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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32 views
Difference between “one-way function” and “cryptographic hash function”
What's the difference between a one-way function and a cryptographic hash function?
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2answers
45 views
Is rainbow table attack applicable to any hash?
Typically when rainbow tables are discussed MD5 hash is used as an example. It's not quite clear whether this attack is specific just to MD5 or to a certain subset of hashes or to just any hash ...
2
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1answer
68 views
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 also be computationally hard to derive the input from ...
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0answers
68 views
Hash function with values in a multiplicative group of prime order
I have to implement a cryptographic protocol which involves a cryptographic hash function $H: \{0,1\}^* \to G$. It is viewed as random oracle. $G$ is a multiplicative group of prime order.
I want to ...
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0answers
24 views
Reverse engineering from known ID inputs and 19/20-digits outputs
I am currently having few samples of a hash encryption function as per following
ID Hash
542591 6896202108404922949
3210 7693203936755907764
2455 14877541378540063734
2275 ...
2
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1answer
65 views
Why is H(message||secret_key) not vulnerable to length-extension attack?
Given a Merkle-Damgård hash function $H$, I know that an attacker can forge a message protected by a MAC computed as $H(\textrm{secret_key}||\textrm{message})$.
Why can't he perform the same ...
7
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2answers
623 views
understanding a length extension attack
I have been trying to understand exactly how a length extension attack works on SHA-1. I'll detail below what I've understood so far, so that I can convey my understanding of the same and hopefully ...
0
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1answer
64 views
Is there a problem with constructing a CSPRNG using MD5 or SHA1?
Is there any security concerns with building a CSPRNG using a broken hash function like MD5 or SHA1? The design is such that a CRC-like function is used for mixing entropy and MD5 is used as the ...
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2answers
195 views
Why does HOTP use such a complex truncate function?
In the HOTP protocol after calculating a 20 byte hash it is truncated to 4 bytes.
For this first an offset is calculated (low-order 4 bits of the last byte) which determines the four bytes to be ...
9
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2answers
1k views
Why use an Initialization Vector (IV)?
Why use an Initialization Vector (IV)?
How are IV's used?
What are the advantages/disadvantages of using an IV?
Why use an IV instead of a longer key in which some section of the key is pubic?
What ...
0
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1answer
80 views
Preimage resistance hash in digital signature
I'm studying about preimage resistance property of the hash functions.
In particularly I'm reading as the missing of this property can be fatal in digital signatures that use RSA.
Further details:
...
0
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1answer
51 views
length extension attack on an MD5 hash of a text file of about 10K Bytes?
How much computing time on a typical desktop computer would it take to find a new length and new data to extend a text file of about 10K Bytes with a given MD5 hash?
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2answers
64 views
Is it possible to anonymize web traffic so that the IP Address cannot be determined while still being able to determine distinct IPs?
I'd like to find a way to anonymize IP addresses in web logs to ensure user privacy given the following requirements:
If given an IP address, there is no way to look up which requests came from the ...
6
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2answers
113 views
When using HMAC, does key encoding matter?
Given:
H is a good hash function with block size L.
K is a key of length >= L (recommended by RFC 2104).
Khex and Kbase64 are ASCII encodings of K.
In the HMAC algorithm, is there a good reason to ...
2
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1answer
48 views
“proof of access” schemes
What is the state of the art way when implementing a scheme challenging a party to prove they have access to certain data?
What I'm looking for something along the lines of
Give them ...
1
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2answers
95 views
How to compare two datasets „anonymously”?
Ok, I hope this question makes some sense because I am not so sure how to word it any differently… Imagine the following situation:
There are 10 defined colors (blue, orange, yellow etc.)
There are ...
2
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2answers
64 views
What does “message schedule” mean in SHA-256?
I am trying to understand the sha-256 algorithm from FIPS 180-2. I understood the padding and parsing of the message string. However after that it states (page 15):
For $i = 1$ to $N$:
{
...
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2answers
88 views
Does collision resistance stay when extending a hash function to a set domain?
