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  • 3 hash functions to best hash sliding window strings for a bloom filter with minimum collisions

    - by Duaa
    Hi all: I need 3 hash functions to hash strings of a sliding window moving over a text, to be used later to search within a bloom vector. I'm using C# in my programming I read something about rolling hash functions and cyclic polynomials, they are used for sliding window applications. But really, I did not find any codes, they are just descriptions So please, if anyone have any idea about 3 best C# hash functions to use with sliding window strings of fixed size (5-char), that consume less time and have minimum number of collisions, either they are rolling hash functions or others, please help me with some C# codes or links to hash functions names Duaa

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  • Distributions and hashes

    - by Don Mackenzie
    Has anyone ever had an incidence of downloading software from a genuine site, where an MD5 or SHA series hash for the download is also supplied and then discovered that the hash calculated from the downloaded artifact doesn't match the published hash? I understand the theory but am curious how prevalent the problem is. Many software publishers seem to discount the threat.

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  • Standard way to hash an RSA key?

    - by Adam J.R. Erickson
    What's the algorithm for creating hash (sha-1 or MD5) of an RSA public key? Is there a standard way to do this? Hash just the modulus, string addition of both and then take a hash? Is SHA-1 or MD5 usually used? I want to use it to ensure that I got the right key (have the sender send a hash, and I calculate it myself), and log said hash so I always know which exact key I used when I encrypt the payload.

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  • passing hashes to a subroutine

    - by Vishalrix
    In one of my main( or primary) routines,I have two or more hashes. I want the subroutine foo() to recieve these possibly-multiple hashes as distinct hashes. Right now I have no preference if they go by value, or as references. I am struggling with this for the last many hours and would appreciate help, so that I dont have to leave perl for php! ( I am using mod_perl, or will be) Right now I have got some answer to my requirement, shown here From http://forums.gentoo.org/viewtopic-t-803720-start-0.html # sub: dump the hash values with the keys '1' and '3' sub dumpvals { foreach $h (@_) { print "1: $h->{1} 3: $h->{3}\n"; } } # initialize an array of anonymous hash references @arr = ({1,2,3,4}, {1,7,3,8}); # create a new hash and add the reference to the array $t{1} = 5; $t{3} = 6; push @arr, \%t; # call the sub dumpvals(@arr); I only want to extend it so that in dumpvals I could do something like this: foreach my %k ( keys @_[0]) { # use $k and @_[0], and others } The syntax is wrong, but I suppose you can tell that I am trying to get the keys of the first hash ( hash1 or h1), and iterate over them. How to do it in the latter code snippet above?

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  • MySQL Unique hash insertion

    - by Jesse
    So, imagine a mysql table with a few simple columns, an auto increment, and a hash (varchar, UNIQUE). Is it possible to give mysql a query that will add a column, and generate a unique hash without multiple queries? Currently, the only way I can think of to achieve this is with a while, which I worry would become more and more processor intensive the more entries were in the db. Here's some pseudo-php, obviously untested, but gets the general idea across: while(!query("INSERT INTO table (hash) VALUES (".generate_hash().");")){ //found conflict, try again. } In the above example, the hash column would be UNIQUE, and so the query would fail. The problem is, say there's 500,000 entries in the db and I'm working off of a base36 hash generator, with 4 characters. The likelyhood of a conflict would be almost 1 in 3, and I definitely can't be running 160,000 queries. In fact, any more than 5 I would consider unacceptable. So, can I do this with pure SQL? I would need to generate a base62, 6 char string (like: "j8Du7X", chars a-z, A-Z, and 0-9), and either update the last_insert_id with it, or even better, generate it during the insert. I can handle basic CRUD with MySQL, but even JOINs are a little outside of my MySQL comfort zone, so excuse my ignorance if this is cake. Any ideas? I'd prefer to use either pure MySQL or PHP & MySQL, but hell, if another language can get this done cleanly, I'd build a script and AJAX it too. Thanks!

