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The Update Framework Specification

Last modified: 19 June 2018

Version: 1.0 (Draft)

We strive to make the specification easy to implement, so if you come across any inconsistencies or experience any difficulty, do let us know by sending an email to our mailing list, or by reporting an issue in the specification repo.

Table of Contents

1. Introduction

  • 1.1. Scope

    This document describes a framework for securing software update systems.

    The keywords "MUST," "MUST NOT," "REQUIRED," "SHALL," "SHALL NOT," "SHOULD," "SHOULD NOT," "RECOMMENDED," "MAY," and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

  • 1.2. Motivation

    Software is commonly updated through software update systems. These systems can be package managers that are responsible for all of the software that is installed on a system, application updaters that are only responsible for individual installed applications, or software library managers that install software that adds functionality such as plugins or programming language libraries.

    Software update systems all have the common behavior of downloading files that identify whether updates exist and, when updates do exist, downloading the files that are required for the update. For the implementations concerned with security, various integrity and authenticity checks are performed on downloaded files.

    Software update systems are vulnerable to a variety of known attacks. This is generally true even for implementations that have tried to be secure.

  • 1.3. History and credit

    Work on TUF began in late 2009. The core ideas are based off of previous work done by Justin Cappos and Justin Samuel that identified security flaws in all popular Linux package managers. More information and current versions of this document can be found at https://www.updateframework.com/

    The Global Environment for Network Innovations (GENI) and the National Science Foundation (NSF) have provided support for the development of TUF.

    TUF's reference implementation is based heavily on Thandy, the application updater for Tor. Its design and this spec are also largely based on Thandy's, with many parts being directly borrowed from Thandy. The Thandy spec can be found at https://gitweb.torproject.org/thandy.git/tree/specs/thandy-spec.txt

    Whereas Thandy is an application updater for an individual software project, TUF aims to provide a way to secure any software update system. We're very grateful to the Tor Project and the Thandy developers as it is doubtful our design and implementation would have been anywhere near as good without being able to use their great work as a starting point. Thandy is the hard work of Nick Mathewson, Sebastian Hahn, Roger Dingledine, Martin Peck, and others.

  • 1.4. Non-goals

    We are not creating a universal update system, but rather a simple and flexible way that applications can have high levels of security with their software update systems. Creating a universal software update system would not be a reasonable goal due to the diversity of application-specific functionality in software update systems and the limited usefulness that such a system would have for securing legacy software update systems.

    We won't be defining package formats or even performing the actual update of application files. We will provide the simplest mechanism possible that remains easy to use and provides a secure way for applications to obtain and verify files being distributed by trusted parties.

    We are not providing a means to bootstrap security so that arbitrary installation of new software is secure. In practice this means that people still need to use other means to verify the integrity and authenticity of files they download manually.

    The framework will not have the responsibility of deciding on the correct course of action in all error situations, such as those that can occur when certain attacks are being performed. Instead, the framework will provide the software update system the relevant information about any errors that require security decisions which are situation-specific. How those errors are handled is up to the software update system.

  • 1.5. Goals

    We need to provide a framework (a set of libraries, file formats, and utilities) that can be used to secure new and existing software update systems.

    The framework should enable applications to be secure from all known attacks on the software update process. It is not concerned with exposing information about what software is being updated (and thus what software the client may be running) or the contents of updates.

    The framework should provide means to minimize the impact of key compromise. To do so, it must support roles with multiple keys and threshold/quorum trust (with the exception of minimally trusted roles designed to use a single key). The compromise of roles using highly vulnerable keys should have minimal impact. Therefore, online keys (keys which are used in an automated fashion) must not be used for any role that clients ultimately trust for files they may install.

    The framework must be flexible enough to meet the needs of a wide variety of software update systems.

    The framework must be easy to integrate with software update systems.

    • 1.5.1 Goals for implementation

      • The client side of the framework must be straightforward to implement in any programming language and for any platform with the requisite networking and crypto support.

      • The process by which developers push updates to the repository must be simple.

      • The framework must be secure to use in environments that lack support for SSL (TLS). This does not exclude the optional use of SSL when available, but the framework will be designed without it.

    • 1.5.2. Goals to protect against specific attacks

      Note: When saying the framework protects against an attack, it means the attack will be unsuccessful. It does not mean that a client will always successfully update during an attack. Fundamentally, an attacker positioned to intercept and modify a client's communication can always perform a denial of service. Nevertheless, the framework must detect when a client is unable to update.

      • Arbitrary installation attacks. An attacker cannot install anything they want on the client system. That is, an attacker cannot provide arbitrary files in response to download requests.

      • Endless data attacks. An attacker cannot respond to client requests with huge amounts of data (extremely large files) that interfere with the client's system.

      • Extraneous dependencies attacks. An attacker cannot cause clients to download or install software dependencies that are not the intended dependencies.

      • Fast-forward attacks. An attacker cannot arbitrarily increase the version numbers of metadata files, listed in the snapshot metadata, well beyond the current value and thus tricking a software update system into thinking any subsequent updates are trying to rollback the package to a previous, out-of-date version. In some situations, such as those where there is a maximum possible version number, the perpetrator cannot use a number so high that the system would never be able to match it with the one in the snapshot metadata, and thus new updates could never be downloaded.

      • Indefinite freeze attacks. An attacker cannot respond to client requests with the same, outdated metadata without the client being aware of the problem.

      • Malicious mirrors preventing updates. A repository mirror cannot prevent updates from good mirrors.

      • Mix-and-match attacks. An attacker cannot trick clients into using a combination of metadata that never existed together on the repository at the same time.

      • Rollback attacks. An attacker cannot trick clients into installing software that is older than that which the client previously knew to be available.

      • Slow retrieval attacks. An attacker cannot prevent clients from being aware of interference with receiving updates by responding to client requests so slowly that automated updates never complete.

