Comment découvrir des fichiers identiques sans les comparer les uns aux autres?

Question

Je construis un site où les utilisateurs peuvent télécharger du contenu. Comme toujours, je vise la domination mondiale, je voudrais donc éviter de stocker le même fichier deux fois. Par exemple, si un utilisateur tente de télécharger le même fichier deux fois (en renommant ou en oubliant simplement ce qu'elle a fait dans le passé).

Mon approche actuelle consiste à avoir la base de données qui suit chaque fichier téléchargé stocker les informations suivantes sur chaque fichier:

• Taille du fichier en octets
• MD5 Somme du contenu du fichier
• SHA1 Somme du contenu du fichier

  et ensuite un indice unique sur ces trois colonnes. En utilisant deux hachages pour minimiser le risque de faux positifs.

  Alors, ma question est vraiment: Quelle est la probabilité de deux fichiers différents ("réels-worlds") de la même taille ayant identique MD5 et sha1 hachage?

  ou: existe une méthode plus intelligente de complexité similaire (ONU)?

  (Je comprends que la probabilité pourrait dépendre de la taille du fichier).

  merci!

Answer 1

The probability of two real-world files of the same size having the same SHA1 hash is zero for all practical purposes. Some weaknesses in SHA1 have been found, but creating a file from a SHA1 hash and a size (1) is incredibly expensive in terms of computing power and (2) produces either garbage or the original file.

Adding MD5 to the mix is total overkill. If you don't trust SHA-1, then a better option is to switch to SHA-2.

If you're really paranoid, try comparing files with identical (size, SHA1) signatures. That will, however, have to read both the files entirely if they are equal.

Answer 2

I believe storing MD5 and SHA1 hashes is adding unnecessary complexity and not good design. I would say storing the tuple of (SHA1, file size) would be by far good enough. Especially if you're starting a new community site, I'd safely use that solution and only create something more clever once it becomes a problem. As the saying goes, premature optimization is the root of all evil, and it's arguable if it'll be `optimizing'.

edit: I did not quantify the odds of you getting a MD5+SHA1 collision. I'd say it's zero. By a crude, back of the envelope calculation, the odds of two different files of arbitrary file sizes having identical (SHA1,MD5) tuple is 2^-288, which is zero as far as I'm concerned. Having to require identical file size reduces that even further.

Answer 3

You can use Broders implementation of the Rabin fingerprinting algorithm. It is faster to compute than sha1 and md5 and it is proven to be collision resistant. However, it is not considered to be safe against malicious attacks, it is possible fot someone to purposefuly alter the file in question sithout changing the fingerprint itself. If you just want to check the similarity of files, it is s pretty good solution.

C# implementation, not tested:

http://www.developpez.net/forums/d863959/dotnet/general-dotnet/contribuez/algorithm-rabin-fingerprint/