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  • storing a sorted array of hashes

    - by srk
    use strict; use warnings; my @aoh =( { 3 => 15, 4 => 8, 5 => 9, }, { 3 => 11, 4 => 25, 5 => 6, }, { 3 => 5, 4 => 18, 5 => 5, }, { 0 => 16, 1 => 11, 2 => 7, }, { 0 => 21, 1 => 13, 2 => 31, }, { 0 => 11, 1 => 14, 2 => 31, }, ); #declaring a new array to store the sorted hashes my @new; print "\n-------------expected output------------\n"; foreach my $href (@aoh) { #i want a new array of hashes where the hashes are sorted my %newhash; my @sorted_keys = sort {$href->{$b} <=> $href->{$a} || $b <=> $a} keys %$href; foreach my $key (@sorted_keys) { print "$key => $href->{$key}\n"; $newhash{$key} = $href->{$key}; } print "\n"; push(@new,\%newhash); } print "-----------output i am getting---------------\n"; foreach my $ref(@new) { my @skeys = sort {$ref->{$a} <=> $ref->{$b} } keys %$ref; foreach my $key (@skeys) { print "$key => $ref->{$key}\n" } print "\n"; } The output of the program : -------------expected output------------ 3 => 15 5 => 9 4 => 8 4 => 25 3 => 11 5 => 6 4 => 18 5 => 5 3 => 5 0 => 16 1 => 11 2 => 7 2 => 31 0 => 21 1 => 13 2 => 31 1 => 14 0 => 11 -----------output i am getting--------------- 4 => 8 5 => 9 3 => 15 5 => 6 3 => 11 4 => 25 3 => 5 5 => 5 4 => 18 2 => 7 1 => 11 0 => 16 1 => 13 0 => 21 2 => 31 0 => 11 1 => 14 2 => 31 Please tell me what am i doing wrong in storing the hashes into a new array.. how do i achieve what i want.. ? Thanks in advance...

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  • reference to specific hash key

    - by dave
    How do I create a reference to the value in a specific hash key. I tried the following but $$foo is empty. Any help is much appreciated. $hash->{1} = "one"; $hash->{2} = "two"; $hash->{3} = "three"; $foo = \${$hash->{1}}; $hash->{1} = "ONE"; #I want "MONEY: ONE"; print "MONEY: $$foo\n";

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  • Hashes vs Numeric id's

    - by Karan Bhangui
    When creating a web application that some how displays the display of a unique identifier for a recurring entity (videos on YouTube, or book section on a site like mine), would it be better to use a uniform length identifier like a hash or the unique key of the item in the database (1, 2, 3, etc). Besides revealing a little, what I think is immaterial, information about the internals of your app, why would using a hash be better than just using the unique id? In short: Which is better to use as a publicly displayed unique identifier - a hash value, or a unique key from the database? Edit: I'm opening up this question again because Dmitriy brought up the good point of not tying down the naming to db specific property. Will this sort of tie down prevent me from optimizing/normalizing the database in the future? The platform uses php/python with ISAM /w MySQL.

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  • Oracle Hash Cluster Overflow Blocks

    - by Andrew
    When inserting a large number of rows into a single table hash cluster in Oracle, it will fill up the block with any values that hash to that hash-value and then start using overflow blocks. These overflow blocks are listed as chained off the main block, but I can not find detailed information on the way in which they are allocated or chained. When an overflow block is allocated for a hash value, is that block exclusively allocated to that hash value, or are the overflow blocks used as a pool and different hash values can then start using the same overflow block. How is the free space of the chain monitored - in that, as data is continued to be inserted, does it have to traverse the entire chain to find out if it has some free space in the current overflow chain, and then if it finds none, it then chooses to allocate a new block?

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  • From string to hex MD5 hash and back

    - by Pablo Fernandez
    I have this pseudo-code in java: bytes[] hash = MD5.hash("example"); String hexString = toHexString(hash); //This returns something like a0394dbe93f bytes[] hexBytes = hexString.getBytes("UTF-8"); Now, hexBytes[] and hash[] are different. I know I'm doing something wrong since hash.length() is 16 and hexBytes.length() is 32. Maybe it has something to do with java using Unicode for chars (just a wild guess here). Anyways, the question would be: how to get the original hash[] array from the hexString. The whole code is here if you want to look at it (it's ~ 40 LOC) http://gist.github.com/434466 The output of that code is: 16 [-24, 32, -69, 74, -70, 90, -41, 76, 90, 111, -15, -84, -95, 102, 65, -10] 32 [101, 56, 50, 48, 98, 98, 52, 97, 98, 97, 53, 97, 100, 55, 52, 99, 53, 97, 54, 102, 102, 49, 97, 99, 97, 49, 54, 54, 52, 49, 102, 54] Thanks a lot!