      • Vulnerability to key compromises. An attacker, who is able to compromise a single key or less than a given threshold of keys, cannot compromise clients. This includes compromising a single online key (such as only being protected by SSL) or a single offline key (such as most software update systems use to sign files).

      • Wrong software installation. An attacker cannot provide a file (trusted or untrusted) that is not the one the client wanted.

    • 1.5.3. Goals for PKIs

      • Software update systems using the framework's client code interface should never have to directly manage keys.

      • All keys must be easily and safely revocable. Trusting new keys for a role must be easy.

      • For roles where trust delegation is meaningful, a role should be able to delegate full or limited trust to another role.

      • The root of trust must not rely on external PKI. That is, no authority will be derived from keys outside of the framework.

2. System overview

The framework ultimately provides a secure method of obtaining trusted files. To avoid ambiguity, we will refer to the files the framework is used to distribute as "target files". Target files are opaque to the framework. Whether target files are packages containing multiple files, single text files, or executable binaries is irrelevant to the framework.

The metadata describing target files is the information necessary to securely identify the file and indicate which roles are trusted to provide the file. As providing additional information about target files may be important to some software update systems using the framework, additional arbitrary information can be provided with any target file. This information will be included in signed metadata that describes the target files.

The following are the high-level steps of using the framework from the viewpoint of a software update system using the framework. This is an error-free case.

   Polling:
        Periodically, the software update system using the framework
        instructs the framework to check each repository for updates.  If
        the framework reports to the application code that there are
        updates, the application code determines whether it wants to
        download the updated target files.  Only target files that are
        trusted (referenced by properly signed and timely metadata) are
        made available by the framework.

   Fetching:
        For each file that the application wants, it asks the framework to
        download the file.  The framework downloads the file and performs
        security checks to ensure that the downloaded file is exactly what
        is expected according to the signed metadata.  The application code
        is not given access to the file until the security checks have been
        completed.  The application asks the framework to copy the
        downloaded file to a location specified by the application.  At
        this point, the application has securely obtained the target file
        and can do with it whatever it wishes.
  • 2.1. Roles and PKI

    In the discussion of roles that follows, it is important to remember that the framework has been designed to allow a large amount of flexibility for many different use cases. For example, it is possible to use the framework with a single key that is the only key used in the entire system. This is considered to be insecure but the flexibility is provided in order to meet the needs of diverse use cases.

    There are four fundamental top-level roles in the framework:

    • Root role
    • Targets role
    • Snapshot role
    • Timestamp role

    There is also one optional top-level role:

    • Mirrors role

    All roles can use one or more keys and require a threshold of signatures of the role's keys in order to trust a given metadata file.

    • 2.1.1. Root Role

      • The root role delegates trust to specific keys trusted for all other top-level roles used in the system.

      • The client-side of the framework must ship with trusted root keys for each configured repository.

      • The root role's private keys must be kept very secure and thus should be kept offline. If less than a threshold of Root keys are compromised, the repository should revoke trust on the compromised keys. This can be accomplished with a normal rotation of root keys, covered in section 6.1 (Key management and migration). If a threshold of root keys is compromised, the Root keys should be updated out-of-band, however, the threshold should be chosen so that this is extremely unlikely. In the unfortunate event that a threshold of keys are compromised, it is safest to assume that attackers have installed malware and taken over affected machines. For this reason, making it difficult for attackers to compromise all of the offline keys is important because safely recovering from it is nearly impossible.

    • 2.1.2 Targets role

      The targets role's signature indicates which target files are trusted by clients. The targets role signs metadata that describes these files, not the actual target files themselves.

      In addition, the targets role can delegate full or partial trust to other roles. Delegating trust means that the targets role indicates another role (that is, another set of keys and the threshold required for trust) is trusted to sign target file metadata. Partial trust delegation is when the delegated role is only trusted for some of the target files that the delegating role is trusted for.

      Delegated roles can further delegate trust to other delegated roles. This provides for multiple levels of trust delegation where each role can delegate full or partial trust for the target files they are trusted for. The delegating role in these cases is still trusted. That is, a role does not become untrusted when it has delegated trust.

      Any delegation can be revoked at any time: the delegating role need only sign new metadata that no longer contains that delegation.

    • 2.1.3 Snapshot role

      The snapshot role signs a metadata file that provides information about the latest version of all of the other metadata on the repository (excluding the timestamp file, discussed below). This information allows clients to know which metadata files have been updated and also prevents mix-and-match attacks.

    • 2.1.4 Timestamp role

      To prevent an adversary from replaying an out-of-date signed metadata file whose signature has not yet expired, an automated process periodically signs a timestamped statement containing the hash of the snapshot file. Even though this timestamp key must be kept online, the risk posed to clients by compromise of this key is minimal.

    • 2.1.5 Mirrors role

      Every repository has one or more mirrors from which files can be downloaded by clients. A software update system using the framework may choose to hard-code the mirror information in their software or they may choose to use mirror metadata files that can optionally be signed by a mirrors role.

      The importance of using signed mirror lists depends on the application and the users of that application. There is minimal risk to the application's security from being tricked into contacting the wrong mirrors. This is because the framework has very little trust in repositories.

  • 2.2. Threat Model And Analysis

    We assume an adversary who can respond to client requests, whether by acting as a man-in-the-middle or through compromising repository mirrors. At worst, such an adversary can deny updates to users if no good mirrors are accessible. An inability to obtain updates is noticed by the framework.

    If an adversary compromises enough keys to sign metadata, the best that can be done is to limit the number of users who are affected. The level to which this threat is mitigated is dependent on how the application is using the framework. This includes whether different keys have been used for different signing roles.

    A detailed threat analysis is outside the scope of this document. This is partly because the specific threat posted to clients in many situations is largely determined by how the framework is being used.

