[sword-devel] How to validate a Sword module unlock key?

[Jaak Ristioja]

Hi!

On 12.01.20 20:53, Greg Hellings wrote:
> On Sun, Jan 12, 2020 at 10:32 AM Tobias Klein <contact at tklein.info> wrote:
> 
>> 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?
>>
> 
> 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.

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