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• DSA signature verification input

  - by calccrypto

  What is the data inputted into DSA when PGP signs a message? From RFC4880, i found A Signature packet describes a binding between some public key and some data. The most common signatures are a signature of a file or a block of text, and a signature that is a certification of a User ID. im not sure if it is the entire public key, just the public key packet, or some other derivative of a pgp key packet. whatever it is, i cannot get the DSA signature to verify here is a sample im testing my program on: -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 abcd -----BEGIN PGP SIGNATURE----- Version: BCPG v1.39 iFkEARECABkFAk0z65ESHGFiYyAodGVzdCBrZXkpIDw+AAoJEC3Jkh8+bnkusO0A oKG+HPF2Qrsth2zS9pK+eSCBSypOAKDBgC2Z0vf2EgLiiNMk8Bxpq68NkQ== =gq0e -----END PGP SIGNATURE----- Dumped from pgpdump.net Old: Signature Packet(tag 2)(89 bytes) Ver 4 - new Sig type - Signature of a canonical text document(0x01). Pub alg - DSA Digital Signature Algorithm(pub 17) Hash alg - SHA1(hash 2) Hashed Sub: signature creation time(sub 2)(4 bytes) Time - Mon Jan 17 07:11:13 UTC 2011 Hashed Sub: signer's User ID(sub 28)(17 bytes) User ID - abc (test key) <> Sub: issuer key ID(sub 16)(8 bytes) Key ID - 0x2DC9921F3E6E792E Hash left 2 bytes - b0 ed DSA r(160 bits) - a1 be 1c f1 76 42 bb 2d 87 6c d2 f6 92 be 79 20 81 4b 2a 4e DSA s(160 bits) - c1 80 2d 99 d2 f7 f6 12 02 e2 88 d3 24 f0 1c 69 ab af 0d 91 -> hash(DSA q bits) and the public key for it is: -----BEGIN PGP PUBLIC KEY BLOCK----- Version: BCPG v1.39 mOIETTPqeBECALx+i9PIc4MB2DYXeqsWUav2cUtMU1N0inmFHSF/2x0d9IWEpVzE kRc30PvmEHI1faQit7NepnHkkphrXLAoZukAoNP3PB8NRQ6lRF6/6e8siUgJtmPL Af9IZOv4PI51gg6ICLKzNO9i3bcUx4yeG2vjMOUAvsLkhSTWob0RxWppo6Pn6MOg dMQHIM5sDH0xGN0dOezzt/imAf9St2B0HQXVfAAbveXBeRoO7jj/qcGx6hWmsKUr BVzdQhBk7Sku6C2KlMtkbtzd1fj8DtnrT8XOPKGp7/Y7ASzRtBFhYmMgKHRlc3Qg a2V5KSA8PohGBBMRAgAGBQJNM+p5AAoJEC3Jkh8+bnkuNEoAnj2QnqGtdlTgUXCQ Fyvwk5wiLGPfAJ4jTGTL62nWzsgrCDIMIfEG2shm8bjMBE0z6ngQAgCUlP7AlfO4 XuKGVCs4NvyBpd0KA0m0wjndOHRNSIz44x24vLfTO0GrueWjPMqRRLHO8zLJS/BX O/BHo6ypjN87Af0VPV1hcq20MEW2iujh3hBwthNwBWhtKdPXOndJGZaB7lshLJuW v9z6WyDNXj/SBEiV1gnPm0ELeg8Syhy5pCjMAgCFEc+NkCzcUOJkVpgLpk+VLwrJ /Wi9q+yCihaJ4EEFt/7vzqmrooXWz2vMugD1C+llN6HkCHTnuMH07/E/2dzciEYE GBECAAYFAk0z6nkACgkQLcmSHz5ueS7NTwCdED1P9NhgR2LqwyS+AEyqlQ0d5joA oK9xPUzjg4FlB+1QTHoOhuokxxyN =CTgL -----END PGP PUBLIC KEY BLOCK----- the public key packet of the key is mOIETTPqeBECALx+i9PIc4MB2DYXeqsWUav2cUtMU1N0inmFHSF/2x0d9IWEpVzEkRc30PvmEHI1faQi t7NepnHkkphrXLAoZukAoNP3PB8NRQ6lRF6/6e8siUgJtmPLAf9IZOv4PI51gg6ICLKzNO9i3bcUx4ye G2vjMOUAvsLkhSTWob0RxWppo6Pn6MOgdMQHIM5sDH0xGN0dOezzt/imAf9St2B0HQXVfAAbveXBeRoO 7jj/qcGx6hWmsKUrBVzdQhBk7Sku6C2KlMtkbtzd1fj8DtnrT8XOPKGp7/Y7ASzR in radix 64 i have tried many different combinations of sha1(< some data + 'abcd'),but the calculated value v never equals r, of the signature i know that the pgp implementation i used to create the key and signature is correct. i also know that my DSA implementation and PGP key data extraction program are correct. thus, the only thing left is the data to hash. what is the correct data to be hashed?

