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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871
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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 secure,...
0
votes
1answer
74 views

How does Pearson hashing compare with other non-cryptographic hashing algorithms?

FNV-1, Murmur2, and DJB2 are examples of non-cryptographic hashing functions used in actual applications (see Which hashing algorithm is best for uniqueness and speed?). These are all similar in that ...
0
votes
1answer
29 views

Secure communication between a mobile app and a microcontroller

I have a MCU-based design that essentially controls other hardware and this MCU is controlled through an app. I was hoping someone could help me figure out how to secure this communication between the ...
4
votes
1answer
50 views

Reproducible uniqueness

So, I have a bunch of calls that are all being generated with a UUID1 throughout the day. At the end of the call the call is processed and some metrics around that call are generated and stored in ...
2
votes
2answers
121 views

Does it make sense to check multiple hashes when detecting a file duplicate?

Please forgive me if this question belongs on different StackExchange site. A much more experienced colleague (like, 8 times more experienced than me) has vigorously defended an idea that you should ...
31
votes
6answers
10k 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 ...
2
votes
2answers
106 views

How can I tell if a web sign up form actually encrypts my password?

Recently I signed up for new account for a service provider's web site. I entered the usual the name, email , password (strong) , repeat password and I was signed up. Great everything works well. A ...
1
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0answers
22 views

Consistent hash ring with subsets in different buckets

I have N buckets. I have M users. Each user can buy anywhere between 1 and N tickets to an upcoming lottery. I would like to distribute the tickets more or less evenly between the buckets, with the ...
0
votes
1answer
47 views

Hash system to generate same (specified) hash from multiple datasets?

I have a hashed value X and need to ensure that any of the data sets D1...Dn can be concatenated with a correctional value C1...Cn and passed into a hashing algorithm to result in the value of X. ...
0
votes
1answer
87 views

Efficient irreversible “serialization”?

Suppose we have a function f that gets a complex (non-flat) object as an input, serializes it, calculates hash of serialization results (e.g. md5) and returns calculated hash. It may use some exiting ...
1
vote
1answer
48 views

Hashing a tuple of maximum-valued integers

I'm considering a hash function for a tuple of and maximum-valued integers for example (3 ,2, 5 ) -- where each element of the tuple has some maximum value N. My plan is to treat the elements of the ...
0
votes
0answers
10 views

Partition stream of data between n servers, with m duplicates

I'm partitioning a stream of input data between n servers. The simple way of doing it is to use the hash of some property of each input data packet to assign a single server, using mod or similar, and ...
7
votes
1answer
147 views

What kind of members should be used in a GetHashCode() implementation

We're building some Roslyn analyzers concerning GetHashCode(), including an analyzer that implements it for you in a given class. While researching the subject we've found that there are many ...
-1
votes
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?
2
votes
2answers
193 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
votes
1answer
73 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
vote
3answers
148 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 ...
-2
votes
2answers
138 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
75 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
vote
3answers
243 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
votes
1answer
124 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 (...
-1
votes
1answer
85 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
votes
1answer
195 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
votes
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
votes
0answers
65 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
vote
0answers
75 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
vote
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
148 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
64 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
vote
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: ...
1
vote
0answers
59 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
vote
1answer
358 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
457 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
196 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
votes
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
vote
0answers
57 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
135 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
98 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 ...
5
votes
2answers
1k 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 ...
-1
votes
1answer
531 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
votes
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
votes
1answer
104 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
vote
1answer
294 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
245 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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votes
1answer
142 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 ...
37
votes
8answers
35k 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
305 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
169 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 value(...