Re: Securing hashing algorithm

From: William Stacey [MVP] (staceywREMOVE_at_mvps.org)
Date: 11/24/04 

Date: Wed, 24 Nov 2004 01:24:33 -0500

When using one way hashing like this, you always have one issue - securing
the password/key passed to the keyed hash. I assume your using a "keyed"
hash like HMACSHA1 or a keyed MD5 to hash your secret. Some options are:
1) string encrypted in your program using obfuscator (better then nothing,
but probably not great.)
2) Protected via ACLs in registry, file or smartcard (or others).
3) Use DPAPI to set and get your password in the OS. See
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/secmod/html/secmod21.asp

Of the three, DPAPI is probably the best. It is still not perfect as a user
logged in using same user account as app used can decrypt the data. If you
use the machine store, any user could decrypt the data unless you use
additional entropy. So possible a slightly better approach may be to use
machine store so any user on local machine could use your program and keep
your "entropy" encrypted in your program (via obfuscator encryption.) So
not a hacker needs to hack your assembly to figure out general idea of what
your doing, somehow decrypt your entropy (i.e. second pw) and then figure
out how to get data from DPAPI and what store your used, etc. As you can
see, you could keep jumping through hoops hiding data until your blue and
still not get 100% protection.
Possibly a better approach is to use RSA private/public key and client only
knows public key. That way, you have nothing you need to hide. You still
need to protect your client apps public key so it can't be replaced by a
hackers public key, etc.

Naturally, none of this matters if your code is plain .net as hacker can use
ildasm and ilasm to round trip your code and do remove your protections and
public key removing the strong name checking at assembly load time. Only
thing that helps this is obfuscating using a good one (I use and like
XenoCode which also prevents ildasm on your assemblies). Getting to a point
where the only option for the hacker is to hack your code is probably the
best effort and should be the goal I would think. 

-- 
William Stacey, MVP
http://mvp.support.microsoft.com
"Wm. Scott Miller" <Scott.Miller@spam.killer.wvinsurance.gov> wrote in
message news:OPhhtfa0EHA.3744@TK2MSFTNGP10.phx.gbl...
> Hello all!
>
> We are building applications here and have hashing algorithms to secure
> secrets (e.g passwords) by producing one way hashes.  Now, I've read alot
> and I've followed most of the advice that made sense.  One comment I've
seen
> alot about is "securing the hashing routine" but no-one explains how to
> accomplish this.  So how do I secure my hashing routine?  Do I use code
> access security, role based security, ACLs, etc or combination?  And if
> combination what combination is the best?  The routines will be used by
two
> "applications."  A ASP.NET and a Windows application.  It already has a
> strong name and is installed in the GAC.  How do I prevent it from being
run
> by any code besides our two applications?  Should it be installed in the
> GAC?  And if not, how to I guarantee the two applications are using the
same
> version?
>
> Thanks for your help,
> Scott
>
>