eap Mailing List: questions about PRF in eap-sim-11.txt

[Michael Richardson (mcr]

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Section  17, page 50, says:

   Key derivation is based on the random number generation specified in 
   NIST Federal Information Processing Standards (FIPS) Publication 
   186-2 [12]. The pseudo-random number generator is specified in the 
   change notice 1 (2001 October 5) of [12] (Algorithm 1). As specified 
   in the change notice (page 74), when Algorithm 1 is used as a 
   general-purpose pseudo-random number generator, the "mod q" term in 
   step 3.3 is omitted. The function G used in the algorithm is 
   constructed via Secure Hash Standard as specified in Appendix 3.3 of 
*  the standard. For convenience, the random number algorithm with the 
   correct modification is cited in Annex B.  
    
   160-bit XKEY and XVAL values are used, so b = 160. On each full 
   authentication, the Master Key is used as the initial secret seed-
   key XKEY. The optional user input values (XSEED_j) in step 3.1 are 
   set to zero.  
    
May I suggest that annex B be actually fully edited to reflect all of
these settings?

In *, I assume it is a reference to 186-2?

We need a total of K_encr(128 bits), K_aut(128 bits), MSK(64 bytes), EMSK(64
bytes). A total of 1280 bytes, or m = 4.

So, the algorithm would become:

        let XKEY := MK,
            XSEED_j := 0

   Step 3: For j = 0 to 3 do 
             a. XVAL = XKEY 
             b. w_0 = SHA1(XVAL) 
             c. XKEY = (1 + XKEY + w_0) mod 2^160
             d. XVAL = XKEY 
             e. w_1 = SHA1(XVAL) 
             f. XKEY = (1 + XKEY + w_1) mod 2^160
         3.3 x_j = w_0|w_1 

Assuming that I'm correct, I would strongly suggest that this be documented
in this way. This makes it trivial to code without wandering through 150
pages of FIPS documents. 

]      Out and about in Ottawa.    hmmm... beer.                |  firewalls  [
]   Michael Richardson, Sandelman Software Works, Ottawa, ON    |net architect[
] mcr [at] sandelman.ottawa.on.ca http://www.sandelman.ottawa.on.ca/ |device 
driver[
] panic("Just another Debian/notebook using, kernel hacking, security guy");  [


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