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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Proving knowledge of a preimage of a hash without disclosing it?

We consider a public hash function $H$, assumed collision-resistant and preimage-resistant (for both first and second preimage), similar in construction to SHA-1 or SHA-256. Alice discloses a value ...
9
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3answers
601 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. ...
8
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2answers
2k views

Use of salt to hash a password

In a few implementations of hashed passwords, I have seen that the length of the random salt is chosen to be, say, 10 or "some constant". Is there any specific reason why the salt is chosen to have a ...
6
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4answers
505 views

Does MD5 generate 128 independent bits?

I heard that there are 128 stochastically independent bits in an MD5 output. Is that true? If so, are there any citations or proofs for that?
3
votes
1answer
195 views

Is there a format preserving cryptographically secure hash?

I want a way to 'scramble' strings (like words, names) in a predictable way but I don't ever want to (myself or anyone else) be able to 'unscramble' the strings. I found that perhaps what I am ...
2
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2answers
242 views

Why is MAC using nonce+message+hash(nonce+message+identifier) not the standard?

Given that we know that nonce + message + hash(nonce + message + mac-key) all inside the encryption makes it possible to detect if any bits of the message have been ...
2
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1answer
580 views

Why are these specific values used to initialise the hash buffer in SHA-512?

I'm reading the book Network Security Essentials written by William Stallings. To create a message digest with SHA-512, we have to go through some steps: append padding bits. append length ...
14
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2answers
918 views

Are AES-256's related-key weaknesses exploitable if it is used to build a hash?

Assume it is made a hash based on AES-256 encryption (perhaps because this is hardware-accelerated, but no standard hash is); and it is used the Merkle–Damgård structure, that is padding of the ...
9
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2answers
305 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 ...
8
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1answer
190 views

Can Skein be used as a secure MAC in format H(k || m)?

Can Skein be used as a secure MAC in format H(k || m)? I understand it can work with HMAC, but it is fine as a one-pass MAC in the format above and resists length extension attacks. Is this ...
8
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7answers
946 views

Turning a cipher into a hashing function

This is theoretical question. I'd like to know if it's possible (and what are eventually the consequences), not that I'm going to do it in one of my projects. ;) The first hashing functions created ...
7
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1answer
458 views

Why are the initial states of hashes functions (like SHA-1) often non-zero?

There is already a question asking "Why initialize SHA1 with specific buffer?" and my question follows on from this: Why are the initial states of hash functions often non-zero? For most, I have ...
7
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2answers
272 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 ...
5
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0answers
252 views

LT codes with Homomorphic hashing

I have been working on a project implementing LT codes with Homomorphic hashing (inspired from http://blog.notdot.net/2012/08/Damn-Cool-Algorithms-Homomorphic-Hashing and ...
5
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1answer
788 views

How does a “Tiger Tree Hash” handle data whose size isn't a power of two?

Constructing a hash tree is simple enough if the data fits into a number of blocks that is a power of two. ...
4
votes
2answers
2k views

Using a Hash as a secure PRNG

I was just looking at some NIST PRNG recommendations, specifically at Hash_DRBG. I read briefly through the algorithm, and even though it is not overly complex, it still seems unnecessary to me. I ...
4
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1answer
656 views

Recommended way of adding a pepper/secret key to password before hashing?

There have been several questions regarding password hashing here and on Security.SE. A "pepper" is sometimes mentioned – an application-specific secret key. The canonical answer on password hashing ...
3
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2answers
183 views

Associative standard cryptographic hash function

I am looking for a standard hash function which satisfies the following property: A hash function $H(a,b) = F(h(a),h(b))$ with $h$ (within $F$) any standard cryptographic hash function and $F$ an ...
2
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2answers
187 views

Using hash for one time pad key

Recently I've been delving into security algorithms, I already knew some of the (easy) math behind AES and RSA and how to and not to implement it. But well, i got a bit bored so i thought I'd just do ...
8
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6answers
852 views

How can I improve a password generation scheme based on a shared secret and URL?

I currently use the following method to generate a different password on every website I have to login: password = SHA1 ( mainPassword . domainName . number ) ...
6
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3answers
440 views

How many bits of hash are realistically needed for key verification?

Say I'm connecting to a web server secured with TLS but with a self-signed certificate. Accordingly, I call the owners of the server and ask them what SHA1 fingerprint they have. They start reading ...
5
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1answer
631 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, ...
4
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1answer
332 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, ...
4
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1answer
200 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. ...
4
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2answers
584 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 ...
3
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1answer
738 views

Why does Openssh use only SHA1 for signing and verifying of digital signatures?

I am learning SSH protocol. With my current understanding of SSH protocol, I think that message digest algorithms for using in digital signature should be derived from Key Exchange. But Openssh ...
3
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2answers
229 views

Parallel-resistant proof-of-work scheme?

I am looking for a proof-of-work scheme which cannot be effectively parallelized. For example, in hashcash (and by extension bitcoin) you have some collision-resistant hash function $f()$, a target ...
3
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1answer
814 views

What happens to the entropy of a password when you hash it?