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• Python print statement prints nothing with a carriage return

  - by Jonathan Sternberg

  I'm trying to write a simple tool that reads files from disc, does some image processing, and returns the result of the algorithm. Since the program can sometimes take awhile, I like to have a progress bar so I know where it is in the program. And since I don't like to clutter up my command line and I'm on a Unix platform, I wanted to use the '\r' character to print the progress bar on only one line. But when I have this code here, it prints nothing. # Files is a list with the filenames for i, f in enumerate(files): print '\r%d / %d' % (i, len(files)), # Code that takes a long time I have also tried: print '\r', i, '/', len(files), Now just to make sure this worked in python, I tried this: heartbeat = 1 while True: print '\rHello, world', heartbeat, heartbeat += 1 This code works perfectly. What's going on? My understanding of carriage returns on Linux was that it would just move the line feed character to the beginning and then I could overwrite old text that was written previously, as long as I don't print a newline anywhere. This doesn't seem to be happening though. Also, is there a better way to display a progress bar in a command line than what I'm current trying to do?

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• Is it immoral to write crappy code even if readability and correctness is not a requirement?

  - by mafutrct

  There are cases when crappy (i.e. unreadable and buggy) code is not much of a problem. For instance, imagine you need to generate a big text file that mostly follows a simple pattern with a few very complex exceptions. What do you do? You quickly write a simple algorithm and insert the exceptional bits in the output manually to save 4 hours. The code is unreadable, and the output is flawed, but it's still the correct way since it is way faster. But let's get this straight: I hate bad code. I've had to read and work with code that caused my stomach to hurt. I care a lot about good code. And actually, I caught myself thinking that it is immoral to write bad code even though the dirty approach is sometimes superior. I was surprised by myself and found my idea to be very irrational. Did you ever experience this? Should I just get rid of this stupid idea and use the most efficient approach to coding?

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• Java Bucket Sort on Strings

  - by Michael

  I can't figure out what would be the best way to use Bucket Sort to sort a list of strings that will always be the same length. An algorithm would look like this: For the last character position down to the first: For each word in the list: Place the word into the appropriate bucket by current character For each of the 26 buckets(arraylists) Copy every word back to the list I'm writing in java and I'm using an arraylist for the main list that stores the unsorted strings. The strings will be five characters long each. This is what I started. It just abrubdly stops within the second for loop because I don't know what to do next or if I did the first part right. ArrayList<String> count = new ArrayList<String>(26); for (int i = wordlen; i > 0; i--) { for (int j = 0; i < myList.size(); i++) myList.get(j).charAt(i) } Thanks in advanced.

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• Radix Sort in Python [on hold]

  - by Steven Ramsey

  I could use some help. How would you write a program in python that implements a radix sort? Here is some info: A radix sort for base 10 integers is a based on sorting punch cards, but it turns out the sort is very ecient. The sort utilizes a main bin and 10 digit bins. Each bin acts like a queue and maintains its values in the order they arrive. The algorithm begins by placing each number in the main bin. Then it considers the ones digit for each value. The rst value is removed and placed in the digit bin corresponding to the ones digit. For example, 534 is placed in digit bin 4 and 662 is placed in the digit bin 2. Once all the values in the main bin are placed in the corresponding digit bin for ones, the values are collected from bin 0 to bin 9 (in that order) and placed back in the main bin. The process continues with the tens digit, the hundreds, and so on. After the last digit is processed, the main bin contains the values in order. Use randint, found in random, to create random integers from 1 to 100000. Use a list comphrension to create a list of varying sizes (10, 100, 1000, 10000, etc.). To use indexing to access the digits rst convert the integer to a string. For this sort to work, all numbers must have the same number of digits. To zero pad integers with leading zeros, use the string method str.zfill(). Once main bin is sorted, convert the strings back to integers. I'm not sure how to start this, Any help is appreciated. Thank you.

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• Verizon SongID - How is it programmed?

