Can you break my encryption protocol ?

[Futplex]

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Mark writes:
[description of threat model elided]
> Client and server both have copies of a passphrase, of any length.
> 
> When starting the connection, client sends 128 random bits to the server.
> 
> Both ends take this data, append the passphrase, and use MD5 to generate 
> a session key. If a key of less than 128 bits is required for legal 
> reasons, then the appropriate number of bits are retained, and the rest 
> replaced with bits from the random data that was sent in the clear.
> 
> That is, if you're only allowed 40 bit security, you take the first 88
> bits that you were sent, and append the last 40 bits of the generated key 
> to give you the session key to use.

So this is:

Salt = RNG(128,Seed);
SessionKey = MD5(Salt | PassPhrase);

Or for export:

Salt = RNG(128,Seed);
Temp = MD5(Salt | PassPhrase);
SessionKey = 
    (Salt[1..(128 - NumExportBits)] | Temp[(128 - NumExportBits + 1)..128]);

Sounds good, assuming the passphrase is nice and long (i.e. 128 bits, or
NumExportBits) and MD5 holds up.

If you haven't already, you might want to look at some of the work Hugo
Krawczyk and some others have been doing on keyed MD5. Their application is
different (primarily, authentication) but I think many of the concerns are
similar. Look for draft-krawczyk-keyed-md5-01.txt in the usual places. "It
is not appropriate to use Internet Drafts as reference material, or to cite
them other than as a ``working draft'' or ``work in progress''."

Futplex <futplex at pseudonym.com>

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