Internet Engineering Task Force (IETF) Phillip Hallam-Baker
Internet-Draft Comodo Group Inc.
Intended Status: Standards Track September 11, 2013
Expires: March 15, 2014
PRISM-Proof Security Considerations
draft-hallambaker-prismproof-req-00
Abstract
PRISM is reputed to be a classified US government that involves
covert interception of a substantial proportion of global Internet
traffic. This document describe the security concerns such a program
raises for Internet users and security controls that may be employed
to mitigate the risk of pervasive intercept capabilities regardless
of source.
Status of This Memo
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Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
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Table of Contents
1. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Attack Degree . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Content Disclosure . . . . . . . . . . . . . . . . . . . 3
2.2. Meta Data Analysis . . . . . . . . . . . . . . . . . . . 4
2.3. Traffic Analysis . . . . . . . . . . . . . . . . . . . . 4
2.4. Denial of Service . . . . . . . . . . . . . . . . . . . . 4
2.5. Protocol Exploit . . . . . . . . . . . . . . . . . . . . 5
3. Attacker Capabilities . . . . . . . . . . . . . . . . . . . . 5
3.1. Passive Observation . . . . . . . . . . . . . . . . . . . 5
3.2. Active Modification . . . . . . . . . . . . . . . . . . . 5
3.3. Cryptanalysis . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Kleptography . . . . . . . . . . . . . . . . . . . . . . 6
3.4.1. Covert Channels in RSA . . . . . . . . . . . . . . . 6
3.4.2. Covert Channels in TLS, S/MIME, IPSEC . . . . . . . 6
3.4.3. Covert Channels in Symmetric Ciphers . . . . . . . . 7
3.4.4. Covert Channels in ECC Curves . . . . . . . . . . . 7
3.4.5. Unusable Cryptography . . . . . . . . . . . . . . . 7
3.5. Lawful Intercept . . . . . . . . . . . . . . . . . . . . 7
3.6. Subversion or Coercion of Intermediaries . . . . . . . . 7
3.6.1. Physical Plant . . . . . . . . . . . . . . . . . . . 8
3.6.2. Internet Service Providers . . . . . . . . . . . . . 8
3.6.3. Router . . . . . . . . . . . . . . . . . . . . . . . 8
3.6.4. End Point . . . . . . . . . . . . . . . . . . . . . 8
3.6.5. Cryptographic Hardware Providers . . . . . . . . . . 8
3.6.6. Certificate Authorities . . . . . . . . . . . . . . 8
3.6.7. Standards Organizations . . . . . . . . . . . . . . 9
4. Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Confidentiality . . . . . . . . . . . . . . . . . . . . . 9
4.1.1. Perfect Forward Secrecy . . . . . . . . . . . . . . 10
4.2. Policy, Audit and Transparency . . . . . . . . . . . . . 10
4.2.1. Policy . . . . . . . . . . . . . . . . . . . . . . 10
4.2.2. Audit . . . . . . . . . . . . . . . . . . . . . . . 10
4.2.3. Transparency . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Requirements
PRISM is reputed to be a classified US government that involves
covert interception of a substantial proportion of global Internet
traffic. While the precise capabilities of PRISM are unknown the
program is believed to involve traffic and meta-data analysis and
that the intercepts are obtained with the assistance of
intermediaries trusted by Internet end users. Such intermediaries may
or may not include ISPs, backbone providers, hosted email providers
or Certificate Authorities.
Government intercept capabilities pose a security risk to Internet
users even when performed by a friendly government. While use of the
intercept capability may be intended to be restricted to counter-
terrorism and protecting national security, there is a long and
abundant history of such capabilities being abused. Furthermore an
agency that has been penetrated by an Internet privacy activist
seeking to expose the existence of such programs may be fairly
considered likely to be penetrated by hostile governments.
The term 'PRISM-Proof' is used in this series of documents to
describe a communications architecture that is designed to resist or
prevent all forms of covert intercept capability. The concerns to be
addressed are not restricted to the specific capabilities known or
suspected of being supported by PRISM or the NSA or even the US
government and its allies.
2. Attack Degree
Some forms of attack are much harder to protect against than others
and providing protection against some forms of attack may make
another form of attack easier.
The degrees of attack that are of concern depend on the security
concerns of the parties communicating.
