Internet DRAFT - draft-hallambaker-prismproof-trust


Internet Engineering Task Force (IETF)              Phillip Hallam-Baker
Internet-Draft                                         Comodo Group Inc.
Intended Status: Standards Track                        October 27, 2014
Expires: April 30, 2015

                        PRISM Proof Trust Model


   This paper extends Shanon's concept of a 'work factor' to provide an 
   objective measure of the practical security offered by a protocol or 
   infrastructure design. Considering the hypothetical work factor based
   on an informed estimate of the probable capabilities of an attacker 
   with unknown resources provides a better indication of the relative 
   strength of protocol designs than the computational work factor of 
   the best known attack.

   The social work factor is a measure of the trustworthiness of a 
   credential issued in a PKI based on the cost of having obtained the 
   credential through fraud at a certain point in time. Use of the 
   social work factor allows evaluation of Certificate Authority based 
   trust models, peer to peer (Web of Trust) models to be evaluated in 
   the same framework. The analysis shows that each model has clear 
   benefits over the other for some classes of user but most classes of 
   user are served better by a combination of both.

Status of This Memo

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

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Copyright Notice

   Copyright (c) 2014 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 
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   ( in effect on the date of 
   publication of this document. Please review these documents 
   carefully, as they describe your rights and restrictions with respect

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   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.  Work Factor  . . . . . . . . . . . . . . . . . . . . . . . . .  4
      1.1.  Computational Work Factor . . . . . . . . . . . . . . . .  4
      1.2.  Hypothetical Work Factor  . . . . . . . . . . . . . . . .  5
         1.2.1.  Known Unknowns . . . . . . . . . . . . . . . . . . .  6
         1.2.2.  Defense in Depth . . . . . . . . . . . . . . . . . .  7
         1.2.3.  Mutual Reinforcement . . . . . . . . . . . . . . . .  8
         1.2.4.  Safety in Numbers  . . . . . . . . . . . . . . . . .  8
      1.3.  Cost Factor . . . . . . . . . . . . . . . . . . . . . . .  9
      1.4.  Social Work Factor  . . . . . . . . . . . . . . . . . . . 11
   2.  The Problem of Evaluating Trust  . . . . . . . . . . . . . . . 13
      2.1.  Probability and Risk  . . . . . . . . . . . . . . . . . . 13
      2.2.  Reputation  . . . . . . . . . . . . . . . . . . . . . . . 14
      2.3.  Curated Spaces  . . . . . . . . . . . . . . . . . . . . . 15
      2.4.  Trustworthy Time  . . . . . . . . . . . . . . . . . . . . 15
   3.  Maximizing Social Work Factor to Maximize Trust  . . . . . . . 15
      3.1.  Trust Specifiers  . . . . . . . . . . . . . . . . . . . . 16
         3.1.1.  Key Identifiers  . . . . . . . . . . . . . . . . . . 16
         3.1.2.  Self Signed Certificates . . . . . . . . . . . . . . 17
      3.2.  Trust Assertions  . . . . . . . . . . . . . . . . . . . . 17
         3.2.1.  Certificate Authority Issued Certificates  . . . . . 17
         3.2.2.  Key Signingey Signing  . . . . . . . . . . . . . . . 19
         3.2.3.  Adding Key Endorsement to PKIX . . . . . . . . . . . 19
      3.3.  Trust Meta Assertions . . . . . . . . . . . . . . . . . . 22
         3.3.1.  Revocation and Status Checkings Checking . . . . . . 22
         3.3.2.  Notarization . . . . . . . . . . . . . . . . . . . . 22
         3.3.3.  Transparency . . . . . . . . . . . . . . . . . . . . 22
      3.4.  Other Approaches  . . . . . . . . . . . . . . . . . . . . 22
         3.4.1.  DNSSEC . . . . . . . . . . . . . . . . . . . . . . . 23
         3.4.2.  SPKI / SDSI  . . . . . . . . . . . . . . . . . . . . 23
         3.4.3.  Identity Based Cryptography  . . . . . . . . . . . . 23
   4.  Maximizing Social Work Factor in a Notary Infrastructure . . . 24
   5.  Conclusions and Related Work . . . . . . . . . . . . . . . . . 25
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25

