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Add text for TLS session resumption

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This commit is contained in:
Tobias Eidelpes 2020-03-31 15:48:44 +02:00
parent f85fa101a4
commit 50e9bdac73
2 changed files with 67 additions and 1 deletions
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5
abbrev/acronym.tex
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@ -6,6 +6,7 @@
\newacronym {URI} {URI} {Uniform Resource Identifier}
\newacronym {WWW} {WWW} {World Wide Web}
\newacronym {HTTP} {HTTP} {Hypertext Transfer Protocol}
\newacronym {HTTPS} {HTTPS} {Hypertext Transfer Protocol Secure}
\newacronym {HTML} {HTML} {Hypertext Markup Language}
\newacronym {DOM} {DOM} {Document Object Model}
\newacronym {API} {API} {Application Programming Interface}
@ -26,4 +27,8 @@
\newacronym {ETag} {ETag} {Entity Tag}
\newacronym {DNS} {DNS} {Domain Name System}
\newacronym {IP} {IP} {Internet Protocol}
\newacronym {IPv6} {IPv6} {Internet Protocol version 6}
\newacronym {OS} {OS} {Operating System}
\newacronym {DNSSEC} {DNSSEC} {Domain Name System Security Extensions}
\newacronym {TTL} {TTL} {Time To Live}
\newacronym {PSK} {PSK} {Pre-Shared Key}

63
chapters/methods.tex
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@ -857,9 +857,70 @@ authoritative \gls{DNS} server, the identifier that is assembled by the script
is unique and thus allows identification of not only the browser but the client
machine itself.
\todo{+ \& -}
Advantages of this tracking method are that it works across browsers in most
cases. \citeauthor{kleinDNSCacheBasedUser2019} found that it survives browser
restarts and is resistant to the privacy mode employed by modern browsers.
Futhermore, \glspl{VPN} do not affect the method and it works with different
protocols (\gls{HTTPS}, \gls{IPv6}, \gls{DNSSEC}).
Disadvantages are that the tracking identifiers do not survive a computer
restart. Additionally, switching the network causes the identifiers to be
obsoleted and identifiers generally only live as long as the \gls{TTL} limit
allows.
Due to these constraining factors, \gls{DNS} tracking is best combined with
methods that are intended for tracking over longer time periods such as cookies
for example.
\subsection{TLS Session Resumption}
\label{subsec:tls session resumption}
\gls{TLS} is widely used today to securely encapsulate communication across the
web. For bandwidth savings and better performance it is possible to cache a
\gls{TLS} session to allow reusing an already established secure connection at a
later point in time. Versions prior to \gls{TLS} 1.3 used two mechanisms to
accomplish this: \gls{TLS} session identifiers and session tickets. Session
identifiers are sent by the server along with the initial handshake with the
user agent. The identifier is randomly generated and saved by the server so that
the current session can be found later. To resume a session, the user agent
sends the identifier with the ClientHello message to the server. The server can
then match the identifier to the previously initiated session and responds with
the same session identifier to signal to the user agent that the session can be
resumed. Session tickets are only issued by the server when the client has
expressed support for them. They are encrypted and provided by the server after
a successful handshake via an out-of-band message. The ticket contains all the
necessary information to reestablish a secure connection. When the user agent
wishes to resume a connection, the session ticket is sent along with the first
ClientHello message and the server can decrypt the ticket and resume the
session.
In \gls{TLS} version 1.3 the session identifiers and tickets have been replaced
with a \gls{PSK}. Instead of sending a ticket which is not encapsulated in the
\gls{TLS}-secured connection, a \gls{PSK} identity is sent from the server after
the initial handshake, usually avoiding out-of-band communication. The \gls{PSK}
identity provides a mechanism by which information associated with a secure
connection (certificates, keys) can be restored.
Because resuming a connection reuses information that has been exchanged before
to establish secure communication, individual sessions can be linked together to
form a history of information exchanges. This tracking method is described by
\citeauthor{syTrackingUsersWeb2018} in \cite{syTrackingUsersWeb2018}. Even
though \gls{TLS} session resumption can be mitigated by restarting the browser
because that clears the cache, the authors argue that due to mobile devices
being online without restarts for long periods the attack remains viable.
Futhermore, despite browsers imposing limits on the lifetime of session
identifiers and \glspl{PSK}, it is possible to maintain a session indefinitely
by carrying out a \emph{prolongation attack}. \citeauthor{syTrackingUsersWeb2018}
define a prolongation attack as an attack where the client asks for a session
resumption by sending the identifier of a previously initiated connection and
the server responds with a new handshake instead of resuming the old one. This
effectively resets the time limit as long as the user is initiating new (or
trying to resume old) connections to the server within the imposed time limit.
The authors present an empirical evaluation of server and browser configurations
with respect to session resumption lifetime by crawling the top 1M web sites as
determined by Alexa. Their results indicate that only 4\% of those sites do not
allow session resumption at all, while the majority (78\%) allows session
identifiers as well as tickets.
\end{document}