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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BLAKE2 input parameters

Being a newbie, I was reading papers on the recent crop of hash functions and BLAKE2 intrigued me. Then I wanted to play around with the "blake2s" code in the code package. If I were to implement a ...
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PGP String to Key specifiers

I've been reading through the PGP Standard and here I'm a little confused. This section is discussing converting string data to a session key. I'm confused about the paragraph in bold. First off, what ...
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Why is C_digest defined in PKCS#11?

Digesting doesn't require any key and can be performed on the client side. Why does the PKCS#11 token interface standard have a C_digest method? For example, why should I send my data to a PKCS11 HSM ...
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How to choose an algorithm

I have some understanding of Cryptography and I would like to know if there is a tool or website to help me choose an algorithm based on my own needs (I'm being vague in purpose). I'm looking for a ...
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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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56 views

Hashes and Ciphers [on hold]

So I'm brand new to cryptography, I'm confused on a few fundamental issues, and my normal avenues of research aren't bearing any fruit. I'm hoping someone can describe the relationship between ...
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Why are the outputs of the md5sum tool and Crypto++'s MD5 different?

Could you please tell me what is the difference of coreutils' md5sum and sha*sum tools (...
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4answers
12k views

Can you help me understand what a cryptographic “salt” is?

I'm a beginner to cryptography and looking to understand in very simple terms what a cryptographic "salt" is, when I might need to use it, and why I should or should not use it. Can anyone offer me a ...
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2answers
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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 ...
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Calculating hash of hash in C [closed]

I've been working with Openssl libcrypto for a long time and I'm quite familiar with it but now I've got a problem I cannot solve in my own, so I need your help. I'm creating a hash of a short string ...
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63 views

Reducing size of hash function

If I have some data I hash with SHA256 as hash=SHA256(data) and then copy only the first 8 bytes of the hash instead of the whole 32 bytes. How easy is it to find a ...
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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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67 views

How to implement the Salsa20 hash function?

I am currently implementing Salsa20 from the specification as an exercise in learning and self-flagellation. I have Sections 1-7 passing the test examples provided. I am now stuck on Section 8, The ...
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Full Domain hash library

I need to know if any of the well-known crypto libraries (openssl, crypto++) supports full domain hash function. If not, what library I can use?
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91 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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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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3answers
626 views

Can I jettison MAC if I already have SHA1(M)?

I'm currently using SSL with AES-CBC and HMAC for a file transfer containing string M. Now suppose Alice already knows SHA1(M) (and the adversary does not), and she downloads M from Bob using only ...
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670 views

Good enough deterministic PRNG based on hashes

Lets say I have a seed named iv which has a decent amount of entropy. Are there any glaring issues with the following? First 64 bytes are generated by SHA512(iv) Next 64 bytes are generated by ...
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Possible hashing scheme

Plaintext(binary) : 10101100 //very short for easy understanding Suppose we have 4 predefined schemes/functions, rotate (101 -> 011) flip (1101 -> 1011) exchange (11011 -> 10111) stretch (1100 -> ...
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Anonymous Gravatar Problem

Gravatar is a service that serves avatars for users based on md5(email_address). The hash is embedded on websites, so it's public. One of the problems with this ...
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How does SafeNet MobilePASS generate passwords? (TOTP Variant)

This question is similar to How does SafeNet MobilePASS generate passwords? The question and answer there is related to the HMAC Variant of SafeNet Mobilepass. I'm looking for details on the TOTP ...
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44 views

Hashing - Digital Signing and Trivial Stretching

Two very basic questions: 1. Digital Signing Why is it not possible to just simply replace the hash value from the request? Examples: ...
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Can a tree hash with leaf-size of 1 byte support verification of content with elided ranges, at a cost of at most 2 supplied internal-node hashes?

Perhaps someone can confirm my intuition: A tree hash with leaves of size 1 (for example Blake2 in a tree-hash mode) would of course not be very efficient. But it seems that it might be a suitable ...
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1answer
62 views

Save the last cycle in GCM GHASH calculation

I need to implement GCM GHASH in hardware, using FPGA. The data bus is 640 bits, so I will use 5 adder/multiplier blocks in parallel. The message size is fixed and AAD is 0. A little example: ...
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95 views

Hash function as secure as one-time pad?

We know that the one-time pad is provably secure as a cipher to encrypt some data. Is there an algorithm which does the same just as a hash function? Can we get a provably secure hash function? Maybe ...
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221 views

How hard is it to find plaintexts whose hashes satisfy $h(a)\oplus h(b)=h(c)$?

