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• TC hashing filters - single rule deletion

  - by exa

  For traffic shaping I'm currently using a setup that looks exactly like the setup from LARTC, on this page: http://lartc.org/howto/lartc.adv-filter.hashing.html I have a simple problem with that - everytime I want to modify something in the hash table (like assign a IP to different flowid), I need to delete the whole filter table and add it again filter by filter. (I actually don't do it by hand, I have a nice program that does it for me... but still...) There is a problem - I got roughly 10k filters allocated this way and deleting and refilling the whole filtertable can get pretty lengthy, which is not exactly good for traffic shaping. My program could easily manage to delete only the rules that need to be deleted (thus reducing the whole problem to several commands and miliseconds), but I simply don't know the command that deletes only the one hashing rule. My tc filter show: filter parent 1: protocol ip pref 1 u32 filter parent 1: protocol ip pref 1 u32 fh 2: ht divisor 256 filter parent 1: protocol ip pref 1 u32 fh 2:a:800 order 2048 key ht 2 bkt a flowid 1:101 match 0a0a0a0a/ffffffff at 16 filter parent 1: protocol ip pref 1 u32 fh 2:c:800 order 2048 key ht 2 bkt c flowid 1:102 match 0a0a0a0c/ffffffff at 16 filter parent 1: protocol ip pref 1 u32 fh 800: ht divisor 1 filter parent 1: protocol ip pref 1 u32 fh 800::800 order 2048 key ht 800 bkt 0 link 2: match 00000000/00000000 at 16 hash mask 000000ff at 16 The wish: 'tc filter del ...' command that removes only one specific filter (for example the 0a0a0a0a IP match (IP address 10.10.10.10)). Removal of some small subgroup would also be good - for example I could still recreate a bucket (bkt a) pretty fast. My attempts: I tried to number all the filters using prio, but with no help -- they just create something unusuable (but deletable) below, but the bucketed filters remain there after that gets deleted. Any ideas? edit - I'm adding a simplified tl;dr description of the problem: I created hash filter on some interfce just like in this http://lartc.org/howto/lartc.adv-filter.hashing.html I want to find a command that deletes one rule (e.g. 1.2.1.123) from the table, leaving the rest untouched and working.

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• Hi, I have a C hashing routine which is behaving strangely?