3. The repository

An application uses the framework to interact with one or more repositories. A repository is a conceptual source of target files of interest to the application. Each repository has one or more mirrors which are the actual providers of files to be downloaded. For example, each mirror may specify a different host where files can be downloaded from over HTTP.

The mirrors can be full or partial mirrors as long as the application-side of the framework can ultimately obtain all of the files it needs. A mirror is a partial mirror if it is missing files that a full mirror should have. If a mirror is intended to only act as a partial mirror, the metadata and target paths available from that mirror can be specified.

Roles, trusted keys, and target files are completely separate between repositories. A multi-repository setup is a multi-root system. When an application uses the framework with multiple repositories, the framework does not perform any "mixing" of the trusted content from each repository. It is up to the application to determine the significance of the same or different target files provided from separate repositories.

  • 3.1 Repository layout

    The filesystem layout in the repository is used for two purposes:

    • To give mirrors an easy way to mirror only some of the repository.
    • To specify which parts of the repository a given role has authority to sign/provide.
  • 3.1.1 Target files

    The filenames and the directory structure of target files available from a repository are not specified by the framework. The names of these files and directories are completely at the discretion of the application using the framework.

  • 3.1.2 Metadata files

    The filenames and directory structure of repository metadata are strictly defined. The following are the metadata files of top-level roles relative to the base URL of metadata available from a given repository mirror.

    /root.json

      Signed by the root keys; specifies trusted keys for the other
      top-level roles.
    

    /snapshot.json

      Signed by the snapshot role's keys.  Lists the version numbers of all
      metadata files other than timestamp.json.
    

    /targets.json

      Signed by the target role's keys.  Lists hashes and sizes of target
      files.
    

    /timestamp.json

      Signed by the timestamp role's keys.  Lists hash(es), size, and version
      number of the snapshot file.  This is the first and potentially only
      file that needs to be downloaded when clients poll for the existence
      of updates.
    

    /mirrors.json (optional)

      Signed by the mirrors role's keys.  Lists information about available
      mirrors and the content available from each mirror.
    
  • 3.1.2.1 Metadata files for targets delegation

When the targets role delegates trust to other roles, each delegated role provides one signed metadata file. As is the case with the directory structure of top-level metadata, the delegated files are relative to the base URL of metadata available from a given repository mirror.

A delegated role file is located at:

/DELEGATED_ROLE.json

where DELEGATED_ROLE is the name of the delegated role that has been specified in targets.json. If this role further delegates trust to a role named ANOTHER_ROLE, that role's signed metadata file is made available at:

/ANOTHER_ROLE.json

4. Document formats

All of the formats described below include the ability to add more attribute-value fields for backwards-compatible format changes. If a backwards incompatible format change is needed, a new filename can be used.

  • 4.1. Metaformat

    All documents use a subset of the JSON object format, with floating-point numbers omitted. When calculating the digest of an object, we use the "canonical JSON" subdialect as described at http://wiki.laptop.org/go/Canonical_JSON

  • 4.2. File formats: general principles

    All signed metadata objects have the format:

     { "signed" : ROLE,
       "signatures" : [
          { "keyid" : KEYID,
            "sig" : SIGNATURE }
          , ... ]
     }
    

    where:

        ROLE is a dictionary whose "_type" field describes the role type.
    
        KEYID is the identifier of the key signing the ROLE dictionary.
    
        SIGNATURE is a signature of the canonical JSON form of ROLE.
    

    All keys have the format:

      { "keytype" : KEYTYPE,
        "scheme" : SCHEME,
        "keyval" : KEYVAL
      }
    

    where:

        KEYTYPE is a string denoting a public key signature system, such
        as RSA or ECDSA.
    
        SCHEME is a string denoting a corresponding signature scheme.  For
        example: "rsassa-pss-sha256" and "ecdsa-sha2-nistp256".
    
        KEYVAL is a dictionary containing the public portion of the key.
    

    The reference implementation defines three signature schemes, although TUF is not restricted to any particular signature scheme, key type, or cryptographic library:

     "rsassa-pss-sha256" : RSA Probabilistic signature scheme with appendix.
      The underlying hash function is SHA256.
      https://tools.ietf.org/html/rfc3447#page-29
    
     "ed25519" : Elliptic curve digital signature algorithm based on Twisted
      Edwards curves.
      https://ed25519.cr.yp.to/
    
      "ecdsa-sha2-nistp256" : Elliptic Curve Digital Signature Algorithm
       with NIST P-256 curve signing and SHA-256 hashing.
       https://en.wikipedia.org/wiki/Elliptic_Curve_Digital_Signature_Algorithm
    

    We define three keytypes below: 'rsa', 'ed25519', and 'ecdsa', but adopters can define and use any particular keytype, signing scheme, and cryptographic library.

    The 'rsa' format is:

      { "keytype" : "rsa",
        "scheme" : "rsassa-pss-sha256",
        "keyval" : {"public" : PUBLIC}
      }
    

    where PUBLIC is in PEM format and a string. All RSA keys must be at least 2048 bits.

    The 'ed25519' format is:

      { "keytype" : "ed25519",
        "scheme" : "ed25519",
        "keyval" : {"public" : PUBLIC}
      }
    

    where PUBLIC is a 32-byte string.

    The 'ecdsa' format is:

      { "keytype" : "ecdsa-sha2-nistp256",
        "scheme" : "ecdsa-sha2-nistp256",
        "keyval" : {"public" : PUBLIC}
      }
    

    where:

      PUBLIC is in PEM format and a string.
    

    The KEYID of a key is the hexdigest of the SHA-256 hash of the canonical JSON form of the key.

    Metadata date-time data follows the ISO 8601 standard. The expected format of the combined date and time string is "YYYY-MM-DDTHH:MM:SSZ". Time is always in UTC, and the "Z" time zone designator is attached to indicate a zero UTC offset. An example date-time string is "1985-10-21T01:21:00Z".

