A hash function is any algorithm that maps data of arbitrary length to data of a fixed length. The values returned by a hash function are called hash values, hash codes, hash sums, checksums or simply hashes. From Wikipedia: http://en.wikipedia.org/wiki/Hash_function
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1answer
89 views
Can someone please explain reverse look up tables? [closed]
Im finding it difficult to understand reverse look up tables and how it works, and the concept of hash chains. This is apart of a computer security model i am taking.
So i understand that brute force, ...
3
votes
1answer
66 views
Should changes to FNV-1A's input exhibit the avalanche effect?
This is kind of related to Which hashing algorithm is best for uniqueness and speed?. In that question, the excellently written top answer notes,
Randomnessification
The other subjective ...
1
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1answer
75 views
Garbage collection & memory leaks on hash tables
I was reading R. Read's How to be a programmer, and I came accross something I didn't understand:
...even with garbage collection, you can fill up all memory with
garbage. A classic mistake is ...
3
votes
3answers
102 views
sorting / merging hashtable
For my project I have a hashtable with 1M buckets (sometimes more).
Each bucket contains lots of values and they are stored sorted in a dynamic array.
I am looking for easy enough way to store all ...
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1answer
75 views
What are some hashmaps we can easily implement? [duplicate]
At many places, I read that we can use HashMap here for O(1) search. Actually, I want to ask how I can implement easy hashmaps which can satisfy this property. Can anyone tell few hashmaps including ...
3
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1answer
165 views
Hash function with uniqueness guarantees and controllable entropy
Is there a class of hash functions that meets the following specs:
Upper and lower bound can be specified
Uniqueness is guaranteed as long as the input is between the upper and lower bounds
Amount ...
0
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2answers
95 views
How early does a hashable type need to become immutable?
Suppose I have a class that looks vaguely like this:
class Foo(object):
_value = None # XXX: see below
def __init__(self, value):
self._value = value
@property
def ...
0
votes
2answers
101 views
Track a series of choices with a single number
I want to track a series of scored questions in a single number (the total score). Is that possible? and how?
For example, I have 30 questions with either 3 or 2 choices. For any given question, the ...
0
votes
0answers
66 views
Does multiple Bloom filters make sense?
In order to decrease the number of false positives, I came up with a couple of possible solutions.
Create two different types of Bloom filters; eg. filter1 using 3 different hash algorithms, filter2 ...
4
votes
0answers
126 views
How to get a one-way hash function that is collision safe for about 1 million unique inputs? [duplicate]
Forgive me if this is a noob question - My CS education is a somewhat incomplete
Basically, I need a way to hash an input, so that someone seeing the output doesn't see the original input value. ...
1
vote
2answers
96 views
Is hash calculated before/after compression?
I had a question regarding compression and calculation of checksum/hash of data.
I would like to know if checksum has to be calculated before or after the compression of data before transmission. ...
1
vote
2answers
389 views
Why a hashtable? Why not just a non-hashed associative array?
I've been learning about using a hashtable to efficiently check for items in a list without looping through the whole thing, but there's one thing that I don't get:
Why hashed keys?
It seems like:
...
0
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1answer
31 views
Hash function for progressive changes
I am looking for a hashing algorithm that works like this. I start with a text file, and compute its hash. Now I know that I will remove one character at, say, position 67 and this character is a "m", ...
1
vote
2answers
400 views
What's the shortest generating one-way hash algorithm?
I have a 24 character long id which is guaranteed to be unique. I would like to shorten that to 7-10 (or even shorter) characters long. If I generate an short id randomly then I would have to check ...
1
vote
2answers
116 views
Is there a known standard data structure which is a hash table that resolves collisions using a binary tree?
Is there a known standard data structure which is a hash table that resolves collisions using a binary tree?
If so what is the name of this data structure?
I imagine such a structure would be useful ...
0
votes
1answer
680 views
Complexity of ArrayList of LinkedHashSet
I get input strings from the console like this:
while ((currentLine = bufferedReader.readLine()) != null ) {
StringTokenizer string = new StringTokenizer(currentLine, " ");
while ...
