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• Combine MD5 hashes of multiple files

  - by user685869

  I have 7 files that I'm generating MD5 hashes for. The hashes are used to ensure that a remote copy of the data store is identical to the local copy. Unfortunately, the link between these two copies of the data is mind numbingly slow. Changes to the data are very rare but I have a requirement that the data be synchronized at all times (or as soon as possible). Rather than passing 7 different MD5 hashes across my (extremely slow) communications link, I'd like to generate the hash for each file and then combine these hashes into a single hash which I can then transfer and then re-calculate/use for comparison on the remote side. If the "combined hash" differs, then I'd start sending the 7 individual hashes to determine exactly which file(s) have been changed. For example, here are the MD5 hashes for the 7 files as of last week: 0709d609d69385255c496436eb50402c 709465a74411bd596595c7b9b158ae6a 4ab657320ef33e3d5eb498e4c13d41b7 3b49c6ab199994fd776bb63761414e72 0fc28c5a010fc3c06c0c930c88e31a15 c4ecd214662cac5aae0e53f6f252bf0e 8b086431e43148a2c2d943ba30d31cc6 I'd like to combine these hashes together such that I get a single unique value (perhaps another MD5 hash?) that I can then send to the remote system. On the remote system, I'd then perform the same calculation to determine if the data as a whole has been changed. If it has, then I'd start sending the individual hashes, etc. The most important factor is that my "combined hash" be short enough so that it uses less bandwidth than just sending all 7 hashes in the first place. I thought of writing the 7 MD5 hashes to a file and then hashing that file but is there a better way?

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• How do you embed a hash into a file recursively?

  - by oasisbob

  Simplest case: You want to make a text file which says "The MD5 hash of this file is FOOBARHASH". How do you embed the hash, knowing that the embedded hash value and the hash of the file are inter-related? eg, Cisco embeds hash values into their IOS images, which can be verified like this: cisco# verify s72033-advipservicesk9_wan-mz.122-33.SXH7.bin Embedded Hash MD5 : D2BB0668310392BAC803BE5A0BCD0C6A Computed Hash MD5 : D2BB0668310392BAC803BE5A0BCD0C6A IIRC, Ubuntu also includes a txt file in the root of their ISOs which have the hash of the entire ISO. Maybe I'm mistaken, but trying to figure out how to do this blows my mind.

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• Optimal strategy to make a C++ hash table, thread safe

  - by Ajeet

  (I am interested in design of implementation NOT a readymade construct that will do it all.) Suppose we have a class HashTable (not hash-map implemented as a tree but hash-table) and say there are eight threads. Suppose read to write ratio is about 100:1 or even better 1000:1. Case A) Only one thread is a writer and others including writer can read from HashTable(they may simply iterate over entire hash table) Case B) All threads are identical and all could read/write. Can someone suggest best strategy to make the class thread safe with following consideration 1. Top priority to least lock contention 2. Second priority to least number of locks My understanding so far is thus : One BIG reader-writer lock(semaphore). Specialize the semaphore so that there could be eight instances writer-resource for case B, where each each writer resource locks one row(or range for that matter). (so i guess 1+8 mutexes) Please let me know if I am thinking on the correct line, and how could we improve on this solution.

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• Find all duplicate files by md5 hash

  - by Jamie Curran

  I'm trying to find all duplicate files based upon md5 hash and ordered by file size. So far I have this: find . -type f -print0 | xargs -0 -I "{}" sh -c 'md5sum "{}" | cut -f1 -d " " | tr "\n" " "; du -h "{}"' | sort -h -k2 -r | uniq -w32 --all-repeated=separate The output of this is: 1832348bb0c3b0b8a637a3eaf13d9f22 4.0K ./picture.sh 1832348bb0c3b0b8a637a3eaf13d9f22 4.0K ./picture2.sh 1832348bb0c3b0b8a637a3eaf13d9f22 4.0K ./picture2.s d41d8cd98f00b204e9800998ecf8427e 0 ./test(1).log Is this the most efficient way?