  - by CheeseConQueso

  For anyone not familiar with Verizon's SongID program, it is a free application downloadable through Verizon's VCast network. It listens to a song for 10 seconds at any point during the song and then sends this data to some all-knowing algorithmic beast that chews it up and sends you back all the ID3 tags (artist, album, song, etc...) The first two parts and last part are straightforward, but what goes on during the processing after the recorded sound is sent? I figure it must take the sound file (what format?), parse it (how? with what?) for some key identifiers (what are these? regular attributes of wave functions? phase/shift/amplitude/etc), and check it against a database. Everything I find online about how this works is something generic like what I typed above. From audiotag.info This service is based on a sophisticated audio recognition algorithm combining advanced audio fingerprinting technology and a large songs' database. When you upload an audio file, it is being analyzed by an audio engine. During the analysis its audio “fingerprint” is extracted and identified by comparing it to the music database. At the completion of this recognition process, information about songs with their matching probabilities are displayed on screen.

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• Encryption puzzle / How to create a ProxyStub for a Remote Assistance ticket

  - by Jon Clegg

  I am trying to create a ticket for Remote Assistance. Part of that requires creating a PassStub parameter. As of the documenation: http://msdn.microsoft.com/en-us/library/cc240115(PROT.10).aspx PassStub: The encrypted novice computer's password string. When the Remote Assistance Connection String is sent as a file over e-mail, to provide additional security, a password is used.<16 In part 16 they detail how to create as PassStub. In Windows XP and Windows Server 2003, when a password is used, it is encrypted using PROV_RSA_FULL predefined Cryptographic provider with MD5 hashing and CALG_RC4, the RC4 stream encryption algorithm. As PassStub looks like this in the file: PassStub="LK#6Lh*gCmNDpj" If you want to generate one yourself run msra.exe in Vista or run the Remote Assistance tool in WinXP. The documentation says this stub is the result of the function CryptEncrypt with the key derived from the password and encrypted with the session id (Those are also in the ticket file). The problem is that CryptEncrypt produces a binary output way larger then the 15 byte PassStub. Also the PassStub isn't encoding in any way I've seen before. Some interesting things about the PassStub encoding. After doing statistical analysis the 3rd char is always a one of: !#$&()+-=@^. Only symbols seen everywhere are: *_ . Otherwise the valid characters are 0-9 a-z A-Z. There are a total of 75 valid characters and they are always 15 bytes. Running msra.exe with the same password always generates a different PassStub, indicating that it is not a direct hash but includes the rasessionid as they say. Some other ideas I've had is that it is not the direct result of CryptEncrypt, but a result of the rasessionid in the MD5 hash. In MS-RA (http://msdn.microsoft.com/en-us/library/cc240013(PROT.10).aspx). The "PassStub Novice" is simply hex encoded, and looks to be the right length. The problem is I have no idea how to go from any hash to way the ProxyStub looks like.

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• converting a Tree to newick format. java

  - by Esmond

  I'm having problems converting a binary rooted tree to newick format. The full explanation for such a format can be found: http://code.google.com/p/mrsrf/wiki/NewickTree An example of a newick format would be as follows: for a tree T such as http://www.cs.mcgill.ca/~cs251/OldCourses/1997/topic8/images/completetreetwo.gif the newick representation would be: (((8,9),(10,11)),((12,13),(14,15))) the internal node will become the commas while the leaves will be retained. such trees have internal nodes which will always have 2 children. I have a problem using recursion to come out with this newick format. The output contains far too many nodes and braces. Any comments to resolve this problem is appreciated or even an iterative algorithm would be welcomed import java.util.Stack; public class Tree { .... public String inOrderNewick(Node root, String output) throws ItemNotFoundException { if (root.hasChild()) { output += "("; output += inOrderNewick(root.child1, output); output += ","; output += inOrderNewick(root.child2, output); output += ")"; return output; } else { output += root.getSeq(); return output; } } }

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• CUDA: When to use shared memory and when to rely on L1 caching?

  - by Roger Dahl

  After Compute Capability 2.0 (Fermi) was released, I've wondered if there are any use cases left for shared memory. That is, when is it better to use shared memory than just let L1 perform its magic in the background? Is shared memory simply there to let algorithms designed for CC < 2.0 run efficiently without modifications? To collaborate via shared memory, threads in a block write to shared memory and synchronize with __syncthreads(). Why not simply write to global memory (through L1), and synchronize with __threadfence_block()? The latter option should be easier to implement since it doesn't have to relate to two different locations of values, and it should be faster because there is no explicit copying from global to shared memory. Since the data gets cached in L1, threads don't have to wait for data to actually make it all the way out to global memory. With shared memory, one is guaranteed that a value that was put there remains there throughout the duration of the block. This is as opposed to values in L1, which get evicted if they are not used often enough. Are there any cases where it's better too cache such rarely used data in shared memory than to let the L1 manage them based on the usage pattern that the algorithm actually has?