38
votes
2answers
5k views
Why do so many hashed and encrypted strings end in an equals sign?
I work in C# and MSSQL and as you'd expect I store my passwords salted and hashed.
When I look at the hash stored in an nvarchar column (for example the out the box aspnet membership provider). I've ...
3
votes
4answers
109 views
Can I save & store a user's submission in a way that proves that the data has not been altered, and that the timestamp is accurate?
There are many situations where the validity of the timestamp attached to a certain post (submission of information) might be invaluable for the post owner's legal usage. I'm not looking for a service ...
0
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2answers
50 views
Hashing + security as it pertains to a theoretical file sharing site
These days it's possible to hash a file client-side, send the hash to the server, and have the server check whether or not that file is already uploaded. If it is, we can skip the file upload and make ...
4
votes
2answers
535 views
Overriding GetHashCode in a mutable struct - What NOT to do?
I am using the XNA Framework to make a learning project. It has a Point struct which exposes an X and Y value; for the purpose of optimization, it breaks the rules for proper struct design, since its ...
0
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2answers
226 views
Randomized Hash function with no collisions
Related to the question Which hashing algorithm is best for uniqueness and speed?
Is there a way to create a hash function, or find one, whose hash length depends completely on the input length, has ...
1
vote
3answers
300 views
what is the javascript internal data structure?
Consider a basic js object:
var obj={x:1,y:'2'};
Is this stored internally as a hashtable or does js use a different mechanism for key value pairs? If they are hash tables does anyone know how they ...
2
votes
2answers
1k views
Design Hash table with simple hash function
I want to learn to Design Hash table with simple hash function for better understanding. I understand that the hash table will work as long as the hash function maps each key to a non-negative integer ...
0
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1answer
128 views
Why Num&sizeMinusOne faster than num&(size-1)
I've been told that when I have a hash table of size m and m=2^k, I can use the & operator as num & (size-1) instead of num % size, to fit the hashCode to my table size.
I've also been told ...
1
vote
1answer
158 views
Measuring “novelty” of data
I have a heuristic in mind that should allow me to "score" data based on "novelty" that I would like to work in real-ish time.
In this case, I mean novelty in the sense that the data source is ...
0
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2answers
109 views
How do crackers determine number of iterations of a Hashing algorithm?
Does a cracker need to know the number of iterations a hashing algorithm uses to compute a hash?
If they don't know it, how do they figure it out? How much longer does it take to figure it out, than ...
2
votes
3answers
899 views
Optimal way to implement this specific lookup table in C#?
I want to create a lookup table for this data:
The "input variables" (what is used to "lookup") are 4 different doubles that can each take on 1 of 200 numbers (the numbers range from 1-1000 but there ...
0
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2answers
252 views
How deterministic are SessionIDs from SHA'd GUIDs?
Assume I'm using the following code to generate pseudo-random sessionID's:
sessionID = SHA-512(GENERATE-GUID())
The GUIDs are pretty deterministic, i.e. I see lots of GUIDs with a lot of the same ...
3
votes
2answers
303 views
Would using rainbow tables to detect weak user passwords feasible?
As I understand in most security breach where the list of hashed password are compromised, attackers do use brute-force to try to find weak password and, invariably, they always find quite some (like ...
2
votes
4answers
415 views
Looking for monotonically increasing (integer) hash function
I'm looking for a HashFunction(X,Y: Integer): Integer that is monotonically increasing on X, then Y.
So:
HashFunction(x1,y1) > HashFunction(x2,y2) if x1>x2
HashFunction(x,y1) > HashFunction(x,y2) if ...
4
votes
4answers
243 views
Is there a name for the concept of a “cumulative checksum”?
Insofar as I understand it, tools like Git and Mercurial derive checksums from their data, and those checksums are used to derive other checksums used in aggregate, leading to a kind of accumulative ...
5
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1answer
834 views
Hashing Algorithm: Deleting an element in linear probing
While using Linear probing method to implement hashing, when we delete and element, the position of the deleted element is declared as a tombstone/ mark it as deleted. Why can't we just shift all the ...
