A cryptographic hash algorithm is a function which takes a variable size input and produces a fixed size output. The algorithm makes it difficult to predict the output for a given input, find two inputs with the same output, or reconstruct the input from the output.

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How were shift amount constants in MD5 found?

The md5 specification gives a series of 4 rounds to execute over a 16-word block. Each round has a repeating sequence of 4 shift amounts (s in ...
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1answer
214 views

Best hash-algorithm [closed]

What is the most secure hash-algorithm. Can you also give the second best and the third best like this: 1. Test 2. test 3. TEST I dont care about the performance
3
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81 views

Signature verification with only hash (without full data)

I am looking for a way to sign a file, such that someone else can verify that I had this file, even if they only know a hash. I want to prove that I have the full file to someone who only has the ...
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0answers
43 views

The purpose of the final xor in Davies–Meyer scheme [duplicate]

In Davies–Meyer hash construction scheme each block is xor'ed with the previous block cipher text. Why? The only obvious flaw I see in a scheme without this xor is the possibility to reconstruct a ...
2
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1answer
369 views

Finding a collision for a hash function

I'm trying to find a collision for the following (modified) Merkle–Damgård hash function. Suppose we already have a hash function $h : \mathbb{Z}_2^{2·n} \to \mathbb{Z}_2^n$ for fixed length bit ...
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1answer
158 views

N way collision of hashes

For a collision $H(A_1) = H(A_2)$, the number of queries is $T^{1/2}$ where $\log_2(T)$ is length of hash output. Then, what would be the number of queries requried to find an $n$-collision ($H(A_1) = ...
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0answers
53 views

Recovering a constant salt for MD5 [duplicate]

I have STRING and MD5(STRING+SALT) for any infinitely many STRING's. Is their any way that I can recover the SALT itself? (SALT is constant) I actually know SALT, but this is an intellectual exercise. ...
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3answers
378 views

Statistical properties of hash functions when calculating modulo

When using SHA-1 to hash an input, the result is a pseudo-random number in the numeric ID space $\{0\dots2^{160}-1\}$. Do I loose any statistical property in the result if I use modulo to restrict the ...
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2answers
2k views

Can a salt for a password hash be public?

For my understanding salts in password hashes are used to prevent the precomputation of $plaintext \to hashvalues$ (rainbow tables). I know from different threads, it is not necessary to keep the ...
0
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1answer
52 views

Is there (this) a case in which salting is dispensable?

tl;dr this is not a duplicate of "what is a salt/why use salt?" question. Instead it is specificly asking when salt may not be necessary for hashing securly? (i.e. when the data being hashed is ...
3
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2answers
130 views

Memory-expensive hash from an array of hashes?

Why can't people make a hash function that requires dozens of mega bytes of memory instead of cpu to avoid all the cracking and hacking? What I have in mind myself is this: We have an input $X$, we ...
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1answer
78 views

MD-compliant hashes don't really accept arbitary length input, do they?

When people talk about hash functions, they usually say that they accept arbitrary-length input, but if you actually look at the padding (eg MD-strengthening padding), you see it's like ...
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1answer
471 views

HMAC ipad and opad choice

It is written in the HMAC paper that the $ipad=0x36$ and $opad=0x5c$ were chosen such that it maximize the Hamming-Distance of the part of the key used in the inner and outer part of the HMAC process. ...
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2answers
472 views

Definition of “pepper” in hash functions

I am confused about the notion of "pepper" in the context of storing hashes of users' passwords. Definition 1: A pepper is a secret key Looking around the Internet, for example here or here, a ...
3
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3answers
346 views

Stream cipher with Diffie-Hellman and Cryptographic Hash Function? [duplicate]

I know: Never do your own crypto. But this question is just theoretical. Assume you do a Diffie-Hellman Key Exchange with a server to produce a shared secret $x$. Then use a cryptographic hash ...
0
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1answer
110 views

Is any group of bits in a SHA-1 hash more/less unique than another?

