[program-l] Re: Text file checksum code
- From: "Tyler Littlefield" <tyler@xxxxxxxxxxxxx>
- To: <program-l@xxxxxxxxxxxxx>
- Date: Wed, 25 Mar 2009 05:36:36 -0600
such, and then also use the file size/length to make sure nothing had ben
changed.
The hash should be good enough; if the length changes, the whole hash
changes.
It's like a fingerprint of sorts. Md5 can be changed a bit easier, but
something like sha256 would be rough.
Thanks,
Tyler Littlefield
Web: tysdomain.com
email: tyler@xxxxxxxxxxxxx
My programs don't have bugs, they're called randomly added features.
----- Original Message -----
From: "Jacob Kruger" <jacobk@xxxxxxxxxxxx>
To: <program-l@xxxxxxxxxxxxx>
Sent: Wednesday, March 25, 2009 3:59 AM
Subject: [program-l] Re: Text file checksum code
Ok.
Checked a bit more, and seems it's not possible to revert back to
unencrypted value, so you would have to simply compare encrypted values as
such, and then also use the file size/length to make sure nothing had ben
changed.
Stay well
Jacob Kruger
Blind Biker
Skype: BlindZA
'...fate had broken his body, but not his spirit...'
----- Original Message -----
From: "Mark Long" <markalong64@xxxxxxxxxxx>
To: "Program-l" <program-l@xxxxxxxxxxxxx>
Sent: Wednesday, March 25, 2009 10:43 AM
Subject: [program-l] Re: Text file checksum code
If the file is of arbitrary length then it can be padded to have the same
checksum. This is called a forced collision.
If you hashed it using two algorithms then it would be very much harder
to do. I suggest that a sha1 and an MD5 would defeat most attackers.
Sample code:
http://dotnetpulse.blogspot.com/2007/12/sha1-hash-calculation-in-c.html
http://blog.brezovsky.net/en-text-2.html
Hope that helps
Mark
--------------------------------------------------
From: "Jacob Kruger" <jacobk@xxxxxxxxxxxx>
Sent: Wednesday, March 25, 2009 7:53 AM
To: "Program-l" <program-l@xxxxxxxxxxxxx>
Subject: [program-l] Text file checksum code
Hi there
Currently looking into implementing a form of custom licensing system
for some apps for work, and the one thing we might need to do is to use
some form of checksum to make sure that the text/XML file to be used for
storing licensing information hasn't been modified as such, so am
currently looking for some sample code/examples related to doing this
with text files since it doesn't as such seem to be naturally supported
within the CLR framework/C#/VB.Net.
Any ideas?
Apart from that, here's a link to MS's own updated C# code samples file
download page:
http://code.msdn.microsoft.com/Release/ProjectReleases.aspx?ProjectName=csharpsamples&ReleaseId=8
Stay well
Jacob Kruger
Blind Biker
Skype: BlindZA
'...fate had broken his body, but not his spirit...'
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