  • 4.3. File formats: root.json

    The root.json file is signed by the root role's keys. It indicates which keys are authorized for all top-level roles, including the root role itself. Revocation and replacement of top-level role keys, including for the root role, is done by changing the keys listed for the roles in this file.

    The "signed" portion of root.json is as follows:

     { "_type" : "root",
       "spec_version" : SPEC_VERSION,
       "consistent_snapshot": CONSISTENT_SNAPSHOT,
       "version" : VERSION,
       "expires" : EXPIRES,
       "keys" : {
           KEYID : KEY
           , ... },
       "roles" : {
           ROLE : {
             "keyids" : [ KEYID, ... ] ,
             "threshold" : THRESHOLD }
           , ... }
     }
    

    SPEC_VERSION is the version number of the specification. Metadata is written according to version "spec_version" of the specification, and clients MUST verify that "spec_version" matches the expected version number. Adopters are free to determine what is considered a match (e.g., the version number exactly, or perhaps only the major version number (major.minor.fix).

    CONSISTENT_SNAPSHOT is a boolean indicating whether the repository supports consistent snapshots. Section 7 goes into more detail on the consequences of enabling this setting on a repository.

    VERSION is an integer that is greater than 0. Clients MUST NOT replace a metadata file with a version number less than the one currently trusted.

    EXPIRES determines when metadata should be considered expired and no longer trusted by clients. Clients MUST NOT trust an expired file.

    A ROLE is one of "root", "snapshot", "targets", "timestamp", or "mirrors". A role for each of "root", "snapshot", "timestamp", and "targets" MUST be specified in the key list. The role of "mirror" is optional. If not specified, the mirror list will not need to be signed if mirror lists are being used.

    The KEYID must be correct for the specified KEY. Clients MUST calculate each KEYID to verify this is correct for the associated key. Clients MUST ensure that for any KEYID represented in this key list and in other files, only one unique key has that KEYID.

    The THRESHOLD for a role is an integer of the number of keys of that role whose signatures are required in order to consider a file as being properly signed by that role.

    A root.json example file:

     {
     "signatures": [
      {
       "keyid": "f2d5020d08aea06a0a9192eb6a4f549e17032ebefa1aa9ac167c1e3e727930d6",
       "sig": "a312b9c3cb4a1b693e8ebac5ee1ca9cc01f2661c14391917dcb111517f72370809
               f32c890c6b801e30158ac4efe0d4d87317223077784c7a378834249d048306"
      }
     ],
     "signed": {
      "_type": "root",
      "spec_version": "1",
      "consistent_snapshot": false,
      "expires": "2030-01-01T00:00:00Z",
      "keys": {
       "1a2b4110927d4cba257262f614896179ff85ca1f1353a41b5224ac474ca71cb4": {
        "keytype": "ed25519",
        "scheme": "ed25519",
        "keyval": {
         "public": "72378e5bc588793e58f81c8533da64a2e8f1565c1fcc7f253496394ffc52542c"
        }
       },
       "93ec2c3dec7cc08922179320ccd8c346234bf7f21705268b93e990d5273a2a3b": {
        "keytype": "ed25519",
        "scheme": "ed25519",
        "keyval": {
         "public": "68ead6e54a43f8f36f9717b10669d1ef0ebb38cee6b05317669341309f1069cb"
        }
       },
       "f2d5020d08aea06a0a9192eb6a4f549e17032ebefa1aa9ac167c1e3e727930d6": {
        "keytype": "ed25519",
        "scheme": "ed25519",
        "keyval": {
         "public": "66dd78c5c2a78abc6fc6b267ff1a8017ba0e8bfc853dd97af351949bba021275"
        }
       },
       "fce9cf1cc86b0945d6a042f334026f31ed8e4ee1510218f198e8d3f191d15309": {
        "keytype": "ed25519",
        "scheme": "ed25519",
        "keyval": {
         "public": "01c61f8dc7d77fcef973f4267927541e355e8ceda757e2c402818dad850f856e"
        }
       }
      },
      "roles": {
       "root": {
        "keyids": [
         "f2d5020d08aea06a0a9192eb6a4f549e17032ebefa1aa9ac167c1e3e727930d6"
        ],
        "threshold": 1
       },
       "snapshot": {
        "keyids": [
         "fce9cf1cc86b0945d6a042f334026f31ed8e4ee1510218f198e8d3f191d15309"
        ],
        "threshold": 1
       },
       "targets": {
        "keyids": [
         "93ec2c3dec7cc08922179320ccd8c346234bf7f21705268b93e990d5273a2a3b"
        ],
        "threshold": 1
       },
       "timestamp": {
        "keyids": [
         "1a2b4110927d4cba257262f614896179ff85ca1f1353a41b5224ac474ca71cb4"
        ],
        "threshold": 1
       }
      },
      "version": 1
     }
    }
    
  • 4.4. File formats: snapshot.json

    The snapshot.json file is signed by the snapshot role. It lists the version numbers of all metadata on the repository, excluding timestamp.json and mirrors.json. For the root role, the hash(es), size, and version number are listed.

    The "signed" portion of snapshot.json is as follows:

     { "_type" : "snapshot",
       "spec_version" : SPEC_VERSION,
       "version" : VERSION,
       "expires" : EXPIRES,
       "meta" : METAFILES
     }
    

    METAFILES is an object whose format is the following:

     { METAPATH : {
           "version" : VERSION }
       , ...
     }
    

    METAPATH is the the metadata file's path on the repository relative to the metadata base URL.

    VERSION is listed for the root file and all other roles available on the repository.