Given a Cryptographic hash function $h$ for element $x$, let's extend it to sets via $H(S)=\prod_{x\in{S}}{h(x)}$. I am asking if the new hash $H$ (in domain of set) is still collision resistant?
To ...
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1answer
42 views
When making public key fingerprints - is a sha1 hash still a good idea?
I'm thinking about trying to save some space (and readability) when referencing 2k and 4k public keys (millions of them) by storing the fingerprint in some places instead of the full public key.
...
3
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1answer
37 views
Distributing blocks with validation and non-dependant list generation
Problem
Suppose I have a system of nodes that can communicate with a parent node, but not among each other. Suppose then a file on the parent node is split up into blocks and divided among the ...
2
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2answers
119 views
HMAC and assumptions on the cryptographic hash
According to Wikipedia, a cryptographic hash function has the following properties:
Pre-image resistance: Given $h$, it's difficult to find any message $m$ such that $h = H(m)$.
Second pre-image ...
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3answers
105 views
Why crypto hash functions must be collision resistant and how to find resistant?
Why cryptographic hash functions must be collision-free and is there any methods to evaluate whether a function is not resistant to collision?
Thanks,
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49 views
How to calculate byte length of a hash [closed]
The following :- c4ca4238a0b923820dcc509a6f75849b is an MD5 hash and we know that it is 16 bytes in length.
My question is how do we understand that it is 16 bytes. How the bytes are calculated. ...
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2answers
123 views
Reversing SHA1 (don't know the correct term)
Given sha1(pad(A) || pad(B)), where B is known, can I calculate sha1(pad(A))? pad(A) means its length is exactly 1 block (64 bytes for SHA-1)
If yes, for which other hash functions it will work too?
...
2
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1answer
83 views
Encrypt user email but be able to find user by email
Sorry for my dumb question, but it's better to ask dumb question than to do dumb things silently.
I want to encrypt user email in my DB so that if someone stole the DB (and not the key) - he won't be ...
2
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1answer
202 views
Are there any practical implementation of a homomorphic hashing or signature scheme?
A homomorphic hash function is a function $H : A \to B$ between two sets with some algebraic structure $(A, *)$ and $(B, \star)$ such that
$H$ is collision resistant, i.e. it is hard to find $x \neq ...
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1answer
128 views
Do I need to keep a 64-bit version number secret?
Assume the following structure:
version | iv | ciphertext | HMAC(version | iv | ciphertext | ..., key)
The version starts out ...
3
votes
2answers
105 views
What are the potential security impacts of using CRAM-MD5 for Emails, when not using an SSL connection?
Background: My current server-provider tells me it's no problem to store the passwords in plain-text in the database, saying he has to do so because they use CRAM-MD5 for email authentication. But ...
8
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1answer
380 views
Is Wikipedia's table about SHA-2 collisions correct?
I was looking a Wikipedia article on SHA-2, and the "Comparison of SHA functions" table seems to indicate that SHA-2 is less secure than SHA-1.
Is this true, or is the table wrong / misleading?
...
-1
votes
1answer
75 views
What key length is required to keep simple keyed “hash” secure?
In a previous question, I described a particular keyed "hash" that mapped a 5-digit input code into a 5-digit output code. It used a 8-bit key which is very insecure - more than 99% of the time, you ...
0
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2answers
132 views
Physical Level Encryption
What types of algorithms that are capable of signing a message are out there that run on a physical level, e.g. lacking the infrastructure of a standard PC, no memory, processor or motherboard in the ...
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3answers
632 views
What is the recommended replacement for MD5?
Since MD5 is broken for purposes of security, what hash should I be using now for secure applications?
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1answer
65 views
How to calculate cycles per byte [closed]
I have this data:
processor clock frequency: 2,1 ghz
message length: 16 byte
Speed: 4,3 Mbytes/s
how can calculate cycles and cycles per byte?
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2answers
203 views
SHA-1:Is there any mathematical result that gives us the minimum number of 1's in a 160-bit SHA-1 hash output?
