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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2answers
90 views

Does sql use hastables for indexes? [closed]

Are there other ways of indexing?, Which are the more used? Does sql have an standard for indexes, it uses hash tables?
832
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10answers
302k views

Which hashing algorithm is best for uniqueness and speed?

Which hashing algorithm is best for uniqueness and speed? Example (good) uses include hash dictionaries. I know there are things like SHA-256 and such, but these algorithms are designed to be ...
2
votes
2answers
151 views

ASP.NET storing encrypted & salted Password

I have a question about storing encrypted passwords in your database to help secure them. I have a class that encrypts the password passed in and returns a string. This string has 3 parts too it, all ...
2
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1answer
64 views

Does murmur3 hash have memory?

This implementation of Murmur hash yields different results when called twice in a row: Murmur3 m3 = new Murmur3(); var la = m3.ComputeHash(new byte[] { 1, 2 }); var lb = m3.ComputeHash(new byte[] { ...
1
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3answers
129 views

How do I generate multiple hashes that can be resolved to a single value?

I need multiple "random" URLs that just mask the original URL, i.e.: /ABC /CBA /CAB All resolve to /6 I came up with matching a letter to a number and then you can sum them to the single value. A ...
28
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6answers
9k views

Is it more secure to hash a password multiple times?

I've read a few times that when storing passwords, it's good practice to 'double hash' the strings (eg. with md5 then sha1, both with salts, obviously). I guess the first question is, "is this ...
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votes
2answers
131 views

How do you hash 4 doubles into a size_t?

I have bounding boxes the key type. Box { double mins[2]; double maxs[2]; } And I want to have Box as the key type in the D programming language, so I have to implement: size_t toHash() const ...
0
votes
1answer
71 views

Question on password hashing security

I'm about to add some user account and password functionality to my software, so I'm reading about password hashing. So my understanding is Hashing is a one way function, and it works like this: ...
1
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3answers
230 views

Why would I need `equals` if I have already `hashcode`?

I got a question about why we need equals if we have hashcode. My first attempt was the answer because collision. But we corrected starting point with the assumption that we have not many objects so ...
0
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1answer
97 views

Strategy for Public / Private API, Encrypted (or Hashed) Data, and Server Compromise

In this very simplistic but realistic scenario, I have 2 combo web/database servers (A) behind a load balancer and a single IP address. They also permit access only from one IP address -- the client ...
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1answer
81 views

What is the hash value of each member of the group? Before modding?

This isn't an actual code related question but it relates to C. We are given: Amy's club has grown and now includes the following members: Abel, Abigail, Abraham, Ada, Adam, Adrian, Adrienne, ...
3
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1answer
133 views

Why would a Rest Api demand a hash of the parameters?

I'm working with a Rest Api that demands to send with any request a hash of the parameters of the request: Hashkey Calculation This hash key has to be generated by the device itself and has to ...
17
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4answers
2k views

Why is the Git .git/objects/ folder subdivided in many SHA-prefix folders?

Git internally stores objects (Blobs, trees) in the .git/objects/ folder. Each object can be referenced by a SHA1 hash that is computed from the contents of the object. However, Objects are not ...
0
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0answers
50 views

Web service security token storage

I have two operations in my JAX-WS web service: First takes username and password as parameters and generates random alphanumeric token, second takes this token and username as parameters, verifies ...
1
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0answers
70 views

Generate a fixed length number hash from a number

Let say I've a fixed length number, e.g. 6 digit: 837493, I want to apply some process (with a salt), and generate a new number of another 6 digits, e.g. number = 837493 another = hash(number + ...
1
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2answers
2k views

What are the advantages of linear probing over separate chaining or vice-versa when implementing hash tables?

I've been brushing up on algorithms and reviewed these two methods of implementing hash tables. It seems like they largely have similar performance characteristics and memory requirements. I can ...
4
votes
3answers
143 views

How to store (and find) updated values in a Distributed Hash Table?

Distributed Hash Tables are a great way to store immutable data in a resilient way. However, I am wondering about how it would be possible to store new versions of old data in this system. Because ...
1
vote
0answers
62 views

Metrics for when to use a hash value?

