Question

Salt values prevent dictionary password attacks?

Asked by: solution46

Hi,

I am working on a web app with an ASP.NET front end (forms authentication), a VB.NET COM layer and a SQL Server database. I am storing the password as an MD5 hash value, then re-computing the value each time the user logs in. So far this is working fine, but I have been reading that it is better to add a random salt value to the end of the password, and store this in the user table for use when re-computing the password hash (see http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnnetsec/html/SecNetHT03.asp for example). Apparently this helps against dictionary attacks but I cannot see how. My view is that a dictionary attack through the front end wont be affected as the salt value wil be appended to each password tried, and if anybody has access to the back end they can read the salt value for themselves.

Full marks for the first person who gives an unequivocal reason why I should use a salt value as described in the above link.

s46.

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Asked On
2005-06-27 at 02:16:46ID21471588
Tags

password

,

salt

,

attack

,

prevent

,

value

Topic

Programming for ASP.NET

Participating Experts
1
Points
500
Comments
2

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Answers

 

by: NeedPointsPosted on 2005-06-27 at 07:03:56ID: 14308679

The article doesn't explain the concept much, so it is normal that you didn't understand the point.

"In addition, storing just the password, even hashed, isn't good enough. If an interloper can get access to your database, and if many users share the same password (no matter how hard you try, users still use stupid words like "password" as their password), a determined hacker might be able to ascertain a pattern and attempt a dictionary attack, trying common combinations of letters and numbers, based on patterns seen in the hashed passwords. To work around this potential security hole, most systems employ a second-level of misdirection in the form of a random "salt" value hashed along with the actual password. "
From : http://www.code-magazine.com/articleprint.aspx?quickid=0411121&printmode=true

So, this salt thing is for someone who somehow accesses the hashed passwords.
Hacker grouped the hashed passwords in the table, and saw that five users have the same password, so their passwords are probably 'password' or '' something common and stupid like that.

But when you salt them, probably every hashed password in the database will be different. So a hacker can't estimate something out of it.


 

by: solution46Posted on 2005-06-27 at 07:19:20ID: 14308850

That's great - thanks. Points on their way.

s46.

20120131-EE-VQP-002

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