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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Could completely public passphrase hashes ever be reliably secure?

This is a hypothetical question and I only have a basic understanding of Cryptography. If one were to follow the very best cryptographic practices for storing passphrases, could it ever be possible ...
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1k views

Using Whirlpool hashing function to encrypt data

I've read that the Whirlpool hash function can produce footprints that could be used as a pseudorandom generator. Is it "OK" to use it to encrypt some data using something like the following? ...
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659 views

Counter mode secure hash algorithm

Ever since the SHA-3 competition, I've been wondering if it is possible to create a hash algorithm that is easier to parallelize. The current algorithms all seem to require building a tree of hashes. ...
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167 views

Is it possible to find a preimage for a reduced size hash?

Given a message $M$ and a 256 bit key $K$, perform HMAC($K$, $M$) with a 256 bit hash function resulting in: $a$ = ...
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1answer
994 views

HASH Algorithm for 8 bits MCU

I need to implement the HASH algorithm (MD5 & SHA-1) on an 8 bit MCU. I hear it can only be implemented on 32bit and sometimes 16bits MCU. Is that possible? I will appreciate link where possible. ...
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0answers
168 views

Precise meaning of various terms related to universal hash functions

I've been reading about universal hashing, but I'm confused by all these different terms and notations. Could someone help me understand the precise meaning or relation between the following terms: ...
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394 views

bcrypt and pbkdf2 double hashing

If I want to protect myself from implementation of algorithm issues and inherent design flaws of algorithms in password hashing, what would be most secure way to combine two hashes? bcrypt(pbkdf2(pw)...
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9k views

What is wrong with using SHA1 in digital signatures? Why is a robust hash function needed?

For the purposes of signing and verifying signatures, what is the value of the hash function? Why would it matter if SHA1 is later determined to be easy to break? Since a Public/Private key process ...
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629 views

Meaning of “family” in “family of hash functions”

Many definitions related to universal hashing mention the term "a family of hash functions, say $H$". What does this 'family' mean exactly? A numerical example would be appreciated. Also, what does it ...
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351 views

Is it possible to anonymize web traffic so that the IP Address cannot be determined while still being able to determine distinct IPs?

I'd like to find a way to anonymize IP addresses in web logs to ensure user privacy given the following requirements: If given an IP address, there is no way to look up which requests came from the ...
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Is using slow password hashing on the client side easier attackable than on the server side?

As we know, one should use a slow password hashing algorithm instead of a fast one for storing passwords, to hinder brute force attacks when the database is compromised. The problem with this is that ...
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1answer
774 views

Will rehashing an SHA256 hash continually, eventually produce every possible value?

So let's say you had infinite time and energy. You have a hashed string of some sort. Because you have infinite time and energy, you can produce a collision(or the original value) easily enough. But, ...
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Are there hash algorithms with variable length output?

I understand that for example MD5 produces a 128 bit hash value from a given text of variable size. My question is if there is a hash-like algorithm that will produce a hash value where one can ...
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5answers
2k views

Using one-way hash functions as the encryption method

Suppose two parties want to communicate securely with each other (Bob and Alice) using a simple messaging system in English. There are approximately 180,000 currently used words in the English ...
3
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2answers
154 views

Feedback requested on a method of posting a message without revealing the author

So I was thinking about variations on the Dining Cryptographers problem - In some cases, it's useful to be able to post a message without revealing the source, but with the additional constraint of ...
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1answer
784 views

Why restricting SHA3 to have only two possible capacities?

I just read the presentation slides of John M. Kesley (from NIST) for his invited talk at CHES 2013 about SHA-3 and learned that NIST is going to standardize Keccak with a possibly modified padding ...
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1k views

How to hash a list of multiple items?

In some protocols, a key is derived from a hash of many parameters (K=H(param1,param2,param3)) Is there a hash function that takes many parameters as input ? Or we can simply use a function like ...
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1answer
78 views

Is size Q equal to size SHA(Q)? [closed]

Assume d is a 128 bit random integer and P is base point of an elliptic curve and Q = dP is a point on the elliptic curve and SHA is a hash function with 128 bit output, my question is: Is size Q ...
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1answer
453 views

Does CBC encryption of a hash provide authenticity?

Given a message $M$ and a cryptographic hash function $H$, let $f(M) = E_K(M || H(M))$ where $E_K$ is AES-128-CBC encryption with PKCS#5 padding. Take $H = \textrm{SHA-256}$ if it matters. In other ...
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415 views

Why have hashes when you have MACs?

It would seem to a naive eye that if you have a MAC, you have a hash function: use a key that all the parties know (such as all-bits-zero). A potential application would be a resource-constrained ...
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1answer
196 views

SHA256 Round Equivalence

I have calculated the average number of operations in 1 round of SHA256 as follows: Additions: 9.25 Bitwise Rotations: 9 Bitwise Shifts: 1.5 Bitwise AND: 5 Bitwise XOR: 10 I have certain ...
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1answer
155 views

Multiple parties must encrypt and get the same result

Hopefully this question is not too simple - I did some research but with encryption I really don't want to make a mistake. Our scenario is that multiple groups need to encrypt a number and arrive at ...
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what kind of hash function can provide a short hash and be collision resistant?

When you try to connect via SSH, you see a signature which is short but I heard it is even stronger than sha256. It is perhaps stronger because it uses more rounds. Is there a hash function or a ...
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Pre-image resistant but not 2nd pre-image resistant?

Are there any cryptographic hash functions for which there is a known pre-image attack, or a known second pre-image attack, but not both? The attack doesn't have to be practical - just anything that ...
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261 views

Allowing the user to choose the hashing formula at the registration

I was thinking of a potential new way to increase someone's account security. The idea was allowing the user to choose one of the hash formulas available in the software (example: "MD5", "SHA512" and ...
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267 views

Why are there $ signs in my passwd file?

