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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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 ...
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2answers
72 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 ...
6
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1answer
161 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?
...
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1answer
63 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 ...
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2answers
107 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
570 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
41 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
156 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?
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36 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? [duplicate]
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 the 160 bit result contains at least 128 1's?
3
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1answer
123 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
844 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
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4answers
171 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 ...
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1answer
148 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
164 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
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1answer
110 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 ...
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2answers
71 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 ...
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1answer
58 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 ...
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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
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4answers
229 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
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3answers
163 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
186 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 ...
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1answer
67 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 ...
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1answer
86 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
95 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 ...
2
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1answer
166 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
74 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 ...
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4answers
191 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 ...
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2answers
184 views
Are cryptographic hash functions perfect hash functions?
For a cryptographic hash function and input values of shorter length than the hash function output, it's pretty obvious that there should be as few collisions as possible. But are there guaranteed to ...
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0answers
17 views
LT codes with Homomorphic hashing [duplicate]
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 ...
2
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2answers
132 views
Why is $h(H, m) = E(m, H) \oplus m$ insecure?
I am taking a cryptography class on Coursera. I learned that the compression function $h(H, m) = E_m(H) \oplus m$ is insecure (even though other variants like Davies-Meyer or Miyaguchi-Preneel are ...
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1answer
54 views
Why are the Davies-Meyer and Miyaguchi-Preneel constructions secure?
The Davies-Meyer compression function $h(H, m) = E_m(H) \oplus H$ is said to be secure. So too is the Miyaguchi-Preneel compression function $h(H, m) = E_m(H) \oplus m \oplus H$. Why are these ...
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0answers
67 views
How can I prove that this encryption scheme from a random oracle is secure?
I am reading this example:
A random oracle is an ideal object.
What makes a random oracle convenient for proofs is the part about knowing nothing on the output for a given input if you do not ...
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1answer
99 views
How to use salt if I am sending hashed password?
If have application that is sending username and hashed password to server for authentication, how would I use salt for this scenario ?
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1answer
245 views
Any point in waiting for the SHA-3 standard?
The Wikipedia article on SHA-3 says that Keccak has been chosen but the standard hasn't been issued. Does this matter?
That is, can we just assume it's going to be as specified by the submitters ...
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2answers
75 views
Using salted hash as password for easy memorization without reuse?
I had an idea earlier:
Secure passwords are a) long, and b) unpredictable. A hash is both of these. Would it be safe to reuse a key between sites, and include the site's name as a salt? For example:
...
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2answers
171 views
Why are the constants so simple in Keccak?
Keccak, the construction selected for SHA-3 is very interesting. It seems unlike other primitives and has chosen very simple constants. (Keccak talk PDF)
The initial values of the state in Keccak is ...
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What security does Keccak offer against quantum attacks, specifically Grover's algorithm?
In the face of non-quantum attacker, Keccak[r=1088,c=512] with 512 bits of output provides:
Collision resistance up to $2^{256}$ operations
Preimage resistance up to $2^{256}$ operations
Second ...
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0answers
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MD4 First preimage - state of the art
What's the state of the attack to get the first preimage on MD4?
Is it still this http://www.di.ens.fr/~leurent/files/MD4_FSE08.pdf in 2^102 ?
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3answers
297 views
“Weaknesses” in SHA-256d?
According to this answer, "SHA-256d" was proposed in one of the Ferguson/Schneier books like so:
SHA-256d(x) = SHA-256(SHA-256(x))
Apparently, the motivation for ...
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2answers
134 views
How can mega store my login details and still be secure?
I understand how Mega's encryption works. For a quick summary of all those in the future looking for an answer on this... here is how it works:
Upon first signing up for an account you make a ...
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1answer
73 views
What's the difference between “HashX-512” and “HashX-1024”?
I need to make a Skein hash's, specifically, Skein-1024; however, I only have access currently to Skein-512. Is there any way to utilize a Skein-512 function to get the result of a Skein-1024 hash?
...
3
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2answers
122 views
RSA digital signature vs authenticated cipher
I want to provide both confidentiality and integrity for data at rest (many large files stored on disk).
I plan to encrypt the data using AES, which will cover the confidentiality requirement.
So ...
6
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1answer
104 views
Is SpookyHash vulnerable to HashDoS?
Aumasson, Bernstein and Bosslet did an excellent job demonstrating fatal weaknesses in MurmurHash and CityHash, but what about SpookyHash?
My first uneducated guess is that it would be much harder to ...
4
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4answers
197 views
What is the difference between a hash and a permutation?
As defined by Wikipedia a hash function is
[...] any algorithm or subroutine that maps large data sets of variable length to smaller data sets of a fixed length. For example, a person's name, ...
4
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4answers
197 views
Could a very long password theoretically eliminate the need for a slow hash?
Before I provide details, I want to clarify that I am not looking to implement this practically, but I'm only asking to get a better understanding.
The way I currently understand it, we use slow ...
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2answers
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What makes a hash function good for password hashing?
Using a cryptographic hash to store e.g. passwords in a database is considered good practice (as opposed to storing them plaintext), but is subject to attacks on said cryptographic hash, assuming the ...
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1answer
33 views
Selecting a large NUMS Safe prime
Suppose I want to use the following simple hash function.
For a mesage $m$, take some public $a$ and prime $p$ and raise $a^m \bmod p$ (never mind the computational expense of this operation).
This ...
4
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2answers
172 views
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 ...
4
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4answers
504 views
Is SHA-256 secure as a CTR block cipher?
Generate a 256-bit random nonce. XOR it with a 256-bit reusable symmetric key. This is x.
We represent numbers in simple binary instead of a counting function. ...
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0answers
54 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 ...