Given a cryptographic hash function $h$, for example SHA256, how hard is it to find plaintexts $a,b,c$ such that $$h(a)\oplus h(b)=h(c) \text?$$
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167 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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149 views

Encrypting with Davies-Meyer hash function scheme

So I need to to illustrate Davies-Meyer hash function for a message “123” with 1-byte blocks. I can use the extended ASCII representation where each character is represented as a byte. And I can pick ...
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Are there two known strings which have the same MD5 hash value?

Is there an example of two known strings which have the same MD5 hash value (representing a so-called "MD5 collision")?
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SHA1 Partial Collision

I'm working on a little python script that reads in two fairly similar English sentences (they're in text files - reading them in) and I'm trying to force a SHA1 partial collision (e.g. first 32 bits ...
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Chosen-prefix collision attack on 24 bit truncated sha-256

I need to answer this question for an exercise: I have a file on which a sha-256 hash is calculated, but only the first 24 bits of the hash are considered. I want to find another file which results ...
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How does BLAKE2 ensure that hash(A) != hash(B) when B = A||0 and both A & B have the same number of blocks?

Suppose I have two arrays : A = a single byte, being zero. B = two bytes, both being zero. B = A||0 (i.e. B starts with A, and differs only by appending a zero byte) BLAKE2(A) != BLAKE2(B) Yet, ...
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How does RSA padding work exactly?

I've read everywhere online and people say plain text RSA is very unsafe. To make it safe you pad it but no examples are shown anywhere on how to do it. It's explained that random data is added to the ...
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How much work is required to detect multiple collisions for a hash function?

Assume an ideal hash function of output size n bits, detecting one collision requires approximately 2^(n/2) evaluations of the ...
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HMAC-Ripemd-160 in TrueCrypt

I'm wondering if it is still secure. I know TrueCrypt closed its doors but I continue using it because the cryptoaudit didn't find any huge bugs or security issues in TrueCrypt. But I have a bad ...
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Hash which can be used to verify one of multiple inputs?

Is there a hash function H1 which, when given inputs a, b, etc., produces output ...
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Abstracting primitives and modes of operation

I am developing a symmetric crypto library and have reached a roadblock. Looking at block ciphers, it is quite obvious that all block ciphers are trivially abstractable as a simple primitive ...
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Help converting sha256 to 32-bit stream

I need some help with hash value calculation. I am a bit confused by this statement that I found on w3 website on calculation of a DigestValue for xmldsig ( ...
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What kind of attack does the current brokenness of SHA-1 allow?

I understand that the following is becoming feasible, or already is: Find any 2 data (d1 and d2), for which SHA1(d1) = SHA1(d2) However, it is not entirely clear ...
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Is double hashing collision resistant?

Double hashing can surely provide more security than only one layer of hashing but does that necessarily mean it is more collision resistant? This question in a more mathematical form: If $H$ is a ...
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Are there any known inverse hash lookups for the zero string?

Just wondering, are there any known inverse hashes / hash lookups in common hashes (md5, sha1, sha2, etc.) for the zero string hash value? (i.e. bytes 0x00 00 00 00 00 ...) Or more specifically, for ...
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Why are cryptographically secure hashes required for digital signing (RSA or DSA)?

I know that a cryptographically secure hashing algorithm is required for proper message signing (SHA-1, or I think SHA-2 is the current suggested algorithm). However, why does the hashing algorithm ...
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135 views

Share encrypted files without giving master key

Imagine I have four files I want to encrypt. I have master key, and from that master key I derive four keys, one for each file: ...
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Are there cryptographic hash functions which do not have any collisions?

I've been studying digital signatures and hash functions recently, and I was wondering: Do hash functions exist which will never produce collisions?
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Is there a hash function which has no collisions?

Is there a hash function which has no collisions? To clarify: it would be some function which would produce variable-length output, and never produce the same output for differing input. It would ...
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Why hash the message before signing it with RSA?

The diagram below illustrates the process of digitally signing a message with RSA: As diagram shows, the message is first hashed, and the signature is then computed on the hash, rather than on the ...
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RSA - Why should we sign hash rather than raw content? [duplicate]

Apart from obvious performance considerations, is there any mathematical or crypto reasons that imply that we sign a hash rather than a raw content ? I read that it was because the hash must fit in ...
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How to use HMAC for authenticating multiple messages

The standard HMAC function, $H(K, m)$, authenticates a single message $m$. Instead, I'd like to authenticate 2 messages, like this: $$H'(K, m_1, m_2)$$ The simplest solution, $H(K, m_1||m_2)$, ...
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Formal proof of theorem concerning the Random Oracle Model

Reading a book on cryptography by Douglas R. Stinson I've met the following theorem, which is stated without proof (see here). Thereby, $\mathcal{F^{X,Y}}$ denotes the set of all functions from ...
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What are the security implications of multiple hashing?

There are plenty of questions on this site about hashing passwords. However, none of them quite cover this topic. One of those links covers using multiple different hash algorithms; one covers general ...