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• [UNIX] Sort lines of massive file by number of words on line (ideally in parallel)

  - by conradlee

  I am working on a community detection algorithm for analyzing social network data from Facebook. The first task, detecting all cliques in the graph, can be done efficiently in parallel, and leaves me with an output like this: 17118 17136 17392 17064 17093 17376 17118 17136 17356 17318 12345 17118 17136 17356 17283 17007 17059 17116 Each of these lines represents a unique clique (a collection of node ids), and I want to sort these lines in descending order by the number of ids per line. In the case of the example above, here's what the output should look like: 17118 17136 17356 17318 12345 17118 17136 17356 17283 17118 17136 17392 17064 17093 17376 17007 17059 17116 (Ties---i.e., lines with the same number of ids---can be sorted arbitrarily.) What is the most efficient way of sorting these lines. Keep the following points in mind: The file I want to sort could be larger than the physical memory of the machine Most of the machines that I'm running this on have several processors, so a parallel solution would be ideal An ideal solution would just be a shell script (probably using sort), but I'm open to simple solutions in python or perl (or any language, as long as it makes the task simple) This task is in some sense very easy---I'm not just looking for any old solution, but rather for a simple and above all efficient solution

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• Help needed in grokking password hashes and salts

  - by javafueled

  I've read a number of SO questions on this topic, but grokking the applied practice of storing a salted hash of a password eludes me. Let's start with some ground rules: a password, "foobar12" (we are not discussing the strength of the password). a language, Java 1.6 for this discussion a database, postgreSQL, MySQL, SQL Server, Oracle Several options are available to storing the password, but I want to think about one (1): Store the password hashed with random salt in the DB, one column Found on SO and elsewhere is the automatic fail of plaintext, MD5/SHA1, and dual-columns. The latter have pros and cons MD5/SHA1 is simple. MessageDigest in Java provides MD5, SHA1 (through SHA512 in modern implementations, certainly 1.6). Additionally, most RDBMSs listed provide methods for MD5 encryption functions on inserts, updates, etc. The problems become evident once one groks "rainbow tables" and MD5 collisions (and I've grokked these concepts). Dual-column solutions rest on the idea that the salt does not need to be secret (grok it). However, a second column introduces a complexity that might not be a luxury if you have a legacy system with one (1) column for the password and the cost of updating the table and the code could be too high. But it is storing the password hashed with a random salt in single DB column that I need to understand better, with practical application. I like this solution for a couple of reasons: a salt is expected and considers legacy boundaries. Here's where I get lost: if the salt is random and hashed with the password, how can the system ever match the password? I have theory on this, and as I type I might be grokking the concept: Given a random salt of 128 bytes and a password of 8 bytes ('foobar12'), it could be programmatically possible to remove the part of the hash that was the salt, by hashing a random 128 byte salt and getting the substring of the original hash that is the hashed password. Then re hashing to match using the hash algorithm...??? So... any takers on helping. :) Am I close?

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• youtube embeded player: change video link with javascript, dinamically

  - by Anthony Koval'

  here is a part of html code (video urls marked with a django-template language variables): <div class="mainPlayer"> <object width="580" height="326"> <param name="movie" value="{{main_video.video_url}}"></param> <param name="allowFullScreen" value="true"></param> <param name="allowscriptaccess" value="always"></param> <embed src="{{main_video.video_url}}" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="580" height="326"></embed> </object> </div> and JS-code (using jQuery 1.4.x) $(document).ready(function(){ ..... $(".activeMovie img").live("click", function(){ video_url = ($(this).parent().find('input').val()); $('.mainPlayer').find('param:eq(0)').val(video_url); $('.mainPlayer').find('embed').attr('src', video_url); }) ... }) such a algorithm works fine in ff 3.6.3, but any luck in chrome4 or opera 10.x., src and value are changed, but youtube player still shows an old video.

