Dietrich Bollmann wrote:
Hi tul,
So this was a very long and informative answer :)
Thank you very much!
On Fri, 2008-06-13 at 12:02 +0200, M. Sokolewicz wrote:
[...] However, people usually write code which may (and will most
of the time) containt exploitable sections which might give a malicious
user the ability to get a dump of the database. A password dump is
always interesting, since it gives a LOT of information. People usually
don't use 1 password per login, but rather have a "standard password"
for most things.
So if the user is allowed to change his password, it should be encrypted
always as there are chances that the same password is used at some other
place? That makes a lot of sense to me :)
If all passwords are generated by the system on the other hand and the
user is not allowed to change his password, if further all the protected
data is in the same database as the password, there would be no need for
encrypting the passwords following your argumentation?
But if some information is stored outside the database - in my case
(simple file server) for example, the database only contains the file
meta-data while the files themselves are stored in some data directory
on the server - some malicious user who would have broken into the
database could get hold of the files if the passwords are stored
unencrypted; if some encryption scheme would have been used on the
other hand the data found in the database wouldn't be of any use at all?
And if the password should be recoverable some encryption with a key
stored somewhere else would force the hacker to break into two systems,
the database itself and the system which is used to store the key.
That makes sense also. I didn't think about the fact that database and
a directory on the server are two different things which would have to
be hacked separately. So I am happy about writing my mail and getting
such a nice answer before implementing some stupid password logic
myself :)
Now, if it were unprotected, the person getting the information can
instantly log in as that user, or if he wants might even take over that
person's identity in other places (rare, but it happens). If it were
protected by encryption of some kind then it would first need to be
decrypted to be usable (unless there is a designflaw which makes this
unnecessery as has been the case in a few messageboards a few years ago).
Now, you can either encrypt or hash your passwords. Hashes are one-way,
encryption two-way. If the malicious user gets hold of a hash: he'll
still not have anything useful in his hands. He might make a reverse
lookup table and figure out the password from that (though there's an
infinite number of possible inputs for each single [hash] output), but
add a salt and don't put that in the database and the user has a low
chance of ever finding out what it was. But, just as the malicious user
can't figure out what the password was, neither can you: so goodby
lost-password feature. Instead you'd have to regenerate a new password
and send that over, or do some other fancy magic which doesn't involve
sending the current password as-is, since you don't know it either.
If you were to use encryption there, you could always decrypt it. If you
have the key. Storing the key separately from the encrypted password
would make this quite safe. enctpyed_string = (data + key), if you know
neither the data nor the key, things get very tough. Because you know
the key, you can figure out the password and make a forgot-password
feature easily which sends out the actual password.
But, because your key is publicly available (if your page has to use it,
then it's automatically publicly available, maybe not easily, but a
malicious user which managed to get hold of a full password table, could
just aswell get hold of the key for the encryption)!
Putting in neither, so just keeping the passwords in their plain form is
safe. As long as noone _ever_ sees them. Guarantee that and you won't
have to bother with hashing/encrypting. If you can't guarantee it, build
in some extra safety in the form of hashing and/or encrypting.
hope that explains it all a bit,
- tul
Yes. A bit. I am actually impressed. But I better read some more
redundant book about intelligent malicious users as I still feel like
not understanding everything of what you said completely.
...any nice book recommendation for naive people like me :?
So how about the following solution to my simple file-server problem:
I generate a new url for every user who is allowed to download a file
and a private password for every new url. Using this approach, the same
file will be downloaded by different users via different urls and
passwords. The password for an url is stored in the database encrypted
and send over to the user unencrypted per email. Of course this makes
some more logic and tables necessary - and a new row for every user also
- but who cares :) What do you think?
Thanks for your interesting explanation!
Dietrich
Considering you're already jailing access by linking a specific url to a
specific password you're making the impact of a hacked password pretty
small. Which is a good thing :)
I would recommend, if you go this way, to add an expiry date to the
url/password combo. So for example you can only use that url/password
combo for 3 days before it expires, after that, you need a new combo.
Doing it this way (with server-generated passwords) you make sure that
_if_ it were ever to fall into hands-it-should-not-be-in, it won't be
there for long.
- Tul
P.S. in other words, sounds fine to me :)
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