Come scoprire file identici senza confrontarli tra loro?

Question

Sto costruendo un sito in cui gli utenti possono caricare contenuti. Come sempre mirare per il dominio del mondo, quindi vorrei evitare di memorizzare lo stesso file due volte. Ad esempio, se un utente tenta di caricare lo stesso file due volte (ridenomendo o semplicemente dimenticando ciò che ha fatto in passato).

Il mio approccio corrente è quello di avere il database che traccia ogni archivio file caricato le seguenti informazioni su ciascun file:

.
• Dimensione del file in byte
• md5 somma del contenuto del file
• SHA1 Somma del contenuto del file

  E poi un indice unico su quelle tre colonne. Utilizzando due hash su minimizzare il rischio di falsi positivi.

  Allora, la mia domanda è davvero: Qual è la probabilità di due file diversi ("Real-World") delle stesse dimensioni aventi identici MD5 e SHA1 hash?

  O: C'è un metodo più intelligente di complessità simile (UN)?

  (Capisco che la probabilità potrebbe dipendere dalla dimensione del file).

  Grazie!

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/