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  • TAP: 4
  • Title: Multiple repository consensus on entrusted targets
  • Version: 1
  • Last-Modified: 15-Dec-2017
  • Author: Trishank Karthik Kuppusamy, Sebastien Awwad, Evan Cordell, Vladimir Diaz, Jake Moshenko, Justin Cappos
  • Status: Accepted
  • Content-Type: text/markdown
  • Requires: TAP 5
  • Created: 09-Sep-2016
  • TUF-Version: 1.0.0

Abstract

This TAP offers guidance for conducting a secure search for particular targets across multiple repositories. It discusses how multiple repositories with separate roots of trust can be required to sign off on the same targets, effectively creating an AND relation and ensuring any files obtained can be trusted. In other words, this TAP demonstrates how target names can be mapped to repositories in a manner similar to the way targets with specific names can be delegated to different roles. Like delegations, these repository entries can be ordered/prioritized, and can "terminate" a search if an entrusted target is unavailable.

Motivation

TAP 4 has been motivated by the following use cases.

Use case 1: obtaining different targets from different repositories

It may be desirable to use the same instance of TUF to download and verify different targets hosted on different repositories. For example, a user might want to obtain some Python packages from their maintainers, and others from PyPI. In this way, one can securely access all Python packages, regardless of where they are hosted, and without the need for a different client tool instance (e.g., copy of pip) for each repository.

Use case 2: hiding sensitive metadata and targets

Extending the previous example, enterprise users may not wish to upload some metadata and targets to a public repository because doing so may reveal sensitive or proprietary information. These users may choose to host the sensitive metadata and targets on a private repository, while still employing a public repository for other files. However, in order to use both private and public repositories, TUF clients need to know how to search for selected targets on the private repository, and all other targets on the public repository.

Use case 3: improving compromise-resilience given multiple repositories

To improve compromise-resilience, a user may wish to have multiple repositories, each with a different root of trust, sign for targets. This means both repository A and repository B must sign for a target file before it can be installed. The effect is similar to the AND relation used in multi-role delegations, only it is applied to repositories instead of target delegations.

Note that if a user is employing multiple repositories with disjointed roots of trust, it is already possible to do something similar. One could have one repository download and use a multi-role delegation to the other repository's target. Thus, if repository A downloaded the targets metadata from repository B, and used a multi-role delegation for the targets metadata, it would achieve a similar result. In this instance, if repository B is compromised, users would not be impacted because repository A's multi-role delegation will prevent the use of repository B's malicious targets files. Unfortunately, if repository A's root role or its top-level targets role were to be compromised, nothing could prevent users from receiving malicious targets files, even if repository B is not compromised. If, though, adopters required valid signatures on the metadata from both repositories, even the compromise described above would not impact users.

Rationale

As our use cases suggest, there are some implementations that may want to allow clients to fetch target files from multiple repositories. Yet, in doing so, users risk receiving malicious files if one of these repositories should be compromised. This TAP presents an implementation strategy to enable a user to securely and precisely control the amount of trust given to different repositories. The guidance here can be applied using any type of data storage/file format mechanism, as long as it follows the implementation logic presented here.

Specification

This section shows how to retrieve a target file with a specific type of name, such as django-2.0.1.tgz, or django*, or *.tar.gzfrom a particular repository. Each repository has its own root of trust (Root role, etc.) so a compromise of one repository does not impact others. Using a scheme similar to targets delegations within a repository, targets may be securely assigned to one or more repositories.

This TAP requires an additional step before a client can request metadata and target files from remote repositories. Specifically, it requires that a client consult a mapping of repositories to file patterns, which lists what repositories should be searched, and in what order, to find particular files. By editing this list of mappings, or assignment instructions, the client, or adopter, can precisely control which repositories are to be trusted for particular target paths. Clients must also keep all of the metadata for each repository in a separate directory of their choice.

The next two sections cover the two main components of this new "pre-update" step. The first explains the mechanism that assigns a target to the specific repository or repositories from which it should be retrieved. The second describes the search logic that uses the mapping mechanism to determine what repositories are visited, and which must sign off on the client's requested target files.

Mechanism that Assigns Targets to Repositories

Adopters must implement a mechanism that directs TUF to the specific repository (or repositories) from which metadata and target files should be downloaded. Assignments of files to repositories are controlled by sets of instructions called mappings.

Each mapping contains the following elements:

A. An ordered list of one or more repositories. If the updater is instructed to contact repositories from this mapping, it tries each repository in the order listed until a threshold of repositories in agreement about the metadata has been reached.

B. A list of file paths associated with the ordered list of one or more repositories. This element supports implementations like the one outlined in use case 3, in which the user requires valid signatures from multiple repositories. The file paths may be condensed as glob patterns. For example, the updater can be instructed to download paths that resemble the glob pattern baz*.tgz from only the third mapping.

C. A "terminating" flag that instructs the updater whether to continue searching subsequent mappings after failing to download requested target files from the repositories specified in the first mapping. The list of repositories within a mapping can indicate/use the terminating flag independent of repositories in other mappings.

D. A threshold that indicates the minimum number of repositories in (A) that are required to sign for the same length and hash of any matching target, as specified in element (B).

The four elements above are all that is required to guide the updater in its search for requested files.

