On Jul 31, 2009, at 7:55 AM, Julio wrote:
> .. And even if you use the same salt for each password It'd still be a
> time-consuming job since for each "clear" password in a rainbow table
> you'd have to "recompute" the new hash based on the salt and scan the
> rainbow table entirely for each record, now I totally agree that
> adding a salt for each password (obviously it has to be predictable so
> we can generate the hash again at log in time) will strengthen the
> passwords greatly.
>
> My suggestion is simply implement the new salting algorithm, move the
> unsalted hashed password to a lookup table (which will be deleted
> eventually), and "expire" the passwords for all your users, and on the
> next logon, they will have to type the old one, then create a new one
> and this time it'll be hashed with the proper salt, this way, even
> though "compatibility" will break, we'll still have a means to
> "correct" this.
>
> The way I do it in pyforum for instance, is this:
>
> passwd = "Hello World"
> hashed_pwd_tmp = hashlib.md5(passwd).hexdigest()
> hashed_passwd = hashlib.md5("%s%s" % (tmp_hashed_pwd,
> passwd)).hexdigest()
>
> So basically I prepend (or append, I don't have the code at this time)
> the hashed password to the clear password and re-hash the whole string
> again, breakable? of course, but exponentially harder.
The problem with this approach (what I've been calling a deterministic
password transform) is that it's no harder to precompute a new rainbow
table, knowing the algorithm, which is public, than it was to compute
the first rainbow table. Well, a little harder, since there are two
md5's per entry instead of one, but that's all.
Consider instead:
passwd = "Hello World"
salt = random.randint(1, 1000000).str
hashed_pwd = hashlib.md5(passwd+salt).hexdigest()+salt
This expands the size of the required rainbow table by a factor of
1,000,000.
The confusion here might be that we're assuming different attacks.
Perhaps:
* you're assuming that the attack is to use an existing md5 rainbow
table
* I'm assuming that the attack is to create a new rainbow table for
web2py passwords
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