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  • Using a password to generate two distinct hashes without reducing password security

    - by Nevins
    Hi there, I'm in the process of designing a web application that will require the storage of GPG keys in an encrypted format in a database. I'm planning on storing the user's password in a bCrypt hash in the database. What I would like to be able to do is to use that bCrypt to authenticate the user then use the combination of the stored bCrypt hash and another hash of the password to encrypt and decrypt the GPG keys. My question is whether I can do this without reducing the security of the password? I was thinking I may be able to use something like an HMAC-SHA256 of a static string using the password and a salt as the secret key. Is there a better way to do this that I haven't thought of? Thanks

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  • Dynamically/recursively building hashes in Perl?

    - by Gaurav Dadhania
    I'm quite new to Perl and I'm trying to build a hash recursively and getting nowhere. I tried searching for tutorials to dynamically build hashes, but all I could find were introductory articles about hashes. I would be grateful if you point me towards the right direction or suggest a nice article/tutorial. I'm trying to read from a file which has paths in the form of one/two/three four five/six/seven/eight and I want to build a hash like VAR = { one : { two : { three : "" } } four : "" five : { six : { seven : { eight : "" } } } } The script I'm using currently is : my $finalhash = {}; my @input = <>; sub constructHash { my ($hashrf, $line) = @_; @elements = split(/\//, $line); if(@elements > 1) { $hashrf->{shift @elements} = constructHash($hashrf->{$elements[0]}, @elements ); } else { $hashrf->{shift @elements} = ""; } return $hashrf; } foreach $lines (@input) { $finalhash = constructHash($finalhash, $lines); }

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  • What's the big deal with brute force on hashes like MD5

    - by Jan Kuboschek
    I just spent some time reading http://stackoverflow.com/questions/2768248/is-md5-really-that-bad (I highly recommend!). In it, it talks about hash collisions. Maybe I'm missing something here, but can't you just encrypt your password using, say, MD5 and then, say, SHA-1 (or any other, doesn't matter.) Wouldn't this increase the processing power required to brute-force the hash and reduce the possibility of collision?

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  • Defining the hash of an object as the sum of hashes of its members

    - by Space_C0wb0y
    I have a class that represents undirected edges in a graph. Every edge has two members vertex1 and vertex2 representing the vertices it connects. The problem is, that an edge can be specified two directions. My idea was now to define the hash of an edge as the sum of the hashes of its vertices. This way, the direction plays no role anymore, the hash would be the same. Are there any pitfalls with that?

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  • Strange ruby behavior when using Hash.new([])

    - by Valentin Vasilyev
    Consider this code: h=Hash.new(0) #new hash pairs will by default have 0 as values h[1]+=1 # {1=>1} h[2]+=2 # {2=>2} that's all fine, but: h=Hash.new([]) #empty array as default value h[1]<<=1 #{1=>[1]} - OK h[2]<<=2 #{1=>[1,2], 2=>[1,2]} # why ?? At this point I expect the hash to be: {1=>[1], 2=>[2]} But something goes wrong. Does anybody know what happens?

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  • How to get the top keys from a hash by value

    - by Kirs Kringle
    I have a hash that I sorted by values greatest to least. How would I go about getting the top 5? There was a post on here that talked about getting only one value. What is the easiest way to get a key with the highest value from a hash in Perl? I understand that so would lets say getting those values add them to an array and delete the element in the hash and then do the process again? Seems like there should be an easier way to do this then that though. My hash is called %words. use strict; use warnings; use Tk; #Learn to install here: http://factscruncher.blogspot.com/2012/01/easy-way-to-install-tk- on-strawberry.html #Reading in the text file my $file0 = Tk::MainWindow->new->Tk::getOpenFile; open( my $filehandle0, '<', $file0 ) || die "Could not open $file0\n"; my @words; while ( my $line = <$filehandle0> ) { chomp $line; my @word = split( /\s+/, lc($line)); push( @words, @word ); } for (@words) { s/[\,|\.|\!|\?|\:|\;|\"]//g; } #Counting words that repeat; put in hash my %words_count; $words_count{$_}++ for @words; #Reading in the stopwords file my $file1 = "stoplist.txt"; open( my $filehandle1, '<', $file1 ) or die "Could not open $file1\n"; my @stopwords; while ( my $line = <$filehandle1> ) { chomp $line; my @linearray = split( " ", $line ); push( @stopwords, @linearray ); } for my $w ( my @stopwords ) { s/\b\Q$w\E\B//ig; } #Comparing the array to Hash and deleteing stopwords my %words = %words_count; for my $stopwords ( @stopwords ) { delete $words{ $stopwords }; } #Sorting Hash Table my @keys = sort { $words{$b} <=> $words{$a} or "\L$a" cmp "\L$b" } keys %words; #Starting Statistical Work my $value_count = 0; my $key_count = 0; #Printing Hash Table $key_count = keys %words; foreach my $key (@keys) { $value_count = $words{$key} + $value_count; printf "%-20s %6d\n", $key, $words{$key}; } my $value_average = $value_count / $key_count; #my @topwords; #foreach my $key (@keys){ #if($words{$key} > $value_average){ # @topwords = keys %words; # } #} print "\n", "The number of values: ", $value_count, "\n"; print "The number of elements: ", $key_count, "\n"; print "The Average: ", $value_average, "\n\n";

