# Tagged Questions

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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### Why hash function are used in digital signatures? [duplicate]

In the lectures I see, hash functions are always used when we want to sign a document $d$. My question is why they are used in digital signatures? Maybe, because it is more easy and fast to calculate ...
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I'll define an example hash-based stream cipher as such: $H: \{0,1\}^* \rightarrow \{0,1\}^{256}$ is the hash function. $K=\{0,1\}^{256}$ is the secret key. $N=\{0,1\}^{128}$ is the nonce. $C=\{0,1\}... 2answers 65 views ### A bijective hash function Suppose:$H: \{0,1\}^{n} \rightarrow \{0,1\}^{n}$.$H$is bijective. It is difficult to derive$x$from$H(x)$. Is this type of function possible? What would the strength of it be? I realize that ... 1answer 77 views ### Reducing the probability that a password will be guessed In a file users' password hashes are stored. There are 2 approaches to reduce the probability that the password is guessed. Increase the size of the salt from 12 bits to 24 bits. Increase the length ... 1answer 32 views ### Encryption strength when using a PBKDF Suppose:$m$is a positive integer.$n=2 \times m$.$H: \{0,1\}^{*} \rightarrow \{0,1\}^n$.$p$is a password.$s$is a salt.$K=\{0,1\}^n$is the derived key.$PBKDF(p,s)$derives a key from$p$... 3answers 546 views ### How to reverse this hash function? I have a function that takes an$m$byte inputs$x_i$and maps it a 32 byte outputs$y_j$. The hash function is defined as: $$y_j = \sum_{i=1}^{m} (x_i)^{i-1} \pmod {127}$$ The input is restricted ... 2answers 120 views ### Understanding Hash function notations I am implementing a Certificateless Cryptography Algorithm by referencing the paper An Efficient Certificateless Encryption for Secure Data Sharing in Public Clouds. In this paper on page no. 4 during ... 1answer 367 views ### Does salt size affect password hash security? I use PBKDF2-SHA512 with an iteration count of 128,000 to hash my passwords. I use a CSPRNG to generate a salt per password. However, I am unsure about the ideal size of the salt. I have read a lot of ... 0answers 32 views ### Increase in security with increase in size of salt [duplicate] Does increasing or doubling the size of salt result in improved security? It will require some extra time to brute force the passwords, but any added advantage? 3answers 74 views ### Are there any hash functions that use integers of arbitrary bit size (e.g., int63)? More precisely, I am looking for hash functions where the internal state can be decomposed as a vector of integers of size 63. That is, machine integers where one of the bits is fixed and unusable. 2answers 169 views ### What may be causing conflicts in Whirlpool implementation versions? I see by layman articles (e.g. at Wikipedia) that the Whirlpool hash function has seemingly three current versions. That is to say, they are referenced as ... 1answer 273 views ### Non-iterative cryptographic hash functions Consider the following cryptographic hash function$H$which maps a message$m$of variable size to$b$bits: $$H:\{0,1\}^{*} \mapsto \{0,1\}^b$$ $$y = H(m) = SPRP(IV||m||padding)\mid_{b}$$ , where: ... 1answer 102 views ### Large Volumes of Asymmetric Encryption First of all, I apologize in advance if I use any incorrect terminology. I am just getting my feet wet when it comes to cryptography and there is a lot to learn. For the past 12 months we have been ... 1answer 54 views ### Do I need to use null bytes and \x80 in hash length extension attack? Say I want to perform a hash length extension attack. All the tools I have used use \x00 and \x80 to pad hashes (hlextend and hashpump). But is it necessary to use these bytes? Based on my ... 2answers 314 views ### Inserting a backdoor into a cryptographic hash function I'm curious as to how a backdoor can be built into a hash function. What might it take the form of? What's probability of getting one that can be backdoored in a way that can actually undermine its ... 0answers 29 views ### Composing hash functions Suppose there is a cryptographic hash function$H : \{0,1\}^* \to \mathbb G$. Now, we want to define a hash function$H' : \{0,1\}^* \to \mathbb G \times \mathbb G $. My first idea is to define$H'$... 1answer 38 views ### Rainbow Table - Number of Chains and Chain Length Is there a difference between having 10,000 of chains (with 1,000 chain length) and 10,000 chain length (with 1,000 of chains)? How does it affect the success rate of the rainbow table? I understand ... 1answer 55 views ### can anyone explain inversion per byte? I am designing the truncating block of SHA3 algorithm , there its given that first we need to do the inversion per byte of the 1600 bit output and then truncating it to give 512 bits output. Can any ... 0answers 24 views ### Filling in MD5 input variables I have a majority of an input string(as well as the total length) and the output for an MD5 hash. Is there a way to calculate the remaining input bytes that I am missing? 1answer 170 views ### Is$H'(M) = 5M + 9H(M) \pmod{2^n}$a secure hash function? theorem: For arbitrary$i≥0$there exists$m,n≥0$such that$35+i=5m+9n$. We can use this theorem for constructing simple hash function. Let$M$be message and$H(M)$be its hashed value with hash ... 1answer 138 views ### How can I figure out a hash algorithm? This may be trivial, and may be impossible... this is outside my field but willing to learn. I have a hardware device that accepts a network MAC address (6 bytes of data) like this: 38-60-77-EC-1D-33 ... 1answer 51 views ### What are fixed points and length extension attack in hash functions? I was recently reading about hash functions, and I came across fixed points (one or more fixed points, interested in both) and length extensions, but I didn't exactly understand how these work. If ... 1answer 177 views ### Provable security of cryptographic hash functions I am working on the following exercise question: Consider the following construction of a “keyed” hash function from Katz & Lindell (ex. 7.22 (1st ed.)/ 8.21(2nd ed.)). Gen : On input$1^...
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I'm studying mechanisms of integrity and authentication in symmetric encryption scenarios. I want to propose some examples to see whether I got the point here: Let $m$ be the message, $c$ the ...
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### What's the safest TrueCrypt algorithm and hash?

