Berkeley CSUA MOTD:Entry 21573
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2025/05/24 [General] UID:1000 Activity:popular
5/24    

2001/6/19 [Computer/SW/Security, Computer/Theory] UID:21573 Activity:high
6/18    I have a question about diffie-hellman. After going through the initial
        key exchange and generating the session key k', how do you use this key
        with 3des or blowfish? Do you just trucate the key to the appropriate
        length (doesn't seem right) or is there some other method?  tia.
        \_ Probably feed the key into a one way hash function (i.e. MD5) that
           outputs the appropriate number of bits.
           \_ This is correct.  You would use a hash function.  However, you
              should not use Diffie-Hellman straight, much the same as you
              should not use plain RSA.  Get a cryptography book and read
              about it.
              \_ Okay, I understand the bit about the hash function, but
                 I don't understand why the session key k' can't be used
                 directly? I've been referring to Applied Cryptography,
                 but I can't seem to find a place where he explains why
                 the session keys should not be used directly.
                 \_ Here's a hand-wavy argument:
                        Your DH key must be larger than your 3DES key since
                        otherwise it's easy to break DH.  This means that
                        you'll have to shrink your DH key to make your 3DES
                        key.  You want to make your 3DES key by using all of
                        the randomness that you've got in your DH key, but
                        you don't know if truncating the DH key will do this.
                        However, you DO know that using a good hash function
                        to make your 3DES key will conserve all of the
                        randomness of your DH key.
                        \_ I guess I wasn't clear. I understand that I
                           need to hash the session key in a way that
                           preserves the randomness of the key and that
                           I need to use the hash value as the key for
                           my crypto algorithm.
                           The bit I don't understand is related to the
                           following: I keep reading that one should use
                           the hashed value of the session key *only* for
                           encrypting a different secret key and then that
                           encrypted secret key should be transmitted so
                           that all other transmissions are encrypted with
                           the secret key rather than the hash of the
                           session key.
                           Why can't I just keep using the hash of the
                           session key? It seems much simpler to do this
                           than to maintain a separate secret key.
2025/05/24 [General] UID:1000 Activity:popular
5/24    

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