Here's a draft of a (rather half-baked) data recovery scheme that I'll be
presenting at a workshop next week. I've included the LaTeX source below;
sorry for the length and for the formatting (which should be reasonably
easy to ignore for those without LaTeX).
Please include me in any response, since I don't read the list these days.
-matt
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\documentstyle[11pt,fullpage]{article}
\begin{document}
\title{Key Escrow without Escrow Agents}
\author{{Matt Blaze}\\
AT\&T Research\\
Murray Hill, NJ 07974\\
{\tt mab(a)research.att.com}}
\date{DRAFT -- 24 May 1996 -- Extended Abstract -- DRAFT}
\maketitle
\begin{abstract}
We propose a simple scheme, based on secret sharing over large-scale
networks, for assuring recoverability of sensitive archived data
(e.g., cryptographic keys). In our model anyone can request a copy of
the archived data but it is very difficult to keep the existence of a
request secret or to subvert the access policy of the data ``owner''.
We sketch an architecture for such a system that might be suitable for
deployment over very large-scale networks such as the Internet.
\end{abstract}
\section{Introduction}
In any system in which sensitive information must be stored for future
use, there is a fundamental tension between ensuring the {\em secrecy}
of data against those who are not authorized for access to it and
ensuring its {\em availability} to those who are. Secrecy is often
best served by making only a small number of carefully-guarded copies
of the data, while availability favors a policy of the widest possible
dissemination in the hope that at least one copy will be intact at the
time it is required. In general, a balance has to be struck between
these two goals based on the requirements of and resources available
to the particular application, but in any case copies of the sensitive
data must be controlled in some careful manner (e.g., through the use
of an off-site, trusted backup facility employing guards and other
effective, if expensive, security practices).
Another approach is ``key escrow'', in which the sensitive data is
encrypted so that the ciphertext can be widely copied and backed-up
via conventional methods, but the decryption keys are controlled in
some careful manner by trusted third parties who assume responsibility
for revealing the keys to authorized entities in the event of an
emergency. One advantage of key escrow over controlled backup of the
data itself is that keys can be escrowed at any time, even prior to
the creation of the actual sensitive data, and one escrowed key can
represent arbitrarily much encrypted information. A number of key
escrow schemes have been proposed for a variety of applications, most
with the aim of facilitating law enforcement access to encrypted data,
but also for commercial data recovery
\cite{clipper}\cite{micali}\cite{tis}\cite{denning}.
Third party backup, whether of data or keys, has a number of
disadvantages, however. The ``escrow agents'' must be highly trusted
and carefully protected, since compromise single escrow site (or small
set of sites, in the case of split data) can result in an irrevocable
loss of security. Since protecting such data is likely to be
expensive, one escrow site can be expected to serve many different
sets of data, making each site an attractive ``fat target'' for
attack. Finally, legal, liability, and conflict-of-interest issues
sometimes make it difficult to ensure that an escrow agent will act
only in the best interests of the data owner, especially when served
with a legal demand to turn over keys or tempted with some inducement
to misbehave. One of the frequently-raised objections to
government-run key escrow systems (e.g., the ``Clipper'' chip) is the
fear that the escrow centers will, perhaps secretly, assist a rogue
government in violating its citizens' privacy.
In this abstract, we propose a different model for assuring both
recoverability and protection of sensitive data based on two concepts:
secret sharing and the decentralized nature of large, heterogeneous
networks such as the Internet. In our model, anyone can request a
copy of anyone's data but it is not possible to keep the existence of
such a request secret or to subvert the access policy of the data
``owner'' without subverting a significant fraction of participants in
the network. There are no explicit ``escrow agents''; instead, key
shares are distributed widely to ordinary networked computers spread
across a wide variety of administrative and geographic boundaries.
\section{``The Net'' as an Escrow Agent}
The goal of our scheme is to make it difficult to recover escrowed
data without the knowledge and consent of the data owner, while still
assuring high availability in an emergency. Its security rests on the
premise that highly distributed systems spread over many
administrative, political, and geographic domains (such as the
Internet), are more robust than any single site or small set of
sites, no matter how well protected. Other systems, such as Eternity
\cite{eternity}, have recognized and exploited this property of
global networks for maintaining information availability; we simply
expand this notion to include secrecy as well.
We assume that each node (or a large number of nodes) in the network
runs an ``escrow server'' that performs most of the steps of the
protocol, and that there is some broadcast mechanism for reaching them
(which could be based on existing mechanisms such as Usenet news).
The first step in using ``the net'' as an escrow agent is to split the
key to be escrowed using some secret sharing scheme \cite{simmons}
with a very large number of shares (e.g., a $k$ out-of $n$ scheme
where $k=500$ and $n=5000$, but we leave the details of determining an
appropriate access structure to the reader). Next, we package each
share along with a key identifier, a digital signature of the share,
and a policy describing the circumstances under which the share should
be disclosed (discussed below). Finally, we select, at random (or
according to some other policy) as many sites as we have shares and
send one share to each site, over a secure channel. We then destroy
the shares and the list of sites to which they were sent.