I have an encrypted file container inside my NTFS pendrive. The root folder Write permission is blocked for everyone. A folder inside it has all permissions unblocked. The dummy file of TrueCrypt is ...
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### 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 ...
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### Weak and Strong Collision Resistance, again

Weak collision resistance (CR), or second-preimage resistance, is the property that given $x$ and $h(x)$ ($h$ a hash function) it's difficult to find $x' \neq x$ such that $h(x') = h(x)$. Strong CR, ...
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### how to verify subset of hashes from hash list

I need to verify a couple hashed (SHA256) integers from a list of approximately 50 hashed integers in a message. The list can be divided into groups. The (plain) integer data is not in the message, ...
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### Is the concept of provably secure hash the same as entropy smoothing hash functions?

Is the concept of provably secure hash the same as entropy smoothing hash functions? In the tutorial Sequences of Games: A Tool for Taming Complexity in Security Proofs V. Shoup shows us a proof of ...
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### “Security” of SHA functions (Wikipedia), what does it mean?

Wikepedia's table Comparison of SHA functions mentions "Security(bits)" for some SHA functions. From the ratio (Output size (bits): Security(bits)), I feel it is something like "collision resistance". ...
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### Generate hash value using public key along with message

I was trying to understand one way signature chaining on message by Saxena and Soh. As the message is passed from one user to other, every user can combine their signature to create one single ...
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### Does encrypting with MGF1/SHA-512/1024-bit seed equal to a 1024-bit key block cipher?

Suppose: Alice uses SHA-512 with MGF1(as in PKCS#1 V2) and a 1024-bit random seed to generate a mask, XORs the mask with a message(M), gets a cipher text (CT), and sends the CT to her old friend ...
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### Why is the compression function in Miyaguchi-Preneel scheme secure?

I was reading about the Miyaguchi-Preneel scheme and had difficulty in understanding why the compression function, $h(H,m)=E(H, H \oplus M) \oplus M \oplus H$ can be called secure. The only resource I ...
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### Is there an algorithm that allows verification that 2 encrypted or hashed bits of data are the same, given that I may only know half of the key/ salt?

We have an interesting problem. Currently, we store a salted hash of some secret key known only to the clients of our clients to allow us (and our clients) to find other records that were generated ...
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### Is truncating a SHA512 hash to the first 160 bits as secure as using SHA1?

I am from a web development background (I don't know an awful lot about cryptography or how the algorithms themselves work), so I am asking this question in simple terms. Consider a hash of the word '...
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### Decrypting SHA-256 with SHA-256 as input?

Is there a way to somewhat decrypt a SHA-256 hash, provided that the initial string was also a SHA-256 hash? If not, could anyone link me to a rainbow table containing some of the $16^{64}$ possible ...