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• removing duplicate strings from a massive array in java efficiently?

  - by Preator Darmatheon

  I'm considering the best possible way to remove duplicates from an (Unsorted) array of strings - the array contains millions or tens of millions of stringz..The array is already prepopulated so the optimization goal is only on removing dups and not preventing dups from initially populating!! I was thinking along the lines of doing a sort and then binary search to get a log(n) search instead of n (linear) search. This would give me nlogn + n searches which althout is better than an unsorted (n^2) search = but this still seems slow. (Was also considering along the lines of hashing but not sure about the throughput) Please help! Looking for an efficient solution that addresses both speed and memory since there are millions of strings involved without using Collections API!

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• Grouping rectangles (getting the bounding boxes of rects)

  - by hyn

  What is a good, fast way to get the "final" bounding boxes of a set of random (up to about 40, not many) rectangles? By final I mean that all bounding boxes don't intersect with any other. Brute force way: in a double for loop, for each rect, test for intersection against every other rect. The intersecting rects become a new rect (replaced), indicating the bounding box. Start over and repeat until no intersection is detected. Because the rects are random every time, and the rect count is relatively small, collision detection using spatial hashing seems like overkill. Is there a way to do this more effectively?

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• Why is only the suffix of work_index hashed?

  - by Jaroslav Záruba

  I'm reading through the PDF that Brett Slatkin has published for Google I/O 2010: "Data pipelines with Google App Engine": http://tinyurl.com/3523mej In the video (the Fan-in part) Brett says that the work_index has to be a hash, so that 'you distribute the load across the BigTable': http://www.youtube.com/watch?v=zSDC_TU7rtc#t=48m44 ...and this is how work_index is created: work_index = '%s-%d' % (sum_name, knuth_hash(index)) ...which I guess creates something like 'mySum-54657651321987' I do understand the basic idea, but is why only one half of work_index is hashed? Is it important to hash only part of it leaving the suffix out? Would it be wrong to do md5('%s-%d' % (sum_name, index)) so that the hash would be like '6gw8....hq6' ? I'm Java guy so I would use md5 to hash, which means I get id like 'mySum' + 32 characters. (Obviously I want my ids/keys to be as short as possible here.) If I could hash the whole string my id would be just 32 chars. Or would you suggest to use something else to do the hashing with?

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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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• Can I encrypt value in C# and use that with SQL Server 2005 symmetric encryption?

  - by Robert Byrne

  To be more specific, if I create a symmetric key with a specific KEY_SOURCE and ALGORITHM (as described here), is there any way that I can set up the same key and algorithm in C# so that I can encrypt data in code, but have that data decrypted by the symmetric key in Sql Server? From the research I've done so far, it seems that the IDENTITY_VALUE for the key is also baked into the cypher text, making things even more complex. I'm thinking about just trying all the various ways I can think of, ie hashing the KEY_SOURCE using different hash algorithms for a key and trying different ways of encrypting the plain text until I get something that works. Or is that just futile? Has anyone else done this, any pointers? UPDATE Just to clarify, I want to use NHibernate on the client side, but theres a bunch of stored procedures on the database side that still perform decryption.

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• How can I compare my PHPASS-hashed stored password to my incoming POST data?

  - by Ygam

  Here's a better example, just a simple checking..stored value in database has password: fafa (hashed with phpass in registration) and username: fafa; i am using the phpass password hashing framework public function demoHash($data) //$data is the post data named password { $hash =new PasswordHash(8, false); $query = ORM::factory('user'); $result = $query ->select('username, password') ->where('username', 'fafa') ->find(); $hashed = $hash->HashPassword($data); $check = $hash->CheckPassword($hashed, $result->password); echo $result->username . "<br/>"; echo $result->password . "<br/>"; return $check; } check is returning false

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• What is the sense of permiting the user to use no passwords longer than xx chars?