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• PHP security regarding login

  - by piers

  I have read a lot about PHP login security recently, but many questions on Stack Overflow regarding security are outdated. I understand bcrypt is one of the best ways of hashing passwords today. However, for my site, I believe sha512 will do very well, at least to begin with. (I mean bcrypt is for bigger sites, sites that require high security, right?) I´m also wonder about salting. Is it necessary for every password to have its own unique salt? Should I have one field for the salt and one for the password in my database table? What would be a decent salt today? Should I join the username together with the password and add a random word/letter/special character combination to it? Thanks for your help!

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• Proper password handling for login

  - by piers

  I have read a lot about PHP login security recently, but many questions on Stack Overflow regarding security are outdated. I understand bcrypt is one of the best ways of hashing passwords today. However, for my site, I believe sha512 will do very well, at least to begin with. (I mean bcrypt is for bigger sites, sites that require high security, right?) I´m also wonder about salting. Is it necessary for every password to have its own unique salt? Should I have one field for the salt and one for the password in my database table? What would be a decent salt today? Should I join the username together with the password and add a random word/letter/special character combination to it? Thanks for your help!

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• Ruby on Rails ActiveSupport or plain Ruby way of merging arrays into a hash

  - by randombits

  What's a smart way of using ActiveSupport or perhaps regular built in Ruby functionality to take two arrays and merge them into a hash where each element in an array matches the element in a parallel array? imagine two arrays: names = ["Danny", "Johnny"] ages = ["25", "32"] The end result should be a hash that looks like: {"Danny" => "25", "Johnny" => "32"}

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• hash of array in objective-c, how?

  - by Horace Ho

  How is a hash of integer array can be represented in objective-c? Here is the ruby hash as an example: hi_scores = { "John" => [1, 1000], "Mary" => [2, 8000], "Bob" => [5, 2000] } such that can be accessed by: puts hi_scores["Mary"][1] => 8000 hopefully easy to serialize too. Thanks!

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• Hash Join require Full Table Scan

  - by Pedro Magalhaes

  So, I want to know if to make a Hash Join between two tables is necessary to make a full table scan on the collumns? If i want to join COL1 wiht COL2, and COL1 is smaller, the It makes a full scan in COL1 creating a Hashmap then makes a full scan in COL2 using the sabe hash function. Is this correct?

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• Git append the current commit hash to result of a commit command

  - by farzan

  I want to append the hash of the ongoing commit to its result. I can retrieve the hash using this command: git log --format=%H | tail -1 Then I try to merge a commit with command above and make an alias in '.gitconfig', like this: [alias] ci = !git commit && git log --format=%H | tail -1 But this does not work; parameters of alias are send to tail command, not git commit. How should I create this alias?

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• How does the hash part in hash maps work?

  - by sub

  So there is this nice picture in the hash maps article on Wikipedia: Everything clear so far, except for the hash function in the middle. How can a function generate the right index from any string? Are the indexes integers in reality too? If yes, how can the function output 1 for John Smith, 2 for Lisa Smith, etc.?

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• Does Windows CE support hash tables?

  - by Bryan

  Quick question of does Windows CE support hash tables? I have a program that I'm modifying and adding to a device that uses Windows CE and I was wondering if CE supported hash tables since it is used in the original software.

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• Perl - getting a value from a hash where the key has a dot

  - by imerez

  I have a hash in Perl which has been dumped into from some legacy code the name of the key has now changed from simply reqHdrs to reqHdrs.bla $rec->{reqHdrs.bla} My problem is now I cant seem to access this field from the hash any ideas? The following is my error Download Script Output: Bareword "reqHdrs" not allowed while "strict subs" in use

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• Remove hash (#) from url in Ajax navigation without refresh

  - by Email

  I have an ajax navigation similar like here. now if a menu is clicked window.location.hash is added like this #about i want to REmove the hash (#) so that people can easily copy and share the link naturally. How this can be done in april 2012 without a pagerefresh crossbrowserwise (IE7+,FF,Opera,Safari) ? For inspiration: Here is actually someone already doing this, click on "portfolio" or "features" and watch the url in your browser. thanks for tips

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• What is a hash map in programming and where can it be used

  - by Saif Bechan

  I have often heard people talking about hashing and hash maps and hash tables. I wanted to know what they are and where you can best use them for.

