I would second that. It would be trivial to identify a known text from the module's printed text and run a brute force<br><br>While $key not identified<br> If module_decrypted($key, John:1:1) begins with "In the beginning"<br> Then Found it!<br> Else increment $key and restart loop<br><br>Peter<br><br>Sent from my mobile. Please forgive shortness, typos and weird autocorrects.<div class="quote" style="line-height: 1.5"><br><br>-------- Original Message --------<br>Subject: Re: [sword-devel] How to validate a Sword module unlock key?<br>From: Tobias Klein <contact@tklein.info><br>To: SWORD Developers' Collaboration Forum <sword-devel@crosswire.org>,Greg Hellings <greg.hellings@gmail.com><br>CC: <br><br><br type="attribution"><blockquote class="quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><html><head></head><body>Hi!<br>
<br>
As has been said before, if somebody really wants to crack an unlock key there are still ways to do that ... so, not having the validation function is then more a lack of usability than actual security.<br>
What's the conclusion to this discussion?<br>
Given the feedback that I saw I would still suggest to move forward with implementing the functionality based on Jaak's proposal.<br>
<br>
Best regards,<br>
Tobias<br><br><div class="gmail_quote">Am 13. Januar 2020 19:41:36 MEZ schrieb Greg Hellings <greg.hellings@gmail.com>:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<div dir="auto"><div><br><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Jan 13, 2020, 11:28 Troy A. Griffitts <<a href="mailto:scribe@crosswire.org">scribe@crosswire.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div>
<p>Hello all,</p>
<p>While the request at face value seems reasonable, let me explain
a bit of the history behind not having a method like this.</p>
<p>The way we've recommended in the past for frontends to build a UI
interface for unlocking is to show some entry in the Bible to the
user after deciphering and ask them if it looks OK. This might
seem kludgy, but it has the same effect as one of those stupid
captchas-- it require human interaction upon each decipher
confirmation.</p>
<p>The reasoning, as you've probably guessed, is to make it more
difficult to brute force an unlock key.</p>
<p>This may or may not be important or effective, but it has been
the policy up until now.</p>
<p>Practically, what happens these days is that deciphering has been
relocated to before decompression, and most enciphered modules use
compression, so the end result is that that the decompression
filter will throw a 'corrupt data during decompression' error and
return a 0 length entry-- which can be used for brute force key
guessing.</p></div></blockquote></div></div><div dir="auto"><br></div><div dir="auto">The last time I looked into this, it wasn't just practice. Enciphering just plain didn't work on uncompressed modules. But that was a while ago.</div><div dir="auto"><br></div><div dir="auto"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>
<p>So, we're in a state where we don't officially provide a means to
programmatically test an unlock key (for reasons stated above),
but in practice, an empty buffer returned for, say John.3.16,
would give a reasonable check for an invalid unlock key, or a
check for a valid UTF-8 stream would also be a reasonable
programmatic check.</p></div></blockquote></div></div><div dir="auto"><br></div><div dir="auto">What about a module like LXX or WLC? I realize those aren't under lock and key, but they're informative that checking for zero length in a particular reference isn't foolhardy. Conversely if it's Genesis 1:1, what about NA28? Particularly of note in commentary modules which could be limited to one book, alone.</div><div dir="auto"><br></div><div dir="auto"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>
<p>So, while in principle, I believe it's a good thing to force user
input into unlock confirmation to discourage brute force guessing,
we don't do a good job with the implementation right now.</p></div></blockquote></div></div><div dir="auto"><br></div><div dir="auto">Being a programmatic API, I'm not sure we can provide anything to prevent brute force attacks. If I know even a single word of any reference in the work, it becomes trivial to attack by just searching for the word's presence in the resulting output. As long as we are using a cryptographic algorithm that is strong against known plain text attacks, we can leave attacks up to the field of crypt analysis and be happy that we've done our due diligence.</div><div dir="auto"><br></div><div dir="auto">Jaak's proposal is reasonable and prevents known plain text attacks by resorting to a hash, but it makes maintaining the config file more of a pain. Storing the text in a module file field solves the maintenance problem with the conf file but could cause problems with existing content, depending on how exactly the file format is laid out - an area you're the expert in.</div><div dir="auto"><br></div><div dir="auto">--Greg</div><div dir="auto"><div class="gmail_quote"><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div>
<p>Thoughts?</p>
<p>Troy<br>
</p>
<div>On 1/12/20 11:42 PM, Tobias Klein
wrote:<br>
</div>
<blockquote type="cite">
<p>I like this idea, Jaak! :)</p>
<p>Can we implement this in the Sword engine with the next release
that also delivers the "individualized unlock key function"?
