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.

learn more… | top users | synonyms (2)

4
votes
2answers
198 views

Public key crypto instead of hashing passwords in a database

Suppose I want to keep a client's password database secure (not in plaintext), while still having access to the plaintext password. Now I generates a public/private keypair. Then I send the public ...
1
vote
1answer
81 views

Taking half of a X bits hash key as secure as using a X/2 bit hash function?

This question comes from a (nitpicking) comment by me in this SO question. I said that using the half of the output from a 256 bit hash function is worse than using a 128 bit hash function. My ...
11
votes
1answer
1k views

Password hashing security of argon2 versus bcrypt/PBKDF2?

I wonder if it can be approximated how much of a security margin the new argon2 hash, winner of the password hashing competition, can give over bcrypt or PBKDF2, for an attacker using large GPU ...
1
vote
1answer
407 views

Decoding base64 encoded message with a key

I am pentesting a database that where I found data which was base64 encoded. I tried the usual method of decoding base64, which didn't work. Later, I found in ...
3
votes
3answers
153 views

Memory hard key derivation (password hash) using AES encryption

I am restricted on a certain environment involving PHP and am currently unable to implement new memory hard hashes such as scrypt (and I am not trying to compete with the likes of scrypt). My ...
2
votes
2answers
260 views

Keyless integrity checking with SHA-256

Currently a program is loading some files from an untrustworthy source (e.g. a CDN) which could have been tampered with. It has a known SHA-256 hash of the file stored locally, then it downloads the ...
1
vote
2answers
347 views

Is there a scheme where two cipher texts can be proven to have the same (unencrypted) content?

Is there a scheme where two ciphertexts (encrypted with different keys) can be proven to have the same (plaintext) content without disclosing the plaintext or any private keys? I assume that the ...
1
vote
1answer
166 views

Proving collision resistance of a hash function?

Let $f$, $g$, and $h$ be hash functions that each map binary strings of length $2n$ to binary strings of length $n$. Suppose that $h(x) = f(g(x)||g(x))$. Prove that if $f$ and $g$ are collision ...
3
votes
1answer
64 views

How is a group element converted into a key?

I've only just started research on cryptography so I apologize if this is a basic question or I'm getting terms confused. I'm researching braid group cryptography and currently looking at the Anshel-...
2
votes
1answer
43 views

Is a hash “reset sequence” possible?

Is there such thing as "hash reset sequence" that brings hash to the same known state? A hash function has a state. In MD5 and others vulnerable to the length extension attack, the attack works by ...
0
votes
2answers
148 views

How does a Hashing algorithm always result in a digest in a fixed size?

This question revolves around the Hashing Algorithm attribute that the resulting digest is always the same size (also known as Fixed Width). For example, whether you hash the word "hello", or 10gb ...
3
votes
2answers
262 views

What is the use case for XOF functions (i.e. SHAKE128/256)?

FIPS 202 defines 2 functions: SHAKE128 SHAKE256 As extendable-output functions (XOFs) that can have variable output length. But in Appendix A.2 marks: it is possible to use an XOF as a hash ...
3
votes
2answers
175 views

Is there any relation between two strings with the same MD5 hash?

Is there any relation between two strings with the same MD5 hash? For example these two strings: ...
3
votes
3answers
215 views

Does complicating a flawed algorithm make it secure?

It's a basic question I guess, but I don't know the answer and I don't think that anyone asked it. Suppose I built a system, it relies on MD5 for security, suddenly I read am article on how easy are ...
1
vote
1answer
53 views

How many Hashes to find a digest of 2^{n−k}?

Given an n-bit cryptographic hash function $H$, how many messages should we expect to hash before finding a message $x$ such that $H(x) < 2^{n−k}$ for some integer k? My take: Shouldn't it simply ...
1
vote
1answer
45 views

Is it possible to have self-authenticated message?