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  • Python hash() can't handle long integer?

    - by Xie
    I defined a class: class A: ''' hash test class a = A(9, 1196833379, 1, 1773396906) hash(a) -340004569 This is weird, 12544897317L expected. ''' def __init__(self, a, b, c, d): self.a = a self.b = b self.c = c self.d = d def __hash__(self): return self.a * self.b + self.c * self.d Why, in the doctest, hash() function gives a negative integer?

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  • Merging hashes into one hash in perl

    - by Nick
    how do I get this: $VAR1 = { '999' => { '998' => [ '908', '906', '0', '998', '907' ] } }; $VAR1 = { '999' => { '991' => [ '913', '920', '918', '998', '916', '919', '917', '915', '912', '914' ] } }; $VAR1 = { '999' => { '996' => [] } }; $VAR1 = { '999' => { '995' => [] } }; $VAR1 = { '999' => { '994' => [] } }; $VAR1 = { '999' => { '993' => [] } }; $VAR1 = { '999' => { '997' => [ '986', '987', '990', '984', '989', '988' ] } }; $VAR1 = { '995' => { '101' => [] } }; $VAR1 = { '995' => { '102' => [] } }; $VAR1 = { '995' => { '103' => [] } }; $VAR1 = { '995' => { '104' => [] } }; $VAR1 = { '995' => { '105' => [] } }; $VAR1 = { '995' => { '106' => [] } }; $VAR1 = { '995' => { '107' => [] } }; $VAR1 = { '994' => { '910' => [] } }; $VAR1 = { '993' => { '909' => [] } }; $VAR1 = { '993' => { '904' => [] } }; $VAR1 = { '994' => { '985' => [] } }; $VAR1 = { '994' => { '983' => [] } }; $VAR1 = { '993' => { '902' => [] } }; $VAR1 = { '999' => { '992' => [ '905' ] } }; to this: $VAR1 = { '999:' => [ { '992' => [ '905' ] }, { '993' => [ { '909' => [] }, { '904' => [] }, { '902' => [] } ] }, { '994' => [ { '910' => [] }, { '985' => [] }, { '983' => [] } ] }, { '995' => [ { '101' => [] }, { '102' => [] }, { '103' => [] }, { '104' => [] }, { '105' => [] }, { '106' => [] }, { '107' => [] } ] }, { '996' => [] }, { '997' => [ '986', '987', '990', '984', '989', '988' ] }, { '998' => [ '908', '906', '0', '998', '907' ] }, { '991' => [ '913', '920', '918', '998', '916', '919', '917', '915', '912', '914' ] } ]}; in Perl?

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  • Are there any working implementations of the rolling hash function used in the Rabin-Karp string sea