To recover escrowed data, we broadcast a request for shares for the
key identifier we want to recover, using some mechanism that is likely
to be received by the shareholders' escrow servers. Upon receipt of a
request for shares, each escrow server logs the request and, if it
holds a share for the key in question, checks the policy contained in
the share package. If the request conforms to the policy we send the
share to the requester. The requester (who can verify the
authenticity of each share by checking the signature) can recover the
key once enough shares have been received.
Whether such a scheme is robust, secure, or otherwise adequate depends
primarily on three factors: the reliability (in terms of continued
existence, security against compromise, and ability to follow
instructions) of the nodes that handle the key shares, the access
structure of the secret sharing scheme, and the nature of the policy
that each node is supposed to follow.
If the nature of today's Internet is any indication, we must assume
that the individual nodes are not very reliable, especially over time.
Some nodes will simply disappear. Others will maliciously fail to
follow instructions. Still others will fail to safeguard their
shares, sometimes due to malice but more often as a result of mistake,
incompetence, or failure of some underlying security mechanism. It is
likely that as the net grows these issues will become even more
pronounced. Therefore, the security of the scheme depends on a choice
of access structures and policies that assumes that a large fraction
of shareholders will not follow the correct protocol.
The secret sharing access structure must be chosen to require enough
shares to prevent key recovery by collusion among a few nodes, yet
with enough redundancy to allow recovery in the likely event that most
nodes are not available or did not retain their shares at the time key
recovery is required. Scale appears to help here; consider, for
example, a 500 out of 5000 threshold scheme, which permits key
recovery even when 90\% of nodes have failed and yet retains its
security until 500 nodes have been compromised. The distribution of
nodes could also play a part here, particularly when the key is split
with a more sophisticated access structure. For example, key shares
could be distributed to nodes selected across a variety of
administrative, legal, political and geographic domains, with the
access structure selected to require that shares be collected from
nodes in several different categories.
Each shareholder is also asked to respect the access policy included
with the share. The policy must be designed to facilitate emergency
access without also permitting undetected disclosure. Because shares
can only be recovered by broadcasting, we can take advantage of the
inherently public nature of requests in formulating the access
policies. For example, the policy might specify a public signature
key to which the real key holder knows the corresponding secret and a
request to delay revealing key shares for some period of time, say one
week. If an unauthorized request for a key is broadcast, the real key
holder would have one week to notice the request and broadcast another
message, signed with this key, requesting that the shareholders ignore
the original request and turn over information that might aid in
tracking down the source of the unauthorized request. Policies might
also include instructions on the minimum identification that share
requests must include and instructions on how share requests should be
logged (e.g., by posting to a news group or even advertisements in
newspapers). They might also include an expiration date beyond which
the share is to be deleted. We defer the question of how policies
should be specified, but it may be sufficient for the server, upon
receipt of a share request, to send a message to its (human) operator
containing instructions (written in English) that were included in the
share package.
Some infrastructure is required. Key holders would need a directory
or other mechanism for identifying and communicating with escrow
servers at the time the shares are created. A broadcast mechanism for
key recovery is also required. It is possible that existing
mechanisms could suffice for both these purposes (e.g., DNS for server
identification and Usenet news for broadcasting) but more specialized
systems would be required if this scheme were to be fielded on a large
scale.
Share distribution must be secure against both eavesdropping and
traffic analysis. The need for security against eavesdropping is
obvious, since observing all the shares allows recovery keys without
the assistance of the shareholders. Resistance to traffic analysis is
required to ensure that shares can only be recovered by broadcasting.
If the identities of the shareholders are known, an attacker could
``target'' the sites believed to be weakest, and, if successful,
recover shares without broadcasting the request and without following
the share access policy. Shares could be distributed via an anonymous
communication network or some other mechanism (such as oblivious
transfer) that obscures the nature of the transaction from an observer
(and perhaps even from the participants themselves). Key identifiers
should be chosen so that an outside attacker cannot derive the purpose
or owner of the key from its identifier and so that shareholders do
not know exactly what their shares are for. In any case, the list of
shareholders should not be retained by the key owner once the shares
have been distributed.
\subsection{Emergency Access -- ``Angry Mob Cryptanalysis''}
In general, the key identifier should be stored with all copies of the
ciphertext (since without the key ID, it is impossible to recover a
key). Under ordinary circumstances when a key recovery is required,
the original key owner will initiate the request. The owner extracts
the key ID and broadcasts the request to the network, performing
whatever (presumably public) logging is required by the policy that
was sent to the shareholders. Upon receipt of the broadcast,
each server checks whether it is a shareholder for the requested key.