    A snapshot.json example file:

     {
     "signatures": [
      {
       "keyid": "fce9cf1cc86b0945d6a042f334026f31ed8e4ee1510218f198e8d3f191d15309",
       "sig": "f7f03b13e3f4a78a23561419fc0dd741a637e49ee671251be9f8f3fceedfc112e4
               4ee3aaff2278fad9164ab039118d4dc53f22f94900dae9a147aa4d35dcfc0f"
      }
     ],
     "signed": {
      "_type": "snapshot",
      "spec_version": "1",
      "expires": "2030-01-01T00:00:00Z",
      "meta": {
       "root.json": {
        "version": 1
       },
       "targets.json": {
        "version": 1
       },
       "project.json": {
        "version": 1
        },
       }
      "version": 1
      },
     }
    
  • 4.5. File formats: targets.json and delegated target roles

    The "signed" portion of targets.json is as follows:

     { "_type" : "targets",
       "spec_version" : SPEC_VERSION,
       "version" : VERSION,
       "expires" : EXPIRES,
       "targets" : TARGETS,
       ("delegations" : DELEGATIONS)
     }
    

    TARGETS is an object whose format is the following:

     { TARGETPATH : {
           "length" : LENGTH,
           "hashes" : HASHES,
           ("custom" : { ... }) }
       , ...
     }
    

    Each key of the TARGETS object is a TARGETPATH. A TARGETPATH is a path to a file that is relative to a mirror's base URL of targets.

    It is allowed to have a TARGETS object with no TARGETPATH elements. This can be used to indicate that no target files are available.

    If defined, the elements and values of "custom" will be made available to the client application. The information in "custom" is opaque to the framework and can include version numbers, dependencies, requirements, and any other data that the application wants to include to describe the file at TARGETPATH. The application may use this information to guide download decisions.

    DELEGATIONS is an object whose format is the following:

     { "keys" : {
           KEYID : KEY,
           ... },
       "roles" : [{
           "name": ROLENAME,
           "keyids" : [ KEYID, ... ] ,
           "threshold" : THRESHOLD,
           ("path_hash_prefixes" : [ HEX_DIGEST, ... ] |
            "paths" : [ PATHPATTERN, ... ]),
           "terminating": TERMINATING,
       }, ... ]
     }
    

    ROLENAME is the name of the delegated role. For example, "projects".

    TERMINATING is a boolean indicating whether subsequent delegations should be considered.

    As explained in the Diplomat paper, terminating delegations instruct the client not to consider future trust statements that match the delegation's pattern, which stops the delegation processing once this delegation (and its descendants) have been processed. A terminating delegation for a package causes any further statements about a package that are not made by the delegated party or its descendants to be ignored.

    In order to discuss target paths, a role MUST specify only one of the "path_hash_prefixes" or "paths" attributes, each of which we discuss next.

    The "path_hash_prefixes" list is used to succinctly describe a set of target paths. Specifically, each HEX_DIGEST in "path_hash_prefixes" describes a set of target paths; therefore, "path_hash_prefixes" is the union over each prefix of its set of target paths. The target paths must meet this condition: each target path, when hashed with the SHA-256 hash function to produce a 64-byte hexadecimal digest (HEX_DIGEST), must share the same prefix as one of the prefixes in "path_hash_prefixes". This is useful to split a large number of targets into separate bins identified by consistent hashing.

    The "paths" list describes paths that the role is trusted to provide. Clients MUST check that a target is in one of the trusted paths of all roles in a delegation chain, not just in a trusted path of the role that describes the target file. PATHPATTERN can include shell-style wildcards and supports the Unix filename pattern matching convention. Its format may either indicate a path to a single file, or to multiple paths with the use of shell-style wildcards. For example, the path pattern "targets/*.tgz" would match file paths "targets/foo.tgz" and "targets/bar.tgz", but not "targets/foo.txt". Likewise, path pattern "foo-version-?.tgz" matches "foo-version-2.tgz" and "foo-version-a.tgz", but not "foo-version-alpha.tgz".

    Prioritized delegations allow clients to resolve conflicts between delegated roles that share responsibility for overlapping target paths. To resolve conflicts, clients must consider metadata in order of appearance of delegations; we treat the order of delegations such that the first delegation is trusted over the second one, the second delegation is trusted more than the third one, and so on. Likewise, the metadata of the first delegation will override that of the second delegation, the metadata of the second delegation will override that of the third one, etc. In order to accommodate prioritized delegations, the "roles" key in the DELEGATIONS object above points to an array of delegated roles, rather than to a hash table.

    The metadata files for delegated target roles has the same format as the top-level targets.json metadata file.

    A targets.json example file:

     {
     "signatures": [
      {
       "keyid": "93ec2c3dec7cc08922179320ccd8c346234bf7f21705268b93e990d5273a2a3b",
       "sig": "e9fd40008fba263758a3ff1dc59f93e42a4910a282749af915fbbea1401178e5a0
               12090c228f06db1deb75ad8ddd7e40635ac51d4b04301fce0fd720074e0209"
      }
     ],
     "signed": {
      "_type": "targets",
      "spec_version": "1",
      "delegations": {
       "keys": {
        "ce3e02e72980b09ca6f5efa68197130b381921e5d0675e2e0c8f3c47e0626bba": {
         "keytype": "ed25519",
         "scheme": "ed25519",
         "keyval": {
          "public": "b6e40fb71a6041212a3d84331336ecaa1f48a0c523f80ccc762a034c727606fa"
         }
        }
       },
       "roles": [
        {
         "keyids": [
          "ce3e02e72980b09ca6f5efa68197130b381921e5d0675e2e0c8f3c47e0626bba"
         ],
         "name": "project",
         "paths": [
          "/project/file3.txt"
         ],
         "threshold": 1
        }
       ]
      },
      "expires": "2030-01-01T00:00:00Z",
      "targets": {
       "/file1.txt": {
        "hashes": {
         "sha256": "65b8c67f51c993d898250f40aa57a317d854900b3a04895464313e48785440da"
        },
        "length": 31
       },
       "/file2.txt": {
        "hashes": {
         "sha256": "452ce8308500d83ef44248d8e6062359211992fd837ea9e370e561efb1a4ca99"
        },
        "length": 39
       }
      },
      "version": 1
      }
     }
    
  • 4.6. File formats: timestamp.json

    The timestamp file is signed by a timestamp key. It indicates the latest versions of other files and is frequently resigned to limit the amount of time a client can be kept unaware of interference with obtaining updates.

