(1)

Philips Research Laboratory, Avenue Van Becelaere, 2, B-1170 Brussels, Belgium.

(1)

CCETT/EPT, BP 59, F-35512 Cesson S&ignP, France.

(3)

Anagram Laboratories, P.O. Box 791, Palo Alto CA 94301, USA.

How to Explain Zero-Knowledge Protocols

to Your Children

QUISQUATER Jean-Jacques(‘), Myziam, Muriel, Micha2;1

GUILLOU Louis(‘), Marie AnnicJc, GaicJ, Anna, Gwenoli, Soazig

in collaboration with Tom BERSON’“) for the English version

0 Know, oh my children, that very long ago, in the

Eastern city of Baghdad, there lived

an old man named Ali Baba. Every day Ali Baba would go to the bazaar to buy or sell

things. This is a story which is partly about Ali Baba, and partly also about a cave, a

strange cave whose secret and wonder exist to this day. But I get ahead of myself . . .

One day in the Baghdad bazaar a thief grabbed a purse from Ali Baba who right away

started to run after him. The thief fled into a cave whose entryway forked into two dark

winding passages: one to the left and the other to the right (The Entry of the Cave).

\

Ali Baba did not see which passage the thief r

into. Ali Baba had to choose which way to go, and

he decided to go to the left. The left-hand passage

ended in a dead end. Ali Baba searched all the

way from the fork to the dead end, but he did

not find the thief. Ali Baba said to himself that

the thief was perhaps in the other passage. So he

searched the right-hand passage, which also came

to a dead end. But again he did not find the thief.

. . . . .._........___...........................

“This cave is pretty strange,”

said Ali Baba to himself, “Where has my thief gone?”

The following day another thief grabbed Ali Baba’s basket and fled, as the first thief

had fled, into the strange cave. Ali Baba pursued him, and again did not see which way

the thief went. This time Ali Baba decided to search to the right. He went all the way

to the end of the right-hand passage, but he did not find the thief. He said to himself

that, like the first thief, the second thief had also been lucky in taking the passage Ali

Baba did not choose to search. This had undoubtedly let the thief leave again and to

blend quietly into the crowded bazaar,

The days went by, and every day brought its thief. Ali Baba always ran after the

thief, but he never caught any of them. On the fortieth day a fortieth thief grabbed

Ali Baba’s turban and fled, as thirty-nine thieves had done before him, into the strange

cave. Ali Baba yet again did not see which way the thief went. This time Ali Baba

decided to search the left-hand passage, but again he did not find the thief at the end

of the passage. Ali Baba was very puzzled.

He could have said to himself, as he had done before, that the fortieth thief had

been as lucky as each of the other thirty-nine thieves. But this explanation was so

G. Brassard (Ed.): Advances in Cryptology - CRYPT0 ‘89, LNCS 435, pp. 628-631, 1990.