Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange
Join us in building a kind, collaborative learning community via our updated Code of Conduct.

Difficulty of finding two different inputs that hash to the same value

1
vote
1answer
73 views

Hashing based on the discrete logarithm problem

At a first look, one could use the elliptic curve discrete logarithm problem to grant for the onewayness of $H(x)=x*G$ (where $G$ is the generator point of the cyclic subgroup). Additionally, $H(x)$ ...
5
votes
2answers
55 views

Can RFC 5649 be used to wrap multiple keys with the same KEK?

I am unable to convince myself that RFC5649 is secure when storing multiple keys with the same KEK. My point is: RFC5649 uses AES-ECB, therefore the usual weakness of this method applies, namely an ...
1
vote
5answers
197 views

What is the mathematical property stating that it is hard to find a collision in the AES algorithm?

Finding a collision in the AES algorithm is (apparently) hard. Why ? What is the mathematical property stating that there is a negligible probability to find a collision by taking two random ...
4
votes
2answers
112 views

Cryptanalysis of MD4 (Wang et al) — how did the authors come up with the collision differential?

Edit: ok, after I re-read the paper a couple of times it is clear that Table 6 is easily derived from Table 5, which describes the collision differential. So the real question is: how did the ...
26
votes
3answers
3k views

Does “Shattered” actually show SHA-1-signed certificates are “unsafe”?

Note: I am not advocating anyone continues using SHA1-signed certificates: they are dead as far as security is concerned and should no longer be used. I'm just trying to clarify my understanding of ...
14
votes
3answers
7k views

After Google's collision attack, is RSA-SHA1 signature still safe?

Google succeeded to get a collision in SHA-1 last year in an attack called shattered. Does this fact make certificates based RSA-SHA1 Signature risky for creating fraud certificates? If RSA-SHA1 ...
2
votes
1answer
38 views

The Collision Differential for MD4 - a question on notation (Wang, et al)

In the paper "Cryptanalysis of the Hash Functions MD4 and RIPEMD" the authors introduce the following notation (paragraph 4.1): $\Delta$$H_0$ = 0 $\xrightarrow{(M_,M')}$ $\Delta$$H$ = 0 What exactly ...
0
votes
0answers
39 views

Anonymized identifier

I'm trying to find an algorithm which would allow me to create anonymized zero-metadata identifiers. I'm creating a system which creates integer identifiers using a counter. Using such identifiers ...
0
votes
2answers
90 views

MD5 hash functions to prove integrity of a download

It doesn't take a lot of time on the crypto StackExchange to realise the sentiment around MD5 hash functions is poor. My questions are: If it's such a bad idea to make use of MD5 hash functions why ...
3
votes
4answers
88 views

Find some hash function with deliberate collisions

I guess this should be well known (or trivially desperate), but I couldn't find any reference. I have a small number (say $k=15$) of messages $m_i$ ,$i=1\cdots k$ (they are fixed length and short, ...
0
votes
0answers
86 views

Security of custom JSON-Object hash

I am currently implementing a signed JSON-Blob, and thus i need to create a hash of any JSON-Object possible. Currently for the hash of the JSON-Object i ended up with the following Java-Code using ...
5
votes
2answers
86 views

Finding k collisions on hash function

Let $n$ be the size of the image-space of a hash function $H$. It is known that you can find a collision on $H$ in $O(\sqrt{n})$ time (by birthday paradox). How can I show that, in order to find $k$ ...
2
votes
2answers
68 views

Is it possible to generate an infinite number of collisions given an infinite amount of strings?

Let us assume that we have a string $w$ such that it belongs the set $C$, where every member of $C$ is not null, and contains every string of characters that can be permuted (numbers, letters etc.). ...
3
votes
0answers
30 views

Ambainis quantum collision-finding alg; triple hash output lengths?

I just stumbled onto a series of 2017 papers about applying the Ambainis quantum collision-finding algorithm to hash functions. (disclaimer: I haven't read all of them in full yet): Post-quantum ...
7
votes
2answers
2k views

How many trials does it take to break HMAC-MD5?

I know that you can find collision in MD5 with 2^64 trials using Birthday paradox. Now everyone is saying that HMAC-MD5 is significantly more secure. How can I ...
0
votes
2answers
50 views

Technical term for hash value being uncorrelated to input message?

