Practical Design of Two-Party Cryptographic Protocols

Pino Caballero-Gil

Department of Statistics, Operations Research and Computation.
University of La Laguna 38271 Tenerife. Canary Islands. Spain


The main objective of this talk is to give a brief overview of the field of two-party cryptographic protocols. The links among several real problems, the algorithms based on different mathematics tools, and their cryptographic application will be illustrated through practical examples.

One of the major contributions of modern cryptography has been the development of advanced protocols created to enable several participants to electronically solve many real world problems. Various solutions that allow to perform in the telecommunication world some usual actions as simple as flipping a coin or putting a message in an envelope, and as complex as signing a contract or sending a certified mail will be described. Furthermore, some proposals will be studied in order to solve new problems such as transferring information or identifying oneself, in both cases probabilistically.

The different solutions will be stated in the form of two-party cryptographic protocols, classified here in two groups. The first of both is formed by Oblivious Transfer and Bit Commitment protocols, which are primitive tools useful for the design of the most complex protocols of the second group, whose representative members here studied are Zero Knowledge Proofs, Contract Signing and Coin Flipping protocols. For every protocol, special attention to the common basic schemes used in the design of the best known algorithms will be paid.

In the description of the analyzed protocols an advisable methodology for practical design will be followed. So, the first step the specification of computation and communication requirements because in most cases protocols include the use of some randomness source, and successive exchanges of messages, and are based on some difficult problem or cryptosystem. Also, since the difficulty of the problems and systems does not guarantee the security of the corresponding protocols, an essential task of their design will be the proof of their correctness and fault tolerance (including the protection of users' secrecy), according to a chosen security model.


Pino Caballero-Gil is professor at the University of La Laguna (ULL) in the Canary Islands. She is author of several books on Cryptography, coordinator of the "ULL Crypto Group", partner of the European Computing Education and Training, and director of Study Plans and Curricula at the ULL Rectory.