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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How to figure out the size of the output of a cryptographic checksum function

There is such a putting in Bishop's Introduction to computer security, page 118 as following: Definition 8–2. A cryptographic checksum function (also called a strong hash function or a strong ...
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Forward Secrecy with pseudorandom functions

Let $H_1$, $H_2$ be keyed hash functions (e.g. $H_i(x) = SHA_{256}(s_i||x)$ for pseudorandom $s_1$, $s_2$). Let $s_n = H_1^k(s_0)$, $k_n = H_2(s_n)$, where $s_0$ is a secret (pseudorandomly chosen ...
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advantages of hashing over elliptic curve signatures for a proof of work protocol

I'm trying to create a proof-of-work protocol for a proof-of-concept software, and it's basically something like this: ...
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How to guarantee every plaintext is in a rainbow table?

This guide says that for a rainbow table or pre-computed hash chain, you can only get a probability that a plaintext is in a table, not a guarantee. Is there a way to guarantee every possible ...
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43 views

Advantages of using intermediate hash over full hash in digital signature application

Description of intermediate hashes Intermediate (or partial) hashes are canonical forms of digest state that can be transferred from one hash implementation to another, so that the other, limited ...
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P = NP security implications [duplicate]

If someone finds an efficient way (ridiculous or not) to turn NP into P (such as creating multiple universes, saving the one with the correct answer), would we have any good algorithms for encryption, ...
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69 views

Can you determine an unknown value when it is combined with a known value and you are given the resulting hash?

Say I have a string that I want to keep hidden. You can then enter a known string which is added to the end of the hidden string and you are given the sha256 hash of the combined string. If you were ...
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32 views

Bit strength of combining hash functions? [duplicate]

Suppose there are hash functions H1 and H2 with bit strength of H1 and H2 is n1 and n2. A new good hash function is generated by combining H1 and H2. What will be the bit strength of these combined ...
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66 views

How to make a crypto-currency robust to 51% attacks?

For Example: In Bitcoin it is theoretically possible to double spend if you control > 50% of the computational power in the network. Also it is possible to apply "selfish mining" with far less than ...
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In Merkle Damgard Constructoin, why do we add another block with value of number of blocks? [duplicate]

I'm trying to understand how can we find collisions, in simpler Merkle-Damgard constructions. consider my question as, what are the collisions for these cases of Merkle-Damgard constructions: we pad ...
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359 views

Would it be possible to create a game where one would need to decrypt a simple encryption?

This might sound pretty weird, but I was thinking of a game a person could play alone with a pen and paper. I got this idea where the person would write a random four letter string on a paper, and ...
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If H(m) = 0 for some m, how can a DSA signature be forged?

If we know for some message $m$ that $H(m) = 0$, how can we forge a DSA signature with only the public key? I got that $g^s*r = (g^x)^r$ where $x$ is the private key, but that's one equation with 2 ...
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43 views

Merkle-Damgard transform: necessity of the Length block [duplicate]

The Merkel-Damgard transform append the message length (in blocks) to the message itself and then computing the 'chained hash' algorithm. Does the last block (which contains the length of the ...
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112 views

Why is appending the key to a mesage and then hashing that insecure if the hash isn't weakly collision resistant? [duplicate]

Suppose I have H(M|K) and that H is not weakly collision resistant. If I have a message mac pair (M,MAC), how is it possible to find another message mac pair (M2,MAC2)? My thinking for this problem is ...
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64 views

Strength of Combining Hash functions [duplicate]

If I combine two hash functions, what will the impact on the strength of the resulting function. If I combine in following way: H1*H2 (multiply) H1 + H2 (concat) H1 Xor H2 H1 (H2) EDIT: Lets say H1 ...
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58 views

What is the exact purpose of length padding in Merkle–Damgård hash functions? [duplicate]

Is a length padding technique in a hash function used to avoid length extension attacks ?
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Could I use an hashing function for Symmetric encryption? [duplicate]

