Introduction to EDH

• The server selects a random prime p and a generator g of the field Z*_p (Let’s ignore the mathematical properties of these values). So, the components p and g are public.
  
• The server picks a secret random number X_s and calculates Y_s=g^X_s mod p. Y_s is public and is sent to the client (just like p and g).
  
• The client does something similar, selecting a secret random number X_c and calculating Y_c=g^X_c mod p too. The client makes Y_c public by sending it to the server.
  
• The shared secret s is the public key of the other part to the exponential of the own private number, all in p modulus. That is, for the client s=Y_s^X_cmod p and for the server s=Y_c^X_smod p.
  
• With this shared secret the parties can encrypt all the following messages in a secure way.
  
• In the Ephemeral Diffie Hellman (EDH), the private numbers are ruled out, so s is mathematically secure and nobody can obtain it even having access to one of the parties after the aforementioned handshake.
  

The “exploit”

• The patch for Wireshark 1.0.2 can be downloaded from here.
  
• Debian packages with the patch applied can be found here.
  
• This is a list of all 2¹⁵ possible 64 and 128 bit DH private keys in systems vulnerable to the predictable OpenSSL PRNG described by DSA-1571.
  
• An example of a pcap file can be found here (it was built with a vulnerable client and one of the Moore toys, a hacked getpid by running $ MAGICPID=101 LD_PRELOAD=‘getpid.so’ ./vulnerable-openssl/apps/openssl s_client -connect db.debian.org:443 )

Issues that can be improved

• When the packets are out-of-order the decipher with stop itself.
  
• The brute force attack should run in a background process (and with a progress bar)
  
• Check the length of the keys before trying to brute force them.
  
• The patch also implements the display of public DH parameters in the packet tree. It’s incomplete.

Credits