2.1. Content Disclosure
Content disclosure is disclosure of the message content. In the case
of an email message disclosure of the subject line or any part of the
message body.
The IETF has a long history of working on technologies to protect
email message content from disclosure beginning with PEM and MOSS. At
present the IETF has two email security standards that address
confidentiality with incompatible message formats and different key
management and distribution approaches.
S/MIME and PGP may both be considered broken in that they reveal the
message subject line and content Meta-data such as the time. This
problem is easily addressed but at the cost of sacrificing backwards
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compatibility.
2.2. Meta Data Analysis
Meta Data is information that is included in a communication protocol
in addition to the content exchanged, This includes the sender and
receiver of a message, the time, date and headers describing the path
the message has taken in the Internet mail service. Meta-data
analysis permits an attacker to uncover the social network of parties
that are in frequent communication with each other.
Preventing disclosure of meta-data is possible through techniques
such as dead drops and onion routing but such approaches impose a
heavy efficiency penalty and it is generally considered preferable to
limit the parties capable of performing meta-data analysis instead.
The IETF STARTTLS extension to email permits the use of TLS to
encrypt SMTP traffic including meta-data. However use of STARTTLS has
two major limitations. First SMTP is a store and forward protocol and
STARTTLS only protects the messages hop-by-hop. Second there is
currently no infrastructure for determining that an SMTP service
offers STARTTLS support or to validate the credentials presented by
the remote server. The DANE Working Group is currently working on a
proposal to address the second limitation.
2.3. Traffic Analysis
Analysis of communication patterns may also leak information about
which parties are communicating, especially in the case of
synchronous protocols such as chat, voice and video.
Traffic analysis of store and forward protocols such as SMTP is more
challenging, particularly when billions of messages an hour may pass
between the major Webmail providers. But clues such as message length
may permit attackers more leverage than is generally expected.
2.4. Denial of Service
Providing protection against denial of service is frequently at odds
with other security objectives. In most situations it is preferable
for a mail client to not send a message in circumstances where there
is a risk of interception. Thus an attacker may be able to perform a
Denial of Service attack by creating the appearance of an intercept
risk.
Whether the potential compromise of confidentiality or service is
preferable depends on the circumstances. If critical infrastructure
such as electricity or water supply or the operation of a port
depends on messages getting through, it may be preferable to accept a
confidentiality compromise over a service compromise even though
confidentiality is also a significant concern.
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2.5. Protocol Exploit
Many protocols are vulnerable to attack at the application layer. For
example the use of JavaScript injection in HTML and SQL injection
attacks.
A recent trend in Internet chat services is to permit the
participants in a group chat to share links to images and other
content on other sites. Introducing a link into the chat session
causes every connected client to retrieve the linked resource, thus
allowing an attacker with access to the chat room to discover the IP
address of all the connected parties.
3. Attacker Capabilities
Some forms of attack are available to any actor while others are
restricted to actors with access to particular resources. Any party
with access to the Internet can perform a Denial of Service attack
while the ability to perform traffic analysis is limited to parties
with a certain level of network access.
A major constraint on most interception efforts is the need to
perform the attack covertly so as to not alert the parties to the
fact their communications are not secure and discourage them from
exchange of confidential information. Even governments that
intentionally disclose the ability to perform intercepts for purposes
of intimidation do not typically reveal intercept methods or the full
extent of their capabilities.
3.1. Passive Observation
Many parties have the ability to perform passive observation of parts
of the network. Only governments and large ISPs can feasibly observe
a large fraction of the network but every network provider can
monitor data and traffic on their own network and third parties can
frequently obtain data from wireless networks, exploiting
misconfiguration of firewalls, routers, etc.
A purely passive attack has the advantage to the attacker of being
difficult to detect and impossible to eliminate the possibility that
an intercept has taken place. Passive attacks are however limited in
the information they can reveal and easily defeated with relatively
simple cryptographic techniques.
3.2. Active Modification
Active attacks are more powerful but are more easily detected. Use of
TLS without verification of the end-entity credentials presented by
each side is sufficient to defeat a passive attack but is defeated by
a man-in-the-middle attack substituting false credentials.
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Active attacks may be used to defeat use of secure after first
contact approaches but at the cost of requiring interception of every
subsequent communication.
While many attackers have the ability to perform ad-hoc active attack
only a few parties have the ability to perform active attack
repeatedly and none can expect to do so with absolute reliability.
