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From Daniel Lescohier <daniel.lescoh...@cbsi.com>
Subject Re: Decrypting mod_session-created cookie
Date Tue, 09 Jul 2013 13:59:05 GMT
Looking at the code, the encoded data block does not have a version number
field in it, so the data format and general algorithm cannot be easily
changed in the future.

The encoded data seems to be comprised of:

base64 encode of:

   - salt (fixed length of sizeof(apr_uuid_t))
   - the Initialization Vector
   - the encrypted data

But to understand the algorithm, one would have to document what all the
apr_crypto_* functions do in the module's encrypt/decrypt functions.  For
example, the apr_crypto_passphrase takes the passphrase and the salt as
parameters (along with other parameters such as block chaining mode [which
mod_session_crypto has set fixed to CBC], number of iterations [which
mod_session_crypto has set fixed to 4096 iterations]).  But what is done
with it when it's passed to APR library?  One would have to look at the
implementation of that function in order to find out.  Taking a quick look
at the apr source, for the openssl implementation, apr_crypto_passphrase is
using PKCS5_PBKDF2_HMAC_SHA1 to generate the key from the passphrase and
salt.

So, it doesn't look like it's an quick and easy thing to document or
re-implement.



On Tue, Jul 9, 2013 at 9:01 AM, André Malo <nd@perlig.de> wrote:

> On Tuesday 09 July 2013 14:51:49 Mikhail T. wrote:
> > 09.07.2013 08:31, Eric Covener написав(ла):
> > > I am not surprised that separately documenting is not a priority for
> > > anyone,  given it's implemented in httpd for anyone to see.
> >
> > Source code is not documentation, is it? In matters of encryption
> > especially the methods chosen should be documented better -- not only
> > for interoperability, but also to allow to better judge the strengths of
> > the method (not that I personally am capable of the latter).
>
> I agree. And everybody is welcome to help fixing that issue.
>
> nd
>

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