  - by aks

  Hi, In this hashing routine: 1.) I am able to add strings. 2.) I am able to view my added strings. 3.) When i try to add a duplicate string, it throws me an error saying already present. 4.) But, when i try to delete the same string which is already present in hash table, then the lookup_routine calls hash function to get an index. At this time, it throws a different hash index to the one it was already added. Hence, my delete routine is failing? I am able to understand the reason why for same string, hash fucntion calculates a different index each time (whereas the same logic works in view hash table routine)? Can someone help me? This is the Output, i am getting: $ ./a Press 1 to add an element to the hashtable Press 2 to delete an element from the hashtable Press 3 to search the hashtable Press 4 to view the hashtable Press 5 to exit Please enter your choice: 1 Please enter the string :gaura enters in add_string DEBUG purpose in hash function: str passed = gaura Hashval returned in hash func= 1 hashval = 1 enters in lookup_string str in lookup_string = gaura DEBUG purpose in hash function: str passed = gaura Hashval returned in hash func= 1 hashval = 1 DEBUG ERROR :element not found in lookup string DEBUG Purpose NULL Inserting... DEBUG1 : enters here hashval = 1 String added successfully Press 1 to add an element to the hashtable Press 2 to delete an element from the hashtable Press 3 to search the hashtable Press 4 to view the hashtable Press 5 to exit Please enter your choice: 1 Please enter the string :ayu enters in add_string DEBUG purpose in hash function: str passed = ayu Hashval returned in hash func= 1 hashval = 1 enters in lookup_string str in lookup_string = ayu DEBUG purpose in hash function: str passed = ayu Hashval returned in hash func= 1 hashval = 1 returns NULL in lookup_string DEBUG Purpose NULL Inserting... DEBUG2 : enters here hashval = 1 String added successfully Press 1 to add an element to the hashtable Press 2 to delete an element from the hashtable Press 3 to search the hashtable Press 4 to view the hashtable Press 5 to exit Please enter your choice: 1 Please enter the string :gaurava enters in add_string DEBUG purpose in hash function: str passed = gaurava Hashval returned in hash func= 7 hashval = 7 enters in lookup_string str in lookup_string = gaurava DEBUG purpose in hash function: str passed = gaurava Hashval returned in hash func= 7 hashval = 7 DEBUG ERROR :element not found in lookup string DEBUG Purpose NULL Inserting... DEBUG1 : enters here hashval = 7 String added successfully Press 1 to add an element to the hashtable Press 2 to delete an element from the hashtable Press 3 to search the hashtable Press 4 to view the hashtable Press 5 to exit Please enter your choice: 4 Index : i = 1 String = gaura ayu Index : i = 7 String = gaurava Press 1 to add an element to the hashtable Press 2 to delete an element from the hashtable Press 3 to search the hashtable Press 4 to view the hashtable Press 5 to exit Please enter your choice: 2 Please enter the string you want to delete :gaura String entered = gaura enters in delete_string DEBUG purpose in hash function: str passed = gaura Hashval returned in hash func= 0 hashval = 0 enters in lookup_string str in lookup_string = gaura DEBUG purpose in hash function: str passed = gaura Hashval returned in hash func= 0 hashval = 0 DEBUG ERROR :element not found in lookup string DEBUG Purpose Item not present. So, cannot be deleted Item found in list: Deletion failed Press 1 to add an element to the hashtable Press 2 to delete an element from the hashtable Press 3 to search the hashtable Press 4 to view the hashtable Press 5 to exit Please enter your choice: My routine is pasted below: include include struct list { char *string; struct list *next; }; struct hash_table { int size; /* the size of the table */ struct list *table; / the table elements */ }; struct hash_table * hashtable = NULL; struct hash_table *create_hash_table(int size) { struct hash_table *new_table; int i; if (size<1) return NULL; /* invalid size for table */ /* Attempt to allocate memory for the table structure */ if ((new_table = malloc(sizeof(struct hash_table))) == NULL) { return NULL; } /* Attempt to allocate memory for the table itself */ if ((new_table->table = malloc(sizeof(struct list *) * size)) == NULL) { return NULL; } /* Initialize the elements of the table */ for(i=0; i<size; i++) new_table->table[i] = '\0'; /* Set the table's size */ new_table->size = size; return new_table; } unsigned int hash(struct hash_table *hashtable, char *str) { printf("\n DEBUG purpose in hash function:\n"); printf("\n str passed = %s", str); unsigned int hashval = 0; int i = 0; for(; *str != '\0'; str++) { hashval += str[i]; i++; } hashval = hashval % 10; printf("\n Hashval returned in hash func= %d", hashval); return hashval; } struct list *lookup_string(struct hash_table *hashtable, char *str) { printf("\n enters in lookup_string \n"); printf("\n str in lookup_string = %s",str); struct list * new_list; unsigned int hashval = hash(hashtable, str); printf("\n hashval = %d \n", hashval); if(hashtable->table[hashval] == NULL) { printf("\n DEBUG ERROR :element not found in lookup string \n"); return NULL; } /* Go to the