A major limitation on active attack is that an attacker can only
perform an active attack if the target is known in advance or the
target presents an opportunity that would compromise previous stored
communications.
3.3. Cryptanalysis
Many parties have the ability to perform cryptanalysis but government
cryptanalytic capabilities may be substantially greater.
3.4. Kleptography
Kleptography is persuading the party to be intercepted to use a form
of cryptography that the attacker knows they can break. Real life
examples of kleptography include the British government encouraging
the continued use of Enigma type cryptography machines by British
colonies after World War II and the requirement that early export
versions of Netscape Navigator and Internet Explorer use 40 bit
symmetric keys.
3.4.1. Covert Channels in RSA
One form of kleptography that is known to be feasible and is relevant
to IETF protocols is employing a RSA modulus to provide a covert
channel. In the normal RSA scheme we choose primes p and q and use
them to calculate n = pq. But the scheme works just as well if we
choose n' and p and look for a prime q in the vicinity of n'/p then
use p and q to calculate the final value of n. Since q ~= n'/p it
follows that n' ~= n. For a 2048 bit modulus, approximately 1000 bits
are available for use as a covert channel.
Such a covert channel may be used to leak some or all of the private
key or the seed used to generate it. The data may be encrypted to
avoid detection.
3.4.2. Covert Channels in TLS, S/MIME, IPSEC
Similar approaches may be used in any application software that has
knowledge of the actual private key. For example a TLS implementation
might use packet framing to leak the key.
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3.4.3. Covert Channels in Symmetric Ciphers
A hypothetical but unproven possibility is the construction of a
symmetric cipher with a backdoor. Such an attack is far beyond the
capabilities of the open field. A symmetric cipher with a perfect
backdoor would constitute a new form of public key cryptography more
powerful than any known to date. For purposes of kleptography however
it would be sufficient for a backdoor to limit the key space that an
attacker needed to search through brute force or have some other
limitation that is considered essential for public key cryptography.
3.4.4. Covert Channels in ECC Curves
Another hypothetical but unproven possibility is the construction of
a weak ECC Curve or a curve that incorporates a backdoor function. As
with symmetric ciphers, this would require a substantial advance on
the public state of the mathematical art.
3.4.5. Unusable Cryptography
A highly effective form of kleptography would be to make the
cryptographic system so difficult to use that nobody would bother to
do so.
3.5. Lawful Intercept
Lawful intercept is a form of coercion that is unique to government
actors by definition. Defeating court ordered intercept by a domestic
government is outside the scope of this document though defeating
foreign lawful intercept requests may be.
While the US government is known to practice Lawful Intercept under
court order and issue of National Security Letters of questionable
constitutional validity, the scope of such programs as revealed in
public documents and leaks from affected parties is considerably more
restricted than that of the purported PRISM program.
While a Lawful Intercept demand may in theory be directed against any
of the intermediaries listed in the following section on subversion
or coercion, the requirement to obtain court sanction constrains the
number and type of targets against which Lawful Intercept may be
sought and the means by which it is implemented. A court is unlikely
to sanction Lawful Intercept of opposition politicians for the
political benefit of current office holders.
3.6. Subversion or Coercion of Intermediaries
Subversion or coercion of intermediaries is a capability that is
almost entirely limited to state actors. A criminal organization may
coerce an intermediary in the short term but has little prospect of
succeeding in the long term.
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3.6.1. Physical Plant
The Internet is at base a collection of data moving over wires,
optical cables and radio links. Every form of interconnect that is a
practical means of high bandwidth communication is vulnerable to
interception at the physical layer. Attacks on physical interconnect
require only a knowledge of where the signal cables are routed and a
back hoe.
Even quantum techniques do not necessarily provide a guarantee of
security. While such techniques may be theoretically unbreakable, the
physical realization of such systems tend to fall short. As with the
'unbreakable' One Time Pad, the theoretical security tends to be
exceptionally fragile.
Attacks on the physical plant may enable high bandwidth passive
intercept capabilities and possibly even active capabilities.
3.6.2. Internet Service Providers
Internet Service Providers have access to the physical and network
layer data and are capable of passive or active attacks. ISPs have
established channels for handling Lawful Intercept requests and thus
any employee involved in an intercept request that was outside the
scope of those programs would be on notice that their activities are
criminal.
