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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Signature and hash function properties

I understand the need for the hash function to be collision resistant and second pre-image resistant. For what reason, exactly, does a hash function need to be pre-image resistant? If this property ...
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Converting a pre-image resistant hash to second pre-image resistant

I am looking for some standard conversion from any hash function $f$, which has pre-image resistance, to another hash function with second preimage resistance. Is there such a standard conversion?
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49 views

Favor hash size or field size when systems are disparate?

I'm working on an implementation of Krawczyk's Hashed MQV (HMQV). I'm using Crypto++, which is a C++ library. C++ has some features where classes that represent the crypto objects can be combined ...
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124 views

How can I generate a good password from a SHA512 hash?

I have to change local administrator passwords on machines. I don't want to store password in a database. I have to generate a password that I can find later to connect to the machine again. So I ...
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1answer
145 views

Keys in HMAC and NMAC

I'm quite new to this topic and have several questions concerning HMAC and NMAC: Why does NMAC need two keys? How can it be attacked if we just used some sort of initialization vector instead of the ...
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1answer
551 views

Can hash functions improve a lousy PRNG?

If I have a poor PRNG, could a cryptographically secure hash function make a better output? Say the PRNG can produce as much numbers as you wish, reasonably random, but not good enough to be used in ...
3
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1answer
218 views

Hash functions to generate one time pad

Newbie question... may be... Stream cypher, will XOR message bits with one time pad bits Suppose I have a pseudo random number generator, like a Mersenne twister or sort of. I could use the ...
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6answers
16k views

Difference between encrypting something and hashing something

What is the difference between encrypting something and hashing something? in what situations would I want one or the other?
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4answers
1k views

What good is a hash accompanying a program?

After studying at length about weaknesses in SHA2, length extension attacks, forged signatures, etc. I have to ask why groups like Gnu provide hashes on their program texts? As long as the public ...
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1answer
333 views

Why isn't outputting only a portion of the hash state a simple defense against length extension attacks?

As I understand length extension attacks, they depend on the coincidental property of most cryptographic hash functions that the hash value is exactly the hash function state after hashing the last ...
5
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2answers
539 views

SHA-256 exhaustive search

How do I calculate the number of computations needed to break SHA256 in case we are using it for safely storing passwords (together with a salt)? Are there any formulas that can be employed? For ...
2
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1answer
67 views

Salting in asp.net?

I have spent the past hour reading up on salting and still don't understand how it is achieved. Forgive me if im wrong, but the way I am thinking of salting is, storing an ArrayList of random strings ...
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177 views

What is degree of randomness in individual bits of MD5 hash?

I am researching on Hashing Algorithms and would like to know if we take individual bits of MD5, say we take first 8 bits or any random 8 bits of MD5 then what is the randomness probability of having ...
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2answers
278 views

How can I map arbitrary group elements to unique integers without using Hash functions?

Let's say, I have a group $G$ of large prime order $p$. A set $S$ consists of $n$ random elements chosen from $G$. Without using a collision resistant hash function $H$, how can I map elements of $G$ ...
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2answers
114 views

How much pre-hashing a password increase security against brute force attack?

I am wondering how much hashing a password used as key for a symmetric key algorithm helps preventing brute force attack on crypted text. Let's assume I want to crypt text A using key B using ...
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1answer
208 views

Problem with testing MD5 collisions

I am trying to test MD5 collisions presented in “Collisions for Hash Functions MD4, MD5, HAVA” (PDF) by Xiaoyun Wang, Dengguo Feng, Xuejia Lai, Hongbo Yu; 2004. I wrote a PHP script to test it, but ...
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2answers
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2
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1answer
69 views

Question about keyed hash functions

Maybe the answer to my question is obvious, but I have doubt... I have noticed that hash functions are studied in two forms, keyed hash functions or unkeyed. In general, lecture notes present hash ...
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2answers
759 views

I hashed my bag and in it I put… “Is there a hash for unordered items?”

(By items I mean hashable objects.) As an example (with obviously way too short hashes for readability), say I packed my bag and in it I put a shovel (hashed to ...
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3answers
409 views

Can passwords be stored securely so that a similarity comparison can be made?

On the Pro Webmasters StackExchange site, someone asked a question about a webapp (in this case cPanel) that refused to allow a password change because it was too similar to a previous password. I ...
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1answer
242 views

Will hashing over and over eventually give the same hash?

There are many cases in maths where doing the same thing over and over again will bring you closer to a value. Derivation of polynomials is the striking example: no matter what you start with, you end ...
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1answer
500 views

Suggestion : CRC vs SHA1

I need some suggestions about the use of the CRC in my application. I need to be sure about the BIOS run by the CPU. I have a FPGA between the BIOS flash and the CPU that sniff every read performed by ...
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1answer
235 views

Collision attacks on digital signatures

After reading this document about MD5 collision attacks, I still don't understand how collisions can make digital signatures insecure. In the paper, the researchers created two files with the same ...
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1answer
607 views

How many attempts does it take to crack a 32-bit password hash with this scenario?

How many attempts does it take to crack (match) a 32-bit password hash from a database of 4 million password hashes? Correct me if I'm wrong, but to crack a 32-bit password hash would take roughly ...
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0answers
36 views

Establish authentication

I have designed a basic authentication scheme where two nodes will mutually authenticate that other holds a shared secret. I have very limited RAM/ROM (its on uC) and bandwidth. Here it goes: ...
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163 views

Accelerated hashing on consumer-grade CPU?

