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\title{Secure Update Framework Repository Key Management and Trust Delegation}
\author{Justin Cappos \and Geremy Condra}
\maketitle
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%--------------------------------- Intro ---------------------------------------
\section{Introduction}
The goal of a TUF repository is to store a collection of targets (the updated files 
to be served to the client) and a set of metadata with which the client can 
ensure that the updates they recieve are complete, timely, and authentic. Meeting 
those goals requires that the respository be able to leverage digital signatures,
and so TUF provides a set of tools that helps manage the complexity of generating 
metadata and improves the security posture of the repository. These tools 
(including mechanisms for key management, delegation of trust, and key revocation) 
form the subject of this document.

\subsection{Scope}
This document specifies the required trust delegation and key management routines
for TUF repositories.
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%-------------------------------- Overview -------------------------------------
\section{Overview}
The Update Framework is a Python library designed to assist software developers
in the task of safely and securely updating their software after its deployment
with an emphasis on resiliance in the face of key compromise. Towards that end,
the issues of key storage, revocation of keys and their accompanying trust, and
the delegation of that trust have been given careful attention. This document
exists to provide guidance on both mandatory behaviors and best practices with 
regard to those.

\subsection{Relationship to Other Documents}
Much of the behavior specified in this document is partially laid out in the
core TUF document. The system's behavior with regard to freeze and replay
attacks is covered in the document entitled "Software Update Security Framework: 
Repository Library Replay and Freeze Attack Protection". The client-side counterpart
to this document contains a large amount of information on the response that
subsystem will demonstrate in many of the same circumstances. In particular, it
contains an overview of how to set up and run an experimental TUF repository and
client.
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%-------------------------------- Details --------------------------------------
\section{Key Management}
Where TUF clients are primarily concerned with appropriately associating roles
to public keys, those maintaining TUF repositories are additionally required to
keep the relevant signing keys private. Mechanically, this means storing them in 
a custom AES-encrypted keystore, but other precautions (detailed below) should 
be observed in order to minimize the risk of a key compromise.

\subsection{Key Storage}
TUF's keystore takes the form of an AES-encrypted key database with two backing
data stores, both Python dictionaries and containing keys (both public and private)
and their roles. In addition to the interface provided by its KeyDB parent class
(primarily responsible for associating roles and key IDs with keys and delegation
information) the keystore also provides three methods:

\begin{itemize}
	\item $set\_password(password)$, sets the password used to generate the AES key.
	\item $clear\_password()$, clears the aforementioned password.
	\item and $save()$, encodes, encrypts, and writes the database to disk.
\end{itemize}

Let's walk through each of these in slightly more detail.
\\\\
Setting or clearing the password to be used does nothing more than set and clear
a password field in the keystore, but note that if a keystore does not have a
password field set when $save()$ is called, it won't encrypt it- your keys will
be on disk in plaintext. Also note that passwords must not be used directly- at
the moment TUF clients use RFC 2440 password mangling to derive a key from
the original password material, but see the future work section at the end of
this document for more information on development of this system.
\\\\
Saving the keystore is done through TUF's usual method of serialization, which
is to say Canonical JSON. Encryption is done using AES-256, after which the result
is written to disk to be decrypted at some later date. Due to recent attacks on
this key length, clients may also opt to use AES-192.

\subsection{Best Practices}
While TUF places a great deal of emphasis on the ability to recover from a key 
compromise and uses strong cryptographic techniques to minimize the attack window, 
avoiding such scenarios in the first place is always preferable. As a result, a
few best practices are recommended for those responsible for storing keying
material:

\begin{enumerate}
	\item Store the root key offline- while a root key compromise is recoverable,
		revoking a root key is more troublesome than revoking other keys.
	\item No single point of failure should permit access to both the target
		and release roles' keys.
	\item The timestamp role's key can be stored online for automated timestamping.
\end{enumerate}

For some, the above will prove to be difficult to provide inside of an
organization. Those users should note that while TUF by no means requries an
external PKI to operate, its design permits their use.

\subsection{Command Line Interface}
To make repository management easier on maintainers, a set of easy-to-use command
line tools has been developed that works to simplify most of the tasks above. 
In particular, the signercli.py script provides an easy way to perform the most
common tasks a maintainer will face.

\subsubsection{Generating and Listing Keys}
Besides generating the initial keystore using the quickstart.py script we saw
earlier, the signercli.py script provides an easy way to generate keys using the
$genkey$ subcommand. Using it is extremely easy:

\begin{lstlisting}
cd demorepo
signercli.py genkey --keystore=../keystore
\end{lstlisting}

This will give you a good deal of output, ultimately including a line something
like the following:

\begin{lstlisting}
[TIME] [tuf] [INFO] Generated new key: KEYID
\end{lstlisting}

Listing key IDs is similarly easy:

\begin{lstlisting}
cd demorepo
signercli.py listkeys --keystore=../keystore
\end{lstlisting}

You can also get more information about a particular key using the dumpkey
subcommand. To print information about a public key, do the following:

\begin{lstlisting}
cd demorepo
signercli.py dumpkey --keystore=../keystore KEYID
\end{lstlisting}

If you pass the "--include-secret" option, it will also print the signing key.

\subsubsection{Changing Keystore Passwords}

To change the keystore password, simply run the signercli.py script with the 
changepass subcommand, type the current password, and then enter the new password.
Here's an example:

\begin{lstlisting}
cd demorepo
signercli.py changepass --keystore=../keystore
\end{lstlisting}

Note that there is not currently a mechanism for providing these options on the
commandline. This is done to prevent the password from being pulled from an
ungaurded shell history file.

\subsubsection{Delegating Trust}

TUF makes delegating trust quite easy, and an example of how to do so is provided
in the companion client-side document. Note that the folder a delegated role's
ROLE.txt metadata file goes into must exist before running the script.

\subsubsection{Revoking Trust}
TUF does not currently provide a command line interface for revoking trust,
however, doing so is simple using existing tools. To revoke a delegated
trust, just delete the accompanying ROLE.txt and create a new update. The
revocation procedure for other keys is described in the companion document, section
7.

\section{Trust Delegation}
The TUF trust delegation model is described in the TUF spec section 4.5 and in
the TUF client key management specification section 6.

\section{Future Work}
Three major areas of future work remain here: first, accounting for improved
cryptographic and cryptanalytic results against AES-256 and the RFC2440 key
derivation algorithm; second, developing additional tools to provide for 
automatic revocation of trust for all roles; and third improving the mechanisms
for automatic integration of other projects with TUF.

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