Here's a new proposal for a thing that Mike wants for TBB. Please review!
Filename: 227-vote-on-package-fingerprints.txt Title: Include package fingerprints in consensus documents Author: Nick Mathewson, Mike Perry Created: 2014-02-14 Status: Open
0. Abstract
We propose extending the Tor consensus document to include digests of the latest versions of one or more package files, to allow software using Tor to determine its up-to-dateness, and help users verify that they are getting the correct software.
1. Introduction
To improve the integrity and security of updates, it would be useful to provide a way to authenticate the latest versions of core Tor software through the consensus. By listing a location with this information for each version of each package, we can augment the update process of Tor software to authenticate the packages it downloads through the Tor consensus.
2. Proposal
We introduce a new line for inclusion in votes and consensuses. Its format is:
"package" SP PACKAGENAME SP VERSION SP URL SP DIGESTS NL
PACKAGENAME = NONSPACE VERSION = NONSPACE URL = NONSPACE DIGESTS = DIGEST | DIGESTS SP DIGEST DIGEST = DIGESTTYPE "=" BASE64
NONSPACE = one or more non-space printing characters
BASE64 = one or more base-64 characters, with trailing =s removed.
SP = " " NL = a newline
Votes and consensuses may include any number of "package" lines, but no vote or consensus may include more than one "package" line with the same PACKAGENAME and VERSION values. All "package" lines must be sorted by PACKAGENAME, and then by VERSION, in lexical (strcmp) order.
If the consensus-method is at least (TBD), then when computing the consensus, package lines for a given PACKAGENAME/VERSION pair should be included if at least three authorities list such a package in their votes. (Call these lines the "input" lines for PACKAGENAME.) That consensus should contain every "package" line that is listed verbatim by more than half of the authorities listing a line for the PACKAGENAME/VERSION pair, and no others.
These lines appear immediately following the client-versions and server-versions lines.
3. Recommended usage
Programs that want to use this facility should pick their PACKAGENAME values, and arrange to have their versions listed in the consensus by at least three friendly authority operators.
Programs may want to have multiple PACKAGENAME values in order to keep separate lists. These lists could correspond to how the software is used (as tor has client-versions and server-versions); or to a release series (as in tbb-alpha, tbb-beta, and tbb-stable); or to how bad it is to use versions not listed (as in foo-noknownexploits, foo-recommended).
Programs MUST NOT use "package" lines from consensuses that have not been verified and accepted as valid according to the rules in dir-spec.txt, and SHOULD NOT fetch their own consensuses if there is a tor process also running that can fetch the consensus itself.
For safety, programs MAY want to disable functionality until confirming that their versions are acceptable.
To avoid synchronization problems, programs that use the DIGEST field to store a digest of the contents of the URL SHOULD NOT use any URLs whose contents are expected to change while any valid consensus lists them.
3.1. Intended usage by the Tor Browser Bundle
Tor Browser Bundle packages will be listed with package names 'tbb-stable, 'tbb-beta', and 'tbb-alpha'. We will list a line for the latest version of each release series.
When the updater downloads a new update, it always downloads the latest version of the Tor Browser Bundle. Because of this, and because we will only use these lines to authenticate updates, we should not need to list more than one version per series in the consensus.
After completing a package download and verifying the download signatures (which are handled independently from the Tor Consensus), it will consult the appropriate current consensus document through the control port.
If the current consensus timestamp is not yet more recent than the proposed update timestamp, the updater will delay installing the package until a consensus timestamp that is more recent than the update timestamp has been obtained by the Tor client.
If the consensus document has a package line for the current release series with a matching version, it will then download the file at the specified URL, and then compute its hash to make sure it matches the value in the consensus.
If the hash matches, the Tor Browser will download the file and parse its contents, which will be a JSON file which lists information needed to verify the hashes of the downloaded update file.
If the hash does not match, the Tor Browser Bundle should display an error to the user and not install the package.
If there are no package lines in the consensus for the expected version, the updater will delay installing the update (but the bundle should still inform the user they are out of date and may update manually).
If there are no package lines in the consensus for the current release series at all, the updater should install the package using only normal signature verification.
4. Limitations and open questions
This proposal won't tell users how to upgrade, or even exactly what version to upgrade to.
If software is so broken that it won't start at all, or shouldn't be started at all, this proposal can't help with that.
This proposal is not a substitute for a proper software update tool.