Délai nécessaire pour casser une clef de chiffrement

samedi 11 juillet 1998

Source : le Monde Informatique du 4 juillet 1997, qui cite un rapport présenté au Sénat américain en juin 1996 (chiffrement symétrique DES).

Depuis, les durées ont été considérablement diminuées (dans un rapport de 100 à 1000 selon les sources). Les prix ont aussi diminué.

Pour un simple hacker (investissement 2000 F)

- 32 bits : quelques minutes
- 40 bits : 5 heures
- 56 bits : 38 ans

Pour une PME (investissemennt 50 KF)

- 40 bits : 12 minutes
- 56 bits : 18 mois

Pour une grande entreprise (investissement 1,5 MF)

- 40 bits : 180 millisecondes
- 56 bits : 3 heures

Pour une banque ou une assurance (investissement 50 MF)

- 40 bits : 5 millisecondes
- 56 bits : 6 minutes

Pour un Etat (investissement 1500 MF)

- 40 bits : 200 millionièmes de secondes
- 56 bits : 12 secondes

La durée de calcul augmente de manière exponentielle avec la longueur de la clef. Il faut certainement aujourd’hui plusieurs années à un Etat pour casser les clefs de 128 bits utilisées par l’algorithme asymétrique inclus en standard dans IE4 aux Etats-Unis et au Canada. Dans ces conditions, l’intérêt du décryptage est pratiquement nul.

Dominique Debray


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