  - by reox

  Its more like a usability question or maybe database, or even maybe security (consider injection attacks) but what is the sense of permiting the user's password to a be not longer than xx chars? It does not make any sense to me, because longer passwords are mostly considered better and even harder to crack, and some users use password safes, so the password length should not matter. I understand that passwords with more than 20 chars are hardly to remember, but if you use diceware or password safe you dont have any problem with that. I really cant understand why there are sites that say "your password need to be between 5 and 8 chars"... also should the password saved as hash, so the length of the field in the database is fixed, so where is the problem? i think that most of the sites where the password is has to be a fixed length are not even using any hashing method...

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• Mapping words to numbers with respect to definition

  - by thornate

  As part of a larger project, I need to read in text and represent each word as a number. For example, if the program reads in "Every good boy deserves fruit", then I would get a table that converts 'every' to '1742', 'good' to '977513', etc. Now, obviously I can just use a hashing algorithm to get these numbers. However, it would be more useful if words with similar meanings had numerical values close to each other, so that 'good' becomes '6827' and 'great' becomes '6835', etc. As another option, instead of a simple integer representing each number, it would be even better to have a vector made up of multiple numbers, eg (lexical_category, tense, classification, specific_word) where lexical_category is noun/verb/adjective/etc, tense is future/past/present, classification defines a wide set of general topics and specific_word is much the same as described in the previous paragraph. Does any such an algorithm exist? If not, can you give me any tips on how to get started on developing one myself? I code in C++.

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• resizing arrays when close to memory capacity

  - by user548928

  So I am implementing my own hashtable in java, since the built in hashtable has ridiculous memory overhead per entry. I'm making an open-addressed table with a variant of quadratic hashing, which is backed internally by two arrays, one for keys and one for values. I don't have the ability to resize though. The obvious way to do it is to create larger arrays and then hash all of the (key, value) pairs into the new arrays from the old ones. This falls apart though when my old arrays take up over 50% of my current memory, since I can't fit both the old and new arrays in memory at the same time. Is there any way to resize my hashtable in this situation Edit: the info I got for current hashtable memory overheads is from here How much memory does a Hashtable use? Also, for my current application, my values are ints, so rather than store references to Integers, I have an array of ints as my values.

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• Convert VB6 Randomize to C#

  - by Thomas G. Mayfield

  I'm writing a C# frontend to a legacy database that uses VB6 Rnd() and Randomize() methods for user password encryption. The encryption function is very simplistic and really not all that secure, but it's what all current passwords are stored with. What I'd like to be able to do is authenticate legacy users from a C# application. I can write new encryption (or preferably hashing) code for VB6 so that all future users have a more secure password, and that can be duplicated in C#. But I don't want to require current users to have had their password reset before they can use the new frontend. Is there any way I can reimplement that algorithm in C# so that it produces identical results to the legacy VB6 code?

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• SHA1CryptoServiceProvider changed in .NET 4

  - by WebDude

  I am currently trying to upgrade a project of mine from .NET 3.5 to .NET 4.0 Everything was going really well, all code compiled, all tests passed. Then I hit a problem deploying to my stagomg environment. Suddenly my logins were no longer working. It seems my SHA1 hashed passwords are being hashed differently in .NET 4. I am using the SHA1CryptoServiceProvider: SHA1CryptoServiceProvidercryptoTransformSHA1 = new SHA1CryptoServiceProvider(); To test I created a new Visual Studio project with 2 console applications. The first targeted at .NET Framework 3.5 and the second at 4.0. I ran exactly the same hashing code in both and different results were produced. Why is this happening and how can I fix this? I obviously cannot go update all of my users passwords considering I do not know what they are. Any help would be greatly appreciated.

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• Creating a unique key based on file content in python

  - by Cawas

  I got many, many files to be uploaded to the server, and I just want a way to avoid duplicates. Thus, generating a unique and small key value from a big string seemed something that a checksum was intended to do, and hashing seemed like the evolution of that. So I was going to use hash md5 to do this. But then I read somewhere that "MD5 are not meant to be unique keys" and I thought that's really weird. What's the right way of doing this? edit: by the way, I took two sources to get to the following, which is how I'm currently doing it and it's working just fine, with Python 2.5: import hashlib def md5_from_file (fileName, block_size=2**14): md5 = hashlib.md5() f = open(fileName) while True: data = f.read(block_size) if not data: break md5.update(data) f.close() return md5.hexdigest()

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• Codeigniter: A nice straight forward tutorial on how to build a reset password/forgotten password?