For example, if the entropy of a password is 30 bits, what is the entropy of the password when you hash it with MD5?
2
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1answer
385 views

How is HKDF-Expand better than a simple hash?

Why do we need HKDF-Expand if we can simply hash the pseudorandom key to make it longer? The docs (in the link) say: The second stage "expands" the pseudorandom key to the desired length... ...
2
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1answer
63 views

Weaker Notion of Target Collision Resistance

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: While it might be easy to find a collision $M,M'$ in $F_K$ by making both $M,M'$ depend on $K$ ...
2
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1answer
76 views

Dependence on Keyed Hash Function

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states: With an ACR hash function $F$ the key $K$ is announced and the adversary wins if she manages to ...
2
votes
3answers
704 views

Salts, how does the script know what the salt is?

I am new to PHP programming and trying to grasp the idea of hashing and encryption for protecting passwords, credit card details and such. I've done a lot of reading about MD5 which I think I ...
1
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1answer
80 views

Hash of multiset of values, which lets me compute the hash of the union

Cryptographic hash functions normally take as input a bitstring. I am looking for a hash function that takes as input a finite multiset of values. In other words, given $S \subset \{0,1\}^*$, I want ...
0
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1answer
122 views

No Birthday Attack to TCR

I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” , which compared TCR (Target Collision Resistant) and ACR (Any collision Resistant). It says we wish to stress ...
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1answer
174 views

Hash Based Encryption (fast & simple), how well would this compare to AES? [duplicate]

First of all, I know it's a very bad idea to invent your own encryption algorithm. It's better to use existing known, trusted, extensively tested and studied algorithms with a proven track record. The ...
27
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1answer
13k views

What are the differences between a digital signature, a MAC and a hash?

A message may be accompanied with a digital signature, a MAC or a message hash, as a proof of some kind. Which assurances does each primitive provide to the recipient? What kind of keys are needed?
28
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1answer
1k views

How is the MD2 hash function S-table constructed from Pi?

For fun, I'm learning more about cryptography and hashing. I'm implementing the MD2 hash function following RFC 1319 (http://tools.ietf.org/html/rfc1319). I'll preface by saying I know there are ...
17
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3answers
2k views

Hashing or encrypting twice to increase security?

Over on the bitcoin forums I asked why the bitcoin client computes SHA-256(SHA-256(x)) as its cryptographic hash for a variety of purposes. The leading theory--since the bitcoin author has ...
16
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2answers
2k views

Is every output of a hash function possible?

Is every output of a hash function (e.g. SHA1, MD5, etc) guaranteed to be possible, or, conversely, are there any output values that cannot possibly be created from any input? If so, what guarantees ...
12
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3answers
6k views

What is pre-image resistance, and how can the lack thereof be exploited?

What is "preimage resistance", and how can the lack thereof be exploited? How is this different from collision resistance, and are there any known preimage attacks that would be considered feasible?
19
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9k views

Why use an Initialization Vector (IV)?

Why use an Initialization Vector (IV)? How are IV's used? What are the advantages/disadvantages of using an IV? Why use an IV instead of a longer key in which some section of the key is pubic? What ...
11
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4answers
941 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 ...
8
votes
2answers
999 views

What is a hard-core predicate?

I read this article on Wikipedia: Hard-core predicate. Still I don't understand what exactly is a hard-core predicate. Is it possible to put this in simple English terminology, and perhaps with a ...
10
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1answer
587 views

Is it possible to actually verify a “sponge function” security claim?

When using a “sponge function” to create a cryptographic hash, we can look at the flat sponge claim, which flattens the claimed success probabilities of all attacks using a single parameter: the ...
9
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2answers
717 views

Is bcrypt better than GnupPG's iterated+salted hashing method?

GnuPG has slow hash built-in in form of iterated+salted S2K. Does it have disadvantages in comparance with bcrypt or scrypt? Is GnuPG's slow hash method easily automated in GPUs?
6
votes
2answers
823 views

Hash decrypts key, key decrypts cipher… why?

I noticed recently that a couple of pieces of encryption software (TrueCrypt being one of them) don't directly use a hash of the password as the key for the block cipher. Instead, they generate a ...
15
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2answers
619 views

What is the general justification for the hardness of finding preimages for cryptographic hash functions?

Since most cryptographic hash functions are simple, compact constructions does this simplicity impose a limit on the complexity and the size of a function that can generate preimages? That is, given a ...
14
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Are NIST's changes to Keccak/SHA-3 problematic?

NIST is working on standardizing SHA-3. They have selected Keccak as the basis for SHA-3, and they plan to make some small changes to it; the result (with NIST's changes) will be standardized as ...
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4answers
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What security do Cryptographic Sponges offer against generic quantum attacks?

In the face of non-quantum attacker, Keccak[r=1088,c=512] with 512 bits of output provides: Collision resistance up to $2^{256}$ operations Preimage resistance up to $2^{256}$ operations Second ...
8
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1answer
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HMAC-SHA1 vs HMAC-SHA256

I have three questions: Would you use HMAC-SHA1 or HMAC-SHA256 for message authentication? How much HMAC-SHA256 is slower than HMAC-SHA1? Are the security improvements of SHA256 (over SHA1) enough ...