3.6.3. Router
Compromise of a router is an active attack that provides both passive
and active intercept capabilities. such compromise may be performed
by compromise of the device firmware or of the routing information.
3.6.4. End Point
Compromise of Internet endpoints may be achieved through insertion of
malware or coercion/suborning the platform provider.
3.6.5. Cryptographic Hardware Providers
Deployment of the 'kleptography' techniques described earlier
requires that the attacker be capable of controlling the
cryptographic equipment and software available to the end user.
Compromise of the cryptographic hardware provided is one means by
this might be achieved.
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3.6.6. Certificate Authorities
Certificate Authorities provide public key credentials to validated
key holders. While compromise of a Certificate Authority is certainly
possible, this is an active attack and the credentials created leave
permanent evidence of the attack.
3.6.7. Standards Organizations
Another route for deployment of cryptography would be to influence
the standards for use of cryptography although this would only permit
the use of kleptographic techniques that are not publicly known.
Another area of concern is that efforts to make strong cryptography
usable through deployment of key discovery infrastructure or security
policy infrastructure may have been intentionally delayed or
discouraged. The chief security failure of the Internet today is that
insecurity is the default and many attacks are able to circumvent
strong cryptography through a downgrade attack.
4. Controls
Traditionally a cryptographic protocol is designed to resist direct
attack with the assumption that protocols that provide protection
against targeted intercept will also provide protection against
pervasive intercept. Consideration of the specific constraints of
pervasive covert intercept demonstrates that a protocol need not
guarantee perfect protection against a targeted intercept to render
pervasive intercept infeasible.
One of the more worrying aspects of the attempt to defend the
legality of PRISM program is the assertion that passive intercept
does not constitute a search requiring court oversight. This suggests
that the NSA is passively monitoring all Internet traffic and that
any statement that a citizen might make in 2013 could potentially be
used in a criminal investigation that began in 2023.
At present Internet communications are typically sent in the clear
unless there is a particular confidentiality concern in which case
techniques that resist active attack are employed. A better approach
would be to always use encryption that resists passive attack,
recognizing that some applications also require resistance to active
attacks.
4.1. Confidentiality
Encryption provides a confidentiality control when the symmetric
encryption key is not known to or discoverable by the attacker. Use
of strong public cryptography provides a control against passive
attacks but not an active attack unless the communicating parties
have a means of verifying the credentials purporting to identify the
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parties.
4.1.1. Perfect Forward Secrecy
One of the main limitations of simple public key exchange schemes is
that compromise of an end entity decryption key results in compromise
of all the messages encrypted using that key. Perfect Forward Secrecy
is a misnomer for a technique that forces an attacker to compromise a
separate private key for every key exchange. This is usually achieved
by performing two layers of public key exchange using the credentials
of the parties to negotiate a temporary key which is in turn used to
derive the symmetric session key used for communications.
Perfect Forward Secrecy is a misnomer as the secrecy is not
'perfect', should the public key system used to identify the
principals be broken, it is likely that the temporary public key will
be vulnerable to cryptanalysis as well. The value of PFS is not that
it is 'perfect' but that it dramatically increases the cost of an
attack to an attacker.
4.2. Policy, Audit and Transparency
The most underdeveloped area of internet security to date is the lack
of a security policy infrastructure and the audit and transparency
capabilities to support it.
4.2.1. Policy
A security policy describes the security controls that a party
performs or offers to perform. One of the main failings in the
Internet architecture is that the parties have no infrastructure to
inform them of the security policy of the party they are attempting
to communicate with except for the case of Certificate Policy and
Certificate Practices Statements which are not machine readable
documents.
A machine readable policy stating that a party always offers a
minimum level of security provides protection against downgrade
attack.
4.2.2. Audit
Audit is verifying that a party is in compliance with its published
security policy. Some security policies are self-auditing (e.g.
advertising support for specific cryptographic protocols) others may
be audited by automatic means and some may require human
interpretation and evaluation.
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4.2.3. Transparency
A security policy is transparent if it may be audited using only
publicly available information.
An important application of transparency is by trusted intermediaries
to deter attempted coercion or to demonstrate that a coercion attempt
would be impractical.
Author's Address
Phillip Hallam-Baker
Comodo Group Inc.
philliph@comodo.com
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