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1. Work Factor

   Recent events have highlighted both the need for open standards based
   security protocols and the possibility that the design of such 
   protocols may have been sabotaged. The community thus faces two 
   important and difficult challenges, first to design an Internet 
   security infrastructure that offers practical security against the 
   class of attacks revealed, and secondly, to convince potential users 
   that the proposed new infrastructure has not been similarly 

   The security of a system should measured by the difficulty of 
   attacking it. The security of a safe is measured by the length time 
   it is expected to resist attack using a specified set of techniques. 
   The security of a cryptographic algorithm against a known attack is 
   measured by the computational cost of the attack.

   This paper extends Shanon's concept of a 'work factor' to provide an 
   objective measure of the security a protocol or infrastructure offers
   against other forms of attack.

1.1. Computational Work Factor

   The term 'Computational Work Factor' is used to refer to Shanon's 
   original concept. 

   One of Shanon's key insights was that the work factor of a 
   cryptographic algorithm could be exponential. Adding a single bit to 
   the key size of an ideal symmetric algorithm presents only a modest 
   increase in computational effort for the defender but doubles the 
   work factor for the attacker.

   More precisely, the difficulty of breaking a cryptographic algorithm 
   is generally measured by the work-factor ratio. If the cost of 
   encrypting a block with 56 bit DES is x, the worst case cost of 
   recovering the key through a brute force attack is x * 2^56. The 
   security of DES has changed over time because the cost x has fallen 

   While the work factor is traditionally measured in terms of the 
   number of operations, many cryptanalytic techniques permit memory 
   used to be traded for computational complexity. An attack requiring 
   2^64 bytes of memory that reduces the number of operations required 
   to break a 128 bit cipher to 2^64 is a rather lower concern than one 
   which reduces the number of operations to 2^80. The term 'cost' is 
   used to gloss over such distinctions.

   [Note that in the following analysis, the constraints of the IETF 
   document format make use of the established notation impractical and 
   a confusing mess, hence the departure from Shannon's notation.]

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   The Computational Work Factor ratio WF_C (A) of a cryptographic 
   algorithm A, is the cost of the best known attack divided by the cost
   of the algorithm itself.

1.2. Hypothetical Work Factor

   Modern cryptographic algorithms use keys of 128 bits or more and 
   present a work factor ratio of 2^128 against brute force attack. This
   work factor is at least 2^72 times higher than DES and comfortably 
   higher than the work factor of 2^80 operations that is generally 
   believed to be about the limit to current attacks.

   While an exceptionally well resourced attacker may gain performance 
   advances through use of massive parallelism, faster clock rates made 
   possible by operating at super-low temperatures and custom designed 
   circuits, the return on such approaches is incremental rather than 

   Performance improvements may allow an attacker to break systems with 
   a work factor several orders of magnitue greater than the public 
   state of the art. But an advance in cryptanalysis might permit a 
   potentially more significant reduction in the work factor.

   The primary consideration in the choice of a cryptographic algorithm 
   therefore is not the known computational work factor as measured 
   according to the best publicly known attack but the confidence that 
   the computational work factor of the best attack that might be known 
   to the attacker.

   While the exact capabilities of the adversary are unknown, a group of
   informed experts may arrive at a conservative estimate of their 
   likely capabilities. The probability that a government attacker has 
   discovered an attack against AES-128 with a work factor ratio of 
   2^120 might be considered relatively high while the probability that 
   an attack with a work factor ratio of less than 2^64 is very low.

   We define the hypothetical work factor function WF_H (A, p) as 
   follows: If WF is a work factor ratio and p is an informed estimate 
   of the probability that an adversary has developed an attack with a 
   work factor ratio against algorithm A of WF or less then WF_H (A, p) 
   = WF.

   Since the best known public attack is known to the attacker, WF_H(A, 
   1) <= WF_C (A)

   The inverse function WF_H' (A, WF) returns the estimated probability 
   that the work factor of algorithm A is at least WF.

   The hypothetical work factor and its inverse may be used to compare 
   the relative strengths of protocol designs. Given designs A and B, we
   can state that B is an improvement on A if WF_H(A,p) > WF_H (B,p) for

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   all p.