Is there any mathematical result that gives us the minimum number of 1's in a 160-bit SHA-1 hash output? What is the probability that a 160-bit SHA-1 hash output contains at least 128 1's?
4
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1answer
176 views
Using an MD5 hash as a password
Suppose Alice is using a password prompt that only accepts up to 32 characters for any particular password.
Memorization of long strings of random characters is not one of Alice's strengths, so she ...
11
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2answers
891 views
Why is H(k||x) not a secure MAC construction?
If H(m) is a secure hash function, can't we implement a MAC using H(k||m)?
However, it seems the more widely used MACs, such as NMAC and HMAC (both originally defined in Keying hash functions for ...
2
votes
4answers
210 views
Increased CRC collision probability when adding bits to input message
The Scenario
I have a message string I need to transport over a wireless network that may be unreliable. This message string is about 100 bits long, and is packaged with an 8-bit CRC. When the ...
4
votes
1answer
150 views
Why xor the message into the state for sponge hashes?
Sponge hashes like Keccak(SHA-3) and CubeHash, xor a message block into part of the internal state. Why use a reversible operation like xor for that, instead of replacing that part of the state with ...
6
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2answers
190 views
Finding hash almost-collisions
A few months ago, XKCD posted a challenge to find a plaintext which hashed (using Skein 1024 1024) to a specified value. Inputs were scored based on the hamming distance between the hash of the ...
2
votes
1answer
122 views
Recommended way of adding a pepper/secret key to password before hashing?
There have been several questions regarding password hashing here and on Security.SE.
A "pepper" is sometimes mentioned – an application-specific secret key. The canonical answer on password hashing ...
1
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2answers
91 views
Many consecutive hashes to slow down brute force attack?
I've heard that hash algorithms like bcrypt are more secure because they take longer to complete, and therefore take much longer to prute force, without a noticable ...
0
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1answer
62 views
How to hash a structure that needs to include that hash's outcome?
so I am busy writing a TCP protocol in which I send a "packet header" with each packet of data. This packet header is a structure that contains information about the data that was sent. I want to ...
9
votes
4answers
1k views
How can rainbow tables be used for a dictionary attack?
I'm putting together a password policy for my company. I very much want to avoid requiring complex passwords, and would much rather require length.
The maximum length I can enforce is 14 characters. ...
3
votes
4answers
255 views
Knowing a valid salted hash for an unknown secret, is it possible to compute another valid hash?
$H(s,r)=d$ is a function that hashes the secret string $s$ with a salt $r$, and returns a digest $d$.
$r$ may be arbitrarily chosen and each $r$ returns a different $d$.
For any $d$, $r$ is known from ...
2
votes
3answers
179 views
Is it possible to work out the hash algorithm from a list of known message-hash pairs?
For example, in my situation I know hash(20) = 486e9638177faf1f34e49910491b77af.
I also know the hashes for all values from 0 to 20.
Is it possible to work out the ...
3
votes
2answers
207 views
toy hash algorithm
I'm looking for a toy hash function, where the idea is to have high school students break (i.e. find a collision) a hash function by hand, in order to teach them how one way functions and hashing ...
1
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1answer
100 views
Secure order preserving hash function
Is there a construction of an order preserving hash function that keeps the preimage property of a crypto hash function? By order preserving hash function (OPHF) i mean for $x<y$ then $OPHF(x) < ...
2
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0answers
100 views
LT codes with Homomorphic hashing
I have been working on a project implementing LT codes with Homomorphic hashing (inspired from http://blog.notdot.net/2012/08/Damn-Cool-Algorithms-Homomorphic-Hashing and ...
1
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1answer
84 views
How can a key pair be derived from an arbitrary hash?
If I correctly understand the concept of a "brain wallet" in BitCoin, you start with a passphrase, generate the hash of the passphrase, then somehow derive a public / private key from that to use as ...
2
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4answers
203 views
Tamper-proofing log files
Problem Overview
I want to securely store log files so the contents are secret, and they can't be modified without detection.
The files will be encrypted using authenticated encryption (AES in GCM ...
