the *discrete logarithm* of\r\n *h* to the base *g*.

\r\n \r\nThere are key agreement protocols, public-key encryption schemes,\r\n and digital signatures employing the discrete logarithm problem.\r\n One example is the Diffie-Hellman key agreement protocol. It allows\r\n two parties, A and B, to agree on a secret key over an insecure\r\n channel. In order to achieve this goal they fix a finite cyclic\r\n group *G* and a generator *g* of *G*. Then A and B\r\n pick random integers *a*,*b* respectively and exchange\r\n *h*A=*g*ˆ*a*\r\n and *h*B=*g*ˆ*b*. Finally they\r\n compute *h*Bˆ*a*=*g*ˆ*ba*\r\n and *h*Aˆ*b*=*g*ˆ*ab*, and\r\n since *g*ˆ*ab*=*g*ˆ*ba* this element\r\n can be used as their secret key.

\r\n\r\nIt is clear that solving the underlying discrete logarithm problem\r\n is sufficient for breaking the Diffie-Hellman protocol. For this\r\n reason one has been searching for groups in which the discrete\r\n logarithm problem is considered to be a computationally hard\r\n problem. Among the groups that have been proposed as candidates are\r\n the multiplicative group of a finite field and the group over an\r\n elliptic curve. It should however be pointed out that the\r\n infeasibility of the discrete logarithm problem has not been proved\r\n in any concrete group.

\r\n\r\nDiscrete logarithm based cryptosystems can be generalized in the\r\n framework of *semigroup actions* (see e.[...]