    Timestamp files will potentially be downloaded very frequently. Unnecessary information in them will be avoided.

    The "signed" portion of timestamp.json is as follows:

     { "_type" : "timestamp",
       "spec_version" : SPEC_VERSION,
       "version" : VERSION,
       "expires" : EXPIRES,
       "meta" : METAFILES
     }
    

    METAFILES is the same is described for the snapshot.json file. In the case of the timestamp.json file, this will commonly only include a description of the snapshot.json file.

    A signed timestamp.json example file:

     {
     "signatures": [
      {
       "keyid": "1a2b4110927d4cba257262f614896179ff85ca1f1353a41b5224ac474ca71cb4",
       "sig": "90d2a06c7a6c2a6a93a9f5771eb2e5ce0c93dd580bebc2080d10894623cfd6eaed
               f4df84891d5aa37ace3ae3736a698e082e12c300dfe5aee92ea33a8f461f02"
      }
     ],
     "signed": {
      "_type": "timestamp",
      "spec_version": "1",
      "expires": "2030-01-01T00:00:00Z",
      "meta": {
       "snapshot.json": {
        "hashes": {
         "sha256": "c14aeb4ac9f4a8fc0d83d12482b9197452f6adf3eb710e3b1e2b79e8d14cb681"
        },
        "length": 1007,
        "version": 1
       }
      },
      "version": 1
      }
     }
    
  • 4.7. File formats: mirrors.json

    The mirrors.json file is signed by the mirrors role. It indicates which mirrors are active and believed to be mirroring specific parts of the repository.

    The "signed" portion of mirrors.json is as follows:

    { "_type" : "mirrors",
     "spec_version" : SPEC_VERSION,
     "version" : VERSION,
     "expires" : EXPIRES,
     "mirrors" : [
        { "urlbase" : URLBASE,
          "metapath" : METAPATH,
          "targetspath" : TARGETSPATH,
          "metacontent" : [ PATHPATTERN ... ] ,
          "targetscontent" : [ PATHPATTERN ... ] ,
          ("custom" : { ... }) }
        , ... ]
    }
    

    URLBASE is the URL of the mirror which METAPATH and TARGETSPATH are relative to. All metadata files will be retrieved from METAPATH and all target files will be retrieved from TARGETSPATH.

    The lists of PATHPATTERN for "metacontent" and "targetscontent" describe the metadata files and target files available from the mirror.

    The order of the list of mirrors is important. For any file to be downloaded, whether it is a metadata file or a target file, the framework on the client will give priority to the mirrors that are listed first. That is, the first mirror in the list whose "metacontent" or "targetscontent" include a path that indicate the desired file can be found there will the first mirror that will be used to download that file. Successive mirrors with matching paths will only be tried if downloading from earlier mirrors fails. This behavior can be modified by the client code that uses the framework to, for example, randomly select from the listed mirrors.

5. Detailed Workflows

The client application

Note: If a step in the following workflow does not succeed (e.g., the update is aborted because a new metadata file was not signed), the client should still be able to update again in the future. Errors raised during the update process should not leave clients in an unrecoverable state.

0. Load the trusted root metadata file. We assume that a good, trusted copy of this file was shipped with the package manager or software updater using an out-of-band process. Note that the expiration of the trusted root metadata file does not matter, because we will attempt to update it in the next step.

1. Update the root metadata file. Since it may now be signed using entirely different keys, the client must somehow be able to establish a trusted line of continuity to the latest set of keys (see Section 6.1). To do so, the client MUST download intermediate root metadata files, until the latest available one is reached.

  • 1.1. Let N denote the version number of the trusted root metadata file.

  • 1.2. Try downloading version N+1 of the root metadata file, up to some X number of bytes (because the size is unknown). The value for X is set by the authors of the application using TUF. For example, X may be tens of kilobytes. The filename used to download the root metadata file is of the fixed form VERSION_NUMBER.FILENAME.EXT (e.g., 42.root.json). If this file is not available, then go to step 1.8.

  • 1.3. Check signatures. Version N+1 of the root metadata file MUST have been signed by: (1) a threshold of keys specified in the trusted root metadata file (version N), and (2) a threshold of keys specified in the new root metadata file being validated (version N+1). If version N+1 is not signed as required, discard it, abort the update cycle, and report the signature failure. On the next update cycle, begin at step 0 and version N of the root metadata file.

  • 1.4. Check for a rollback attack. The version number of the trusted root metadata file (version N) must be less than or equal to the version number of the new root metadata file (version N+1). Effectively, this means checking that the version number signed in the new root metadata file is indeed N+1. If the version of the new root metadata file is less than the trusted metadata file, discard it, abort the update cycle, and report the rollback attack. On the next update cycle, begin at step 0 and version N of the root metadata file.

  • 1.5. Note that the expiration of the new (intermediate) root metadata file does not matter yet, because we will check for it in step 1.8.

  • 1.6. Set the trusted root metadata file to the new root metadata file.

  • 1.7. Repeat steps 1.1 to 1.7.

  • 1.8. Check for a freeze attack. The latest known time should be lower than the expiration timestamp in the trusted root metadata file (version N). If the trusted root metadata file has expired, abort the update cycle, report the potential freeze attack. On the next update cycle, begin at step 0 and version N of the root metadata file.

  • 1.9. If the timestamp and / or snapshot keys have been rotated, then delete the trusted timestamp and snapshot metadata files. This is done in order to recover from fast-forward attacks after the repository has been compromised and recovered. A fast-forward attack happens when attackers arbitrarily increase the version numbers of: (1) the timestamp metadata, (2) the snapshot metadata, and / or (3) the targets, or a delegated targets, metadata file in the snapshot metadata. Please see the Mercury paper for more details.

