Date: Mon, 7 Aug 2000 14:39:25 +0200 (CEST) From: Marc van Woerkom <marc.vanwoerkom@science-factory.com> To: zzhang@cs.binghamton.edu Cc: freebsd-questions@freebsd.org Subject: Re: How to use checksum.md5 Message-ID: <20000807123925.1637B1EE7@nil.science-factory.com> In-Reply-To: <Pine.SOL.4.21.0008031054470.11190-100000@sol.cs.binghamton.edu> (message from Zhihui Zhang on Thu, 3 Aug 2000 10:58:00 -0400 (EDT)) References: <Pine.SOL.4.21.0008031054470.11190-100000@sol.cs.binghamton.edu>
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> I download the FreeBSD ISO image. I notice there is a checksum.md5 file
> there. Can anyone tell me how to use it (which tools do I need)?
Like you wrote MD5 is a checksum or hash.
The idea is to compute the checksum of the (well known) algorithm yourself
and compare it to the checksum listed.
If they differ it is clear that something fishy is going on, either
transmission errors or even tampering.
That is because hash functions are choosen in a way that this equality
is *very* hard to accomplish:
hash(x) = hash(x + dx + comp)
(meaning: manipulating input first x+dx and then compensating in a way +comp
that one still gets the original hash number)
On a FreeBSD system you have a 'md5' command that computes this checksum.
There might be other programs that provide this calculation (a Perl
built in?)
Porting md5.c is easy as well, I once made it run with Cygwin under
Win32 (just throw a bit unnecessary stuff away)
> Do I absolutely need to do the check? Thanks.
You need not - this is something for the cautious/paranoid
who fear that they got some altered version of the file.
I am not sure, if an ISO image per se has some checksum integrated,
so that you would run into some warning at burning time.
Otherwise running a checksum over a 650MB download seems not a bad
idea.
Regards,
Marc
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