DLP – sa cryptanalyse – son utilisation - PolSys

DLP – sa cryptanalyse – son utilisation
Gu´ ena¨ el Renault
Sorbonne Universit´ es UPMC, INRIA, CNRS LIP6

Part I
Motivation : L’´ echange des cl´ es ` a l’essor
de l’informatique et... plus tard
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R´ eseaux de communication se mettent en place
1950 Les premiers ordinateurs commerciaux, cryptographie
bas´ ee sur la taille des cl´ es, Th´ eorie de Shannon.
1970 R´ eseaux num´ eriques, standardisation du DES (chiffrement
sym´ etrique) par la NSA
1970 L’ordinateur devient un outil pour toutes les entreprises
Probl ` eme : l’´ echange des cl´ es
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Probl ´ ematique
Un nombre ´ enorme de cl´ es ` a g´ erer et ´ echanger !
Bob
Henry
Ken
Carl
Eve
Alice
Gil
Ines
Fred
Dan
Jean
+Il faut un moyen s ˆ ur d’´ echanger chaque cl´ e au pr´ ealable !
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´
Echanger une cl´ e sans jamais se rencontrer !
+En 1976, Diffie, Hellman et Merkle publient le premier sch´ ema de
d’´ echange de cl´ es ` a l’aide de la notion de cl´ e publique qu’ils
´
enonc` erent.
New Directions in Cryptography
Invited Paper
Whitfield Diffie and Martin E. Hellman
Abstract Two kinds of contemporary developments in cryp- communications over an insecure channel order to use cryptog-
tography are examined. Widening applications of teleprocess- raphy to insure privacy, however, it currently necessary for the
ing have given rise to a need for new types of cryptographic communicating parties to share a key which is known to no
systems, which minimize the need for secure key distribution one else. This is done by sending the key in advance over some
channels and supply the equivalent of a written signature. This secure channel such a private courier or registered mail. A
paper suggests ways to solve these currently open problems. private conversation between two people with no prior acquain-
It also discusses how the theories of communication and compu- tance is a common occurrence in business, however, and it is
tation are beginning to provide the tools to solve cryptographic unrealistic to expect initial business contacts to be postponed
problems of long standing. long enough for keys to be transmitted by some physical means.
The cost and delay imposed by this key distribution problem
is a major barrier to the transfer of business communications
1 INTRODUCTION
to large teleprocessing networks.
Section III proposes two approaches to transmitting keying
We stand today on the brink of a revolution in cryptography. information over public (i.e., insecure) channel without compro-
The development of cheap digital hardware has freed it from mising the security of the system. In public key cryptosystem
the design limitations of mechanical computing and brought enciphering and deciphering are governed by distinct keys, E
the cost of high grade cryptographic devices down to where and D, such that computing Dfrom Eis computationally infeasi-
they can be used in such commercial applications as remote ble (e.g., requiring 10
100
instructions). The enciphering key
cash dispensers and computer terminals. In turn, such applica- Ecan thus be publicly disclosed without compromising the
tions create a need for new types of cryptographic systems deciphering key D. Each user of the network can, therefore,
which minimize the necessity of secure key distribution chan- place his enciphering key in a public directory. This enables
nels and supply the equivalent of a written signature. At the any user of the system to send a message to any other user
same time, theoretical developments in information theory and enciphered in such a way that only the intended receiver is
computer science show promise of providing provably secure able to decipher it. As such, a public key cryptosystem is
cryptosystems, changing this ancient art into a science.
multiple access cipher. A private conversation can therefore be
The development of computer controlled communication net-
held between any two individuals regardless of whether they
works promises effortless and inexpensive contact between peo-
have ever communicated before. Each one sends messages to
ple or computers on opposite sides of the world, replacing most
the other enciphered in the receiver public enciphering key
mail and many excursions with telecommunications. For many
and deciphers the messages he receives using his own secret
applications these contacts must be made secure against both
eavesdropping and the injection of illegitimate messages. At deciphering key.
present, however, the solution of security problems lags well We propose some techniques for developing public key crypt-
behind other areas of communications technology. Contempo- osystems, but the problem is still largely open.
rary cryptography is unable to meet the requirements, in that Public key distribution systems offer a different approach to
its use would impose such severe inconveniences on the system eliminating the need for a secure key distribution channel. In
users, as to eliminate many of the benefits of teleprocessing. such a system, two users who wish to exchange a key communi-
The best known cryptographic problem is that of privacy: cate back and forth until they arrive a key in common. A third
preventing the unauthorized extraction of information from party eavesdropping on this exchange must find it computation-
ally infeasible to compute the key from the information over-
Manuscript received June 3, 1976. This work was partially supported by
heard. A possible solution to the public key distribution problem
the National Science Foundation under NSF Grant ENG 10173. Portions of
is given in Section III, and Merkle [1] has a partial solution of
this work were presented at the IEEE Information Theory Workshop, Lenox,
a different form.
MA, June 23–25, 1975 and the IEEE International Symposium on Information
Theory in Ronneby, Sweden, June 21–24, 1976. A second problem, amenable to cryptographic solution which
W. Diffie is with the Department of Electrical Engineering, Stanford Univer-
stands in the way of replacing contemporary business communi-
sity, Stanford, CA, and the Stanford Artificial Intelligence Laboratory, Stanford,
cations by teleprocessing systems is authentication. In current
CA 94305.
business, the validity of contracts guaranteed by signatures. A
M. E. Hellman is with the Department of Electrical Engineering, Stanford
University, Stanford, CA 94305. signed contract serves as gal evidence of an agreement which
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