If it is, it checks whether request satisfies the access policy
(perhaps by transmitting a copy of the English-specified policy to the
server operator, perhaps by automated means if the policy is more
formally specified). If the access policy is satisfied (e.g., a
message announcing the request appeared in some established place, a
certificate of the identity of the requester was included in the
request, or whatever) and after waiting however log the policy
specified to allow for repudiation of the request by the legitimate
key owner, the share is transmitted back to the requestor over a
secure channel. The requestor can then combine the shares to recover
the key; corrupted shares will not affect the protocol since the
shares should have been digitally signed by the original key owner at
the time they were distributed.
Sometimes, however, an extreme emergency might make it necessary to
recover keys in a manner contrary to the policy specified in the
original shares. For example, it may be necessary to recover keys
before the policy-imposed delay has elapsed, or to obtain access in
spite of the objections of the original key owner. Such a situation
is most likely to arise from some kind of law enforcement or public
safety emergency in which the requestor makes the case that public
policy should supersede the access policy of the key holder. Of
course, such a situation is fraught with difficult issues of judgement
and policy, and fears of abuse, fraud, or coercion are among the
primary objections raised against key escrow in general. Our scheme
places the burden of determining whether such an exceptional access
request should be granted on the shareholders.
Indeed, the dependence on the collective judgement of the widely
distributed shareholder operators may be the scheme's most important
property. Under normal circumstances, the shareholders can be
expected to behave approximately as specified in the share policies
(with occasional pathological exceptions, limited in their effect by
the nature of the secret sharing access structure). In exceptional
situations, however, a public appeal can be made in an attempt to
convince the shareholders to reveal their shares in a manner not
permitted by the stated policy (e.g., the police could broadcast an
appeal for key shares on television news, stating the facts of the
case under investigation). In particular, because the identities of
the shareholders are not known, such an appeal must be done publicly
and in a manner designed to attract considerable attention. It is not
possible to secretly induce, through legal means or otherwise,
shareholders to reveal their shares. For some applications (e.g.,
personal information associated with an individual), such a scheme
could be acceptable even when key escrow is not. (We introduce the
rather lighthearted term ``angry mob cryptanalysis'' to refer to the
threat of enough shareholders being convinced to violate the share
access policy to permit key recovery. It is distinguished from
``rubber hose cryptanalysis,'' which involves obtaining keys by legal
or extra-legal intimidation\footnote{The phrase ``rubber hose
cryptanalysis'' appears to be due to Phil Karn.}.)
\section{Conclusions}
Key escrow is a confusing subject, especially so because there is
little general agreement as to even its basic goals and requirements.
We have proposed a scheme that has a number of interesting properties
that may make it appropriate for protecting secrecy and availability in
certain kinds of applications. A number of open problems remain, of
course, before such a scheme could be made completely practical.
Areas for further study include the effects of different access
structures, specification of policy, and economic, performance, and
reliability analysis.
Of course, we do not in complete seriousness propose this scheme as a
general solution to the key recovery problem, but intend instead to
open a new avenue of discussion. In particular, the scheme appears to
address many of the concerns of both opponents of ``government'' key
escrow as well as many of the (stated) concerns of law enforcement.
\section{Acknowledgements}
Much of the inspiration for this scheme arose from Ross Anderson's
description of the motivation and principles behind the Eternity file
service, in conversations at Cambridge and at AT\&T Bell Labs.
\begin{thebibliography}{MMMM00}
\bibitem[Ande96]{eternity}
\newblock Ross Anderson.
\newblock ``The Eternity Service.''
\newblock Invited paper to appear at {\em Pragocrypt 96.} 30 Sep - 3
Oct 1996, Prague.
\bibitem[Denn96]{denning}
\newblock Dorothy Denning.
\newblock ``A Taxonomy for Key Escrow Encryption Systems.''
\newblock {\em CACM.} March 1996.
\bibitem[Mica94]{micali}
\newblock Silvio Micali.
\newblock ``Fair Cryptosystems.''
\newblock {\em MIT/LCS/TR-579.c} Laboratory for Computer Science,
Massachusetts Institute of Technology, Cambridge, MA, August 1994.
\bibitem[NIST94]{clipper}
\newblock National Institute for Standards and Technology.
\newblock Escrowed Encryption Standard, {\em Federal Information
Processing Standards Publication 185}, U.S. Dept. of Commerce, 1994.
\bibitem[Simm92]{simmons}
\newblock G.J. Simmons.
\newblock ``An introduction to Shared Secret and/or Shared Control
Schemes and their Applications.''
\newblock In {\em Contemporary Cryptolgy,} Simmons, ed. IEEE Press,
1992.
\bibitem[WLEB96]{tis}
\newblock Stephen T. Walker, Stephen B. Lipner, Carl M. Ellison, and
David M. Balenson.
\newblock ``Commercial Key Recovery.''
\newblock {\em CACM.} March 1996.
\end{thebibliography}
\end{document}