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• A couple of questions about NHibernate's GuidCombGenerator

  - by Eyvind

  The following code can be found in the NHibernate.Id.GuidCombGenerator class. The algorithm creates sequential (comb) guids based on combining a "random" guid with a DateTime. I have a couple of questions related to the lines that I have marked with *1) and *2) below: private Guid GenerateComb() { byte[] guidArray = Guid.NewGuid().ToByteArray(); // *1) DateTime baseDate = new DateTime(1900, 1, 1); DateTime now = DateTime.Now; // Get the days and milliseconds which will be used to build the byte string TimeSpan days = new TimeSpan(now.Ticks - baseDate.Ticks); TimeSpan msecs = now.TimeOfDay; // *2) // Convert to a byte array // Note that SQL Server is accurate to 1/300th of a millisecond so we divide by 3.333333 byte[] daysArray = BitConverter.GetBytes(days.Days); byte[] msecsArray = BitConverter.GetBytes((long) (msecs.TotalMilliseconds / 3.333333)); // Reverse the bytes to match SQL Servers ordering Array.Reverse(daysArray); Array.Reverse(msecsArray); // Copy the bytes into the guid Array.Copy(daysArray, daysArray.Length - 2, guidArray, guidArray.Length - 6, 2); Array.Copy(msecsArray, msecsArray.Length - 4, guidArray, guidArray.Length - 4, 4); return new Guid(guidArray); } First of all, for *1), wouldn't it be better to have a more recent date as the baseDate, e.g. 2000-01-01, so as to make room for more values in the future? Regarding *2), why would we care about the accuracy for DateTimes in SQL Server, when we only are interested in the bytes of the datetime anyway, and never intend to store the value in an SQL Server datetime field? Wouldn't it be better to use all the accuracy available from DateTime.Now?

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• How can I apply a PSSM efficiently?

  - by flies

  I am fitting for position specific scoring matrices (PSSM aka Position Specific Weight Matrices). The fit I'm using is like simulated annealing, where I the perturb the PSSM, compare the prediction to experiment and accept the change if it improves agreement. This means I apply the PSSM millions of times per fit; performance is critical. In my particular problem, I'm applying a PSSM for an object of length L (~8 bp) at every position of a DNA sequence of length M (~30 bp) (so there are M-L+1 valid positions). I need an efficient algorithm to apply a PSSM. Can anyone help improve performance? My best idea is to convert the DNA into some kind of a matrix so that applying the PSSM is matrix multiplication. There are efficient linear algebra libraries out there (e.g. BLAS), but I'm not sure how best to turn an M-length DNA sequence into a matrix M x 4 matrix and then apply the PSSM at each position. The solution needs to work for higher order/dinucleotide terms in the PSSM - presumably this means representing the sequence-matrix for mono-nucleotides and separately for dinucleotides. My current solution iterates over each position m, then over each letter in word from m to m+L-1, adding the corresponding term in the matrix. I'm storing the matrix as a multi-dimensional STL vector, and profiling has revealed that a lot of the computation time is just accessing the elements of the PSSM (with similar performance bottlenecks accessing the DNA sequence). If someone has an idea besides matrix multiplication, I'm all ears.

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• Matlab Simulation: Point (symbol) Moving from start point to end point and back

  - by niko

  Hi, I would like to create an animation to demonstrate LDPC coding which is based on Sum-Product Algorithm So far I have created a graph which shows the connections between symbol nodes (left) and parity nodes (right) and would like to animate points travelling from symbol to parity nodes and back. The figure is drawn by executing the following method: function drawVertices(H) hold on; nodesCount = size(H); parityNodesCount = nodesCount(1); symbolNodesCount = nodesCount(2); symbolPoints = zeros(symbolNodesCount, 2); symbolPoints(:, 1) = 0; for i = 0 : symbolNodesCount - 1 ji = symbolNodesCount - i; scatter(0, ji) symbolPoints(i + 1, 2) = ji; end; parityPoints = zeros(parityNodesCount, 2); parityPoints(:, 1) = 10; for i = 0 : parityNodesCount - 1 ji = parityNodesCount - i; y0 = symbolNodesCount/2 - parityNodesCount/2; scatter(10, y0 + ji) parityPoints(i + 1, 2) = y0 + ji; end; axis([-1 11 -1 symbolNodesCount + 2]); axis off %connect vertices d = size(H); for i = 1 : d(1) for j = 1 : d(2) if(H(i, j) == 1) plot([parityPoints(i, 1) symbolPoints(j, 1)], [parityPoints(i, 2) symbolPoints(j, 2)]); end; end; end; So what I would like to do here is to add another method which takes start point (x and y) and end point as arguments and animates a travelling circle (dot) from start to end and back along the displayed lines. I would appreciate if anyone of you could show the solution or suggest any useful tutorial about matlab simulations. Thank you!

