full-disclosure - Re: Take the trash-talker challenge!

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Message-ID: <20020827153903.GA1275@insomnia.benzedrine.cx>
From: daniel at benzedrine.cx (Daniel Hartmeier)
Subject: Re: Take the trash-talker challenge!

On Tue, Aug 27, 2002 at 04:35:25AM -0700, aliver@...il.com wrote:

> If my detractors would like to prove what a
> silly ass I am then they should feel free to reverse the encrypted message
> into plaintext.

Your key setup is broken:

   memset(hash,0,16);
   /* lets hash the keys. We get a 256 bit hash    */
   /* from md5, but xxtea takes a 128 bit key so   */
   /* the hash is truncated to the first 128 bits  */
   md5_init(&state);
   md5_append(&state, (const md5_byte_t *) keyphrase, strlen(keyphrase));
   md5_finish(&state, digest);

So far, so good. But

   for (i = 0; i < 8; i++) {
      snprintf(hbuf,3,"%02x",digest[i]);
      memcpy(hash+(i*2),hbuf,2);
   }

Now hash consists of 16 characters 0-9, a-f. That's 16^16 or 2^64
possible hashes. And hash is used as the key. So, effectively, you're
using 64 bit keys.

It's too large a key space to brute-force in 10 minutes for me, but it
should be obvious that it's a severe flaw nonetheless.

Daniel