Re: Revert MD4

On Oct 29, 2:52 pm, Federico Bertola
<federico.bertola....@xxxxxxxxx>
wrote:
2^128 (you write this right?) are teorically
possibles COLLISIONS that means that a file has the
same hash value as the original but with a infinite
range of length.
For example a file of 3 Mb may have a collision in
a 300 Kb file as in a 2 Tb file!!!
If I close the range of possibilities fixing the
length I can find (at leas one if the length is the
same as the original) very fewer collisions!
I mean a MD4 in a not-so-hostile environment where
you have some information about the original file.

there are just under 200,000 3MB files with the same
hash

there an infinite number of files of any size

it is impossible to revert a hash
in the way you mentioned

the cryptographic attacks on md4 are different
they find _different_ messages
with the _same_ hash

being able to do this
means the hash is not safe for validation purposes
as man-in-the-middle interception
can replace messages that still authorise

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
galathaea: prankster, fablist, magician, liar


Yes, I know that there are about 200.000 possibles collision per a file lenght and this alone is enought for stop you to try to revert the hash (I choose MD4 but you can use any easy-to-break hash like CRC32) but many collision-finding algo are based into find two bloks and if you have some information about the original file (an irrealistic situation in the real world but...) you can easily filter the results in a better way than filter the end result.
This is not so difficult to compute because any collision with this method can be founded in 2^8 operations.

I'm sorry for my perseverance but my cryptography teacher is not a dumb and if he says that I think this is possible .

Thankyou for your patience :)

Federico
.

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