Hacker News Reader

Hacker News

Comment by creshal | Hacker News Reader

Parent Full thread

creshal

Or, alternatively:

1. Tag your release with `git tag -s`

2. `git push --follow-tags` (or `git push remotename tagname` in git <1.8.3)

3. Upload your pubkey to github.

Much less effort…

View on HN

Exactly. It has the additional advantage of signing the history, not just one source dump.

Do the above steps generate an actual signed release, or just a signed tag?

A tag, not a release. I didn't realize there was a difference, but in hindsight, I'd argue it's superior anyway: I'm much more comfortable to sign my local repository copy than signing an archive generated by github in a non-reproducible way.

I can verify the contents of github's archives, but I cannot, in good faith, say I've verified that the archives do not contain malicious code that triggers vulnerabilities in zip/tar/gzip.

I'm much more comfortable to sign my local repository copy than signing an archive generated by github

Both can be part of the process.

However, signing the tag doesn't really help someone who isn't cloning the repository. For those people that are just downloading the source tarball directly, having a GPG signature for the release can ensure that github(or other attacker) hasn't modified the contents of the tarball.

Sort of.

I mean if github wants to modify the contents of the tarball, they'll also modify the signature. So unless you're also confirming that it's the right signature, and not just that it's signed, what exactly are you doing?

That is where signing other people's OpenPGP keys comes in, and the web of trust for those who haven't met their upstreams.

github can also modify signed tags.

Reproducing GitHub's archives locally is trivial. They just use a standard git command to do it anyway.

git archive -o project-0.5.tar.gz --prefix project-0.5/ v0.5

I think we have different standards for "reproducible". You can replicate the archive structure and naming, but the archive metadata – and thus checksums – will be different:

$ curl -sL https://github.com/creshal/yspave/archive/0.1.0.zip | sha512sum

375564f127b46f3d1208c30b4eb5c39725985ca09d7bacf940d346727e8bcf9362966f8523c39e6c4f6f1a86b52ceba8823c2f4b016fdedb5947cd6a6bafc930 -

$ git clone git@github.com:creshal/yspave.git

…

$ cd yspave

$ git archive -o 0.1.0.zip --prefix yspave-0.1.0/ 0.1.0

$ sha512sum 0.1.0.zip

aca1163d5270e8ad6796f10511f49b0cf5fe61007fed0786a6643ae522e59a6aa7d6b0cddf4e5b94a435da8d99aa09b2f2ebef7788a82d4c2e7d8d4ad79e0634 0.1.0.zip

–––––

So, I can verify that the contents weren't tampered with, but I cannot verify the archives themselves unless I happen to understand the zip/tar/gz… specifications better than most people implementing it.

you can verify by downloading the archive and comparing the content of the archive with the content of the archive produced by git archive. The issue seems to be that the zip file contains the version of the zip utility used to create the file, so you'd need the exact same version to make a reproducible build.

That's interesting. I'm not able to replicate your results. Cloning that respository and doing the same git archive command produces an identical file to what GitHub serves.

I'm on a fully updated Arch Linux with the latest Git. If you have an older version, that might be making the difference.

I can reproduce the result: The content of the file is identical, the checksum differs.

> I'm on a fully updated Arch Linux with the latest Git.

Same, actually.

According to diffoscope, the (only) differences are in the archive member modification times:

  --- ../0.1.0.zip
  +++ 0.1.0.zip
  ├── zipinfo -v {}
  │ @@ -28,15 +28,15 @@
  │    file system or operating system of origin:      MS-DOS, OS/2 or NT FAT
  │    version of encoding software:                   0.0
  │    minimum file system compatibility required:     MS-DOS, OS/2 or NT FAT
  │    minimum software version required to extract:   1.0
  │    compression method:                             none (stored)
  │    file security status:                           not encrypted
  │    extended local header:                          no
  │ -  file last modified on (DOS date/time):          2016 Apr 8 06:33:12
  │ +  file last modified on (DOS date/time):          2016 Apr 8 14:33:12
  │    file last modified on (UT extra field modtime): 2016 Apr 8 13:33:13 local
  │    file last modified on (UT extra field modtime): 2016 Apr 8 13:33:13 UTC
  │    32-bit CRC value (hex):                         00000000
  │    compressed size:                                0 bytes
  │    uncompressed size:                              0 bytes
  │    length of filename:                             13 characters
  │    length of extra field:                          9 bytes
  │ @@ -63,15 +63,15 @@
  │    version of encoding software:                   0.0
  │    minimum file system compatibility required:     MS-DOS, OS/2 or NT FAT
  │    minimum software version required to extract:   1.0
  │    compression method:                             deflated
  │    compression sub-type (deflation):               normal
  │    file security status:                           not encrypted
  │    extended local header:                          no
  │ -  file last modified on (DOS date/time):          2016 Apr 8 06:33:12
  │ +  file last modified on (DOS date/time):          2016 Apr 8 14:33:12
  │    file last modified on (UT extra field modtime): 2016 Apr 8 13:33:13 local
  │    file last modified on (UT extra field modtime): 2016 Apr 8 13:33:13 UTC
  │    32-bit CRC value (hex):                         570b4391
  │    compressed size:                                52 bytes
  │    uncompressed size:                              78 bytes
  │    length of filename:                             23 characters
  │    length of extra field:                          9 bytes
  │ @@ -97,15 +97,15 @@
  │    file system or operating system of origin:      Unix
  │    version of encoding software:                   2.3
  │    minimum file system compatibility required:     MS-DOS, OS/2 or NT FAT
  │    minimum software version required to extract:   1.0
  │    compression method:                             none (stored)
  │    file security status:                           not encrypted
  │    extended local header:                          no
  │ -  file last modified on (DOS date/time):          2016 Apr 8 06:33:12
  │ +  file last modified on (DOS date/time):          2016 Apr 8 14:33:12
  │    file last modified on (UT extra field modtime): 2016 Apr 8 13:33:13 local
  │    file last modified on (UT extra field modtime): 2016 Apr 8 13:33:13 UTC
  │    32-bit CRC value (hex):                         bc890775
  │    compressed size:                                59 bytes
  │    uncompressed size:                              59 bytes
  │    length of filename:                             20 characters
  │    length of extra field:                          9 bytes

(rest of output omitted as it's too long for an HN comment; the full output can be found at http://paste.debian.net/432708/)

Timezone difference, with extra DST madness mixed in? 13:33 is UTC, 14:33 is UTC+DST, 06:33 is Pacific Time+DST

Thanks for spotting this. If I set TZ before running the command, I actually can reproduce the different result. Apparently I'm in the same time zone as GitHub :)

If you do "TZ=America/Los_Angeles git archive..." for a Zip file, you should then be capable of making a bit-exact file.

ZIP is a fun file format.

Ahh, never mind. The tar.gz is indeed identical, "only" the zip differs.

That does not nearly have the same security level. Signing a tag in git is like signing a commit. A commit is nothing else than a reference to a tree SHA1 (and parent, message and author). That tree SHA1 contains SHA1 of the files and trees in the directory.

Even assuming your client checks all the SHA1 and does not just trust the server (which is not the default), and that you are verifying the git tag signature correctly (which is harder than just checking a tarball), SHA1 collisions are considered practical.

Git is essentially a Merkle DAG, thus a single reference to the root object suffices.

Also collisions are only practical with large amounts of padding which would be visible. That's why nobody really cares too much about them for source control systems.

Reply on news.ycombinator.com