I am working on a project that requires a series of flat files. I will be doing some data checking against simple date like: ID;StatusCode So a typical file might be like: 13993;R 38293;A 19349;I ...
2
votes
2answers
111 views

Why use a unique hashkey for form submissions?

I have multiple forms and use AJAX to submit them. I asked my boss if he needed any specific format for the form ID and he told me to generate a unique hash and keep it in session; check it whenever ...
1
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2answers
1k 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
votes
0answers
56 views

Writing a container that treats nearly-identical images as the same

I want to create a mapping in Python that will use images as keys, but treat similar (but not identical) images as the same key. I have an approach for deciding which images are similar enough: I ...
1
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1answer
296 views

How to generate Generate Hash incorporating time stamp so it can be unhashed to get the timestamp when it was generated?

I want to create hash based on random id, That hash has to be valid for only 15 minutes. Is there a way to do that? I'm using this for form submission. If some unknown submitted form or some one ...
6
votes
3answers
445 views

Hashing growth strategy

What is a good growth strategy for hash tables? If the number of elements exceeds the number of buckets, I increase the number of buckets with the following formula: n = int(n * 1.618033988749895) | ...
3
votes
1answer
188 views

If passwords are stored hashed, does that mean that theoretically there exists another password that could be used to log in? [duplicate]

From what I understand, in most sites, passwords are stored as a hash; not in their original form. This means that if the database is hacked into, the attacker will see the (nearly) useless hashes ...
6
votes
8answers
5k views

Using HashTable without overriding hashcode()

Today I was asked this question during an interview: What will happen if we do not override the hashcode method for our class, then add it to HashTable and then try to get objects? What could ...
9
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1answer
1k views

Hash function classification

On the internet, I've come across this question: Classify the Hashing Functions based on the various methods by which the key value is found. with answers like Direct method Subtraction method ...
1
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0answers
51 views

Is the Rabin fingerprinting scheme with a random polynomial a universal hash function family?

Say I have a greater-than-expected number of collisions in my hash table that has 2^w buckets (half of them empty). So I pick a fresh new random CRC polynomial that generates w-bit check values (i.e., ...
0
votes
1answer
103 views

Uniformly distributing GUIDS to bucket of size N [closed]

How can uniformly and deterministically distribute set of GUIDS to N buckets. N can be as small as 2. Need to make sure the same GUID is always mapped to the same bucket. Can't use any ...
0
votes
1answer
91 views

hashing algorithm to check inclusion in a set

The problem I am trying to solve is to validate if a username is already used before allowing an admin user to create a new user with that username. Currently I am doing an ajax request to check if ...
0
votes
0answers
46 views

Fast comparison of recursive directory tree against a database?

I'm creating basic media player in the style of foobar2000. I want to load Id3 tags into a database because scanning whole directory tree every time is slow. The problem is, when user changes ...
5
votes
2answers
944 views

Is it possible to speed up a hash table by using binary search trees for separate chaining?

I want to implement a Hash Table using Binary Search Trees to reduce the search complexity in the Separate Chaining process from O(n) (using linked list) to O(log n) (using BST). Can this be done, and ...
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1answer
420 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, ...
18
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6answers
2k views

What makes a hashing algorithm “secure”?

After reading this interesting question, I felt like I had a good idea of which insecure hashing algorithm I'd use if I needed one, but no idea why I might use a secure algorithm instead. So what is ...
3
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1answer
100 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
248 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
218 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
127 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 ...
35
votes
8answers
33k views

Why almost no webpages hash passwords in the client before submitting (and hashing them again on the server), as to “protect” against password reuse?

Relevant XKCD There are many sites on the Internet that require login information, and the only way to protect against password reusing is the "promise" that the passwords are hashed on the server, ...
3
votes
1answer
291 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
votes
2answers
163 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
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2answers
142 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
90 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
140 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
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2answers
184 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
126 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 ...
7
votes
2answers
1k 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 ...
1
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2answers
2k 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 ...
0
votes
1answer
48 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", ...
0
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1answer
1k 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 ...
45
votes
2answers
9k 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 ...