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• Idiomatic STL: Iterating over a list and inserting elements

  - by mkilling

  I'm writing an algorithm that iterates over a list of points, calculates the distance between them and inserts additional points if the distance is too great. However I seem to be lacking the proper familiarity with STL to come up with an elegant solution. I'm hoping that I can learn something, so I'll just show you my code. You might have some hints for me. for (std::list<PathPoint>::iterator it = ++points_.begin(); it != points_.end(); it++) { Vector curPos = it->getPosition(); Vector prevPos = (--it)->getPosition(); Vector vecFromPrev = curPos - prevPos; float distance = vecFromPrev.abs(); it++; if (distance > MAX_DISTANCE_BETWEEN_POINTS) { int pointsToInsert = (int)(distance / MAX_DISTANCE_BETWEEN_POINTS); Vector curPos = prevPos; for (int i = 0; i < pointsToInsert; i++) { curPos += vecFromPrev / pointsToInsert; it = points_.insert(it, PathPoint(curPos, false)); it++; } } }

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• Linked List Sorting with Strings In C

  - by user308583

  I have a struct, with a Name and a single Node called nextName It's a Singly Linked list, and my task is to create the list, based on alphabetical order of the strings. So iff i enter Joe Zolt and Arthur i should get my list structured as Joe Than Joe Zolt Than Arthur Joe Zolt I'm having trouble implementing the correct Algorithm, which would put the pointers in the right order. This is What I have as of Now. Temp would be the name the user just entered and is trying to put into the list, namebox is just a copy of my root, being the whole list if(temp != NULL) { struct node* namebox = root; while (namebox!=NULL && (strcmp((namebox)->name,temp->name) <= 0)) { namebox = namebox->nextName; printf("here"); } temp->nextName = namebox; namebox = temp; root = namebox; This Works right now, if i enter names like CCC BBB than AAA I Get Back AAA BBB CCC when i print But if i put AAA BBB CCC , When i print i only get CCC, it cuts the previous off.

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• MySQL Query That Can Pull the Data I am Seeking?

  - by Amy

  On the project I am working on, I am stuck with the table structure from Hades. Two things to keep in mind: I can't change the table structure right now. I'm stuck with it for the time being. The queries are dynamically generated and not hard coded. So, while I am asking for a query that can pull this data, what I am really working toward is an algorithm that will generate the query I need. Hopefully, I can explain the problem without making your eyes glaze over and your brain implode. We have an instance table that looks (simplified) along these lines: Instances InstanceID active 1 Y 2 Y 3 Y 4 N 5 Y 6 Y Then, there are multiple data tables along these lines: Table1 InstanceID field1 reference_field2 1 John 5 2 Sally NULL 3 Fred 6 4 Joe NULL Table2 InstanceID field3 5 1 6 1 Table3 InstanceID fieldID field4 5 1 Howard 5 2 James 6 2 Betty Please note that reference_field2 in Table1 contains a reference to another instance. Field3 in Table2 is a bit more complicated. It contains a fieldID for Table 3. What I need is a query that will get me a list as follows: InstanceID field1 field4 1 John Howard 2 Sally 3 Fred The problem is, in the query I currently have, I do not get Fred because there is no entry in Table3 for fieldID 1 and InstanceID 6. So, the very best list I have been able to get thus far is InstanceID field1 field4 1 John Howard 2 Sally In essence, if there is an entry in Table1 for Field 2, and there is not an entry in Table 3 that has the instanceID contained in field2 and the field ID contained in field3, I don't get the data from field1. I have looked at joins till I'm blue in the face, and I can't see a way to handle the case when table3 has no entry.

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• Encryption puzzle / How to create a PassStub for a Remote Assistance ticket

  - by Jon Clegg

  I am trying to create a ticket for Remote Assistance. Part of that requires creating a PassStub parameter. As of the documentation: http://msdn.microsoft.com/en-us/library/cc240115(PROT.10).aspx PassStub: The encrypted novice computer's password string. When the Remote Assistance Connection String is sent as a file over e-mail, to provide additional security, a password is used.<16 In part 16 they detail how to create as PassStub. In Windows XP and Windows Server 2003, when a password is used, it is encrypted using PROV_RSA_FULL predefined Cryptographic provider with MD5 hashing and CALG_RC4, the RC4 stream encryption algorithm. As PassStub looks like this in the file: PassStub="LK#6Lh*gCmNDpj" If you want to generate one yourself run msra.exe in Vista or run the Remote Assistance tool in WinXP. The documentation says this stub is the result of the function CryptEncrypt with the key derived from the password and encrypted with the session id (Those are also in the ticket file). The problem is that CryptEncrypt produces a binary output way larger then the 15 byte PassStub. Also the PassStub isn't encoding in any way I've seen before. Some interesting things about the PassStub encoding. After doing statistical analysis the 3rd char is always a one of: !#$&()+-=@^. Only symbols seen everywhere are: *_ . Otherwise the valid characters are 0-9 a-z A-Z. There are a total of 75 valid characters and they are always 15 bytes. Running msra.exe with the same password always generates a different PassStub, indicating that it is not a direct hash but includes the rasessionid as they say. Some other ideas I've had is that it is not the direct result of CryptEncrypt, but a result of the rasessionid in the MD5 hash. In MS-RA (http://msdn.microsoft.com/en-us/library/cc240013(PROT.10).aspx). The "PassStub Novice" is simply hex encoded, and looks to be the right length. The problem is I have no idea how to go from any hash to way the PassStub looks like.