Consider a text with $n$ words which $m$ words of it can be replaced with an equivalent word. Therefore we have $2^m$ different texts with the same meaning. For example "You are {so/very} {keen/eager}...
2
votes
2answers
85 views

Easy hash function collision

I'm designing a cryptography-breaking assignment for a college-level introductory security course, and I'm looking for a hash function which is reasonably easy (but not too easy) to generate ...
0
votes
0answers
19 views

Encrypted vs unencrypted digital signatures [duplicate]

Is there a difference in terms of security between An encrypted stream of bits obtained by encrypting a plaintext concatenated with it's signature $ C = E_k(plaintext||\sigma)$ where $E$ is a ...
2
votes
3answers
204 views

Hash email address with a non-random salt for moderate security?

Crypto noob here. I have an application that has only moderate security requirements (internal use, limited audience). I want to send a unique url with a token in it to each user. If anyone has the ...
21
votes
7answers
4k views

Why can't I reverse a hash to a possible input?

I'm going to provide “proof” why a hash function can be reversed, and I hope you can tell my why I'm wrong So, a hash function can be implemented as a series of logic gates. All logic gates can be ...
8
votes
0answers
95 views

Is there a standardized tree hash?

SHA-1, SHA-2, and the standardized version of SHA-3 are all sequential. This is impractical for hashing very large files distributed across machines. Any sequential hash can be straightforwardly ...
2
votes
1answer
106 views

Cracking a hash which descends to a single value after repeated hashes

Assume I have a hash function (such as the one below) and a cipher, and I know that if I repeatedly hash any string over and over again, it will eventually descend to one unique value. Is there any ...
5
votes
2answers
428 views

Why do cryptographic hashes need to be fixed length?

Why do cryptographic hashes need to have a fixed length output? I know that the shallow answer is that an output that varies by key size or file size can leak information somehow, leading to ...
1
vote
1answer
71 views

Is it safe to generate ECDSA keys from the hash of the previous, over and over again?

I have memorized a very long and secure passphrase, that when hashed with sha256, I can use the result as an ECDSA private key, and use it as a brain-address for Bitcoin. Now I need more bitcoin ...
0
votes
1answer
151 views

SHA-256 Hash over multiple SHA-256 Hashes

I am trying to implement a checksum over multiple files. We can assume, that i have a package containing N files. I want to identify such a package uniquely by following calculation: SHA256-Hash ...
2
votes
1answer
74 views

Can the salting function complexity improve the hash robustness?

I'm quite novice in security, and all examples I saw seem to salt the password by concatenating it, or by applying a simple binary OR or XOR. So the hash is based on "salt + password", "salt | ...
1
vote
0answers
112 views

MD5 hash different between sources - different hash for the same string [closed]

I've implemented the md5 source from a crypt lib from http://sourceforge.net/projects/libcrypt/ The string I'm passing to md5 is ...
2
votes
0answers
44 views

Signing integers and UUIDs and restoring them from a third party server without tampering

I'm educating myself about cryptography. I know this is a long one, but please, bear with me. :) Now I have a scenario in my mind and I'd like to ask if what I've thought is sound (enough), what more ...
2
votes
0answers
64 views

Discrete logarithms: large prime modulus vs. large semi prime modulus

I have a cryptography homework question, in the question it says a cryptographic hash function in the form of $f(x) = g^x \bmod n$ , where $n$ is a very large prime (1024bits and more), $g$ is a ...
2
votes
3answers
108 views

Proving that a node is the only child

Say I have a graph of messages like this: A | B / \ C D By including the parent hash in the message to be hashed I can show that B is a child of A, ...
3
votes
1answer
143 views

Double randomised HMAC verification to prevent timing attack

A way to prevent timing attacks for hash string comparison is to perform additional HMAC signing in order to randomize the verification process (see Double HMAC Verification). In addition to the ...
3
votes
1answer
109 views

Is md5(x)&md5(y&x) secure?

I was wondering if the following hash function f(x) := md5(x) & md5('abc'&x) (with & as concatenation operator) is secure. This schema can be even ...
1
vote
1answer
100 views

How bad is it to use the identity function as hash for ECDSA?

I was recently asked whether a certain library supports the ECDSAwithNone Signature algorithm. Clearly this would mean ECDSA with the identity function as the hash function. I know this is a really ...
1
vote
1answer
164 views

Triple DES as cryptographic hash function

I need to generate a hash for a small piece of data (~50 bytes). I'd normally use MD5 or SHA but that's not available in the platform I'm programming for. Luckily, it provides Triple DES and I ...
3
votes
2answers
136 views

Small Quantum Signatures - Reality check needed

I've been thinking a bit lately about how to get quantum resistant signatures fast and (relatively) small. One idea I've been keen on exploring is finding a crypto PRNG that allows fast-forwarding, e....
3
votes
1answer
134 views

Can the round function $F$ in a Feistel Network be practically any non-invertible function?

