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# MD5 RNG

Posted on 2001-06-29
Medium Priority
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Hello,

Can anybody explain me how does the MD5 RNG method to generate random numbers work? I hear that it uses hash tables, but I don't know much about it.
I just want to know a simple explanation, I don't need any actual implementation.

Thanks
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Question by:elito
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AlexVirochovsky earned 400 total points
ID: 6237381
You can find article +source code in
http://www.pbm.com/dice/random.html
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Author Comment

ID: 6237424
As I said I don't need source code. Could you explain me what are hash tables and how they're used to produce better random numbers?

cheers,
elito
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Expert Comment

ID: 6237692
I am not familar with this algorithm.  But it appears to me that it does not use hash tables, but only a hash operation.

a hash operation is used to convert data, usually of a reasonably large size, like a string, to a smaller, seemingly random binary value (a number).   For example, to hash a string, you might add up the numerical values of all the ASCII characters in the string to produce a hash value, a single number.  to hash a 3d point, you might take the product of the 3 coordinates to produce a single number.   etc.     While the value produced by the hash operation appears to be random it is not.   Givien the same initial data you can repeat the process to produce the exact same hash value.  However the reverse is not true.  Ther is no way to take a hash value and deduce the original data.

Hash values are used in hash tables.   If you need to store data for rapid searching, you hash the data and then place it in a hash table at a position determined by its hash value.   If you latter need to find that item, you can produce a hash value for that item, then look it up in the table.   hash searches tend to be approximately constant complxity.  That is, no matter how many items are in the table, it always takes the same amount of time (which is very little) to find an item.  Other mechanisms, like binary trees (or other trees) tend to get slower as the number of items increase.    However they have features that hash tables don't.  Trees, sorted arrays, heaps can be iterated in order.  hash tables cannot.  Trees etc, can also be used to find near matches.  i.e search for "the" and find :"these".   hash table tables cann only be used for exact matches.      But they are perfect for example, for storing symbols a compiler finds in a program.  Whent he compile sees a symbol being used like "FileCount" it can search the hash table to see if the symbol was declared and what its type is etc.   This search only requires an exact match.

Anyways that code seems to be using a hash operation to develop pseudo-random data that is then used to generate the pseodo-random number.
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Expert Comment

ID: 6237782
this random generator uses MD5 digest.
Message digests are secure one-way hash functions (NOT hashtable) that take arbitrary-sized data and output a fixed-length hash value. This hash value truncate to your  random interval.
for producing better random numbers you could use some heuristic rules:
1. set any simple integer function f(i) = (i+25)^12+i*7542+...
2. generate MD5 digest for each i = 1000 ... 10000.
3. truncate digest for random interval

Do you want to know mathematics principles of crypto?? i know some russian articles for this.

Andrey
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change
2. generate MD5 digest for each f(i) (where i = 1000 ... 10000).

Andrey
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Expert Comment

ID: 6237814
Do plynomials like that one in "1." work to produce decent results (numbers well deestributed over the range and with no discernable patern)?   My intuition would tell me no.
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>> Message digests are SECURE ONE-WAY hash functions (NOT hashtable) that take arbitrary-sized data(may be any successive numbers) and output a fixed-length hash value...

hacker cannot to know the following random number, if he know all preceding random numbers (one condition : he don't know polynom; user could select any polynom every time).

best regards
Andrey
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elito,

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