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September 16 2013


PRISM-Proof Security Considerations

(Copy-paste of the all document.)

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


  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

  This Internet-Draft is submitted in full conformance with the
  provisions of BCP 78 and BCP 79.

  Internet-Drafts are working documents of the Internet Engineering
  Task Force (IETF).  Note that other groups may also distribute
  working documents as Internet-Drafts.  The list of current Internet-
  Drafts is at

  Internet-Drafts are draft documents valid for a maximum of six months
  and may be updated, replaced, or obsoleted by other documents at any
  time.  It is inappropriate to use Internet-Drafts as reference
  material or to cite them other than as "work in progress."

Copyright Notice

  Copyright (c) 2013 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  ( in effect on the date of
  publication of this document. Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document. Code Components extracted from this document must
  include Simplified BSD License text as described in Section 4.e of
  the Trust Legal Provisions and are provided without warranty as
  described in the Simplified BSD License.

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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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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

  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

  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

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

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

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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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

  A machine readable policy stating that a party always offers a
  minimum level of security provides protection against downgrade

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.

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