correct list based on the hash value and see if str is * in the list. If it is, return return a pointer to the list element. * If it isn't, the item isn't in the table, so return NULL. */ for(new_list = hashtable->table[hashval]; new_list != NULL;new_list = new_list->next) { if (strcmp(str, new_list->string) == 0) return new_list; } printf("\n returns NULL in lookup_string \n"); return NULL; } int add_string(struct hash_table *hashtable, char *str) { printf("\n enters in add_string \n"); struct list *new_list; struct list *current_list; unsigned int hashval = hash(hashtable, str); printf("\n hashval = %d", hashval); /* Attempt to allocate memory for list */ if ((new_list = malloc(sizeof(struct list))) == NULL) { printf("\n enters here \n"); return 1; } /* Does item already exist? */ current_list = lookup_string(hashtable, str); if (current_list == NULL) { printf("\n DEBUG Purpose \n"); printf("\n NULL \n"); } /* item already exists, don't insert it again. */ if (current_list != NULL) { printf("\n Item already present...\n"); return 2; } /* Insert into list */ printf("\n Inserting...\n"); new_list->string = strdup(str); new_list->next = NULL; //new_list->next = hashtable->table[hashval]; if(hashtable->table[hashval] == NULL) { printf("\n DEBUG1 : enters here \n"); printf("\n hashval = %d", hashval); hashtable->table[hashval] = new_list; } else { printf("\n DEBUG2 : enters here \n"); printf("\n hashval = %d", hashval); struct list * temp_list = hashtable->table[hashval]; while(temp_list->next!=NULL) temp_list = temp_list->next; temp_list->next = new_list; // hashtable->table[hashval] = new_list; } return 0; } int delete_string(struct hash_table *hashtable, char *str) { printf("\n enters in delete_string \n"); struct list *new_list; struct list *current_list; unsigned int hashval = hash(hashtable, str); printf("\n hashval = %d", hashval); /* Does item already exist? */ current_list = lookup_string(hashtable, str); if (current_list == NULL) { printf("\n DEBUG Purpose \n"); printf("\n Item not present. So, cannot be deleted \n"); return 1; } /* item exists, delete it. */ if (current_list != NULL) { struct list * temp = hashtable->table[hashval]; if(strcmp(temp->string,str) == 0) { if(temp->next == NULL) { hashtable->table[hashval] = NULL; free(temp); } else { hashtable->table[hashval] = temp->next; free(temp); } } else { struct list * temp1; while(temp->next != NULL) { temp1 = temp; if(strcmp(temp->string, str) == 0) { break; } else { temp = temp->next; } } if(temp->next == NULL) { temp1->next = NULL; free(temp); } else { temp1->next = temp->next; free(temp); } } } return 0; } void free_table(struct hash_table *hashtable) { int i; struct list *new_list, *temp_list; if (hashtable==NULL) return; /* Free the memory for every item in the table, including the * strings themselves. */ for(i=0; i<hashtable->size; i++) { new_list = hashtable->table[i]; while(new_list!=NULL) { temp_list = new_list; new_list = new_list->next; free(temp_list->string); free(temp_list); } } /* Free the table itself */ free(hashtable->table); free(hashtable); } void view_hashtable(struct hash_table * hashtable) { int i = 0; if(hashtable == NULL) return; for(i =0; i < hashtable->size; i++) { if((hashtable->table[i] == 0) || (strcmp(hashtable->table[i]->string, "*") == 0)) { continue; } printf(" Index : i = %d\t String = %s",i, hashtable->table[i]->string); struct list * temp = hashtable->table[i]->next; while(temp != NULL) { printf("\t %s",temp->string); temp = temp->next; } printf("\n"); } } int main() { hashtable = create_hash_table(10); if(hashtable == NULL) { printf("\n Memory allocation failure during creation of hash table \n"); return 0; } int flag = 1; while(flag) { int choice; printf("\n Press 1 to add an element to the hashtable\n"); printf("\n Press 2 to delete an element from the hashtable\n"); printf("\n Press 3 to search the hashtable\n"); printf("\n Press 4 to view the hashtable\n"); printf("\n Press 5 to exit \n"); printf("\n Please enter your choice: "); scanf("%d",&choice); if(choice == 5) flag = 0; else if(choice == 1) { char str[20]; printf("\n Please enter the string :"); scanf("%s",&str); int i; i = add_string(hashtable,str); if(i == 1) { printf("\n Memory allocation failure:Choice 1 \n"); return 0; } else if(i == 2) { printf("\n String already prsent in hash table : Couldnot add it again\n"); return 0; } else { printf("\n String added successfully \n"); } } else if(choice == 2) { int i; struct list * temp_list; char str[20]; printf("\n Please enter the string you want to delete :"); scanf("%s",&str); printf("\n String entered = %s", str); i = delete_string(hashtable,str); if(i == 0) { printf("\n Item found in list: Deletion success \n"); } else printf("\n Item found in list: Deletion failed \n"); } else if(choice == 3) { struct list * temp_list; char str[20]; printf("\n Please enter the string :"); scanf("%s",&str); temp_list = lookup_string(hashtable,str); if(!temp_list) { printf("\n Item not found in list: Deletion failed \n"); return 0; } printf("\n Item found: Present in Hash Table \n"); } else if(choice == 4) { view_hashtable(hashtable); } else if(choice == 5) { printf("\n Exiting ...."); return 0; } else printf("\n Invalid choice:"); }; free_table(hashtable); return 0; }

