Date: Thu, 27 Dec 2001 21:01:32 -0600 (CST) From: Ryan Thompson <ryan@sasknow.com> To: <sysop_blast@hotmail.com> Cc: <freebsd-questions@FreeBSD.ORG> Subject: Re: From M. Callihan Message-ID: <20011227205415.N21746-100000@catalyst.sasknow.net> In-Reply-To: <DURONKum3v7RlySvTD800000162@HIVAIDSsearch.com>
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sysop_blast@hotmail.com wrote to freebsd-questions@FreeBSD.ORG: > M. Callihan says: > > I have found that once a given file becomes corrupted md5 will > give a different checksum on that file each time it is ran against > it. Unless I'm reading you wrong, I think you have misunderstood MD5. The MD5 algorithm is designed to always return the same message digest, given the same input. So, if the input (file) changes, the MD5 digest will change as well. If the input file does not change, the MD5 digest will be the same. If the input file becomes corrupted, the MD5 digest will be different than the digest for the un-corrupted version. But calculating MD5 again on the *same* corrupted version will produce the *same* incorrect checksum. In other words, MD5 has no concept of "corruption". It reads bits and calculates a checksum based on the input it observes. If the bits are the same, the checksum will be the same. If the bits are different, the checksum will (almost certainly) be different. Try it... echo foo > bar md5 bar echo led >>bar md5 bar md5 bar (twice.. notice same checksum both times) Clear? - Ryan -- Ryan Thompson <ryan@sasknow.com> Network Administrator, Accounts SaskNow Technologies - http://www.sasknow.com #106-380 3120 8th St E - Saskatoon, SK - S7H 0W2 Tel: 306-664-3600 Fax: 306-664-1161 Saskatoon Toll-Free: 877-727-5669 (877-SASKNOW) North America To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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