I am trying to get access to my eReader and I managed to get the passwd file. ...
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818 views

What is the purpose of using different hash functions for the leaves and internals of a hash tree?

I just learned that the THEX hash tree specification which is widely used in P2P requires that two different hash functions be used: one for the leaf nodes (hashes of input data) and one for the ...
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417 views

Why is feed-forward mechanism used in hash functions?

The compression function of SHA-1 when used in Davies-Meyer mode adds its input to the chaining values at the final step. For the first message block, the IV is used as the input and in the next step, ...
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230 views

Did Merkle invent cryptographic hashing?

Chapter II of Merkle's 1979 PhD thesis is titled "One Way Hash Functions." The chapter appears to be the first reference to cryptographic hashing. The chapter has no references. Is there an earlier ...
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2k views

Is a second preimage attack on MD5 feasible?

What's the practical status of MD5 w.r.t. second-preimage? Integrity of a piece of data is protected by an MD5 hash, itself assumed genuine. The data (and thus the hash) is known to the adversary. ...
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1answer
824 views

Why is H(message||secret_key) not vulnerable to length-extension attack?

Given a Merkle-Damgård hash function $H$, I know that an attacker can forge a message protected by a MAC computed as $H(\textrm{secret_key}||\textrm{message})$. Why can't he perform the same ...
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643 views

How did LinkedIn “salt” all their passwords?

First, just to make sure I understand "salting" correctly: You randomly generate a string to append to the password before hashing it, so as to increase its length and make precomputed tables much ...
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277 views

split up sha256 hash and compare indiv block results

We have a h/w accelerator in our SoC and it can do sha256. The restrictions are it's input descriptors (which defines the input packet and len) must not straddle 64K boundary. When it does we can ...
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Compare two hashes with different salt

I'm storing the salted hash of a credit card number in a database. What I'd like to be able to do is determine if two different entries in the same database correspond to the same credit card. ...
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1answer
226 views

why hash(hash(str)) is bad?

Why do people say that using hash twice or more is bad? yes there is key derivation that do that job, but in a low bandwidth network countries, the solution i see is to use a simple hashing system on ...
10
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1answer
493 views

Hash function from narrower block cipher operated in CBC-encryption mode?

I am trying to build a public hash function (thus collision-resistant and preimage-resistant, and more generally behaving like a random oracle), with input a message $M$ of fixed size $|M|=m\cdot b$ ...
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2answers
138 views

What prevents continued hashing of a key from being used as a cipher when xored with plaintext? [duplicate]

This is just an idea I had while looking through some threads so take it as you will. Given a plaintext of any length and a key that a specific hash function is capable of taking as input(I mention ...
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1answer
237 views

Length of data to hash for PGP

I have finally managed to verify some simple PGP signed message blocks. However, I discovered that for some reason, my implementation limits me to verifying data that is 9-16 bytes long. no less. no ...
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1answer
110 views

Best hash size for hash trees

I want to know if having a 8-byte hash value is better than 16-byte hash value for use within hash trees. My mind tells me that the shorter the hash value, the better. But doesn't a smaller hash ...
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3answers
189 views

Hash Family Example

I am reading my lecture and I found this definition: Let be $F$ be a family of hash functions, that is, a parameterized set $$F=\{g_k:\{0,1\}^{*}\rightarrow\{0,1\}^{n}|k\in K\},$$ where $n\in \...
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1answer
269 views

Tor's integrity verification system?

I am trying to understand the design of Tor(onion router). I am reading the original paper on the 2nd generation onion routing system. Where, under integrity checking on streams, they say, "When Alice ...
5
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1answer
334 views

md5: is reverse length-extension attack possible?

If I know H = md5( SECRET_KEY | DATA ) then I can calculate H' = md5( SECRET_KEY | DATA | DATA' ) That's length-extension ...
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2answers
247 views

Digital Signature Scheme Count

My question is How I will be able to count digital signature Scheme that can sign many documents with one private key. There are signature scheme as long as exists private keys in the keys spaces? or ...
11
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1answer
417 views

Which MACs can be converted into a secure unkeyed hash function?

It is known that setting the secret key to a fixed, public value does not make MACs like CBC-MAC or GMAC into secure unkeyed cryptographic hash functions that could be used - for instance - for ...
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3k views

Difference between “one-way function” and “cryptographic hash function”

What's the difference between a one-way function and a cryptographic hash function?
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171 views

Hash function with values in a multiplicative group of prime order [closed]

I have to implement a cryptographic protocol which involves a cryptographic hash function $H: \{0,1\}^* \to G$. It is viewed as random oracle. $G$ is a multiplicative group of prime order. I want to ...
22
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understanding a length extension attack

I have been trying to understand exactly how a length extension attack works on SHA-1. I'll detail below what I've understood so far, so that I can convey my understanding of the same and hopefully ...
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1answer
301 views

Is there a problem with constructing a CSPRNG using MD5 or SHA1?

Is there any security concerns with building a CSPRNG using a broken hash function like MD5 or SHA1? The design is such that a CRC-like function is used for mixing entropy and MD5 is used as the ...
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552 views

Why does HOTP use such a complex truncate function?

In the HOTP protocol after calculating a 20 byte hash it is truncated to 4 bytes. For this first an offset is calculated (low-order 4 bits of the last byte) which determines the four bytes to be ...
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275 views

Preimage resistance hash in digital signature

I'm studying about preimage resistance property of the hash functions. In particularly I'm reading as the missing of this property can be fatal in digital signatures that use RSA. Further details: ...