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Andrey “Rarst” Savchenko —  — Software — dropbox, freeware, hash, portable, virustotal

Easy hash tricks

hash_icon Hashes are very interesting and deep topic… If you are into math or programming. Otherwise here are few down to earth rules that work with hashes and how can you benefit from them.

  1. Hash is bunch of letters and numbers that can be calculated for any file.
  2. Files with different hashes are different.
  3. Files with equal hashes are most probably equal.

This creates opportunities for some easy but useful tricks.

Check file integrity

hash_ubuntu

Pretty often file downloads in Internet are accompanied by hash values (they look like someone slept on keyboard). If you calculate hash of file downloaded and compare to hash provided on site - you can be sure it is not corrupted.

Check for viruses

hash_virustotal

VirusTotal (online service that checks files with multiply antiviruses) allows to submit file’s hash. If that file was checked already and result is in the system - you get it without need to upload file.

Instantly upload file to cloud

Dropbox (file storage and synchronizing service) uses a lot of hashing behind the curtains. For example if you try to put huge but common file in your Dropbox (large patch or whatever) its hash is calculated. If file is already present on servers it is linked to your account. Without wasting time and bandwidth.

How to calculate hash?

hash_interface

Easy! I use simple program called Hash. You simply drop one (or several) file into it and it calculates MD5, SHA1 and CRC32 (most common ones) for them. It is tiny, freeware and portable.

Home&download page http://keir.net/hash.html

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8 Comments

  • MK #

    Thanks for the enlightenment. I need educational posts like this ^_^

  • Rarst #

    @MK No problem. Thanks for visiting and commenting. By the way if you want such posts about some specific topic I urge you to try that link to my suggestions taking Skribit page. :) http://skribit.com/blogs/www-rarst-net

  • Brad Blogging.com - Personal Blog Tips And Blog Help #

    Awesome post Rarst. Thanks for the useful tools.. I've added you to my Blogroll. (On the main page) Brad.

  • Rarst #

    @Brad As always I am glad to be of use. :) Hashes are very interesting and widely used concept.

  • Peter #

    do you have also a possibility to use these hashes to get the file which it was generated from into the dropbox without really having the file, but only the hash? and which hashing algorith does dropbox use?

  • Rarst #

    @Peter Nope, there is no mechanism to operate hashes instead of files in Dropbox. I don't think I saw any public information on what specific hash mechanism they use.

  • fapestniegd #

    @Rarst Peter was correct. https://github.com/driverdan/dropship

  • Rarst #

    @fapestniegd Yep, comments above were made long before that project. Guess with Dropbox's popularity it was a matter of time before something like that was implemented by fans. :)