Ideally directly with a convenient API function that has the
purpose to validate a given unlock key, with a signature like
this:</p>
<p><b><font size="+1"><tt>bool isSwordUnlockKeyValid(std::string
key)</tt></font></b><br>
</p>
<p>In my view, having a mechanism for validating the unlock key is
essential for having a professional unlock frontend. Without the
availability of such a mechanism I see the following issues:</p>
<p>- Users need to go through full installation of a module before
knowing that the unlock key they entered works. This is a rather
lengthy feedback loop.<br>
</p>
<p>- Since there is a possibility for input errors when entering
the key, the frontend must provide extra functions to "correct
the key" after the installation has already happened (this
wouldn't be necessary with a validation function).</p>
<p>Best regards,<br>
Tobias<br>
</p>
<div>On 1/12/20 11:46 PM, Jaak Ristioja
wrote:<br>
</div>
<blockquote type="cite">
<pre>Hi!
On 12.01.20 20:53, Greg Hellings wrote:
</pre>
<blockquote type="cite">
<pre>On Sun, Jan 12, 2020 at 10:32 AM Tobias Klein <a href="mailto:contact@tklein.info" target="_blank" rel="noreferrer"><contact@tklein.info></a> wrote:
</pre>
<blockquote type="cite">
<pre>Hi,
I'm adding Sword module unlock support to Ezra Project and I've been
wondering how you would validate a given unlock key?
Basically the dialog for entering the unlock key is shown when a locked
module is selected for installation. Before going through the effort of
installing a module I would like to make sure that the given unlock key
actually works with the selected module. Is there something in the SWORD
API that supports the validation of the unlock key entered by the user?
</pre>
</blockquote>
<pre>The last time this came up, I believe the answer was that you just have to
try it and display it to the user and they have to decide if the results
are human readable.
It would be possible to include a field in modules with a known-good value,
then the API could test if that value matched what was expected when it was
decrypted. Unless that functionality already exists, I don't know of any
other way you could accomplish this.
</pre>
</blockquote>
<pre>I've thought about this many times myself and as far as I know Greg is
right that there is currently no other way besides trial and error to
verify the unlock key.
Greg: Do I understand you correctly, that there would need to be an
extra field in every such module, and extra logic must be added to SWORD
so that this extra field does not show up in frontends? If this is so,
it might slightly break compatiblity of modules with older versions of
SWORD which do not contain such enhancements.
As an alternative, I suggest for consideration the following approach:
Add in the module configuration file the two extra pieces of
information (presented here as two configuration options with bad names):
UnlockKeyVerifyValue=<Some sufficiently long random ASCII string>
UnlockKeyVerifyHash=<Hash of field value>
When a newer version of SWORD detects these configuration options in the
module configuration, it can verify the unlock key using the following
algorithm:
1) Decrypt the value of the UnlockKeyVerifyValue configuration option
(after whitespace trimming) with the unlock key
2) Verify that the hash of the value decrypted in step 1 matches the
value of the UnlockKeyVerifyHash configuration option.
Pros:
* Modules can easily be amended by adding new entries to their
configuration files.
* No extra field in the module text is needed, so modules amended with
these configuration options will continue to work with older versions of
SWORD.
* Anyone with the key can generate this verification information.
* Only access to the module configuration file is needed to verify the
unlock key, so no expensive seeking/reading/parsing the encrypted module
content is necessary.
* Doesn't too leak much about the key.
Cons:
* A hash function must be implemented, but I think this would not need
to be cryptographically secure, but would act more like checksum, so
even something as simple as CRC-32 might do.
Notes:
* Another alternative would be to use a ciphertext/plaintext pair
instead so that no checksum/has must be implemented at all, but this
might potentially leak too much about the key, and will likely require
the configuration options to include binary values (i.e.
escaping/encoding would be needed).
* Another alternative would be to decrypt and verify a field from the
encrypted module itself, but reading the ciphertext from the module file
might be a more expensive operation.
Hope this helps.
Best regards,
J
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