Verifying the Integrity of an Easynews Download without a Checksum File
Suppose you had to download a huge file as a single piece because a multiple archive version wasn’t available. If the poster didn’t supply a checksum file (MD5, SFV) or Parchive files, you can’t readily tell if the file you’ve downloaded is intact. Here’s a method I came up with for verifying the integrity of an ISO image file that didn’t come with a checksum. I didn’t want to burn the ISO image without knowing that the file was intact, to avoid an interrrupted or corrupt installation.
Take a look at the URL of a typical Easynews download:
http://downloads.members.easynews.com/news/7/a/3 *snip* /7a38dcc18a67ac805cff5abc5ef36d12011d39e6b.iso/foo.ISO
The file is inside a folder whose name contains a bunch of hexadecimal digits. Let’s break down the digits as follows:
7a38dcc18a67ac805cff5abc5ef36d12 0 11d39e6b
I discovered that the first block of 32 characters (7a38dcc18a67ac805cff5abc5ef36d12) is the MD5 checksum for the file, and the last block of 8 characters (11d39e6b) is the CRC32 checksum. The zero in between probably has no significance.
Comparing either of these to the checksum you get for the downloaded copy should give you enough confidence that your download is intact. For getting the MD5 checksum, you could use Microsoft’s File Checksum Integrity Verifier utility on Windows:
fciv -a "foo.ISO" -md5 // // File Checksum Integrity Verifier version 2.05. // 7a38dcc18a67ac805cff5abc5ef36d12 foo.iso
If this matches what’s in the folder name, it is safe to assume that your download is intact. Alternatively, you can use a SFV utility to compare the CRC32 checksums. The number following the filename in the SFV file below is the CRC32 checksum for that file:
; Generated by WIN-SFV32 v1.0 ; (Compatible: Total Commander 6.51) foo.ISO 11D39E6B
February 9th, 2007 at 1:01 pm
Very clever method!
If you are a user of Total Commander or similarly capable tools then there is an easier method for checking integrity; ie. you can selected the file(s) you have downloaded and from the “Files” menu choose “Create CRC Checksums”. If you select the MD5 option in the following dialog, you can use the output to compare against the MD5 value(s) from the URL(s).