2. Download the timestamp metadata file, up to Y number of bytes (because the size is unknown.) The value for Y is set by the authors of the application using TUF. For example, Y may be tens of kilobytes. The filename used to download the timestamp metadata file is of the fixed form FILENAME.EXT (e.g., timestamp.json).

  • 2.1. Check signatures. The new timestamp metadata file must have been signed by a threshold of keys specified in the trusted root metadata file. If the new timestamp metadata file is not properly signed, discard it, abort the update cycle, and report the signature failure.

  • 2.2. Check for a rollback attack. The version number of the trusted timestamp metadata file, if any, must be less than or equal to the version number of the new timestamp metadata file. If the new timestamp metadata file is older than the trusted timestamp metadata file, discard it, abort the update cycle, and report the potential rollback attack.

  • 2.3. Check for a freeze attack. The latest known time should be lower than the expiration timestamp in the new timestamp metadata file. If so, the new timestamp metadata file becomes the trusted timestamp metadata file. If the new timestamp metadata file has expired, discard it, abort the update cycle, and report the potential freeze attack.

3. Download snapshot metadata file, up to the number of bytes specified in the timestamp metadata file. If consistent snapshots are not used (see Section 7), then the filename used to download the snapshot metadata file is of the fixed form FILENAME.EXT (e.g., snapshot.json). Otherwise, the filename is of the form VERSION_NUMBER.FILENAME.EXT (e.g., 42.snapshot.json), where VERSION_NUMBER is the version number of the snapshot metadata file listed in the timestamp metadata file. In either case, the client MUST write the file to non-volatile storage as FILENAME.EXT.

  • 3.1. Check against timestamp metadata. The hashes and version number of the new snapshot metadata file MUST match the hashes and version number listed in timestamp metadata. If hashes and version do not match, discard the new snapshot metadata, abort the update cycle, and report the failure.

  • 3.2. Check signatures. The new snapshot metadata file MUST have been signed by a threshold of keys specified in the trusted root metadata file. If the new snapshot metadata file is not signed as required, discard it, abort the update cycle, and report the signature failure.

  • 3.3. Check for a rollback attack.

    • 3.3.1. Note that the trusted snapshot metadata file may be checked for authenticity, but its expiration does not matter for the following purposes.

    • 3.3.2. The version number of the trusted snapshot metadata file, if any, MUST be less than or equal to the version number of the new snapshot metadata file. If the new snapshot metadata file is older than the trusted metadata file, discard it, abort the update cycle, and report the potential rollback attack.

    • 3.3.3. The version number of the targets metadata file, and all delegated targets metadata files (if any), in the trusted snapshot metadata file, if any, MUST be less than or equal to its version number in the new snapshot metadata file. Furthermore, any targets metadata filename that was listed in the trusted snapshot metadata file, if any, MUST continue to be listed in the new snapshot metadata file. If any of these conditions are not met, discard the new snaphot metadadata file, abort the update cycle, and report the failure.

  • 3.4. Check for a freeze attack. The latest known time should be lower than the expiration timestamp in the new snapshot metadata file. If so, the new snapshot metadata file becomes the trusted snapshot metadata file. If the new snapshot metadata file is expired, discard it, abort the update cycle, and report the potential freeze attack.

4. Download the top-level targets metadata file, up to either the number of bytes specified in the snapshot metadata file, or some Z number of bytes. The value for Z is set by the authors of the application using TUF. For example, Z may be tens of kilobytes. If consistent snapshots are not used (see Section 7), then the filename used to download the targets metadata file is of the fixed form FILENAME.EXT (e.g., targets.json). Otherwise, the filename is of the form VERSION_NUMBER.FILENAME.EXT (e.g., 42.targets.json), where VERSION_NUMBER is the version number of the targets metadata file listed in the snapshot metadata file. In either case, the client MUST write the file to non-volatile storage as FILENAME.EXT.

  • 4.1. Check against snapshot metadata. The hashes (if any), and version number of the new targets metadata file MUST match the trusted snapshot metadata. This is done, in part, to prevent a mix-and-match attack by man-in-the-middle attackers. If the new targets metadata file does not match, discard it, abort the update cycle, and report the failure.

  • 4.2. Check for an arbitrary software attack. The new targets metadata file MUST have been signed by a threshold of keys specified in the trusted root metadata file. If the new targets metadat file is not signed as required, discard it, abort the update cycle, and report the failure.

  • 4.3. Check for a rollback attack. The version number of the trusted targets metadata file, if any, MUST be less than or equal to the version number of the new targets metadata file. If the new targets metadata file is older than the trusted targets metadata file, discard it, abort the update cycle, and report the potential rollback attack.

  • 4.4. Check for a freeze attack. The latest known time should be lower than the expiration timestamp in the new targets metadata file. If so, the new targets metadata file becomes the trusted targets metadata file. If the new targets metadata file is expired, discard it, abort the update cycle, and report the potential freeze attack.

  • 4.5. Perform a preorder depth-first search for metadata about the desired target, beginning with the top-level targets role. Note: If any metadata requested in steps 4.5.1 - 4.5.2.3 cannot be downloaded nor validated, end the search and report that the target cannot be found.

    • 4.5.1. If this role has been visited before, then skip this role (so that cycles in the delegation graph are avoided). Otherwise, if an application-specific maximum number of roles have been visited, then go to step 5 (so that attackers cannot cause the client to waste excessive bandwidth or time). Otherwise, if this role contains metadata about the desired target, then go to step 5.

    • 4.5.2. Otherwise, recursively search the list of delegations in order of appearance.

      • 4.5.2.1. If the current delegation is a multi-role delegation, recursively visit each role, and check that each has signed exactly the same non-custom metadata (i.e., length and hashes) about the target (or the lack of any such metadata).

