md5

Mark Smith mark at maseurope.net
Thu Feb 21 13:24:02 CST 2008


Alejandro, I don't think the process described by Mark Waddingham  
will produce the same result as getting the digest of the whole file,  
no matter what chunk size you use. But I'm sure he's right that the  
result will produce very little chance of collision.

If your computer has openssl installed, you can us a shell call:

get shell("openssl md5" && pathToFile)

Best,

Mark


On 21 Feb 2008, at 19:12, capellan wrote:

>
> Hi Mark,
>
> This has been discussed before ;-)
>
> In 2005, september 25
> Mark Waddingham wrote,
> in the message:
> put url some url into someVar...
>
> Mark Schonewille was trying to do something very similar a while  
> ago and
> filed an enhancement request about being able to do md5 digests on  
> large
> files *without* them needing to be loaded. For interest see here:
>
>   <http://support.runrev.com/bugdatabase/show_bug.cgi?id=2410>
>
> As suggested (by me) in the bug-report, you needn't take the md5digest
> of the entire file at once and can do something like this instead:
>
> function quasiMD5 pFile
>   local tMD5s
>   open file pFile for binary read
>   repeat
>     read from file pFile for CHUNK_SIZE chars
>     if the result is EOF then
>       exit repeat
>     end if
>     put the md5digest of it after tMD5s
>   end repeat
>   close file pFile
>   return the md5Digest of tMD5s
> end quasiMD5
>
> Where you can make CHUNK_SIZE some suitable size (perhaps 256k/512k).
>
> [ My intuitive analysis of the impact of doing this on the integrity
> (i.e. potential for collision) of the digest is that it will be  
> minimal
> - but perhaps someone more knowledgeable in this area could comment. ]
>
> Hope this helps,
>
> Mark Waddinham.
>
>
> Well, i do not try this code myself, but i am sure that no  
> md5digest command
> line tool tries to load the whole Linux Installer image CD in  
> memory to
> verify
> the file. Surely, they use an aproach similar to this suggested by  
> Mark
> Waddingham.
> But there is only one thing to know for sure... What exact size are  
> the
> chunks to process?
>
> alejandro
>
>
>
> masmit wrote:
>>
>> I need to get the md5 of potentially very large
>> files on disc. I'm hoping to avoid having to load them into memory in
>> order get the digest.
>>
>> Mark
>>
>
> -- 
> View this message in context: http://www.nabble.com/md5- 
> tp15536091p15618684.html
> Sent from the Revolution - User mailing list archive at Nabble.com.
>
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