Re: Hi again, and an invitation to kibitz

[daw@cs.berkeley.edu (David Wagner)]

In article <199701140755.CAA04514@mail.intercon.com>,
Amanda Walker <amanda@intercon.com> wrote:
> (a) Server sends 8-byte challenge to client
> 
> (b) Client sends Microsoft NT authentication response to the server
>     (take the password in Unicode form, do an MD4 hash, pad with 0s to 21
>     bytes, split into 3 7-byte groups, use these as DES keys to encrypt
>     the challenge three times, send the 24-byte result as the response).
> 
> (c) If authentication fails, close the connection.
> 
> (d) If authentication succeeds, all subsequent traffic is enccrypted with
>     DES in CFB mode.  Until April :), the DES key used is taken from the
>     first 7 bytes of the MD4 hash of the password (after April, we expect
>     to switch to Diffie-Hellman key exchange first, followed by a revised
>     authentication handshake).

Some weaknesses:

- It doesn't resist dictionary attacks (no salt) when the attacker can make
    one active probe (forge a fixed challenge and get the client's response).
- It doesn't stop replay attacks (replay a fixed challenge, now the same DES
    key is used, so replay DES-encrypted session data).
- DES-encryption doesn't provide message authentication against active
    attacks; use a MAC too.
- You should use independent DES keys for each direction of the connection.
- Also the DES encryption key doesn't change for each connection.  It should.