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• Secure hash and salt for PHP passwords

  - by luiscubal

  It is currently said that MD5 is partially unsafe. Taking this into consideration, I'd like to know which mechanism to use for password protection. Is “double hashing” a password less secure than just hashing it once? Suggests that hashing multiple times may be a good idea. How to implement password protection for individual files? Suggests using salt. I'm using PHP. I want a safe and fast password encryption system. Hashing a password a million times may be safer, but also slower. How to achieve a good balance between speed and safety? Also, I'd prefer the result to have a constant number of characters. The hashing mechanism must be available in PHP It must be safe It can use salt (in this case, are all salts equally good? Is there any way to generate good salts?) Also, should I store two fields in the database(one using MD5 and another one using SHA, for example)? Would it make it safer or unsafer? In case I wasn't clear enough, I want to know which hashing function(s) to use and how to pick a good salt in order to have a safe and fast password protection mechanism. EDIT: The website shouldn't contain anything too sensitive, but still I want it to be secure. EDIT2: Thank you all for your replies, I'm using hash("sha256",$salt.":".$password.":".$id) Questions that didn't help: What's the difference between SHA and MD5 in PHP Simple Password Encryption Secure methods of storing keys, passwords for asp.net How would you implement salted passwords in Tomcat 5.5

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• MD5 and SHA1 checksum uses for downloading

  - by Zac

  I notice that when downloading a lot of open source tools (Eclipse, etc.) there are links for MD5 and SHA1 checksums, and didn't know what these were or what their purpose was. I know these are hashing algorithms, and I do understand hashing, so my only guess is that these are used for hashing some component of the download targets, and to compare them with "official" hash strings stored server-side. Perhaps that way it can be determined whether or not the targets have been modified from their correct version (for security and other purposes). Am I close or completely wrong, and if wrong, what are they?!?! Thanks!

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• hashing password giving different results

  - by geoff

  I am taking over a system that a previous developer wrote. The system has an administrator approve a user account and when they do that the system uses the following method to hash a password and save it to the database. It sends the unhashed password to the user. When the user logs in the system uses the exact same method to hash what the user enters and compares it to the database value. We've run into a couple of times when the database entry doesn't match the user's entry whey they should. So it appears that the method isn't always hashing the value the same. Does anyone know if this method of hashing isn't reliable and how to make it reliable? Thanks. private string HashPassword(string password) { string hashedPassword = string.Empty; // Convert plain text into a byte array. byte[] plainTextBytes = Encoding.UTF8.GetBytes(password); // Allocate array, which will hold plain text and salt. byte[] plainTextWithSaltBytes = new byte[plainTextBytes.Length + SALT.Length]; // Copy plain text bytes into resulting array. for(int i = 0; i < plainTextBytes.Length; i++) plainTextWithSaltBytes[i] = plainTextBytes[i]; // Append salt bytes to the resulting array. for(int i = 0; i < SALT.Length; i++) plainTextWithSaltBytes[plainTextBytes.Length + i] = SALT[i]; // Because we support multiple hashing algorithms, we must define // hash object as a common (abstract) base class. We will specify the // actual hashing algorithm class later during object creation. HashAlgorithm hash = new SHA256Managed(); // Compute hash value of our plain text with appended salt. byte[] hashBytes = hash.ComputeHash(plainTextWithSaltBytes); // Create array which will hold hash and original salt bytes. byte[] hashWithSaltBytes = new byte[hashBytes.Length + SALT.Length]; // Copy hash bytes into resulting array. for(int i = 0; i < hashBytes.Length; i++) hashWithSaltBytes[i] = hashBytes[i]; // Append salt bytes to the result. for(int i = 0; i < SALT.Length; i++) hashWithSaltBytes[hashBytes.Length + i] = SALT[i]; // Convert result into a base64-encoded string. hashedPassword = Convert.ToBase64String(hashWithSaltBytes); return hashedPassword; }

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• Can I save & store a user's submission in a way that proves that the data has not been altered, and that the timestamp is accurate?

  - by jt0dd

  There are many situations where the validity of the timestamp attached to a certain post (submission of information) might be invaluable for the post owner's legal usage. I'm not looking for a service to achieve this, as requested in this great question, but rather a method for the achievement of such a service. For the legal (in most any law system) authentication of text content and its submission time, the owner of the content would need to prove: that the timestamp itself has not been altered and was accurate to begin with. that the text content linked to the timestamp had not been altered I'd like to know how to achieve this via programming (not a language-specific solution, but rather the methodology behind the solution). Can a timestamp be validated to being accurate to the time that the content was really submitted? Can data be stored in a form that it can be read, but not written to, in a proven way? In other words, can I save & store a user's submission in a way that proves that the data has not been altered, and that the timestamp is accurate? I can't think of any programming method that would make this possible, but I am not the most experienced programmer out there. Based on MidnightLightning's answer to the question I cited, this sort of thing is being done. Clarification: I'm looking for a method (hashing, encryption, etc) that would allow an average guy like me to achieve the desired effect through programming. I'm interested in this subject for the purpose of Defensive Publication. I'd like to learn a method that allows an every-day programmer to pick up his computer, write a program, pass information through it, and say: I created this text at this moment in time, and I can prove it. This means the information should be protected from the programmer who writes the code as well. Perhaps a 3rd party API would be required. I'm ok with that.

