Elgato_dark/python-tuf
1
0
Fork 0
mirror of https://github.com/theupdateframework/python-tuf synced 2026-05-24 10:08:28 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 3
python-tuf/docs/tuf-spec.txt
dachshund 99ae000645 Clarify priority tags.
2013-06-19 01:35:35 +08:00

736 lines
31 KiB
Text
Raw Blame History

TUF: The Update Framework
1. Introduction
1.1. Scope
This document describes a framework for securing software update systems.
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 development of TUF is supported by GENI (http://www.geni.net/).
TUF's Python implementation is based heavily on Thandy, the application
updater for Tor (http://www.torproject.org/). 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 here:
https://gitweb.torproject.org/thandy.git?a=blob_plain;f=specs/thandy-spec.txt;hb=HEAD
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 aren't 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 updating (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 framework should be easily customizable for use with any crypto
libraries.
The process by which developers push updates to the repository must be
simple.
The repository must serve only static files and be easy to mirror.
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 for specific attacks to protect against
Note: When saying the framework protects against an attack, this means that
the attack will not be successful. It does not mean that a client will
always be able to successfully update during an attack. Fundamentally, an
attacker positioned to intercept and modify a client's communication will
always be able to perform a denial of service. The part we have control
over is not allowing an inability to update to go unnoticed.
Rollback attacks. Attackers should not be able to trick clients into
installing software that is older than that which the client previously knew
to be available.
Indefinite freeze attacks. Attackers should not be able respond to client
requests with the same, outdated metadata without the client being aware of
the problem.
Endless data attacks. Attackers should not be able to respond to client
requests with huge amounts of data (extremely large files) that interfere
with the client's system.
Slow retrieval attacks. Attackers should not be able to prevent clients
from being aware of interference with receiving updates by responding to
client requests so slowly that automated updates never complete.
Extraneous dependencies attacks. Attackers should not be able to cause
clients to download or install software dependencies that are not the
intended dependencies.
Mix-and-match attacks. Attackers should not be able to trick clients into
using a combination of metadata that never existed together on the
repository at the same time.
Malicious repository mirrors should not be able to prevent updates from good
mirrors.
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 will 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 identity 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
- Release 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.
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 developer 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.
Delegated trust can be revoked at any time by the delegating role signing
new metadata that indicates the delegated role is no longer trusted.
2.1.3 Release role
The release 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 the hash of the release 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.txt
Signed by the root keys; specifies trusted keys for the other
top-level roles.
/release.txt
Signed by the release role's keys. Lists hashes and sizes of all
metadata files other than timestamp.txt.
/targets.txt
Signed by the target role's keys. Lists hashes and sizes of target
files.
/timestamp.txt
Signed by the timestamp role's keys. Lists hashes and size of the
release file. This is the first and potentially only file that needs
to be downloaded when clients poll for the existence of updates.
/mirrors.txt (optional)
Signed by the mirrors role's keys. Lists information about available
mirrors and the content available from each mirror.
An implementation of the framework may optionally choose to make available
any metadata files in compressed (e.g. gzip'd) format. In doing so, the
filename of the compressed file should be the same as the original with the
addition of the file name extension for the compression type (e.g.
release.txt.gz). The original (uncompressed) file should always be made
available, as well.
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. This file is located at:
/targets/DELEGATED_ROLE.txt
where DELEGATED_ROLE is the name of the delegated role that has been
specified in targets.txt. If this role further delegates trust to a role
named ANOTHER_ROLE, that role's signed metadata file is made available at:
/targets/DELEGATED_ROLE/ANOTHER_ROLE.txt
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 files are of the format:
{ "signed" : X,
"signatures" : [
{ "keyid" : K,
"method" : M,
"sig" : S }
, ... ]
}
where: X is a list whose first element describes the signed object.
K is the identifier of a key signing the document
M is the method to be used to make the signature
S is a signature of the canonical encoding of X using the
identified key.
We define one signing method at present:
sha256-pkcs1 : A base64 encoded signature of the SHA256 hash of the
canonical encoding of X, using PKCS-1 padding.
All times are given as strings of the format "YYYY-MM-DD HH:MM:SS",
in UTC.
All keys are of the format:
{ "keytype" : KEYTYPE,
"keyval" : KEYVAL }
where KEYTYPE is a string describing the type of the key and how it's
used to sign documents. The type determines the interpretation of
KEYVAL.
The KEYID of a key is the hex representation of the SHA-256 hash of the
canonical encoding of the key.
We define one keytype at present: 'rsa'. Its format is:
{ "keytype" : "rsa",
"keyval" : { "e" : E,
"n" : N }
}
where E and N are the binary representations of the exponent and
modulus, encoded as big-endian numbers in base64. All RSA keys must
be at least 2048 bits long.
4.3. File formats: root.txt
The root.txt 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 format of root.txt is as follows:
{ "_type" : "Root",
"ts" : TIME,
"expires" : EXPIRES,
"keys" : {
KEYID : KEY
, ... },
"roles" : {
ROLE : {
"keyids" : [ KEYID, ... ] ,
"threshold" : THRESHOLD }
, ... }
}
The "ts" line describes when this file was updated. Clients
MUST NOT replace a file with an older one, and SHOULD NOT accept a
file too far in the future.
