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11 September 2013

PRISM-Proof Security Considerations

More on this topic:

Date: Wed, 11 Sep 2013 16:30:50 -0400
From: Phillip Hallam-Baker <hallam[at]>
To: "cryptography[at]" <cryptography[at]>
Subject: [Cryptography] Summary of the discussion so far

I have attempted to produce a summary of the discussion so far for use as a requirements document for the PRISM-PROOF email scheme. This is now available as an Internet draft.

I have left out acknowledgements and references at the moment. That is likely to take a whole day going back through the list and I wanted to get this out.

If anyone wants to claim responsibility for any part of the doc then drop me a line and I will have the black helicopter sent round.




The cryptography mailing list

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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