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• Hash Algorithm Randomness Visualization

  - by clstroud

  I'm curious if anyone here has any idea how the images were generated as shown in this response: Which hashing algorithm is best for uniqueness and speed? Ian posted a very well-received response but I can't seem to understand how he went about making the images. I hate to make a new question dedicated to this, but I can't find any means to ask him more directly. On the other hand, perhaps someone has an alternative perspective. The best I can personally come up with would be to have it almost like a bar graph, which would illustrate how evenly the buckets of the hash table are being generated. I have a working Cocoa program that does this, but it can't generate anything like what he showed there. So the question is two fold I suppose: A) How does one truly interpret the data he shows? Is it more than "less whitespace = better"? B) How does one generate such an image based on some set of inputs, a hash, and an index? Perhaps I'm misunderstanding entirely, but I really would like to know more about this particular visualization technique. Or maybe I'm mis-applying this to hash tables rather than just hashes in general, but in that case I don't know how it would be "bounded" for the image.

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• nginx hashing on GET parameter

  - by Sparsh Gupta

  I have two Varnish servers and I plan to add more varnish servers. I am using a nginx load balancer to divide traffic to these varnish servers. To utilize maximum RAM of each varnish server, I need that same request reaches same varnish server. Same request can be identified by one GET parameter in the request URL say 'a' In a normal code, I would do something like- (if I need to divide all traffic between 2 Varnish servers) if($arg_a % 2 == 0) { proxy_pass varnish1; } if($arg_a % 2 == 1) { proxy_pass varnish2; } This is basically doing a even / odd check on GET parameter a and then deciding which upstream pool to send the request. My question are- What is the nginx equivalent of such a code. I dont know if nginx accepts modulas Is there a better/ efficient hashing function built in with nginx (0.8.54) which I can possibly use. In future I want to add more upstream pools so I need not to change %2 to %3 %4 and so on Any other alternate way to solve this problem

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• Automate hashing for each file in a folder?

  - by Kennie R.

  I have quite a few FTP folders, and I add a few each month and prefer to leave some sort of method of verifying their integrity, for example the files MD5SUMS, SHA256SUMS, ... which I could create using a script. Take for example: find ./ -type f -exec md5sum $1 {} \; This works fine, but when I run it each time for each shaxxx sum afterwards, it creates a sum of the MD5SUMs file which is really not wanted. Is there a simpler way, or script, or common way of hashing all the files in to their sums file without causing problems like that? I could really use a better option.

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• Using hashing to group similar records

  - by Neil Dobson

  I work for a fulfillment company and we have to pack and ship many orders from our warehouse to customers. To improve efficiency we would like to group identical orders and pack these in the most optimum way. By identical I mean having the same number of order lines containing the same SKUs and same order quantities. To achieve this I was thinking about hashing each order. We can then group by hash to quickly see which orders are the same. We are moving from an Access database to a PostgreSQL database and we have .NET based systems for data loading and general order processing systems, so we can either do the hashing during the data loading or hand this task over to the DB. My question firstly is should the hashing be managed by DB, possibly using triggers, or should the hash be created on-the-fly using a view or something? And secondly would it be best to calculate a hash for each order line and then to combine these to find an order-level hash for grouping, or should I just use a trigger for all CRUD operations on the order lines table which re-calculates a single hash for the entire order and store the value in the orders table? TIA

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• Can't get Postfix Admin to use Dovecot password hashing

  - by Paul

  I'm setting up Postfix Admin 2.91 and trying to use dovecot:SHA512-CRYPT for password hashing. In config.inc.php I have set: // dovecot:CRYPT-METHOD = use dovecotpw -s 'CRYPT-METHOD'. Example: dovecot:CRAM-MD5 // (WARNING: don't use dovecot:* methods that include the username in the hash - you won't be able to login to PostfixAdmin in this case) $CONF['encrypt'] = 'dovecot:SHA512-CRYPT'; // If you use the dovecot encryption method: where is the dovecotpw binary located? // for dovecot 1.x // $CONF['dovecotpw'] = "/usr/sbin/dovecotpw"; // for dovecot 2.x (dovecot 2.0.0 - 2.0.7 is not supported!) $CONF['dovecotpw'] = "/usr/sbin/doveadm pw"; I have also tried SHA256-CRYPT and MD5-CRYPT with same results (as I understand it, these do not include usernames in the hash) In running setup.php, I get the following message when trying to create an admin account: can't encrypt password with dovecotpw, see error log for details Server error log reports: 1624#0: *6 FastCGI sent in stderr: "PHP message: dovecotpw password encryption failed. PHP message: STDERR output: sh: 1: /usr/sbin/doveadm: not found" while reading response header from upstream <...> upstream: "fastcgi://unix:/var/run/php5-fpm.sock:" <...> A couple quick checks: # ll /usr/sbin/doveadm -rwxr-xr-x 1 root root 423264 Feb 13 23:23 /usr/bin/doveadm* # doveadm pw -l CRYPT MD5 MD5-CRYPT SHA SHA1 SHA256 SHA512 SMD5 SSHA SSHA256 SSHA512 PLAIN CLEAR CLEARTEXT PLAIN-TRUNC CRAM-MD5 SCRAM-SHA-1 HMAC-MD5 DIGEST-MD5 PLAIN-MD4 PLAIN-MD5 LDAP-MD5 LANMAN NTLM OTP SKEY RPA SHA256-CRYPT SHA512-CRYPT # doveadmin pw -s SHA512-CRYPT Enter new password: Retype new password: {SHA512-CRYPT}$6$<long string here>/ Using Dovecot 2.2, PHP 5.5, MariaDB 10, Postfix 2.11, nginx 1.6.0, Ubuntu 12.04.

