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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What is the shortest fingerprint of a Curve25519 public key that is secure [duplicate]

My clients have to exchange public keys. To ensure security they are exchanging fingerprints and typing them into the computer by hands. I want the fingerprints to be as short as possible. I am ...
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36k views

How is SHA1 different from MD5?

On the surface, SHA1 and MD5 look pretty similar. Their diagrams include chunks of bits, bit rotation, xor and special functions. Their implementations are roughly the same length (at least the ones ...
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Hash functions vs Stream Ciphers in terms of Speed

We know that it's possible to create a hash using a stream cipher (for instance, RC4 or AES in Counter Mode). As stated here: "A Pseudo Random Generator (PRG) and a hash function are both Pseudo ...
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311 views

Difference between actual attacks and theoretical attacks on SHA cryptographic series

Could anyone tell me what the difference is between a theoretical attack (Like the one done on SHA-1) and an actual, practical attack (Like the one done on SHA-0)? Is a theoretical attack a proposed ...
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779 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 ...
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NTRUSign Hashing

I have some trouble understanding the inner workings of NTRUSign. So what I have understood so far is, that when Alice wants to sign a message she does the following steps Convert Alice's message ...
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1answer
67 views

Fast Forward Hash Signatures

As a follow up to the Q/A session: Small Quantum Signatures - Reality check needed I have implemented the discussed idea for a new short-quantum-safe signature system using fast-forwarding hash. It ...
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335 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 ...
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203 views

Why Merkle Tree doesn't suffer Birthday attack?

I am trying to understand why Merkle Tree doesn't suffer the birthday attack? Can you help me? Thank you!
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1answer
87 views

Unconditionally Secure Authentication

I'm trying to add the authentication for my One-Time Pad implementation. I know that to provide Unconditionally Secure Authentication I need to use the One-Time MAC authentication. But I don't ...
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2answers
85 views

Probability approximation of no collisions

I am comparing the actual probability of no collisions to the probability approximation formula of no collisions from the Understanding Cryptography text. The approximation is as follows: ...
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Can knowing part of what was used to generate an md5 hash make it vulnerable? [duplicate]

Say I am able to ask a black box MD5 generator to generate 1,000,000 hashes given input $STRING$ Where $STRING$ is an arbitrary ...
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74 views

Does including the size of the hashed data improve security over just the hash

Trying to settle a discussion at work. We are mostly concerned with pre-image resistance. Suppose we have a common secure hash function $H$ (e.g. SHA-1/SHA-2) and some message $M$ we want to hash. ...
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Does OEAP padding limit the size of my modulous to the size fo the hash function?

im looking at wiki at the moment as how to use oeap padding and it appears, help me out if im wrong as G(r) - expands r to the size of the padded message, I am then limited by the size of my hash ...
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1answer
64 views

Cryptographic hash with ability to ignore specific bits based on a hidden pattern?

I'm looking to use password-style hashing to validate an environment, where some parts may be irrelevant. For example: ...
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1answer
77 views

Which hash function is usually used in RSA?

I know there are several available, however I can't seem to find whether there is one standard one or not. I think I understand the process of digital signatures, just wondering which has function is ...
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1answer
62 views

Rogue CA Certificate MD5 Collision Control

Papers from Marc Stevens et al show that X509 certificates that rely on MD5 signatures can break the assumed security model, that is, it allows for a forged certificate to have the same signature as a ...
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1answer
173 views

Backdoorable hash function in password database

I want to use a (preferably established) hash function that is backdoorable in a password database. I want to do so because I need password escrow (being able to get the cleartext back. I want to ...
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66 views

Memory hardness of key derivation function through XOR-ring multiple matrix values

For theoretical purposes in order to enhance my own understanding, and NOT in order to create my own cryptography, I am asking a question about the memory-hardness of a key derivation function ...
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Reversible hash function

I met the following 'reversible hash' function applied on a data $d$ consisting of $d_1, \ldots, d_n$ words: $h_0(d) = 0, h_i(d) = c ∗ (d_i + h_{i−1}(d))$, for $0 < i \leq n$. $c$ is a small, odd ...
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173 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 ...
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3answers
148 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 ...
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1answer
75 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 ...
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argon2 versus bcrypt/PBKDF2 password hashing security gain

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 ...
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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 ...
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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 ...
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230 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 ...
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144 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 ...
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1answer
42 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 ...
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3answers
204 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 ...
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1answer
59 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 ...
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228 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 ...
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2answers
172 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: ...
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1answer
107 views

Hash and salt or salt and hash?

People say "hash and salt", but aren't you supposed to salt password the first, and then hash it? So that means its called "salt and hash" right?
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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 ...
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3answers
688 views

Computational feasible to reverse MD5SUM?

This might be out of ignorance, I apologize, but how complex of a problem might it be to generate a file of size $N$ whose MD5SUM is $X$? For example, ...
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2answers
81 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 ...
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3answers
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Can I use a key-derivation-function as the hash function H in SRP?

In the Secure Remote Password Protocol, the verifier must be stored on the server. In the case of a server compromise, an attacker could obtain these verifiers. If nobody reused passwords, this ...
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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} ...
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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
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3answers
197 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 ...
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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 ≥ ...
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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 ...
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1answer
103 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 ...
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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 ...
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1answer
203 views

Hashing passwords

The common way to store passwords in web applications is this form: $$hash(password||salt)$$ Does it make sense to store them in the following form instead:? ...
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1answer
135 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 ...
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1answer
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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 ...
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1answer
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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 | ...
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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 ...