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• Classification: Dealing with Abstain/Rejected Class

  - by abner.ayala

  I am asking for your input and/help on a classification problem. If anyone have any references that I can read to help me solve my problem even better. I have a classification problem of four discrete and very well separated classes. However my input is continuous and has a high frequency (50Hz), since its a real-time problem. The circles represent the clusters of the classes, the blue line the decision boundary and Class 5 equals the (neutral/resting do nothing class). This class is the rejected class. However the problem is that when I move from one class to the other I activate a lot of false positives in the transition movements, since the movement is clearly non-linear. For example, every time I move from class 5 (neutral class) to 1 I first see a lot of 3's before getting to the 1 class. Ideally, I will want my decision boundary to look like the one in the picture below where the rejected class is Class =5. Has a higher decision boundary than the others classes to avoid misclassification during transition. I am currently implementing my algorithm in Matlab using naive bayes, kNN, and SVMs optimized algorithms using Matlab. Question: What is the best/common way to handle abstain/rejected classes classes? Should I use (fuzzy logic, loss function, should I include resting cluster in the training)?

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• Password Cracking in 2010 and Beyond

  - by mttr

  I have looked a bit into cryptography and related matters during the last couple of days and am pretty confused by now. I have a question about password strength and am hoping that someone can clear up my confusion by sharing how they think through the following questions. I am becoming obsessed about these things, but need to spend my time otherwise :-) Let's assume we have an eight-digit password that consists of upper and lower-case alphabetic characters, numbers and common symbols. This means we have 8^96 ~= 7.2 quadrillion different possible passwords. As I understand there are at least two approaches to breaking this password. One is to try a brute-force attack where we try to guess each possible combination of characters. How many passwords can modern processors (in 2010, Core i7 Extreme for eg) guess per second (how many instructions does a single password guess take and why)? My guess would be that it takes a modern processor in the order of years to break such a password. Another approach would consist of obtaining a hash of my password as stored by operating systems and then search for collisions. Depending on the type of hash used, we might get the password a lot quicker than by the bruteforce attack. A number of questions about this: Is the assertion in the above sentence correct? How do I think about the time it takes to find collisions for MD4, MD5, etc. hashes? Where does my Snow Leopard store my password hash and what hashing algorithm does it use? And finally, regardless of the strength of file encryption using AES-128/256, the weak link is still my en/decryption password used. Even if breaking the ciphered text would take longer than the lifetime of the universe, a brute-force attack on my de/encryption password (guess password, then try to decrypt file, try next password...), might succeed a lot earlier than the end of the universe. Is that correct? I would be very grateful, if people could have mercy on me and help me think through these probably simple questions, so that I can get back to work.

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• A Digg-like rotating homepage of popular content, how to include date as a factor?

  - by Ferdy

  I am building an advanced image sharing web application. As you may expect, users can upload images and others can comments on it, vote on it, and favorite it. These events will determine the popularity of the image, which I capture in a "karma" field. Now I want to create a Digg-like homepage system, showing the most popular images. It's easy, since I already have the weighted Karma score. I just sort on that descendingly to show the 20 most valued images. The part that is missing is time. I do not want extremely popular images to always be on the homepage. I guess an easy solution is to restrict the result set to the last 24 hours. However, I'm also thinking that in order to keep the image rotation occur throughout the day, time can be some kind of variable where its offset has an influence on the image's sorting. Specific questions: Would you recommend the easy scenario (just sort for best images within 24 hours) or the more sophisticated one (use datetime offset as part of the sorting)? If you advise the latter, any help on the mathematical solution to this? Would it be best to run a scheduled service to mark images for the homepage, or would you advise a direct query (I'm using MySQL) As an extra note, the homepage should support paging and on a quiet day should include entries of days before in order to make sure it is always "filled" I'm not asking the community to build this algorithm, just looking for some advise :)