Is there a technical term for the desirable property of hash functions. That a small change to the input message will produce a massive unpredictable and seemingly uncorrelated change to the output ...
2
votes
2answers
450 views

collisions on hash functions

Why do we say that collision-resistance is a "harder" property than second pre-image for hash functions whereas if you have an attack on the second pre-image then you find a collision ? Moreover, a ...
0
votes
1answer
74 views

Security of XORing hashes vs concatenating

On this question https://stackoverflow.com/questions/5889238/why-is-xor-the-default-way-to-combine-hashes a couple of the answers say that xoring is a bad/insecure choice for combining hashes. However,...
-1
votes
2answers
96 views

(strong) collision resistance Example

I'm studying Cryptography & Network Security. It has the following example for collision resistance: "Suppose Bob writes an IOU message, sends it to Alice, and she signs it. Bob finds two ...
4
votes
1answer
72 views

For a random permutation $P$, what's the probability of the following event?

For a random permutation $P$ and $q$ distinct inputs $x_1,\ldots,x_q\in\{0,1\}^n$, what's the probability of the event that there exists at least one collision among $\{P(x_1)\oplus x_1,\ldots,P(x_q)\...
1
vote
2answers
186 views

Rolling your own crypto

I know this is usually frowned upon, but supposing you were to roll your own encryption cipher (completely on your own or with a small group of friends, with no peer review possible), what could you ...
1
vote
0answers
41 views

Hash Function SHA-3 [duplicate]

I would like to discuss the minimum input of hash function to consider strong (e.g Shack128 (SHA-3)). I have a mechanism based on SHA-3, but the input is just 40-bit, some friends told me 40-bit input ...
0
votes
1answer
56 views

Taking $n-1$ bits from hash function will also be hash function?

If $\mathcal{H}=\left(\text{Gen},H\right)$ is a collision resistant hash family does $\mathcal{W}=\left(\text{Gen},W\right)$ where $W_{s}\left(x\right)$ denote the $n-1$ left most bits of $H_{s}\left(...
3
votes
2answers
496 views

Attack MD5 with partial information?

If I generate an MD5 hash from 2 pieces of information – i.e. by concatenating two strings – how easy is it to guess what that hash is, assuming an attacker has knowledge of only 1 piece of those info?...
5
votes
1answer
143 views

What is the collision resistance of a 2048-bit hash based on SHA-3?

Let $M$ denote the message. Let $B_i$ denote eight different bitstrings (chosen arbitrarily or randomly) such that $$0 \le \text{length}(B_i) \le 12345678.$$ Consider the following function (it ...
3
votes
2answers
267 views

Is there are a metric based on collisions to compare bad hash functions?

Bad hash functions are not so perfect as in the "general collision probability" hypothesis... And a general concept of "collision resistence" not need the constraint of independence between the ...
0
votes
1answer
48 views

The collision rate probability decreases when using compression?

Some standard compression procedures, like the IANA's gzip provided by HTTP protocol, will consume CPU-time anyway... So we can reuse compressed file, $Z(x)$, in the checksum procedure. That is, we ...
0
votes
2answers
181 views

Does the double-hash H(H(x)) have greater collision probability than H(x)?

Let $H$ be a collision resistant hash function and $P_c[H](S)$ the collision probability about a sample set $S$ of input elements (eg. random numbers). It increases with double hash? That is, $P_c[H\...
1
vote
1answer
58 views

Is exponentiation of a fixed generator modulo a prime collision-resistant?

Let $p$ be a prime number, and $g$ a generator of $\mathbb Z/p\mathbb Z$. For a message $m$, define the hash function $$h(m) = g^m \pmod p.$$ Is $h$ collision-resistant?
0
votes
0answers
93 views

Davies-Meyer with DES insecure

I'm stuck in coming up with a collision for a hash function $H$, which is built using a Davies-Meyer construction chained in a Merkle-Damgard fashion. I I guess its actually easy using the fact that ...
1
vote
1answer
129 views

Is there a deterministic one-way collision-free crypto algorithm?

I use usernames encrypted using a function f as ids of records. Users see records identified by f(username). I must not know real usernames. I want to be protected from attack when adversary who has ...
0
votes
1answer
51 views

6 Byte for HMAC in wireless system secure?

A wireless system has 8 byte payload and 6 byte for message authentication. Scheme: ...
0
votes
2answers
102 views

Security of password hash with known prefix and appendix

Assume I have a password that is exactly 32 characters long. I hash it the following way: Prepend 1524 bytes (known to an attacker) Append 3356 bytes (known to an attacker) Do the checksum It is ...
1
vote
1answer
36 views

Randomized public-key encryption as binding commitment / “collision-resistance”?