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• SHA512 vs. Blowfish and Bcrypt

  - by Chris

  I'm looking at hashing algorithms, but couldn't find an answer. Bcrypt uses Blowfish Blowfish is better than MD5 Q: but is Blowfish better than SHA512? Thanks.. Update: I want to clarify that I understand the difference between hashing and encryption. What prompted me to ask the question this way is this article, where the author refers to bcrypt as "adaptive hashing" http://chargen.matasano.com/chargen/2007/9/7/enough-with-the-rainbow-tables-what-you-need-to-know-about-s.html Since bcrypt is based on Blowfish, I was led to think that Blowfish is a hashing algorithm. If it's encryption as answers have pointed out, then seems to me like it shouldn't have a place in this article. What's worse is that he's concluding that bcrypt is the best. What's also confusing me now is that the phpass class (used for password hashing I believe) uses bcrypt (i.e. blowfish, i.e. encryption). Based on this new info you guys are telling me (blowfish is encryption), this class sounds wrong. Am I missing something?

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• What's the recommended implemenation for hashing OLE Variants?

  - by Barry Kelly

  OLE Variants, as used by older versions of Visual Basic and pervasively in COM Automation, can store lots of different types: basic types like integers and floats, more complicated types like strings and arrays, and all the way up to IDispatch implementations and pointers in the form of ByRef variants. Variants are also weakly typed: they convert the value to another type without warning depending on which operator you apply and what the current types are of the values passed to the operator. For example, comparing two variants, one containing the integer 1 and another containing the string "1", for equality will return True. So assuming that I'm working with variants at the underlying data level (e.g. VARIANT in C++ or TVarData in Delphi - i.e. the big union of different possible values), how should I hash variants consistently so that they obey the right rules? Rules: Variants that hash unequally should compare as unequal, both in sorting and direct equality Variants that compare as equal for both sorting and direct equality should hash as equal It's OK if I have to use different sorting and direct comparison rules in order to make the hashing fit. The way I'm currently working is I'm normalizing the variants to strings (if they fit), and treating them as strings, otherwise I'm working with the variant data as if it was an opaque blob, and hashing and comparing its raw bytes. That has some limitations, of course: numbers 1..10 sort as [1, 10, 2, ... 9] etc. This is mildly annoying, but it is consistent and it is very little work. However, I do wonder if there is an accepted practice for this problem.

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• Good PHP / MYSQL hashing solution for large number of text values

  - by Dave

  Short descriptio: Need hashing algorithm solution in php for large number of text values. Long description. PRODUCT_OWNER_TABLE serial_number (auto_inc), product_name, owner_id OWNER_TABLE owner_id (auto_inc), owener_name I need to maintain a database of 200000 unique products and their owners (AND all subsequent changes to ownership). Each product has one owner, but an owner may have MANY different products. Owner names are "Adam Smith", "John Reeves", etc, just text values (quite likely to be unicode as well). I want to optimize the database design, so what i was thinking was, every week when i run this script, it fetchs the owner of a proudct, then checks against a table i suppose similar to PRODUCT_OWNER_TABLE, fetching the owner_id. It then looks up owner_id in OWNER_TABLE. If it matches, then its the same, so it moves on. The problem is when its different... To optimize the database, i think i should be checking against the other "owner_name" entries in OWNER_TABLE to see if that value exists there. If it does, then i should use that owner_id. If it doesnt, then i should add another entry. Note that there is nothing special about the "name". as long as i maintain the correct linkagaes AND make the OWNER_TABLE "read-only, append-new" type table - I should be able create a historical archive of ownership. I need to do this check for 200000 entries, with i dont know how many unique owner names (~50000?). I think i need a hashing solution - the OWNER_TABLE wont be sorted, so search algos wont be optimal. programming language is PHP. database is MYSQL.

