4.18 Consider the following protocol:

A KDC: IDA IDB|| N₁

KDCA: E(K. [Ks|| IDB|N₁|| E(Kb. [Ks|IDA]))

A → B:

E(Kb. [Ks || IDA])

B→A:

E(KS, N₂)A → B:

E(Ks. f(N₂))

a. Explain the protocol.

b. Can you think of a possible attack on this protocol? Explain how it can be done.

c. Mention a possible technique to get around the attack-not a detailed mechanism,

just the basics of the idea.

Note: The remaining problems deal with a cryptographic product developed by IBM,

which is briefly described in a document at this book's Web site in IBMCrypto.pdf.

Try these problems after reviewing the document.