Is there any bias in certain parts of the hash? E.g., first n bits are more/less unique than the last/middle n bits? Is there ...
0
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1answer
81 views

Complexity of preimage resistance

Why is the complexity of pre-image resistance $2$ raised to the power of $n$? I have been looking in everything lecture about pre-image resistance, but I still didn't know the answer.
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1answer
70 views
3
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2answers
179 views

Can a secret message be securely transmitted within a MAC tag?

A sender wants to transmit an ultra secret code $M$ which could be either 'go', 'stop' or 'wait'. This could be any selection of code words really and adopted for any use such as transmitting short ...
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1answer
206 views

Reversing a Psuedo Crytographic Hash [closed]

I'm trying to get a better understanding of cryptographic hashing. Take the example transformation function below, is it possible to reverse this formula to solve/isolate for message[i]? ...
0
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1answer
121 views

How to figure out the size of the output of a cryptographic checksum function

There is such a putting in Bishop's Introduction to computer security, page 118 as following: Definition 8–2. A cryptographic checksum function (also called a strong hash function or a strong ...
0
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1answer
2k views

SHA1 - SSL/TLS Cipher Suite

SHA-1 is on the way to deprecation, in particular for digital certification (x509). But I can't find ressources about SHA-1 deprecation from SSL/TLS Cipher Suites. Is it implicit, as deprecation ...
1
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2answers
429 views

Different padding rules for Merkle–Damgård and Keccak/sponge function

We learned that length padding was used in Merkle–Damgård where after padding with zeros another block is added that contains the initial length of the input. This is supposed to prevent same hash ...
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0answers
91 views

Less known hash functions producing 128 bit hash [closed]

Good day. While reversing a malware I stumbled upon a function producing 16 byte hash. I tried to determine which one is that by checking every function from wiki. Nothing fits. So, there are two ...
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0answers
80 views

Efficient proof of knowledge using Wegman-Carter hash

A verifier wants to ensure, with only little exchange of data with other systems, that a large block of data $M$ that the verifier holds is also available to some other system(s). It is not an ...
2
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2answers
245 views

Large file validation on an embedded system through hash and encryption

As a preface, I have to say that I am a noob in this area. Having said that, I will ask the question. I have a situation where I need to validate and protect against tampering a handful of large ...
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1answer
497 views

Hash collision using the leading 40-bits of SHA-1

I am trying to implement Floyd's cycle finding algorithm for finding a leading 40-bit hash collision in the SHA-1 algorithm. However, my code will not find a repetition even with 800,000,000 hashes. ...
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41 views

Designing hash function in space-efficient identity based encryption

Let $n$ be an RSA-prime and we know prime factorization of $n(=pq)$. The hash function is the current context maps from a given user identity(id) to a positive integer less than $n$, whose jacobi ...
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1answer
907 views

Collision resistant hash functions

Let (Gen1,H1) and (Gen2,H2) be two hash functions. Define (Gen,H) so that Gen runs Gen1 and Gen2 to obtain keys s1 and s2, respectively. Then define Hs1,s2(x)=Hs1(x)||Hs2(x). a) Prove that if at ...
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109 views

Complexity to find an preimage of a hash function

Suppose a hash function based on Merkle-Damguard construction. Its compression function is given as $H_i=E_{m_i}(H_{i-1})$, where $E_{m_i}()$ denotes the encryption of an ideal block cipher with ...
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5answers
583 views

Can hashing become encrypting?

hashing some data and encrypting data share some similar attributes. Surely this is why "Difference between encrypting something and hashing something" has been asked. The point of this question is ...
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2answers
1k views

Attacks of the MAC construction $\mathcal{H}(m||k)$ for common hashes $\mathcal{H}$?

Consider a common practically-collision-resistant hash function $\mathcal{H}$ (e.g. SHA-1, SHA-256, SHA-512, RIPEMD-160), perhaps based on the Merkle–Damgård construction as are the first three. We ...
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4answers
248 views

Why do we need to compute message digest of a message first while signing it?