I am looking to use randomized public-key encryption in a context where it should also serve as a sort of "binding commitment". That is, I want to encrypt a value $x$ with some randomness $rnd$ under ...
0
votes
0answers
27 views

Can a hash-input be the same as the hash-output? [duplicate]

Could it be that (i.e. for SHA-2) the hash-output is identical to the input or is this deliberately prohibited by some sort of mechanism?
23
votes
4answers
10k views

How can hashes be unique if they are limited in number? [duplicate]

I'm curious, how can for example SHA-256 be unique if there are only a limited number of them?! For clarification: how many MD5 hashes are there? $16^{32}$ MD5 hashes can be produced. $16^{64}$ SHA-...
6
votes
1answer
128 views

Is there a way one can combine two correlated hash outputs to maximize the collision resistance?

While practically diving into collision resistance, I am trying to first write a non-cryptographic hash (to learn from it) which is limited to arithmetic operations on 32-bit integers only. It ...
3
votes
1answer
56 views

Security implications of slow-by-design hashes on relative security vs. hash size

Would it be true to say that that one can get away with much smaller cryptographic hashes if the hashing algorithm is slow and expensive by design? Example: let's say you want 128-bit hashes. This is ...
4
votes
1answer
124 views

How can I determine if a hash function is secure?

I'm doing an exam in computer security and I encountered this problem which I'm unsure of how to attack properly. Should I get down and dirty with the Hash function on paper or is there a more general ...
1
vote
1answer
65 views

Dynamic Length Hash Function?

Sorry if this is elementary, but is there such a thing as a dynamic length (output) hash function? Something that would be a fast way to calculate hashes and output different lengths of strings for ...
1
vote
1answer
61 views

Backward Expansion of SHA1

Given a SHA1 hash, is it now possible to "practically" find the source material that was passed through SHA1 now that the Shattered paper has produced the first "practical" collision? As a follow on ...
2
votes
1answer
50 views

Why do you need padding block at the end of Merkle damgard if the input is multiple of block length?

Why do you need padding block at the end of Merkle Damgard if the input is multiple of block length? I learned that it was not collision resistant if a dummy block is not added to the end but I want ...
0
votes
0answers
62 views

What is a fast good-uniformity hash function that if it's seed and it's hash values are unknown, can be strong as cryptographic hash functions?

The hash function is for an application that the input to the hash function is from a user's, hence the user can control the collisions, but the question is that if the collisions, the hash function's ...
6
votes
2answers
92 views

Collision-resistance of $H\Bigl(\bigl(H(m_1\mathbin\| 1)\oplus H(m_2\mathbin\|2)\oplus\dots\oplus H(m_n\mathbin\|n)\bigr)\mathbin\|n\Bigr)$?

In this answer and discussion, it is considered a distributable hash $$\tilde H(m)\ =\ H\Bigl(\bigl(H(m_1\mathbin\| 1)\oplus H(m_2\mathbin\|2)\oplus\dots\oplus H(m_n\mathbin\|n)\bigr)\mathbin\|n\Bigr)...
3
votes
2answers
329 views

Is it secure to hash an AES key?

Let's say I want a way of generating a deterministic public ID for a specific symmetric cryptographic key (AES, for example). Would it be secure to use SHA-3 (or SHAKE) to hash the key, and then ...
2
votes
1answer
73 views

Create a CR hash function where truncating one bit leads to collisions

I was reading Boneh and Shoups's A Graduate Course in Applied Cryptography, when I stumbled upon the following exercise (I have changed the array indices to start at 1 instead of 0): 8.1 (...
0
votes
1answer
29 views

Convert a state-dependent hash-based signature scheme into a stateless message-dependent one

Suppose we have an XMSSMT instance that is capable of signing up to 2^60 messages securely when given some state-dependent information, I could envision making it stateless as follow: Hash the ...
0
votes
0answers
70 views

What does the < > notation mean in this situation?

I was looking at solutions for a problem but I noticed it said $<2>$ and was unsure of what that meant. From Homework 5 (PDF)
0
votes
0answers
96 views

Merkle-Damgard: Why not use the input length as the IV?

Would the resulting construction still be collision-resistant? Define the construction as follows: Let n be the number of bits in one block. Let L be the length of input x. Set B := the ...
0
votes
0answers
51 views

Why do we pad the length of the message at the end in Merkle–Damgård constructions? [duplicate]

In a Merkle–Damgård construction we pad the hash of the final block with the length of the actual message. Is this necessary? What happens if prepend the message length instead of appending it? Will ...