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• Foosball result prediction

  - by Wolf

  In our office, we regularly enjoy some rounds of foosball / table football after work. I have put together a small java program that generates random 2vs2 lineups from the available players and stores the match results in a database afterwards. The current prediction of the outcome uses a simple average of all previous match results from the 4 involved players. This gives a very rough estimation, but I'd like to replace it with something more sophisticated, taking into account things like: players may be good playing as attacker but bad as defender (or vice versa) players do well against a specific opponent / bad against others some teams work well together, others don't skills change over time What would be the best algorithm to predict the game outcome as accurately as possible? Someone suggested using a neural network for this, which sounds quite interesting... but I do not have enough knowledge on the topic to say if that could work, and I also suspect it might take too many games to be reasonably trained. EDIT: Had to take a longer break from this due to some project deadlines. To make the question more specific: Given the following mysql table containing all matches played so far: table match_result match_id int pk match_start datetime duration int (match length in seconds) blue_defense int fk to table player blue_attack int fk to table player red_defense int fk to table player red_attack int fk to table player score_blue int score_red int How would you write a function predictResult(blueDef, blueAtk, redDef, redAtk) {...} to estimate the outcome as closely as possible, executing any sql, doing calculations or using external libraries?

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• How do you rotate a sprite around its center by caclulating a new x and y position?

  - by Sam152

  I'm using Dark GDK and C++ to create a simple 2d game. I'm rotating an object but it rotates from the top left corner of the sprite. I have the following variables available: PlayerX PlayerY PlayerWidth PlayerHeight RotateAngle (360 x 0) Is there an algorithm that will modify the pivot point of the sprite, preferable to the center? Here is a small code sample: void Player::Move( void ) { if ( checkLeft() ) { PlayerX -= PlayerSpeed; if ( PlayerX < 0 ) PlayerX = 0; } if ( checkRight() ) { PlayerX += PlayerSpeed ; if ( PlayerX > 800 - PlayerWidth ) PlayerX = 800 - PlayerWidth; } if ( checkUp()) { PlayerY -= PlayerSpeed; if ( PlayerY < 0 ) PlayerY = 0; } if ( checkDown()) { PlayerY += PlayerSpeed; if ( PlayerY > 600 - PlayerHeight) PlayerY = 600 - PlayerHeight; } RotateAngle += 5; if(RotateAngle > 360) RotateAngle -=360; dbRotateSprite(Handle,RotateAngle); dbSprite ( 1 , PlayerX , PlayerY , Handle ); } Edit I'm considering opening up some reputation for this question, I have yet to be provided with an answer that works for me. If someone can provide an actual code sample that does the job, I'd be very happy. The problem with Blindy's answer is that no matter how much I simply translate it back or forth, the spirte still rotates around the top left hand corner and moving it somewhere rotating around the top left corner, then moving it back to the same position accomplishes nothing. Here is what I believe to be going on: Just so there is no confusion I have created a an image of what is going on. The left shows what is actually happening and the right shows what I need to happen.

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• Perceptron Classification and Model Training

  - by jake pinedo

  I'm having an issue with understanding how the Perceptron algorithm works and implementing it. cLabel = 0 #class label: corresponds directly with featureVectors and tweets for m in range(miters): for point in featureVectors: margin = answers[cLabel] * self.dot_product(point, w) if margin <= 0: modifier = float(lrate) * float(answers[cLabel]) modifiedPoint = point for x in modifiedPoint: if x != 0: x *= modifier newWeight = [modifiedPoint[i] + w[i] for i in range(len(w))] w = newWeight self._learnedWeight = w This is what I've implemented so far, where I have a list of class labels in answers and a learning rate (lrate) and a list of feature vectors. I run it for the numbers of iterations in miter and then get the final weight at the end. However, I'm not sure what to do with this weight. I've trained the perceptron and now I have to classify a set of tweets, but I don't know how to do that. EDIT: Specifically, what I do in my classify method is I go through and create a feature vector for the data I'm given, which isn't a problem at all, and then I take the self._learnedWeight that I get from the earlier training code and compute the dot-product of the vector and the weight. My weight and feature vectors include a bias in the 0th term of the list so I'm including that. I then check to see if the dotproduct is less than or equal to 0: if so, then I classify it as -1. Otherwise, it's 1. However, this doesn't seem to be working correctly.

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