This question might seem silly, but how true is the fact that the round function $F$ does not have to be invertible? I am curious to know this, because non-invertible functions can be very lossy, i.e....
1
vote
1answer
243 views

Encrypt large data with RSA through blocks in java

I'm working on a communication protocol where a client sends encrypted data to a server. What happens is that in some point, the client creates a symmetric key used to create data digests through the ...
4
votes
0answers
74 views

What level of security is provided when a Feistel Cipher is used as a round function of another Feistel Cipher?

Recently, I was reading: Are there any specific requirements for the function F in a Feistel cipher?, and the answer posted mentions a Feistel Cipher named Turtle, which uses a four-round Feistal ...
0
votes
0answers
56 views

Detecting Software Changes - Verifying untampered code

This question assumes an environment devoid of any internet connections, the software in questions operates in a non-connected environment on a single machine. I have a piece of software that may be ...
4
votes
3answers
117 views

Fast forwarding hash functions

We know that a good hash function is a one way/trapdoor function. Easy to calculate one way, harder to work out a previous state. I was wondering if anyone knew a good hashing system that allowed you ...
2
votes
1answer
49 views

How to detect changes in data while preserving confidentiality (under constraints)?

Suppose party $A$ has two pieces of data, $X$ and $Y$, either of which may change separately, or they may change together. Given the following constraints: $A$ knows its current $X$ and $Y$ but has ...
0
votes
0answers
32 views

Number of qubits and breaking hashes [duplicate]

Noting D-Wave's press release on a 1000 qubits quantum computer had me wondering... Does the number of qubits nonlinearly change the speed/rate at breaking a SHA256 hash? If someone makes a say 10,...
0
votes
1answer
80 views

Why does TLS 1.2 use GCM with additional SHA hash? [duplicate]

I am studying the cipher suites provided by TLS 1.2 and found an interesting question. TLS 1.2 adopts GCM for encryption and SHA256 or SHA384 for hash (e.g. TLS_RSA_WITH_AES_128_GCM_SHA256). But, GCM ...
1
vote
1answer
318 views

SHA256: Padding a 512 bits length message

I am currently implementing the SHA256 hash-algorithm for a "custom-built" embeded-device. Obviously I have a problem with message padding. The routine I wrote does not work with message whose size ...
0
votes
2answers
88 views

The benefit of PBKDF2 for big keyfile?

The man page of cryptsetup says: Whenever a passphrase is added to a LUKS header (luksAddKey, luksFormat), the user may specify how much the time the passphrase processing should consume. The ...
3
votes
3answers
851 views

For any hash value, is there an infinite number of inputs that hash to it?

Bruce Schneier writes (back in 2005) in a post on cryptanalysis of SHA-1: SHA-1 produces a 160-bit hash. That is, every message hashes down to a 160-bit number. Given that there are an ...
2
votes
1answer
267 views

Testing hash functions for collision resistance

I was hoping to implement a software that allows testing a user-defined hash function for cryptographic properties (Meant to pique interest in cryptographic hash function for a school showcase), as ...
1
vote
2answers
161 views

MD5 Collision Attack Explained

In one of my assignments, I came across this line Due to MD5’s length-extension behavior, we can append any suffix to both messages and know that the longer messages will also collide. This lets ...
-1
votes
1answer
100 views

About end to end algorithm SMS from Jo Mehmet Sollihagen Øztarman [closed]

This is a signcryption scheme from End-to-End Data Protection of SMS Messages by Jo Mehmet Sollihagen Øztarman (pdf, section 4.3.1): Public parameters C: an elliptic curve over GF(ph) with p ≥ 2^...
4
votes
1answer
108 views

Effect of message length on collision attacks

I know that message collisions are supposed to be hard to calculate for a good cryptographic hash. Given 1k worth of data, it's hard to find another 1k worth of data that collides with the same hash. ...