Searching for Files on Multiple Repositories

Figure 1 - Mapping In the figure above (figure 1), a request is made for foo-1.0.tgz and an ordered list of mappings is consulted to determine which repositories should be contacted.

To complete a search using this mechanism, a TUF client will follow these steps:

  1. Check each mapping, in the listed order, and identify the first mapping that matches the requested file. In figure 1, the client should choose the second mapping because the glob pattern (foo*.tgz) matches the file.

  2. Once a mapping is identified for the requested file, TUF metadata is downloaded and verified from the assigned repositories it lists. Verification occurs if the length and hashes about the target match across a threshold of repositories (per element D). Custom targets metadata are exempt from this requirement. In figure 1, repositories D and F can be contacted to download metadata, and both repositories must provide matching metadata about foo-1.0.tgz to meet the mapping's threshold of 2.

  3. If the targets metadata is a match across the specified threshold of repositories, return this metadata.

  4. If the metadata is not a match, or if fewer than the threshold of repositories signed metadata about the desired target, then the client should take one of the following actions:

    4.1. If the terminating flag (per element C) is set to true, report that either the repositories do not agree on the target, or that none of them have signed for the target. In figure 1, the terminating flag for the first mapping is True, but since the mapping was not a match, the search continued to the second mapping.

    4.2. Otherwise, go back to step 1 and process the next mapping that matches the requested file. In figure 1, if the second mapping was not a match, or if none of its repositories signed metadata about foo-1.0.tgz, the third mapping will still match because its glob pattern is * (all requested files match).

Example using the Reference Implementation's Map File

To demonstrate the reference implementation's handling of multiple repository consensus on entrusted targets, we employ a file named map.json. This map file comes into play when a TUF client requests targets and adheres to the four elements of the mapping mechanism.

If the map file is to be used to assign targets to repositories, it will either be constructed by a user employing the TUF command-line tools, or distributed by an out-of-band bootstrap process. This file is not intended to be automatically available or refreshed from a repository. In fact, the map file is kept on the client, and is only modified by a user who is trusted to configure the updater instance.

The map file contains a dictionary that holds two keys, "repositories" and "mapping." The value of the "repositories" key is another dictionary that lists the URLs for a set of repositories. Each key in this dictionary is a repository name, and its value is a list of URLs. The repository name also corresponds to the name of the local directory on the TUF client where metadata files would be cached. Crucially, this is where the root metadata file for a repository is located.

The repository will also contain a list of URLs that indicates the location from which files should be retrieved. Each URL points to a root directory containing metadata and target files.

The value of the "mapping" key is a priority-ordered list that maps paths (i.e., target names) to specific repositories, like the mechanism described earlier in this document. Every entry in this list is a dictionary containing the following keys:

  • "paths" specifies a list of target paths of patterns. A desired target must match a pattern in this list for this mapping to be consulted.
  • "repositories" specifies a list of one or more repository names.
  • "terminating" is a Boolean attribute indicating whether or not this mapping terminates backtracking.
  • "threshold" is the minimum number of roles that must sign for any given target under "paths".

The following is an example of a map file:

{
  // For each repository, its key name is the directory where files, including
  // the root metadata file, are cached, and its value is a list of URLs where
  // files may be downloaded.
  "repositories": {
    "Django": ["https://djangoproject.com/"],
    "PyPI":   ["https://pypi.python.org/"]
  },
  // For each set of targets, specify a list of repositories where files may be
  // downloaded.
  "mapping": [
    {
      // Much like target delegation, the order of these entries indicates
      // the priority of the delegation.  The entries listed first will be
      // considered first.

      // Map the targets "/django/django-1.*.tgz" to both Django and PyPI.
      "paths":        ["/django/django-1.*.tgz"],
      "repositories": ["Django", "PyPI"],

      // At least one repository must sign for the same length and hashes
      // of the "/django/django-1.*.tgz" targets.
      "threshold": 1

      // In this case, the "terminating" attribute is set to false.
      "terminating":  false,
      // Therefore, if this mapping has not signed for "/django/django-1.*.tgz"
      // targets, the following mapping will be consulted.

    },
    {
      ...
    {
      // Map all other targets only to PyPI.
      "paths":        ["*"],
      "repositories": ["PyPI"],
      "terminating": true
      "threshold": 1
    }
  ]
}

Security Analysis

Employing this TAP allows users to securely map targets to one or more repositories. However, it does not change the way TUF verifies metadata for any individual repository. Each repository will continue to be treated as it was previously, with TUF performing the necessary verification of repository metadata.

In order to avoid accidental denial-of-service attacks when multiple repositories sign the same targets, there must be a coordinated effort to ensure all are signing the same targets metadata (i.e., length and hashes).

Backwards Compatibility

This specification is backwards-compatible, though older clients will not support multiple repository consensus on entrusted target files, and so will ignore this TAP. These clients may continue to use a single repository. New clients need to add relatively little code to follow the behavior defined by TAP 4. However, they must be careful to store the metadata for each repository separately from others (e.g., by using a different directory for each repository).

A TUF repository does not need to change in any way to support this TAP.

Augmented Reference Implementation

This branch demonstrates how TAP 4 can be implemented without modifying an existing TUF repository.

Copyright

This document has been placed in the public domain.

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