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• What's the recommended implementation for hashing OLE Variants?

  - by Barry Kelly

  OLE Variants, as used by older versions of Visual Basic and pervasively in COM Automation, can store lots of different types: basic types like integers and floats, more complicated types like strings and arrays, and all the way up to IDispatch implementations and pointers in the form of ByRef variants. Variants are also weakly typed: they convert the value to another type without warning depending on which operator you apply and what the current types are of the values passed to the operator. For example, comparing two variants, one containing the integer 1 and another containing the string "1", for equality will return True. So assuming that I'm working with variants at the underlying data level (e.g. VARIANT in C++ or TVarData in Delphi - i.e. the big union of different possible values), how should I hash variants consistently so that they obey the right rules? Rules: Variants that hash unequally should compare as unequal, both in sorting and direct equality Variants that compare as equal for both sorting and direct equality should hash as equal It's OK if I have to use different sorting and direct comparison rules in order to make the hashing fit. The way I'm currently working is I'm normalizing the variants to strings (if they fit), and treating them as strings, otherwise I'm working with the variant data as if it was an opaque blob, and hashing and comparing its raw bytes. That has some limitations, of course: numbers 1..10 sort as [1, 10, 2, ... 9] etc. This is mildly annoying, but it is consistent and it is very little work. However, I do wonder if there is an accepted practice for this problem.

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• Overriding GetHashCode in a mutable struct - What NOT to do?

  - by Kyle Baran

  I am using the XNA Framework to make a learning project. It has a Point struct which exposes an X and Y value; for the purpose of optimization, it breaks the rules for proper struct design, since its a mutable struct. As Marc Gravell, John Skeet, and Eric Lippert point out in their respective posts about GetHashCode() (which Point overrides), this is a rather bad thing, since if an object's values change while its contained in a hashmap (ie, LINQ queries), it can become "lost". However, I am making my own Point3D struct, following the design of Point as a guideline. Thus, it too is a mutable struct which overrides GetHashCode(). The only difference is that mine exposes and int for X, Y, and Z values, but is fundamentally the same. The signatures are below: public struct Point3D : IEquatable<Point3D> { public int X; public int Y; public int Z; public static bool operator !=(Point3D a, Point3D b) { } public static bool operator ==(Point3D a, Point3D b) { } public Point3D Zero { get; } public override int GetHashCode() { } public override bool Equals(object obj) { } public bool Equals(Point3D other) { } public override string ToString() { } } I have tried to break my struct in the way they describe, namely by storing it in a List<Point3D>, as well as changing the value via a method using ref, but I did not encounter they behavior they warn about (maybe a pointer might allow me to break it?). Am I being too cautious in my approach, or should I be okay to use it as is?

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• Convert filenames to their checksum before saving to prevent duplicates. Is is a smart thing to do?

  - by Xananax

  TL;DR:what the title says I am developing some sort of image board in PHP. I was thinking of changing each image's filename to it's checksum prior to saving it. This way, I might be able to prevent duplicates. I know this wouldn't work for two images that are the same but differ in size or level of compression or whatnot, but this method would allow for an early check. What bugs me is that I never saw this method implemented anywhere, so I was wondering if there is a catch to it. Maybe it is just more efficient to keep the original filename and store the hash in DB? Maybe the whole method is just not useful and my question is moot? What do you think? On a side note, I don't really get how hashes are calculated so I was wondering, if my first question checks out, if it would be possible to calculate the likeness that two images are similar by comparing hashes (levenshtein or something of the sort).

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• I'm trying to understand hash tables - can someone explain it to me - clearly?

  - by Stevo

  I want to understand the correct use and implementation of hash tables in php (sorry). I read somewhere that an in-experienced programmer created a hash table and then iterated through it. Now, I understand why that is wrong but I haven't quite got the full knowledge to know if my understanding is correct (if you know what I mean). So could someone explain to me how to implement a hash table in php (presumably an associative array) and perhaps more importantly, how to access the values 'with a hash' and what that actually means?

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• Finding duplicate files?

  - by ub3rst4r

  I am going to be developing a program that detects duplicate files and I was wondering what the best/fastest method would be to do this? I am more interested in what the best hash algorithm would be to do this? For example, I was thinking of having it get the hash of each files contents and then group the hashes that are the same. Also, should there be a limit set for what the maximum file size can be or is there a hash that is suitable for large files?

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• How does a web browser save passwords?