  - by Psychonetics

  I've built a full sign up system with user account activation, login, validation, captcha etc. To complete this I now need to implement a forgot password/reset password feature.. I have created one function that generates a random password 8 characters, another method that takes that random password word and applies sha1 and hashing. Also one that takes that hashed password and stores it in a table in the database. I will keep these methods to one side as they might come in handy later on but for now I would like to know if anyone can point me to a nice tutorial for creating a password reset feature for my website. Thanks in advance

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• MD5 hash differences between Python and other file hashers

  - by Sam

  I have been doing a bit of programming in Python (still a n00b at it) and came across something odd. I made a small program to find the MD5 hash of a filename passed to it on the command line. I used a function I found here on SO. When I ran it against a file, I got a hash "58a...113". But when I ran Microsoft's FCIV or the md5sum.py in \Python26\Tools\Scripts\, I get a different hash, "591...ae6". The actual hashing part of the md5sum.py in Scripts is m = md5.new() while 1: data = fp.read(bufsize) if not data: break m.update(data) out.write('%s %s\n' % (m.hexdigest(), filename)) This looks functionally identical to the code in the function given in the other answer... What am I missing? (This is my first time posting to stackoverflow, please let me know if I am doing it wrong.)

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• How are CD Keys generated?

  - by The Rook

  CD Keys are the defacto-standard as an anti-piracy measure. To be honest this strikes me as Security Though Obscurity, although I really have no idea how CD Keys are generated. What is a good (secure) example of CD Key generation? What cryptographic primitive (if any) are they using? Is it a message digest? If so what data would they be hashing? What methods do developers employ to make it difficult for crackers to build their own key generators?

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• Available options for hosting FTP server in .NET application

  - by duane

  I need to implement an FTP service inside my .NET application (running as a Windows Service) and have not had much luck finding good/current source code or vendors. Ideally it needs to be able to respond to the basic FTP Protocol and accept the data stream from an upload via a stream, enabling me to process the data as it is being received (think on the fly hashing). I need to be able to integrate it into my service because it will stack on top of our current code base with an existing custom TCP/IP communication protocol. I don't want to write (and then spend time debugging and performance testing) my own protocol, or implementation. I have already found plenty of ftp client implementations, I just need an acceptable server solution.

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• [PHP] md5(uniqid) makes sense for random unique tokens?

  - by Exception e

  I want to create a token generator that generates tokens that cannot be guessed by the user and that are still unique (to be used for password resets and confirmation codes). I often see this code; does it make sense? md5(uniqid(rand(), true)); According to a comment uniqid($prefix, $moreEntopy = true) yields first 8 hex chars = Unixtime, last 5 hex chars = microseconds. I don't know how the $prefix-parameter is handled.. So if you don't set the $moreEntopy flag to true, it gives a predictable outcome. QUESTION: But if we use uniqid with $moreEntopy, what does hashing it with md5 buy us? Is it better than: md5(mt_rand())

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• memcached cluster maintenance

  - by Yang

  Scaling up memcached to a cluster of shards/partitions requires either distributed routing/partition table maintenance or centralized proxying (and other stuff like detecting failures). What are the popular/typical approaches/systems here? There's software like libketama, which provides consistent hashing, but this is just a client-side library that reacts to messages about node arrivals/departures---do most users just run something like this, plus separate monitoring nodes that, on detecting failures, notify all the libketamas of the departure? I imagine something like this might be sufficient since typical use of memcached as a soft-state cache doesn't require careful attention to consistency, but I'm curious what people do.

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• What's a good way to encrypt data using an asymmetric key, that's available to both java and ruby?

  - by Michael Campbell

  I have a customer that wants to encrypt some data in his database (not passwords; this needs actual encryption, not hashing). The application which will be doing the encrypting/writing is in Java, but the process which will DECRYPT it is behind a secure firewall, and is written in ruby. The idea was to use a public/private key scheme; the java system would encrypt it with the public key, then the process on his local box would use the private key to decrypt it as needed. I'm looking for any experience anyone has doing something like that; my main question is what sorts of libraries on java and ruby can interoperate with the same keys and data.