I know there are way better and more tested encryption methods, but I was wondering if the following code could be use for symmetric encryption? ...
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68 views

Base64 for a hash algorithm [closed]

May be a silly question, but I am really curious. If a hash algorithm uses Base64 in the process of hashing a string for example, it is still considered a hash algorithm, even though it uses an ...
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313 views

Recovering SHA1 knowing 2/3 of the hash generated

I am doing some research on cryptography, so I generated a random string of numbers resembling a creditcard number(according to MOD10 validation) and encrypted it with SHA1 algorithm for encryption. ...
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60 views

What is the translation for “hash” in french? [closed]

I am doing a French presentation on basic cryptography, and the generation of random numbers. Most terms are easily translatable, but I can't seem to find a word for hash, as in cryptographic hashing, ...
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62 views

Keccak and compression functions

Keccak follows a sponge construction. Can we say that Keccak employs a compression function? Generally speaking, for sponge constructions, can we say that there is an underlying compression function? ...
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What's wrong with this “order-preserving MAC” function?

Please note: this is purely a thought experiment and not intended for any real-world usage! I came up with a simple function $\mathrm{OPF}$ to map the integers $[0,C)$ (where $C$ is the "ChunkSize") ...
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141 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 ...
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2answers
207 views

What are alternatives to the random oracle model for modelling hash functions? [closed]

I was looking for more realistic alternatives to the ROM for describing hash functions in theoretical proofs. I came across the common reference string model (where hash functions can be modeled as ...
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Cryptographic Primitive Method

Is there any cryptographic primitive bijective (one-to-one and onto) function for creating cryptographic tools like symmetric encryption/decryption, Hash code generator, MAC, HMAC and Random number ...
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Hash function and indifferentiability [closed]

What are the properties derived from the fact that a hash function is indifferentiable from a random oracle?
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125 views

How Does Progressive Hashing Work?

Example: http://jsfiddle.net/KGtUE/ https://code.google.com/p/crypto-js/#Progressive_Hashing sha256.update("abc"); sha256.update("def"); sha256.update("ghi"); ...
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149 views

Reverse engineering a single checksum byte [closed]

I am working to understand a system which saves a few bytes of data together with what appears to be a single checksum byte that provides a very naive guarantee that the data has not been tampered ...
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1answer
243 views

How can I convert numbers into prime numbers?

I'm working with one-way accumulators, but I'm not knowledgable in cryptography. Is there an easy peasy way to hash numbers (or whatever) into prime numbers? Obviously I'd like it to be collision ...
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148 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 ...
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1answer
75 views

Adding “salt” to “doHash” function in Java [closed]

For one of my university projects I am being asked to "Improve the code to satisfy the pre-image resistance requirement." Basically the program is a basic encryption server system, and when I type a ...
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Is size Q equal to size SHA(Q)? [closed]

Assume d is a 128 bit random integer and P is base point of an elliptic curve and Q = dP is a point on the elliptic curve and SHA is a hash function with 128 bit output, my question is: Is size Q ...
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How many possible input files are there who's length can be stored in 63-bits?

I'm trying to calculate how many possible combinations of files there can be using a signed 64-bit file length, but can't seem to find a formula (or I'm using the wrong keywords). For example, the ...
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Python KDF - Master Key and Derivatives

The encrypted file can be opened by following combinations (either any 2 DK, or 1 MK): DK1 + DK2 DK1 + DK3 DK2 + DK3 DK1 + DK2 + DK3 MK Any heads-up with Python ...
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1answer
88 views

Is XORing a SHA256 better than truncating it? [duplicate]

Let say I want to truncate SHA256 to 128 bits. What would be the best way to minimize a probability of a collision and improve collision resistance? Taking the last 16 bytes Taking the first 16 ...
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Is there an encryption that is only reversible with a key?

I'm thinking about man-in-the-middle attacks during a website login, and started wondering about reversible encryption. What I'm imagining is an encryption that takes input ...