# Tag Info

## Hot answers tagged hash

30

If you repeatedly apply a generic function on its result, in a finite domain, you tend to obtain a "rho" structure: at some point, you enter a cycle whose length is (roughly) $\sqrt{N}$, where $N$ is the size of the output space for your function. In the case of MD5, $N = 2^{128}$ (MD5 outputs 128-bit values), so the cycle will have length about $2^{64}$ ...

20

This isn't necessarily unexpected. 32-bit platforms vs 64-bit platforms can make a significant difference, as well as the amount of data you're hashing. $uname -m x86_64$ openssl speed sha256 sha512 The 'numbers' are in 1000s of bytes per second processed. type 16 bytes 64 bytes 256 bytes 1024 bytes 8192 bytes sha256 ...

16

SHA-512 has 25% more rounds than SHA-256. On a 64-bit processor each round takes the same amount of operations, yet can process double the data per round, because the instructions process 64-bit words instead of 32-bit words. Therefore, 2 / 1.25 = 1.6, which is how much faster SHA-512 can be under optimal conditions. Of course there is memory overhead, ...

14

The echo command appends a new line at the end, by default. The -n option omits this character. Compare these two executions: > echo -n "test123" | md5sum cc03e747a6afbbcbf8be7668acfebee5 > echo "test123" | md5sum 4a251a2ef9bbf4ccc35f97aba2c9cbda So the difference between the hash values is simply caused by the new line character.

13

Multiple hashing, in itself, is not a bad idea. What's bad is trying to design your own non-standard password hashing scheme, without understanding what features such a scheme needs in order to be secure. In fact, hashing the password many times can be a very good idea, as long as you do it sufficiently many times. This is one way to slow down the hashing ...

11

I restrict to hash functions $H$ with an output of some fixed size $n\ge1$ bit(s), accepting as input some strings, including all $n$-bit strings; MD5 (resp. SHA-1, SHA-256) is an example of such function for $n=128$ (resp. $n=160$, $n=256$). Whether there exists a solution to $H(x)=x$ depends on the particular hash function. If $H$ is a random function (as ...

11

Expanding then shrinking in SHA-1 refers to the process, performed for each round (each 512-bit block of padded message), of message expansion from 512 bits to 2560 bits; keeping only 160 bits of state for the next round. The later directly follows from the construction of SHA-1 as a Merkle-Damgård hash of 160 bit. The former occurs because SHA-1's ...

10

This is trivially true via the pigeonhole principle. SHA-2/512 has $2^{512}$ possible outputs, but $2^{2^{128}} - 1$ possible inputs. Trying $2^{512}+1$ unique inputs is sufficient to produce at least one collision. That said, SHA-2/512 is designed to be collision resistant, which implies that it should be hard to find two inputs that hash to the same ...

9

The difference is in the choice of $m_1$. In the first case (second preimage resistance), the attacker is handed a fixed $m_1$ to which he has to find a different $m_2$ with equal hash. In particular, he can't choose $m_1$. In the second case (collision resistance), the attacker can freely choose both messages $m_1$ and $m_2$, with the only requirement ...

9

This is impossible for any generally useful hash: a hash must map all inputs to a fixed-length output, but you normally want to be able to take variable (and fairly long) inputs. The problem is that there are more inputs than outputs: you normally want to be able to hash any string up to a fairly big length, but the hash itself should not be too long, and ...

9

Contrary to your assumption, this is done, and it is secure: For instance, the hash functions SHA-224 and SHA-384 are basically the same algorithms as SHA-256 and SHA-512! The only differences are in the initial values for the Merkle-Damgård construction used internally and, of course, in that only the first $224$ or $384$ bits of the resulting hash are ...

9

What choice did they have? F1 is a bitwise function with three inputs and one output. There are $2^8 = 256$ such functions. Only 70 of them are "unbiased" (i.e. have as many 0 and 1 outputs in their image). If you further require that each input, as well as the order of inputs, matters for the output, you are left with only 36. However, those 36 are all ...

8

I believe Thomas Pornin's answer is by far superior to mine, but perhaps this answer can provide a simplification to his answer. When you initially hash some data, the possible input is infinite/limitless. You could input "abcdefghi...", "123456...", etcetera. However, the resulting hash possibilities are finite/limited. One of the beautiful things about ...

7

A "generic attack" against a cryptographical primitive is one that can be run independently of the details of how that cryptographical primitive is implemented. The most obvious case is a cipher that takes an $N$ bit key; the generic attack of brute force takes a ciphertext, and attempts to decrypt it with all $2^N$ keys; when we find the known (or ...

7

You can in principle encrypt using a hash function, in the manner you describe (although what you have described is not necessarily a secure construction). What you are trying to do is generate a keystream from a hash function and a key. You can use counter mode to turn any strong pseudorandom function (PRF) into a stream cipher. CTR mode produces a ...

