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Re: [Cfrg] 答复: Re: [saag] New draft: Hashed Password Exchange

"Blumenthal, Uri - 0668 - MITLL" <uri@ll.mit.edu> Fri, 03 February 2012 15:46 UTC

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From: "Blumenthal, Uri - 0668 - MITLL" <uri@ll.mit.edu>
To: "Igoe, Kevin M." <kmigoe@nsa.gov>, "cfrg@irtf.org" <cfrg@irtf.org>
Date: Fri, 03 Feb 2012 10:45:54 -0500
Thread-Topic: [Cfrg] 答复: Re: [saag] New draft: Hashed Password Exchange
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Subject: Re: [Cfrg] 答复: Re: [saag] New draft: Hashed Password Exchange
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The real benefit of hashing a password is making the result more-or-less
uniformly distributed in the key-space of the subsequent consumer (some
cryptographers argue that all the proofs about block cipher constructs
hold only if the key is random & uniformly distributed :). In this
particular case, it shouldn't matter what kind of input HMAC (or the
underlying hash) is fed. My $0.02.
-- 
Regards,
Uri      uri@ll.mit.edu





On 2/3/12 09:08 AM, "Igoe, Kevin M." <kmigoe@nsa.gov> wrote:

>Applying a function to a random variable can never increase its
>entopy.  See chapter 2 of Cover & Thomas' "Elements of Information
>Theory", exercise 5, which shows that for any function g and random
>variable X in the domain of g,
>
>	H( g(X) ) <= H(X)
>
>(where H(Z) = the entropy of a random variable Z).
> 
>In our case g is a hash function, X is the original password and
>g(X) is the hash of the password. Then a hashed password cannot
>have more entropy than the original password and may in fact
>have less entropy.
>
>
>> -----Original Message-----
>> From: cfrg-bounces@irtf.org [mailto:cfrg-bounces@irtf.org] On Behalf Of
>> Blumenthal, Uri - 0668 - MITLL
>> Sent: Wednesday, February 01, 2012 4:46 PM
>> To: cfrg@irtf.org
>> Subject: Re: [Cfrg] 答复: Re: [saag] New draft: Hashed Password
>> Exchange
>> 
>> On 2/1/12 12:11 , "Rose, Greg" <ggr@qualcomm.com> wrote:
>> 
>> >On 2012 Feb 1, at 0:13 , <zhou.sujing@zte.com.cn>
>> ><zhou.sujing@zte.com.cn> wrote:
>> >> Since passwords are often not too long, and not so random, it is
>> better
>> >> to hash it before using it as a key in a HMAC.
>> >
>> >I'm afraid this is a fallacy. While it will be longer, and will look
>> >random, there is exactly the same (lack of) entropy in a hashed weak
>> >password as there is in the original password. It's still vulnerable
>> to
>> >password search, although with a slightly increased workload due to
>> the
>> >(single) extra hash invocation.
>> 
>> Concur 100%.
>> 
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