On Wed, Apr 16, 2008, Lutz Jaenicke wrote:
> David Erosa García wrote:
> > He???llo all.
> >
> > I tried the openssl-users list but I think this may be a question for
> > the devel list:
> >
> > I'm doing my "homework" about openssl, but *this question has nothing to
> > do with it*. It's just a doubt that arised while doing it.
> >
> > There is one exercise with the following text:
> >
> > --------
> > Con el comando ???openssl enc??? y la siguiente clave AES:
> > 188458A6D15034DFE386F23B61D43774 se puede descifrar cierta información.
> > Podrías decir cual?
> > --------
> > Using the command " openssl enc" and the following AES key:
> > 188458A6D15034DFE386F23B61D43774 you can decode some information, could
> > you say what?
> >
> > I started playing with "openssl enc" and I thought the only thing I
> > could "guess" was the salt (Surely I'm wrong).
> >
> > So I ran the command with a random IV:
> > openssl enc -aes128 -K 188458A6D15034DFE386F23B61D43774 -iv 1 -P
> >
> > I found that the salt varies as it should on two machines with 32 bit
> > CPU (not my main one):
> >
> > Office's computer (openssl 0.9.8g-4ubuntu2):
> > salt=4075DFB76496F2B7
> > salt=4045D8B76466EBB7
> > salt=40C5DAB764E6EDB7
> > salt=4015DEB76436F1B7
> > salt=4025DFB76446F2B7
> >
> > A server I have somewhere else (openssl 0.9.8c-4etch1):
> > salt=50D882BF0C000000
> > salt=B05DD9BF0C000000
> > salt=A0CCC7BF0C000000
> > salt=E0C88BBF0C000000
> > salt=204190BF0C000000
> >
> > But when I run it on my main computer, it always outputs the same salt!
> > This machine is a 64bit CPU, running a 64bits linux distribution
> > (openssl 0.9.8g-4ubuntu2):
> >
> > salt=0004000000000000
> > salt=0004000000000000
> > salt=0004000000000000
> > salt=0004000000000000
> >
> > I've been searching through the openssl lists and found nothing about
> > this behavior.
> >
> > What can be happening? Is it about the 64 bit version of openssl?
> >
> No, the actual output may depend on the system but the reason behind it
> is found in apps/enc.c:
> ...
> if (cipher != NULL)
> {
> /* Note that str is NULL if a key was passed on the command
> * line, so we get no salt in that case. Is this a bug?
> */
> if (str != NULL)
> ...
>
> In the case the "str == NULL" the memory containing the "salt" is an
> uninitialized part of the stack so its content is undefined and the
> behavior will depend on system and compiler (options) used.
>
Note that the salt is used to derive the key an IV from a passphrase in the
enc utility so if a key and IV are specified on the command line the salt
is never used. The bug is that it still prints out the unused salt.
Steve.
--
Dr Stephen N. Henson. Email, S/MIME and PGP keys: see homepage
OpenSSL project core developer and freelance consultant.
Homepage: http://www.drh-consultancy.demon.co.uk
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