Vous connaissez l’exercice: utilisez un mot de passe long et varié, n’utilisez pas le même mot de passe deux fois, utilisez un mot de passe différent pour chaque site. L'utilisation d'un mot de passe court est-elle vraiment dangereuse? La séance de questions et réponses d’aujourd’hui nous est offerte par SuperUser, une sous-division de Stack Exchange, un groupe de sites Web de questions-réponses dirigé par la communauté.
La question
Le lecteur superutilisateur User31073 est curieux de savoir s'il devrait vraiment tenir compte de ces avertissements de mot de passe court:
Using systems like TrueCrypt, when I have to define a new password I am often informed that using a short password is insecure and “very easy” to break by brute-force.
I always use passwords of 8 characters in length, which are not based on dictionary words, which consists of characters from the set A-Z, a-z, 0-9
I.e. I use password like sDvE98f1
How easy is it to crack such a password by brute-force? I.e. how fast.
I know it heavily depends on the hardware but maybe someone could give me an estimate how long it would take to do this on a dual core with 2GHZ or whatever to have a frame of reference for the hardware.
To brute-force attack such a password one needs not only to cycle through all combinations but also try to decrypt with each guessed password which also needs some time.
Also, is there some software to brute-force hack TrueCrypt because I want to try to brute-force crack my own password to see how long it takes if it is really that “very easy”.
Les mots de passe courts de caractères aléatoires sont-ils vraiment en danger?
La réponse
Josh K., contributeur à SuperUser, explique ce dont l’attaquant aurait besoin:
If the attacker can gain access to the password hash it is often very easy to brute force since it simply entails hashing passwords until the hashes match.
The hash “strength” is dependent on how the password is stored. A MD5 hash might take less time to generate then a SHA-512 hash.
Windows used to (and may still, I don’t know) store passwords in a LM hash format, which uppercased the password and split it into two 7 character chunks which were then hashed. If you had a 15 character password it wouldn’t matter because it only stored the first 14 characters, and it was easy to brute force because you weren’t brute forcing a 14 character password, you were brute forcing two 7 character passwords.
If you feel the need, download a program such as John The Ripper or Cain & Abel (links withheld) and test it.
I recall being able to generate 200,000 hashes a second for an LM hash. Depending on how Truecrypt stores the hash, and if it can be retrieved from a locked volume, it could take more or less time.
Brute force attacks are often used when the attacker has a large number of hashes to go through. After running through a common dictionary they will often start weeding passwords out with common brute force attacks. Numbered passwords up to ten, extended alpha and numeric, alphanumeric and common symbols, alphanumeric and extended symbols. Depending on the goal of the attack it can lead with varying success rates. Attempting to compromise the security of one account in particular is often not the goal.
Un autre contributeur, Phoshi, développe l'idée:
Brute-Force is not a viable attack, pretty much ever. If the attacker knows nothing about your password, he isn’t getting it through brute-force this side of 2020. This may change in the future, as hardware advances (For example, one could use all however-many-it-has-now cores on an i7, massively speeding up the process (Still talking years, though))
If you want to be -super- secure, stick an extended-ascii symbol in there (Hold alt, use the numpad to type in a number larger than 255). Doing that pretty much assures that a plain brute-force is useless.
You should be concerned about potential flaws in truecrypt’s encryption algorithm, which could make finding a password much easier, and of course, the most complex password in the world is useless if the machine you’re using it on is compromised.
Nous annotons la réponse de Phoshi en ces termes: «La force brute n’est pas une attaque viable lorsque l’utilisation du cryptage sophistiqué de la génération actuelle est quasi inexistante».
Comme nous l'avons souligné dans notre récent article, Brute-Force Attacks Explained: Tout le cryptage est vulnérable, les systèmes de cryptage vieillissant et la puissance matérielle augmentant, il ne reste plus qu'une question de temps avant ce qui était une cible difficile (comme l'algorithme de cryptage de mot de passe NTLM de Microsoft) est défaitable en quelques heures.
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