    - by c14ppy
    I'm looking to use a rolling hash function so I can take hashes of n-grams of a very large string. For example: "stackoverflow", broken up into 5 grams would be: "stack", "tacko", "ackov", "ckove", "kover", "overf", "verfl", "erflo", "rflow" This is ideal for a rolling hash function because after I calculate the first n-gram hash, the following ones are relatively cheap to calculate because I simply have to drop the first letter of the first hash and add the new last letter of the second hash. I know that in general this hash function is generated as: H = c1ak - 1 + c2ak - 2 + c3ak - 3 + ... + cka0 where a is a constant and c1,...,ck are the input characters. If you follow this link on the Rabin-Karp string search algorithm , it states that "a" is usually some large prime. I want my hashes to be stored in 32 bit integers, so how large of a prime should "a" be, such that I don't overflow my integer? Does there exist an existing implementation of this hash function somewhere that I could already use? Here is an implementation I created: public class hash2 { public int prime = 101; public int hash(String text) { int hash = 0; for(int i = 0; i < text.length(); i++) { char c = text.charAt(i); hash += c * (int) (Math.pow(prime, text.length() - 1 - i)); } return hash; } public int rollHash(int previousHash, String previousText, String currentText) { char firstChar = previousText.charAt(0); char lastChar = currentText.charAt(currentText.length() - 1); int firstCharHash = firstChar * (int) (Math.pow(prime, previousText.length() - 1)); int hash = (previousHash - firstCharHash) * prime + lastChar; return hash; } public static void main(String[] args) { hash2 hashify = new hash2(); int firstHash = hashify.hash("mydog"); System.out.println(firstHash); System.out.println(hashify.hash("ydogr")); System.out.println(hashify.rollHash(firstHash, "mydog", "ydogr")); } } I'm using 101 as my prime. Does it matter if my hashes will overflow? I think this is desirable but I'm not sure. Does this seem like the right way to go about this?

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  • How to map hash keys to methods for an encapsulated Ruby class (tableless model)?

    - by user502052
    I am using Ruby on Rails 3 and I am tryng to map a hash (key, value pairs) to an encapsulated Ruby class (tableless model) making the hash key as a class method that returns the value. In the model file I have class Users::Account #< ActiveRecord::Base def initialize(attributes = {}) @id = attributes[:id] @firstname = attributes[:firstname] @lastname = attributes[:lastname] end end def self.to_model(account) JSON.parse(account) end My hash is hash = {\"id\":2,\"firstname\":\"Name_test\",\"lastname\":\"Surname_test\"} I can make account = Users::Account.to_model(hash) that returns (debugging) --- id: 2 firstname: Name_test lastname: Surname_test That works, but if I do account.id I get this error NoMethodError in Users/accountsController#new undefined method `id' for #<Hash:0x00000104cda410> I think because <Hash:0x00000104cda410> is an hash (!) and not the class itself. Also I think that doing account = Users::Account.to_model(hash) is not the right approach. What is wrong? How can I "map" those hash keys to class methods?

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  • perl array of hashes sorting

    - by srk
    use strict; my @arr; $arr[0][0]{5} = 16; $arr[0][1]{6} = 11; $arr[0][2]{7} = 25; $arr[0][3]{8} = 31; $arr[0][4]{9} = 16; $arr[0][5]{10} = 17; sort the array based on hash values so this should change to $arr[0][0]{6} = 11; $arr[0][1]{9} = 16; $arr[0][2]{5} = 16; $arr[0][3]{10} = 17; $arr[0][4]{7} = 25; $arr[0][5]{8} = 31; first sort on values in the hash.. when the values are same reverse sort based on keys... Please tell me how to do this.. Thank you

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  • How do I design a cryptographic hash function?

    - by Eyal
    After reading the following about why one-way hash functions are one-way, I would like to know how to design a hash function. http://stackoverflow.com/questions/1038307/help-me-better-understand-cryptographic-hash-functions/1047106#1047106 Before everyone gets on my case: Yes, I know that it's a bad idea to not use a proven and tested hash function. I would still like to know how it's done. I'm familiar with Feistel-network ciphers but those are necessarily reversible, horrible for a cryptographic hash. Is there some sort of construction that is well-used in cryptographic hashing? Something that makes it very one-way?

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  • analyzing hashes

    - by calccrypto
    Is anyone willing to devote some time to helping me analyze a (hopefully cryptographically secure) hash? I honestly have no idea what im doing, so i need someone to show me how to, to teach me. almost all of the stuff ive found online have been really long, tedious, and vague the code is in python because for some reason i dont know c/c++. all i know about the hash: 1. there are no collisions (so far) and 2. differences between two similar inputs results in wildly different differences and please dont tell me that if i dont know what im doing, i shouldnt be doing it.

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  • Perl, creating a hash of hashes.

    - by Mike
    Based on my current understanding of hashes in Perl, I would expect this code to print "hello world." It instead prints nothing. %a=(); %b=(); $b{str} = "hello"; $a{1}=%b; $b=(); $b{str} = "world"; $a{2}=%b; print "$a{1}{str} $a{2}{str}"; I assume that a hash is just like an array, so why can't I make a hash contain another?

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