Why it is more efficient to compute a message digest of message and digitally sign it then, rather than sign the message directly?
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1answer
178 views

Davies-Meyer Hash Function

So the Davies-Meyer Hash Function is: Hi = Hi−1 ⊕ exi (Hi−1) Say I pick H0 = 1110 0011 And the message x1 = "5" or "0000 0101" Is this the correct way to compute H1? H1 = 1110 0011 ⊕ (0000 0101 ⊕ ...
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2answers
214 views

What, cryptographically speaking, is a “long message”?

I have read about long message attacks on some cryptographic hash functions. However, I don't quite understand what is being referred to as a "long message". Also, do long message attacks only apply ...
0
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1answer
313 views

RC4 encryption/ decryption with hashing

I am working on RC4 encryption/ decryption algorithm based on the following: A wants to send M to B, for that A did: H = Hash (K||M) C = Ek (M||H) A sent ...
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1answer
256 views

How do I safely anonymize/hash a unique identifier to protect privacy?

I am building a system to collect a unique identifier (MAC addresses from 802.11 probe requests). The system will have several collection points over a large area, submitting to a central database. I ...
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2answers
348 views

CRC32 vs. low 32 bits of cryptographic hash

I was having a discussion with a colleague yesterday whose education and experience exceeded mine. He said something yesterday that sounded incorrect to me intuitively, but I don't have the knowledge ...
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4answers
1k views

Are there cryptographic hash functions that can be computed using only paper and pen without leaking any information about the plaintext?

I am looking for a cryptographic hash function that can be computed by a human using only paper and pen without ever leaking any information about the plaintext on the paper. The cryptographic hash ...
2
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1answer
952 views

Why does openssl use SHA1 in ECC when I use secp384r1 curve

I need a small clarification that why openssl using SHA1 in ECC when I am using secp384r1 curve, but in rfc they are saying we should use SHA2. Thing here is am using nanoECC in my DTLS, nanoECC ...
5
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2answers
349 views

Secure way to derive separate encryption and MAC keys from a single master key?

I want to derive a 256 bit encryption key and a 256 bit MAC key from a single 256 bit master key for an authenticated encryption scheme. I was considering the following construction to derive the two ...
8
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1answer
1k views

Security of KDF1 and KDF2 (hash based KDF's)

It's still common to come across implementations of KDF1 and KDF2. Basically these are KDF's that simply derive multiple keys from the key seed and a counter: $K_i = KDF(K_{master}, i) = H(K_{master} ...
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2answers
102 views

Is it just as difficult to find a desired output in a hash digest, for MSB vs LSB?

This question comes to mind after seeing "custom" Bitcoin addresses with some special string at the beginning of the address. They are generated in the same way Bitcoins are "mined", continually ...
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1answer
365 views

Security of chameleon hash functions

With the knowledge of the trapdoor, we are able to compute the same digest for differents messages. I've read that given two messages $m$, $m'$ and two numbers $r$, $r'$ such that ...
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4answers
5k views

What is the recommended replacement for MD5?

Since MD5 is broken for purposes of security, what hash should I be using now for secure applications?
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0answers
60 views

Forward Secrecy with pseudorandom functions

Let $H_1$, $H_2$ be keyed hash functions (e.g. $H_i(x) = SHA_{256}(s_i||x)$ for pseudorandom $s_1$, $s_2$). Let $s_n = H_1^k(s_0)$, $k_n = H_2(s_n)$, where $s_0$ is a secret (pseudorandomly chosen ...
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1answer
818 views

Where did the SHAKEs come from in SHA3?

Where did SHAKE128 and SHAKE256 originate from? I am trying to find them in the original Keccak documentation but can't find them. Is it some special mode of Keccak referenced in the documentation? ...
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3answers
248 views

Does this guarantee a unique 32 bit Hash?

I came across some source code that loosely does the below in order to achieve a 32 bit hash. The input string is passed through MD5 to get 16 bytes Hash (as usual). Then the 16 bytes are split into ...
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3answers
915 views

Is there a string that's hash is equal to itself?

I was wondering if there's any string that has a hash equal to itself, so that – when using any (none specific) hash function – the hash would be equal to that string? so that: ...
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87 views

advantages of hashing over elliptic curve signatures for a proof of work protocol

I'm trying to create a proof-of-work protocol for a proof-of-concept software, and it's basically something like this: ...