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• REST authentication S3 like hmac sha1 signature vs symetric data encryption.

  - by coulix

  Hello stackers, I was arguing about an S3 like aproach using authorization hash with a secret key as the seed and some data on the request as the message signed with hmac sha1 (Amazon S3 way) vs an other developer supporting symetric encryption of the data with a secret key known by the emiter and the server. What are the advantage of using signed data with hmac sha1 vs symetric key other than the fact that with the former, we do not need to encrypt the username or password. What would be the hardest to break ? symetric encryption or sha1 hashing at la S3 ? If all big players are using oauth and similar without symetric key it is sure that there are obvious advantages, what are those ?

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• Dynamic Custom Fields for Data Model

  - by Jerry Deng

  I am in the process of creating a dynamic database where user will be able to create resource type where he/she can add custom fields (multiple texts, strings, and files) Each resource type will have the ability to display, import, export its data; I've been thinking about it and here are my approaches. I would love to hear what do you guys think. Ideas: just hashing all the custom data in a data field (pro: writing is easier, con: reading back out may be harder); children fields (the model will have multiple fields of strings, fields of text, and fields for file path); fixed number of custom fields in the same table with a key mapping data hash stored in the same row; Non-SQL approach, but then the problem would be generating/changing models on the fly to work with different custom fields;

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• Is there a class like a Dictionary without a Value template? Is HashSet<T> the correct answer?

  - by myotherme

  I have 3 tables: Foos, Bars and FooBarConfirmations I want to have a in-memory list of FooBarConfirmations by their hash: FooID BarID Hash 1 1 1_1 2 1 2_1 1 2 1_2 2 2 2_2 What would be the best Class to use to store this type of structure in-memory, so that I can quickly check to see if a combination exists like so: list.Contains("1_2"); I can do this with Dictionary<string,anything>, but it "feels" wrong. HashSet looks like the right tool for the job, but does it use some form of hashing algorithm in the background to do the lookups efficiently?

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• How to I count key collisions when using boost::unordered_map?

  - by Nikhil

  I have a data structure with 15 unsigned longs, I have defined a hash function using hash_combine as follows: friend std::size_t hash_value(const TUPLE15& given) { std::size_t seed = 0; boost::hash_combine(seed, val1); boost::hash_combine(seed, val2); ... return seed; } I insert a large number of values into a boost::unordered_map but the performance is not good enough. Probably, I could do better with an alternative hashing function. To confirm this, I need to check how many collisions I am getting. How do I do this?

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• What algorithm would you use to code a parrot?

  - by Phil H

  A parrot learns the most commonly uttered words and phrases in its vicinity so it can repeat them at inappropriate moments. So how would you create a software version? Assuming it has access to a microphone and can record sound at will, how would you code it without requiring infinite resources? The best I can imagine is to divide the stream using silences in the sound, and then use some pattern recognition to encode each one as a list of tokens, storing new ones as you meet them. Hashing the token sequences and counting occurrences in a database, you could build up a picture of the most frequently uttered phrases. But given the huge variety in phrases, how do you prevent this just becoming a huge list? And the sheer number of pairs to match would surely generate lot of false positives from the combinatorial nature of matching. Would you use a neural net, since that's how a real parrot manages it? Or is there another, cleverer way of matching large-scale patterns in analogue data?

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• Looking for an array (vs linked list) hashtable implementation in C

  - by kingusiu

  hi, I'm looking for a hashtable implementation in C that stores its objects in (twodimensional) arrays rather than linked lists. i.e. if a collision happens, the object that is causing the collision will be stored in the next free row index rather than pushed to the head and first element of a linked list. plus, the objects themselves must be copied to the hashtable, rather than referenced by pointers. (the objects do not live for the whole lifetime of the program but the table does). I know that such an implementation might have serious efficiency drawbacks and is not the "standard way of hashing" but as I work on a very special system-architecture i need those characteristics. thanks

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