Re: FW: New Version Notification for draft-rafiee-6man-cga-attack-00.txt

[marcelo bagnulo braun <[e-mail]>]

El 01/12/13 22:53, Hosnieh Rafiee escribió:
>> El 29/11/13 08:28, Hosnieh Rafiee escribió:
>>>
>>> In CGA, we have a hash function or like the above example, we have a
>>> hash password. What is important for us is only find another message
>>> that leads to the same hash. This is where birthday paradox applies.
>>> So, in CGA, the attacker really does not care what is the content of
>>> your message, it cares about the hash value. He only wants to have
>>> another message (or in birthday paradox to have another person with
>>> the same birthday that matches yours) to match yours.
>>>
>> sigh, no.
>>
>> the birthday paradox applies when you want to find two values that match
>> each other and NOT when you have a target and you want to find a hash
>> output that matches the target of your attack.
>>
>> In other words, the birthday paradox is not about finding another person
> that
>> has the same birthday than you but about finding two persons that happen
> to
>> have the same birthday.
>>
>> This is a critical distinction when you want to use this for an attack, as
> you
>> cannot rely on this to attack a specific target, you can only use it for
> finding
>> two random values that clash.
>
> Birthday attack is still possible with the following approach. Check this
> nice article.
> http://eprint.iacr.org/2003/065.pdf

I just skimmed the paper and it seems to state that in some hash 
functions fewer attemps are needed to find a clash.
This does not contradicts my statement. Birthday paradox can be used to 
find tow values that clash, not to find a preimage of a given target value.

Regards, marcelo


> smile,
> Hosnieh
>
>