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Re: Forgotten Password
From: saghar estehghari <s.estehghari () gmail com>
Date: Wed, 21 Aug 2013 10:16:30 +0200

Hi list,

Thanks for the all the replies :)

@Clemens :The system is semi-trusted. This implies that we can't
access to user's data while he is offline (the data is encrypted at
rest). This is because the client is considered as a weakest link and
it is complicated for him  to handle the keys securely and to do the
encryption/decryption. So having this in mind, we can't be involved in
any encryption and decryption related to user's data that is saved on
the server!! And that's why I proposed the solution like that
(encrypting the pass with server's key but saving it on client side).
In this case an internal attacker, who has access to DBMS and server
keys, can't decrypt the user's data (while the user is offline).

@Tudor:  However, I know that my proposed solution has it's own
deficiencies, as if the user looses the certificate, there is no other
soluition for password retrival or as you said if an inside attacker
who has access to server keys perfoms a targeted attack and steels the
certificate then he can decrypt the data.

So I have another idea in mind which might be less complicated and
more secure than the previous one. At the registration stage we can
provide the user with a 3 challenge response questions, putting all
the responses together creates a string of length minimum 10
characters. The using PBKDF2 over the responses + salt can create a
key with which we can encrypt the password (this key can be paired
with our key. this means that the IT manager must be involved in this
process). So at the password retrival stage the same question will be
asked and the if correctly answered the old pass will be retrived and
user will be asked to choose a new password.

Any feedback will be appreciated :)

Saghar

On Wed, Aug 21, 2013 at 3:28 AM, Clemens Lode
<clemens.lode () medisanaspace com> wrote:
Hi Saghar,

That depends on your risk analysis and requirements.  If e.g. nobody at your
company may access the encrypted data, then obviously you need to save the
key at some other place. The ideal place is in the user's head. With your
solution, you allow anyone with access to the person's computer (a less
secure system than your servers - hopefully) access to the encrypted files
on your servers.

I guess it's better to provide that security for the user on your own
premises. For example with a computer mostly disconnected from any network
as a backup system for keys and only offline read access. And secured by
asking for additional details from the user (e.g. copy of identity card if
you will).

If the user doesn't trust you, then your business concept is wrong. Because
even if you don't have any keys saved on your system (in the solution you
are describing), you still have the keys for decryption temporarily. Then
better do all the encryption on the user's side and use the password merely
for authentication.

Best regards,
Clemens

On Aug 21, 2013 2:33 AM, "saghar estehghari" <s.estehghari () gmail com> wrote:

Hi,

In the system that I'm currently working on, the users authenticate
themselves using username and password. As this is kind of a secure
file sharing system, each user has a key that is drived from his
password and all of his data and files are encrypted using this key.

Since the password is not kept clear on the database, I face a problem
where the user forgets his password.  So it means that if we reset the
password we cannot decrypt his files anymore.

My solution to this problem was generating a certifcate at the
registration time that contains the encrypted password (using the
server's key), and ask them to save it. So when he clicks on "forgot
password " link, the server asks him to provide the certificate. After
verify the certificate, an email with a link for reseting the password
or an sms for a secret code will be sent to the user to verfy that
s/he is the legitimate user or not!

However, I'm not sure about the security of such solution! I was
wondering whether you have any better ideas or any feedback over my
solution.

Thanks



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This list is sponsored by Cenzic
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Request Yours Now! 
http://www.cenzic.com/2009HClaunch_Securityfocus
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