  - by marcus

  How do current web browsers (or mobile mail clients and any software in general) save user passwords? All answers about storing passwords say we should store only hashes, not the password themselves. But I'm having a hard time searching the web trying to find the best techniques to store passwords when we know we will need them in plain text later on — without storing them in plain text, without using a weak encryption (known key) and without asking the user for a master password. Any ideas?

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• Salting a public hash

  - by Sathvik

  Does it make any sense at all to salt a hash which might be available publicly? It doesn't really make sense to me, but does anyone actually do that? UPDATE - Some more info: An acquaintance of mine has a common salted-hash function which he uses throughout his code. So I was wondering if it made any sense at-all, to do so. Here's the function he used: hashlib.sha256(string+SALT).hexdigest() Update2: Sorry if it wasn't clear. By available publicly I meant, that it is rendered in the HTML of the project (for linking, etc) & can thus be easily read by a third party. The project is a python based web-app which involves user-created pages which are tracked using their hashes like myproject.com/hash so thus revealing the hash publicly. So my question is, whether in any circumstances would any sane programmer salt such a hash? Question: Using hashlib.sha256(string+SALT).hexdigest() vs hashlib.sha256(string).hexdigest() , when the hash isn't a secret.

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• Concept behind SHA-1 Checksum

  - by Vishwas Gagrani

  What's the basis behind SHA-1 or SHA-2 or other Checksum algorithms? I read about it here http://en.wikipedia.org/wiki/SHA-1#Data_Integrity But I am still wondering about an answer in a layman's language. Can I understand it as a very, very compressed code that can be translated back into original data? Let's say, I have a letter written in notepad. Then the whole of my 1 A4 page size data can be converted into something like this "9b90417b6a186b6f314f0b679f439c89a3b0cdf5". So whenever I want my original data back, I can convert this back into original data? I am very sure that I am wrong here, because it is weird how data that itself contains combination of letters and numbers can be represented by smaller set of letters and numbers. Illogical! Then, what's the basic?

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• Looking for a non-cryptographic hash function that returns a single character

  - by makerofthings7

  Suppose I have a dictionary of ASCII words stored in uppercase. I also want to save those words into separate files so that the total word count of each file is approximately the same. By simply looking at the word I need to know which file it should be in (if it's there at all). Duplicate words should go into the same file and overwrite the last one. My first attempt at solving this problem is to use .NET's object.GetHashCode() function and .Trim() to get one of the "random" characters that pop up. I asked a similar question here If I only use one character of object.GetHashCode() I would get a hash code character of A..Z or 0..9. However saving the result of GetHashCode to disk is a no-no so I need a substitute. Question: What algorithm (or subset of an algorithm) is appropriate for pigeonholing strings into a single character or range of characters (Like hex 0..F offers 16 chars)? Real world usage: I'll use this answer to modify the Partition key used in Azure Table storage as described here

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• Would md5 hashes allow detection of synced files?

  - by codpursue

  We have to develop our own file management system in Java web application. We need to sync files between our main server and client severs and find out whether all the client server has all the latest version of files. Our files are in pdf, doc and xls format they changes every now and then as and when it is required. What we are thinking of using MD5 checksum to find out hashcode of files on Main server and store it in database. Same would be there in Client Servers database. After comparing records on database we would come to know whether client servers are synced or not. Please suggest if there are any better ways to do the same.

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• Security benefits from a second opinion, are there flaws in my plan to hash & salt user passwords vi

  - by Tchalvak

  Here is my plan, and goals: Overall Goals: Security with a certain amount of simplicity & database-to-database transferrability, 'cause I'm no expert and could mess it up and I don't want to have to ask a lot of users to reset their passwords. Easy to wipe the passwords for publishing a "wiped" databased of test data. (e.g. I'd like to be able to use a postgresql statement to simply reset all passwords to something simple so that testers can use that testing data for themselves). Plan: Hashing the passwords Account creation records the original email that an account is created with, forever. A global salt is used, e.g. "90fb16b6901dfceb73781ba4d8585f0503ac9391". An account specific salt, the original email the account was created with, is used, e.g. "[email protected]". The users's password is used, e.g. "password123" (I'll be warning against weak passwords in the signup form) The combination of the global salt, account specific salt, and password is hashed via some hashing method in postgresql (haven't been able to find documentation for hashing functions in postgresql, but being able to use sha-2 or something like that would be nice if I could find it). The hash gets stored in the database. Recovering an account To change their password, they have to go through standard password reset (and that reset email gets sent to the original email as well as the most recent account email that they have set). Flaws? Are there any flaws with this that I need to address? And are there best practices to doing hashing fully within postgresql?

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