On Fri, Apr 5, 2019, 18:06 Thomas Schmitt <scdbac...@gmx.net> wrote: > Hi, > > Chris XX wrote: > > I was trying to Verify the authenticity of Debian CDs on your website, > but I > > don't see instructions that will guide me through the process > > (step-by-step). > > (We are the users. But some Debian Developers are watching, too.) > > Obviously there is a gap between checksum file verification and .iso image > verification. > > Let's first look at the files offered for download: > https://cdimage.debian.org/debian-cd/current/amd64/iso-cd/ > has among others > > SHA512SUMS.sign > SHA512SUMS > debian-9.8.0-amd64-netinst.iso > > > https://www.debian.org/CD/verify > > This publishes the key "fingerprints" by which you can recognize authentic > pairs of SHA512SUMS.sign and SHA512SUMS. > > It points to > https://keyring.debian.org/ > where you probably shall learn how to obtain the keys in question, > namely by the shell commands > > gpg --keyserver keyring.debian.org --recv-keys 64E6EA7D > gpg --keyserver keyring.debian.org --recv-keys 6294BE9B > gpg --keyserver keyring.debian.org --recv-keys 09EA8AC3 > > Experienced users of gpg would know that one can check authenticity by > > gpg --verify SHA512SUMS.sign SHA512SUMS > > which should say something like > > gpg: Signature made Sun 17 Feb 2019 04:10:30 PM CET using RSA key ID > 6294BE9B > gpg: Good signature from "Debian CD signing key < > debian...@lists.debian.org>" > gpg: WARNING: This key is not certified with a trusted signature! > gpg: There is no indication that the signature belongs to the > owner. > Primary key fingerprint: DF9B 9C49 EAA9 2984 3258 9D76 DA87 E80D 6294 > BE9B > > The reported fingerprint must be one of the published fingerprints, > or else something is fishy. > Here it is the Debian one of 2011-01-05. I.e. all is well so far. > > If you change some character in SHA512SUMS and run above command again > then you will see > > gpg: Signature made Sun 17 Feb 2019 04:10:30 PM CET using RSA key ID > 6294BE9B > gpg: BAD signature from "Debian CD signing key < > debian...@lists.debian.org>" > > > So you can trust the content of SHA512SUMS, if gpg --verify says it is > good and if the key fingerprint matches one of the Debian fingerprints. > > Now you have to follow the tiny link "faq" at the bottom to > https://www.debian.org/CD/faq/ > where you hop to > https://www.debian.org/CD/faq/#verify > > Between the lines you read that there is a text line in SHA512SUMS which > shows the name of the .iso file which you actually want to verify: > > > cc4a6bd50925c1c4af98049060e304494bc9da61eb5eb272c556d67608de14d4e6a4b8bc1c9412a0f810083912e228569f3771ffffa7174538f3e26f45a05245 > debian-9.8.0-amd64-netinst.iso > > More explicite is the hint to use program "sha512sum". A run of > > sha512sum debian-9.8.0-amd64-netinst.iso > > puts out > > > cc4a6bd50925c1c4af98049060e304494bc9da61eb5eb272c556d67608de14d4e6a4b8bc1c9412a0f810083912e228569f3771ffffa7174538f3e26f45a05245 > debian-9.8.0-amd64-netinst.iso > > which you should compare with the line in SHA512SUMS. > > Alternatively you could run > > sha512sum --check SHA512SUMS 2>/dev/null > > to get > > debian-9.8.0-amd64-netinst.iso: OK > debian-9.8.0-amd64-xfce-CD-1.iso: FAILED open or read > debian-mac-9.8.0-amd64-netinst.iso: FAILED open or read > > Or you could download > > https://people.debian.org/~danchev/debian-iso/check_debian_iso > > and run > > chmod u+x ./check_debian_iso > ./check_debian_iso SHA512SUMS debian-9.8.0-amd64-netinst.iso > > to get > > Piping 149504 blocks of 'debian-9.8.0-amd64-netinst.iso' through > 'sha512sum' > to verify checksum list item 'debian-9.8.0-amd64-netinst.iso'. > 149504+0 records in > 149504+0 records out > 306184192 bytes (306 MB) copied, 0.882765 s, 347 MB/s > Ok: 'debian-9.8.0-amd64-netinst.iso' matches > 'debian-9.8.0-amd64-netinst.iso' in 'SHA512SUMS' > > > Now let's see what happens if a single byte is altered in the .iso > > dd if=/dev/zero bs=1 count=1 conv=notrunc seek=511 > of=debian-9.8.0-amd64-netinst.iso > > Now the proposed verifyier runs yield: > > > 0b0a75b8a0c8dc05a4b43273e44d7b5e3b0ecec6d9b4e1c88a95d9c886cba5ae0dbeb4b7a5a3016106096a9071572b9a3d8b54dd91a50abce15f713fa22ff229 > debian-9.8.0-amd64-netinst.iso > > which does obviously not match the line in SHA512SUMS, or > > debian-9.8.0-amd64-netinst.iso: FAILED > ... > > or > > ... > Found: > > 0b0a75b8a0c8dc05a4b43273e44d7b5e3b0ecec6d9b4e1c88a95d9c886cba5ae0dbeb4b7a5a3016106096a9071572b9a3d8b54dd91a50abce15f713fa22ff229 > Expected: > cc4a6bd50925c1c4af98049060e304494bc9da61eb5eb272c556d67608de14d4e6a4b8bc1c9412a0f810083912e228569f3771ffffa7174538f3e26f45a05245 > MISMATCH: 'debian-9.8.0-amd64-netinst.iso' checksum differs from > 'debian-9.8.0-amd64-netinst.iso' in 'SHA512SUMS' > > So you know that the checksumers really detect nearly all damages of > debian-9.8.0-amd64-netinst.iso. > > -------------------------------------------------------------------------- > > @ Steve McIntyre (maintainer of debian-cd): > > Do you agree with the instructions above ? > > Is there a consolidated wiki page with such instructions which i failed > to find ? If not: shall we make such a page ? > > > Have a nice day :) > > Thomas >
Thomas, thank you for posting this. It is a good "walk-through" of the verification process. Unfortunately, proper verification can seem too complicated for some users, especially newer ones. So often they just: sha512 sum debian-9.8.0-amd64-netinst.iso say, "looks about the same", and call it a day. Hopefully this will help someone. Thanks again.