7

Yes. Let $H$ be a collision resistant hash function and assume that one can find a collision $(x,y)$ for $H\circ H$, that is, $x$ and $y$ with $x\neq y$ and $H(H(x))=H(H(y))$. Consider the results $H(x)$ and $H(y)$ of applying $H$ once to both inputs. Then either $H(x)=H(y)$, hence $(x,y)$ is a collision for $H$; or $H(x)\neq H(y)$, hence $(H(x),H(y))$ is ...

7

The entropy for the output of SHA-256 truncated to its first $128$ bits when fed a random $128$-bit input is about $127.173$ bit, down from very close to $128$ bit before truncation (see final note). The truncation does not halve the entropy, because the halves are not independent. The right line of thought is that SHA-256 truncated to its first $128$ bits ...

7

SHA-512 (and SHA-384) is usually faster on 64-bit platforms, and SHA-256 is usually faster on 32-bit platforms.

7

Well, if you can assume that the website hasn't been hacked, then providing a SHA2 hash of a program would allow you do make sure you downloaded (from anywhere on the internet) a good version of the software. You are right, however, that since the method is publicly known, if someone can replace the binary on the site, they can also replace the hash on the ...

6

The cost of finding collision for SHA-1 is currently estimated as $2^{61}$ SHA-1 calls. To understand how much (or how little) it is, we could look at Bitcoin mining. Right now (September 2014) the entire mining network computes 200,000,000 giga-double-hashes of SHA-256 per second, or $2^{61}$ hashes in three seconds.

6

The definition of perfect hash functions do not have any security/cryptographic requirements. For example, the hash function that simply outputs the first n bits of any string is a perfect hash function on the set of n bit strings. Obviously this function does not hide the input at all. So the answer to your questions is that it depends on the hash ...

6

You could use a hash tree for this purpose. It wouldn't have the properties of your "hash", exactly, but it would work for the game. Use a secret key to derive individual keys for the leaves, e.g. $k_i = H(k, i)$. Have each leaf be a keyed hash of an item in the bag, $h_i = H(k_i, v_i)$. Then produce a normal hash tree by hashing two leaf values, then ...

6

Possible password search space = $36^5$ = 6.05 million possible combinations or ~$2^{26}$. If the passwords were randomly generated it would be 26 bits of entropy which isn't just weak it is pointless. To put that into perspective the throughput on modern GPUs is on the order 1 billion SHA-256 hashes per second. So the exhaustive search time to break an ...

6

Yes, you can certainly do this, and there has been a lot of theoretical work in the area. At a high level, what this is called is a randomness extractor (Wikipedia): A randomness extractor, often simply called an "extractor", is a function, which being applied to output from a weakly random entropy source, together with a short, uniformly random seed, ...

6

SHA-1 is still thought to be secure whenever collision resistance isn't required. The hash is both used for signing certificates and ECDHE public keys. There's however a difference with regard to collision attacks. It is possible for an attacker to attack the collision resistance with certificates by getting their own certificate signed by a CA. In ECDHE ...

6

With respect to collisions, hashing twice can not increase security, because if $x$ and $x'\ne x$ collide for $H$, that is $H(x)=H(x')$, then $H(H(x))=H(H(x'))$. Otherwise said, any collision for $H$ is a collision for the double hash $H\circ H$. It is therefore trivial to exhibit collisions for $\operatorname{MD5}\circ\operatorname{MD5}$. Hence the answer ...

5

Please bear in mind that this information is all secondhand. I have not looked closely at the original drafts of Hash DRBG (although you might find a draft that's early enough if you peruse the FOIA results in [1]). However, during conversations with folks at NIST I was told that there were certain weaknesses in early drafts of Hash DRBG that were very ...

5

I prefer using definitions that explicitly specify who does what. Weak collision resistance: After Bob creates some message x1, it is "computationally infeasible" for an attacker Mallory to compute some other message x2 such that h(x1) == h(x2). Strong collision resistance: It is "computationally infeasible" for an attacker Mallory to find any two messages ...

5

If you had an algorithm that used a Base64 conversion as a part of its processing, that would not be considered grounds for disqualifying it as a hash function (be it a cryptographical or noncryptographical hash). We consider a function to be a hash solely on the properties it has (statistical in the case of a noncryptographical hash; preimage, second ...

5

Without the specific reference I can't be sure this is what you are talking about, but generally a "long message" attack is a way to defeat second preimage resistance with less complexity than expected. It uses a time-space tradeoff to find a second preimage with complexity $2^{n/2}$ for a $n$-bit hash function (normally you would expect $2^n$). In the ...

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