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10

MD5, like other hash functions, uses the Merkle-Damgard construction. You take the message and break it up into fixed-size blocks. You start with an intialization vector (IV), which you feed into a compression function along with the first block. Take the output (it will be the same length as the IV), and feed it into the compression function along with the ...


7

The Merkle–Damgård hash construction customarily pads the message $M$ to be hashed with a single bit set to 1, a minimal number of bit(s) set to 0, and the representation of the length of the message in binary over some fixed number of bits. The padded message is then formed of a number of blocks $B_i$. The hash is computed by repeatedly applying a ...


6

As far as I am aware, there are no practical known second pre-image attacks on MD5, under the conditions you listed. However: if the attacker can control any part of the original, I would worry about using MD5 in this setting. Its security in this setting may be fragile and there may well be cleverer attacks than anything currently in the literature. I ...


6

How does the length extension attack against $H(k||m)$ work? For Merkle-Damgård hashes, if you know $H(x)$ but not $x$ you can still choose an $e$ and then compute $H(x||p||e)$. With $x=k||m$ you can compute $H((k||m||p)||e)=H(k||(m||p||e))$ which is a valid authentication tag for $m||p||e$. Why doesn't it work against $H(m||k)$? With a length extension ...


6

One issue with this construction is described in section 6 of the original HMAC paper, "Keying hash functions for message authentication" by Bellare, Canetti and Krawczyk, where they note that finding a collision on $\mathcal H$, i.e. two inputs $x \ne x'$ such that $\mathcal H(x) = \mathcal H(x')$, directly yields a collision on $\mathcal C$ such that ...


4

There is a beautiful characterization for the collision preserving padding rule of any Merkle–Damgård-construction: the padding rule should be suffix free. See the 2009 paper Characterizing Padding Rules of MD Hash Functions Preserving Collision Security by Mridul Nandi for more details. The length of the message, as it turns out to be, is the simplest ...


4

Actually, the Merkle–Damgård construction also specifies a padding bit after the message. The length is there the ensure that a padded message cannot be the suffix of a different longer message. A collision at the prefix leads to a collision in both messages. With a padding bit, a singe byte message 0x30 vs a 2 byte message 0x30 0x00 are padded to 0x30 0x80 ...


2

Isn't it still possible to find two different inputs that will be padded to the same value and then deliver the same hash? Well, no, it isn't. Given a padded message (that is, padded by adding a 1 bit, and then as many 0 bits as needed to fill it out to a multiple of the internal block size), we can unambiguously recover the original message -- by ...


2

The answer is that you can't; if you have a collision in $h^*$, you also have a collsion in $h$. The standard way to prove a collision-resistant hash based on a hash-resistant primitive is to show that if we are given a collision in the full hash, we can show that gives us a collision in the primitive. Hence if we believe we can't find a collision in the ...


2

The MAC you created is what's commonly called a keyed hash function. The way you have done it has a couple of issues. One is that you're hashing the message and then the key, but it's better to do the key and then the message. The reason for that is that if someone finds a collision with your message, then they are going to end up with the same MAC. It is ...


2

Yes, a Merkle–Damgård hash can be one-way. Even MD5 hasn't been broken in that way. Target collision resistance (TCR) is a notion similar to second preimage resistance (but for keyed functions). Preimage resistance, i.e. one-wayness, does not imply second preimage resistance, so a hash function can be one-way without even having second preimage resistance. ...


1

In most Merkle–Damgård hash functions, it uses a block cipher in the compression function, with the inputs being the padded message blocks and the IV. The IV is the fixed width input plaintext to the cipher, and the message becomes the key. The hashing of the input block x1 works by expanding it (using a key expansion) so that there are as many subkeys as ...



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