# A Classical Introduction to Cryptography Exercise BookSecurity Protocols with Conventional Cryptography

A Classical Introduction to Cryptography Exercise Book: Security Protocols with Conventional... Chapter 5 SECURITY PROTOCOLS WITH CONVENTIONAL CRYPTOGRAPHY Exercises Two persons are responsible for correcting an exam on cryptography, none wants to do it, but one, and only one, has to do it. Thus, they want to decide who will correct the exam by flipping a coin. One person chooses "head" or "tail". The other person flips a coin, and the decision is made upon the face up. One problem is that one of them is traveling so they can only communicate remotely over some channel (e.g., Internet or telephone). In order to solve this problem, somebody proposes the following protocol. w Participant A chooses x ="headv or x ="tailn and picks a random key K. He encrypts x with DES by using K and obtains y. Participant A sends y to participant B. w Participant B flips a coin and tells which face is up to participant A. w Participant A reveals K Participant B decrypts y with DES by using K and obtains the bet of participant A. This person claims that it is impossible for participant A "to change his mind" due to the commitment y. 126 EXERCISE BOOK 1 By using a birthday-like attack, show that http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png

# A Classical Introduction to Cryptography Exercise BookSecurity Protocols with Conventional Cryptography

Springer Journals — Jan 1, 2006
9 pages

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Publisher
Springer US
ISBN
978-0-387-27934-3
Pages
125 –134
DOI
10.1007/0-387-28835-X_5
Publisher site
See Chapter on Publisher Site

### Abstract

Chapter 5 SECURITY PROTOCOLS WITH CONVENTIONAL CRYPTOGRAPHY Exercises Two persons are responsible for correcting an exam on cryptography, none wants to do it, but one, and only one, has to do it. Thus, they want to decide who will correct the exam by flipping a coin. One person chooses "head" or "tail". The other person flips a coin, and the decision is made upon the face up. One problem is that one of them is traveling so they can only communicate remotely over some channel (e.g., Internet or telephone). In order to solve this problem, somebody proposes the following protocol. w Participant A chooses x ="headv or x ="tailn and picks a random key K. He encrypts x with DES by using K and obtains y. Participant A sends y to participant B. w Participant B flips a coin and tells which face is up to participant A. w Participant A reveals K Participant B decrypts y with DES by using K and obtains the bet of participant A. This person claims that it is impossible for participant A "to change his mind" due to the commitment y. 126 EXERCISE BOOK 1 By using a birthday-like attack, show that

Published: Jan 1, 2006

Keywords: Block Cipher; Security Protocol; Authentication Server; Ascii Character; Exercise Book