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• How to hash a password and store for later verification with another digest

  - by oxygen8

  I am using gsoap's wsseapi plugin and would like to store hashed sha1 passwords rather than plain text. I have spent a rediculous amount of time experimenting with various methods of hashing the plain text password for storage. Can anyone suggest a way to hash a password so it can be later verified against a username token digest sent by the client. I can't seem to get the client password to authenticate against my stored hash

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• Hash Table question [closed]

  - by Fatimah

  I need your help to solve this program ... Implement a separate chaining hash table that stores strings. You’ll need a hash function that converts string into an index number. Assume the strings will be lowercase words, so 26 characters will suffice.

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• Problem with initializing a hash in ruby

  - by Cyborgo

  Hi, I have a text file from which I want to create a Hash for faster access. My text file is of format (space delimited) author title date popularity I want to create a hash in which author is the key and the remaining is the value as an array. created_hash["briggs"] = ["Manup", "Jun,2007", 10] Thanks in advance.

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• C++ hash table w/o using STL

  - by Bobby

  I need to create a hash table that has a key as a string, and value as an int. I cannot use STL containers on my target. Is there a suitable hash table class for this purpose?

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• C++ hash table implementation with collision handling

  - by myahya

  What is a good C++ library for hash tables / hash maps similar to what java offers. I have worked with Google Sparsehash, but it has no support for collisions.

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• Perl, "closure" using Hash

  - by Mike

  I would like to have a subroutine as a member of a hash which is able to have access to other hash members. For example sub setup { %a = ( txt => "hello world", print_hello => sub { print ${txt}; }) return %a } my %obj = setup(); $obj{print_hello}; Ideally this would output "hello world"

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• Changing keys in a "for ( keys %hash ) {}"-loop

  - by sid_com

  Hello! I remember something about not changing the keys in a for my $key ( keys %hash ) { ... for example for my $key ( keys %hash ) { $key = "$key_x"; } But deleting keys and changing values would be fine. Are my memories OK?

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• How can I get access to password hashing in postgresql? Tried installing postgresql-contrib in ubun

  - by Tchalvak

  So I'm trying to just hash some passwords in postgresql, and the only hashing solution that I've found for postgresql is part of the pgcrytpo package ( http://www.postgresql.org/docs/8.3/static/pgcrypto.html ) that is supposed to be in postgresql-contrib ( http://www.postgresql.org/docs/8.3/static/contrib.html ). So I installed postgresql-contrib, (sudo apt-get install postgresql-contrib), restarted my server (as a simple way to restart postgresql). However, I still don't have access to any of the functions for hashing that are supposed to be in postgresql-contrib, e.g.: ninjawars=# select crypt('global salt' || 'new password' || 'user created date', gen_salt('sha256')); ERROR: function gen_salt(unknown) does not exist ninjawars=# select digest('test', 'sha256') from players limit 1; ERROR: function digest(unknown, unknown) does not exist ninjawars=# select hmac('test', 'sha256') from players limit 1; ERROR: function hmac(unknown, unknown) does not exist So how can I hash passwords in postgresql, on ubuntu?

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• perl sorting an array of hashes

  - 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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• In Windows XP, is it possible to disable user credential caching for particular users

  - by kdt

  I understand that when windows caches user credentials, these can sometimes be used by malicious parties to access other machines once a machine containing cached credentials is compromised, a method known as "pass the hash"[1]. For this reason I would like to get control over what's cached to reduce the risk of cached credentials being used maliciously. It is possible to prevent all caching by zeroing HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon\CachedLogonsCount, but this is too indiscriminate: laptops users need to be able to login when away from the network. What I would like to do is prevent the caching of credentials of certain users, such as administrators -- is there any way to do that in Windows XP? http://www.lbl.gov/cyber/systems/pass-the-hash.html

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