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Updated autoupdater-readme.txt with manifest signing details.

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rcgordon committed Jun 4, 2017
1 parent e04bfd4 commit ced74370426e63d751ee64ed46cec9f6d1ac7eb9
Showing with 46 additions and 6 deletions.
  1. +46 −6 autoupdater-readme.txt
@@ -1,7 +1,8 @@
The updater program's code is public domain. The rest of ioquake3 is not.
The source code to the autoupdater is in the code/autoupdater directory.
There is a small piece of code in ioquake3 itself at startup, too.
There is a small piece of code in ioquake3 itself at startup, too; this is
in code/sys/sys_autoupdater.c ...
(This is all Unix terminology, but similar approaches on Windows apply.)
@@ -20,8 +21,14 @@ The basic flow looks like this:
- The updater downloads a manifest from a known URL over https://, using
libCurl. The base URL is platform-specific (it might be
https://example.com/mac/, or https://example.com/linux-x86/, whatever).
The url might have other features, like a updater version or a specific
product name, etc.
The manifest is at $BASEURL/manifest.txt
- The manifest looks like this: three lines per file...
- The updater also downloads $BASEURL/manifest.txt.sig, which is a digital
signature for the manifest. It checks the manifest against this signature
and a known public RSA key; if the manifest doesn't match the signature,
the updater refuses to continue.
- The manifest looks like this: three lines per item...
Contents/MacOS/baseq3/uix86_64.dylib
332428
@@ -105,13 +112,46 @@ Failure points:
- If an update bricks ioquake3 to the point where it can't run the updater,
running the updater directly should let it recover (assuming a future update
fixes the problem).
- If the download server is compromised, they would need the private key
(not stored on the download server) to alter the manifest to serve
compromised files to players. If they try to change a file or the manifest,
the updater will know to abort without updating anything.
- If the private key is compromised, we generate a new one, ship new
installers with an updated public key, and re-sign the manifest with the
new private key. Existing installations will never update again until they
do a fresh install, or at least update their copy of the public key.
How manifest signing works:
Some admin will generate a public/private key with the rsa_make_keys program,
keeping the private key secret. Using the private key and the rsa_sign
program, the admin will sign the manifest, generating manifest.txt.sig.
The public key ships with the game (adding 270 bytes to the download), the
.sig is downloaded with the manifest by the autoupdater (256 bytes extra
download), then the autoupdater checks the manifest against the signature
with the public key. if the signature isn't valid (the manifest was tampered
with or corrupt), the autoupdater refuses to continue.
If the manifest is to be trusted, it lists sha256 checksums for every file to
download, so there's no need to sign every file; if they can't tamper with the
manifest, they can't tamper with any other file to be updated since the file's
listed sha256 won't match.
If the private key is compromised, we generate new keys and ship new
installers, so new installations will be able to update but existing ones
will need to do a new install to keep getting updates. Don't let the private
key get compromised. The private key doesn't go on a public server. Maybe it
doesn't even live on the admin's laptop hard drive.
If the download server is compromised and serving malware, the autoupdater
will reject it outright if they haven't compromised the private key, generated
a new manifest, and signed it with the private key.
Items to consider for future revisions:
- GPG sign the manifest; if we can be confident that the manifest isn't
compromised, then the sha256 hashes of each file it contains should protect
the rest of the process. As it currently stands, we trust the download
server isn't compromised.
- Maybe put a limit on the number manifest downloads, so we only check once
every hour? Every day?
- Channels? Stable (what everyone gets by default), Nightly (once a day),

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