14
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6answers
953 views
How to assure users that website and passwords are secure
On reliable websites I always see claims such as "All data is encrypted" or "All passwords are encrypted using 128bit encryption" and etc. However I have never come across a claim such as "All ...
1
vote
2answers
895 views
Data Aggregation of CSV files java
I have k csv files (5 csv files for example), each file has m fields which produce a key and n values. I need to produce a single csv file with aggregated data.
I'm looking for the most efficient ...
5
votes
1answer
740 views
Sorting versus hashing
My problem is as follows. I have an array of n strings with m < n of them distinct. I want to create a one-to-one function which assigns each of the m distinct strings to the numbers 0 ... m-1. ...
5
votes
3answers
2k views
How can I reverse engineer a hash code?
I am building an application in C# that works with a Progress database. The passwords that are stored in this database are stored using a hash algorithm that Progress has not made public. However, I ...
2
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4answers
286 views
Preventing password hashing algorithm from overloading CPU
These days password hashing algorithms are designed to be slow. While it prevents black hats from guessing the password (at least partially), it also gives additional work for the server.
I can ...
1
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1answer
600 views
ruby-idiomatic hashes vs arrays
So I am still fairly new to ruby, though I have noticed that it is very hard to create 2d-array and that hashes seem to be more of the go to data structure than arrays.
I was wondering why the Ruby ...
1
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2answers
1k views
Finding duplicate files? [duplicate]
I am going to be developing a program that detects duplicate files and I was wondering what the best/fastest method would be to do this? I am more interested in what the best hash algorithm would be ...
5
votes
1answer
334 views
How can I benchmark concurrent key-value stores?
I have some concurrent key-value store implementations that are implemented with hash tables and search trees that I would like to compare. I would like to benchmark them with a real world application ...
0
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1answer
187 views
Is there any design pattern to remove elements from a hash map?
In my application I keep track of the running threads via a hash map, in that way I can retrieve the correct thread and do any actions. The app evolved in a way that new threads are created and old ...
0
votes
1answer
406 views
Linear probing hashing collision resolution
So I have a quick question about the linear probing method of collision resolution in hash tables. So by definition a linear probing method would look like:
while (hashTable[hash] != null)
hash = ...
1
vote
1answer
473 views
Max number of items mapped to same location in hashmap
I have a question about hashmaps.
If you have this hashmap with m slots, and need to map n items to it, and n > m. There will be collisions for sure. But assuming there is simple uniform hashing ...
4
votes
6answers
1k views
Would md5 hashes allow detection of synced files?
We have to develop our own file management system in Java web application. We need to sync files between our main server and client severs and find out whether all the client server has all the latest ...
4
votes
2answers
966 views
How can “hash functions” be used to implement hash maps at all?
My understandment is that hash maps allow us to link, say, a string, to certain memory location. But if every string were to be linked to a unique place in memory it would need a huge block of empty ...
6
votes
2answers
963 views
How do scalable bloom filters work?
I was reading up on scalable bloom filters and could not understand how each time a constituent bloom filters fills up, a new bloom filter with larger size is added.
The elements that contributed to ...
8
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1answer
930 views
Hash function classification interview question
In several places on the Internet there's the interview question
Classify the Hashing Functions based on the various methods by which the key value is found.
with answers like
Direct method
...
4
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4answers
320 views
Concept behind SHA-1 Checksum
What's the basis behind SHA-1 or SHA-2 or other Checksum algorithms?
I read about it here http://en.wikipedia.org/wiki/SHA-1#Data_Integrity
But I am still wondering about an answer in a layman's ...
1
vote
1answer
525 views
How to test if a hashing algorithm is good? [duplicate]
Possible Duplicate:
Which hashing algorithm is best for uniqueness and speed?
I have created a hash algorithm and would like to test if it is usable. What do I need to test and how?
0
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3answers
1k views
Basic memcached question
I have been reading up on distributed hashing. I learnt that consistent hashing is used for distributing the keys among cache machines. I also learnt that, a key is duplicated on mutiple caches to ...