The "expires" line states when the metadata should be considered expired
and no longer trusted by clients. Clients MUST NOT trust an expired file.
A ROLE is one of "root", "release", "targets", "timestamp", or "mirrors".
A role for each of "root", "release", "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.
4.4. File formats: release.txt
The release.txt file is signed by the release role. It lists hashes and
sizes of all metadata on the repository, excluding timestamp.txt and
mirrors.txt.
The format of release.txt is as follows:
{ "_type" : "Release",
"ts" : TIME,
"expires" : EXPIRES,
"meta" : METAFILES
}
METAFILES is an object whose format is the following:
{ METAPATH : {
"length" : LENGTH,
"hashes" : HASHES,
("custom" : { ... }) }
, ...
}
METAPATH is the the metadata file's path on the repository relative to the
metadata base URL.
4.5. File formats: targets.txt and delegated target roles
The format of targets.txt is as follows:
{ "_type" : "Targets",
"ts" : TIME,
"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.
The HASH and LENGTH are the hash and length of the target file. 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" : {
ROLE : {
"keyids" : [ KEYID, ... ] ,
"threshold" : THRESHOLD,
"paths" : [ PATHPATTERN, ... ] }
, ... }
}
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. The format of a PATHPATTERN may be either a path to a
single file or a path to a directory and end with "/**" to indicate all
files under that directory. The value of "/**" by itself therefore means
all files.
We are currently investigating a few "priority tag" schemes to resolve
conflicts between delegated roles that share responsibility for overlapping
target paths. One of the simplest of such schemes is for the client to
consider metadata in order of appearance of delegations; we treat the order
of delegations such that the first delegation is trusted more than the
second one, the second delegation is trusted more than the third one, and so
on. 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
delegation, and so on. In order to accommodate this scheme, the "roles" key
in the DELEGATIONS object above should point to an array, instead of a hash
table, of delegated roles, like so:
"roles" : [{
"role": ROLE,
"keyids" : [ KEYID, ... ] ,
"threshold" : THRESHOLD,
"paths" : [ PATHPATTERN, ... ]
}, ... ]
Another priority tag scheme would have the clients prefer the delegated role
with the latest metadata for a conflicting target path. Similar ideas were
explored in the Stork package manager (University of Arizona Tech Report
08-04)[https://isis.poly.edu/~jcappos/papers/cappos_stork_dissertation_08.pdf].
The metadata files for delegated target roles has the same format as the
top-level targets.txt metadata file.
4.6. File formats: timestamp.txt
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 format of the timestamp file is as follows:
{ "_type" : "Timestamp",
"ts" : TIME,
"expires" : EXPIRES,
"meta" : METAFILES
}
METAFILES is the same is described for the release.txt file. In the case of
the timestamp.txt file, this will commonly only include a description of the
release.txt file.
4.7. File formats: mirrors.txt
The mirrors.txt file is signed by the mirrors role. It indicates which
mirrors are active and believed to be mirroring specific parts of the
repository.
The format of mirrors.txt is as follows:
{ "_type" : "Mirrorlist",
"ts" : TIME,
"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
5.1. The client application
Note: At any point in the following process there is a problem (e.g. only
expired metadata can be retrieved), the software update system using the
framework must decide how to proceed.
The client code instructs the framework to check for updates. The framework
downloads the timestamp.txt file from a mirror and checks that the file is
properly signed by the timestamp role, is not expired, and is not older than
the last timestamp.txt file retrieved. If the timestamp file lists the same
release.txt file as was previously seen, the client code is informed that no
updates are available and the update checking process stops.
If the release.txt file has changed, the framework downloads the file and
verifies that it is properly
signed by the release role, is not expired, has a newer timestamp than the
last release.txt file seen, and matches the description (hashes and size)
in the timestamp.txt file. The framework then checks which metadata files
listed in release.txt differ from those described in the last release.txt
file the framework had seen. If the root.txt file has changed, the
framework updates this (following the same security measures as with the
other files) and starts the process over. If any other metadata files have
changed, the framework downloads and checks those.
By comparing the trusted targets from the old trusted metadata with the new
metadata, the framework is able to determine which target files have
changed. The framework ensures that any targets described in delegated
targets files are allowed to be provided by the delegated role.
When the client code asks the framework to download a target file, the
framework downloads the file from (potentially trying multiple mirrors),
checks the downloaded file to ensure that it matches the information
described in the targets files, and then makes the file available to the
client code.
6. Usage
See https://www.updateframework.com/ for discussion of recommended usage in
various situations.
6.1. Key management and migration
All keys except the timestamp file signing key and the mirror list signing
key should be stored securely offline (e.g. encrypted and on a separate
machine, in special-purpose hardware, etc.).
To replace a compromised root key or any other top-level role key, the root
role signs a new root.txt file that lists the updated trusted keys for the
role. When replacing root keys, an application will sign the new root.txt
file with both the new and old root keys until all clients are known to have
obtained the new root.txt file (a safe assumption is that this will be a
very long time or never). There is no risk posed by continuing to sign the
root.txt file with revoked keys as once clients have updated they no longer
trust the revoked key. This is only to ensure outdated clients remain able
to update.
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.
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.
Reference in a new issue View git blame Copy permalink