      • 4.5.2.2. If the current delegation is a terminating delegation, then jump to step 5.

      • 4.5.2.3. Otherwise, if the current delegation is a non-terminating delegation, continue processing the next delegation, if any. Stop the search, and jump to step 5 as soon as a delegation returns a result.

5. Verify the desired target against its targets metadata.

  • 5.1. If there is no targets metadata about this target, abort the update cycle and report that there is no such target.

  • 5.2. Otherwise, download the target (up to the number of bytes specified in the targets metadata), and verify that its hashes match the targets metadata. (We download up to this number of bytes, because in some cases, the exact number is unknown. This may happen, for example, if an external program is used to compute the root hash of a tree of targets files, and this program does not provide the total size of all of these files.) If consistent snapshots are not used (see Section 7), then the filename used to download the target file is of the fixed form FILENAME.EXT (e.g., foobar.tar.gz). Otherwise, the filename is of the form HASH.FILENAME.EXT (e.g., c14aeb4ac9f4a8fc0d83d12482b9197452f6adf3eb710e3b1e2b79e8d14cb681.foobar.tar.gz), where HASH is one of the hashes of the targets file listed in the targets metadata file found earlier in step 4. In either case, the client MUST write the file to non-volatile storage as FILENAME.EXT.

6. Usage

See https://www.theupdateframework.com/ for discussion of recommended usage in various situations.

  • 6.1. Key management and migration

    All keys, except those for the timestamp and mirrors roles, should be stored securely offline (e.g. encrypted and on a separate machine, in special-purpose hardware, etc.). This document does not prescribe how keys should be encrypted and stored, and so it is left to implementers of this document to decide how best to secure them.

    To replace a compromised root key or any other top-level role key, the root role signs a new root.json file that lists the updated trusted keys for the role. When replacing root keys, an application will sign the new root.json file with both the new and old root keys. Any time such a change is required, the root.json file is versioned and accessible by version number, e.g., 3.root.json. Clients update the set of trusted root keys by requesting the current root.json and all previous root.json versions, until one is found that has been signed by a threshold of keys that the client already trusts. This is to ensure that outdated clients remain able to update, without requiring all previous root keys to be kept to sign new root.json metadata.

    In the event that the keys being updated are root keys, it is important to note that the new root.json must at least be signed by the keys listed as root keys in the previous version of root.json, up to the threshold listed for root in the previous version of root.json. If this is not the case, clients will (correctly) not validate the new root.json file. For example, if there is a 1.root.json that has threshold 2 and a 2.root.json that has threshold 3, 2.root.json MUST be signed by at least 2 keys defined in 1.root.json and at least 3 keys defined in 2.root.json. See step 1 in Section 5.1 for more details.

    To replace a delegated developer key, the role that delegated to that key just replaces that key with another in the signed metadata where the delegation is done.

7. Consistent Snapshots

So far, we have considered a TUF repository that is relatively static (in terms of how often metadata and target files are updated). The problem is that if the repository (which may be a community repository such as PyPI, RubyGems, CPAN, or SourceForge) is volatile, in the sense that the repository is continually producing new TUF metadata as well as its targets, then should clients read metadata while the same metadata is being written to, they would effectively see denial-of-service attacks. Therefore, the repository needs to be careful about how it writes metadata and targets. The high-level idea of the solution is that each snapshot will be contained in a so-called consistent snapshot. If a client is reading from one consistent snapshot, then the repository is free to write another consistent snapshot without interrupting that client. For more reasons on why we need consistent snapshots, please see https://github.com/theupdateframework/pep-on-pypi-with-tuf#why-do-we-need-consistent-snapshots

  • 7.1. Writing consistent snapshots

    We now explain how a repository should write metadata and targets to produce self-contained consistent snapshots.

    Simply put, TUF should write every metadata file as such: if the file had the original name of filename.ext, then it should be written to non-volatile storage as version_number.filename.ext, where version_number is an integer.

    On the other hand, consistent target files should be written to non-volatile storage as digest.filename.ext. This means that if the referrer metadata lists N cryptographic hashes of the referred file, then there must be N identical copies of the referred file, where each file will be distinguished only by the value of the digest in its filename. The modified filename need not include the name of the cryptographic hash function used to produce the digest because, on a read, the choice of function follows from the selection of a digest (which includes the name of the cryptographic function) from all digests in the referred file.

    Additionally, the timestamp metadata (timestamp.json) should also be written to non-volatile storage whenever it is updated. It is optional for an implementation to write identical copies at version_number.timestamp.json for record-keeping purposes, because a cryptographic hash of the timestamp metadata is usually not known in advance. The same step applies to the root metadata (root.json), although an implementation must write both root.json and version_number.root.json because it is possible to download root metadata both with and without known version numbers. These steps are required because these are the only metadata files that may be requested without known version numbers.

    Most importantly, no metadata file format must be updated to refer to the names of metadata or target files with their version numbers included. In other words, if a metadata file A refers to another metadata file B as filename.ext, then the filename must remain as filename.ext and not version_number.filename.ext. This rule is in place so that metadata signed by roles with offline keys will not be forced to sign for the metadata file whenever it is updated. In the next subsection, we will see how clients will reproduce the name of the intended file.

    Finally, the root metadata should write the Boolean "consistent_snapshot" attribute at the root level of its keys of attributes. If consistent snapshots are not written by the repository, then the attribute may either be left unspecified or be set to the False value. Otherwise, it must be set to the True value.

    For more details on how this would apply on a community repository, please see https://github.com/theupdateframework/pep-on-pypi-with-tuf#producing-consistent-snapshots

  • 7.2. Reading consistent snapshots

    See Section 5.1 for more details.

F. Future directions and open questions

  • F.1. Support for bogus clocks.

    The framework may need to offer an application-enablable "no, my clock is supposed to be wrong" mode, since others have noticed that many users seem to have incorrect clocks.