Question is a follow-up to this one. The question was about accelerating SHA1. I am writing an application, where I do have a choice of hash algorithm, as long as it's a strong one. I want to be able ...
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Getting the encryption/has algorithm used for modem configuration file [closed]

Hello guys I have backed up my modem configuration file and found the username and password is in clear text; the files shows the username and the password is : ...
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3answers
255 views

Establishing encryption key using shared secret

I need to establish some security on a network of 8 bit microcontrollers. very limited RAM, CPU and packet sizes. I have zeroed in on a shared secret based scheme. Setting up shared secret is out of ...
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29 views

HMAC for encrypted authentication token: recommended iteration number [duplicate]

In the web application I use 10,000+ iterations for hashing passwords stored in database (random salt and all). Password-related operations are not frequent enough to create a noticeable performance ...
3
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1answer
122 views

Do hand-based hash functions / MACs exist?

Is there research on a hash function or a MAC that can be executed without computers? I know there's a lot of history on hand-based ciphers and the OTP can easily be excecuted by hand. (By using ...
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1answer
126 views

Proof that two parties can communicate

Suppose we have three parties, Alice, Bob, and Carol. Alice can receive messages from Bob and Carol individually, but can only send messages to both simultaneously. Additionally, Alice cannot tell if ...
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How are timestamps verified?

You put an input and the hash value comes as an output then when someone puts the input the hash function it is applied to see if it is the same hash original value is stored in some database , that ...
3
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1answer
154 views

Generating PGP key *without* random generator? (using fixed input only)

I understand why in general, a strong random generator or source of entropy is used when generating PGP keys. However, I have a use case where I want to generate PGP keys deterministically, i.e. ...
3
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2answers
704 views

Proofs of collision resistance by reduction

I'm looking for examples of a proof by reduction. For example: Let $A=(Gen, H)$ be a hash function. We define a new Hash function $A'=(Gen',H')$ with $Gen=Gen'$ by $H'_s(x)=H_s(H_s(x))$. It ...
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1answer
141 views

Crypto hash function vs encryption algorithms

Why is a cryptographic hash function generally faster to execute in software than conventional encryption algorithms such as DES?
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Determine safety of exercise hash functions

There have been several attacks against hash functions such as $h_{sha1}$ and $h_{md5}$. Professor A suggests a new function $h_{666}$ with an output length of 666 bits. Professor B suggests ...
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1answer
103 views

Hash “Preimage by product” resistance

Let H() be a hash function that achieves collision resistance as well as first and second preimage resistance. Let's equip the output set of H of a multiplicative group structure, more precisely a ...
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What is search space of this problem? [duplicate]

I have an exam and i have no time to solve this problem(I think about it a lot but i do not solve it).please help me and solve it for me: $A$ is a random number and has $512$ bits.We make $B$ ...
5
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1answer
1k views

What are the differences between collision attack and birthday attack?

From my understanding both types of attack, collision and birthday, are based on the principle of two randomly/pseudo-randomly chosen plaintext to hash to the same value. I don't want to launch any ...
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Brute-force attack given multiple hash prefixes

(Context: I'm auditing some code which I suspect to be insecure, but I'd like to be able to quantify this.) Suppose you have a 56-bit secret key ($secret), and ...
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3answers
271 views

How to quickly detect incorrect password in encrypted file without compromising security?

I am developing a software to encrypt/decrypt files/streams using symmetrical encryption algorithm. During encryption phase data wil be compressed and encrypted. When decrypting data, I want to be ...
19
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3answers
8k views

SHA512 faster than SHA256?

I'm getting this strange result that SHA512 is around 50% faster than SHA256. I'm using .net's SHA512Managed and SHA256Managed ...
0
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1answer
235 views

How HMAC.Update function works?

I know how standard HMAC(key,msg) function works, and I want to write a Delphi port of HMAC class that implements Update function. The problem is that I don't understand how this Update function works ...
0
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1answer
151 views

What is difference between Cryptographic Transformation, Encryption and Hash? [duplicate]

I read in a book that: let $f(K,R)$ is cryptographic transformation of $R$ using $K$, $K\{R\}$ means $R$ is encrypted using $K$ and $h\{R\}$ is the hash of $R$ and then the example follows. I want to ...
0
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1answer
105 views

What's wrong with an iterative (with counter) hashed salted KDF?

It seems like the brain-dead simplest KDF would have a key-stretching structure that is not substantially different from $\text{hash}(S | P | S | P |\dots|S|P)$, where $S$ is a salt and $P$ is a ...
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1answer
407 views

SHA-256 hash of null input?

What is the SHA-256 hash if the input is null, i.e. an empty bitstring? (Not the hash of 0 or "0".)
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2answers
270 views

What makes SHA-2 and SHA-3 have security levels half their output hash length?

The tag wiki for sha-256 says, in part: SHA-256 ... processes the input in 512-bit block and has a 256-bit output, for a 128-bit security level. I'm not a cryptographer, just an interested ...
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0answers
66 views

Anyone familiar with domain of hash functions in bilinear pairing based system?

I need a clarification regarding domain of hash functions. I have defined a bilinear pairing based system as follows: Let G1 and G2 be cyclic multiplicative groups of prime order p generated by g1 ...
2
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1answer
357 views

modular exponentiation as a one-way-hash

As far as I can tell most one-way hashes apply some iterated encryption algorithm to the input data. What would be the issues with a one-way hash based on some fixed large prime p and a generator g ...
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1answer
81 views

Differences between HMAC and HKDF in a specific case

I want to use a key derivation function to derive a key to another one. I think HKDF is the easiest solution in my case. The following picture gives a clear definition of it : ...