   When considering a protocol or infrastructure design we can thus 
   improve a protocol by either:

      *  Increasing WF_H(A,p) for some p, or

      *  Decreasing WF_H'(A,WF)

1.2.1. Known Unknowns

   Unlike the computational work factor, the hypothetical work factor 
   does not provide an objective measure of the security offered by a 
   design. The purpose of the hypothetical work factor is to allow the 
   protocol designer to compare the security offered by different design

   The task that the security engineer faces is to secure the system 
   from all attacks whether the attacks themselves are known or unknown.
   In the current case it is known that an attacker is capable of 
   breaking at least some of the cryptographic algorithms in use but not
   which algorithms are affected or the nature of the attack(s).

   Unlike the computational work factor, the hypothetical work factor 
   does not deliver an academically rigorous, publication and citation 
   worthy measure of the strength of a design. That is not its purpose. 
   the purpose of the hypothetical work factor is to assist the protocol
   designer in designing protocols.

   Design of security protocols has always required the designer to 
   consider attackers whose capabilities are not currently known and 
   thus involved a considerable degree of informed opinion and 
   guesswork. Whether correctly or not, the decision to reject changes 
   to the DNSSEC protocol to enable deployment in 2002 rested in part on
   a statement by a Security Area Director that a proposed change gave 
   him a bad feeling in his gut. The hypothetical work factor permits 
   the security designer to model to quantify such intestinally based 
   assumptions and model the effect on the security of the resulting 

   Security is a property of systems rather than individual components. 
   While it is quite possible that there are no royal roads to 
   cryptanalysis and cryptanalysis of algorithms such as AES 128 is 
   infeasible even for the PRISM-class adversaries, such adversaries are
   not limited to use of cryptanalytic attacks. 

   Despite the rise of organized cyber-crime, many financial systems 
   still employ weak cryptographic systems that are known to be 
   vulnerable to cryptanalytic attacks that are well within the 
   capabilities of the attackers. But fraud based on such techniques 
   remains vanishingly rare as it is much easier for the attackers to 

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   persuade bank customers to simply give their access credentials to 
   the attacker.

   Even if a PRISM-class attacker has a factoring attack which renders 
   an attack on RSA-2048 feasible, it is almost certainly easier for a 
   PRISM-class attacker to compromise a system using RSA-2048 in other 
   ways. For example persuading the target of the surveillance to use 
   cryptographic devices with a random number generator that leaks a 
   crib for the attacker. Analyzing the second form of attack requires a
   different type of analysis which is addressed in the following 
   section on social work factor.

1.2.2. Defense in Depth

   The motivation behind introducing the concept of the hypothetical 
   work factor is a long experience of seeing attempts to make security 
   protocols more robust being deflected by recourse to specious 
   arguments based on the computational work factor. 

   For example, consider the case in which a choice between a single 
   security control and a defense in depth strategy is being considered:

      *  Option A: Uses algorithm X for protection.

      *  Option B: Uses a combination of algorithm X and algorithm Y for
         protection such that the attacker must defeat both to break the
         system and algorithms based on different cryptographic 
         principles are chosen so as to minimize the risk of a common 
         failure mode.

   If the computational work factor for both algorithms X and Y is 
   2^128, both options present the same work factor ratio. Although 
   Option B offers twice the security, it also requires twice the work. 

   The argument that normally wins is that both options present the same
   computational work factor ratio of 2^128, Option A is simpler and 
   therefore Option A should be chosen. This despite the obvious fact 
   that only Option B offers defense in depth.

   If we consider the adversary of being capable of performing a work 
   factor ratio of 2^80 and the probability the attacker has discovered 
   an attack capable of breaking algorithms X and Y to be 10% in each 
   case, the probability that the attacker can break Option A is 10% 
   while the probability that an attack on Option B is only 1%, a 
   significant improvement.

   While Option B clearly offers a significant potential improvement in 
   security, this improvement is only fully realized if the 
   probabilities of a feasible attack are independent. 

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1.2.3. Mutual Reinforcement

   The defense in depth approach affords a significant improvement in 
   security but an improvement that is incremental rather than 
   exponential in character. With mutual reinforcement we design the 
   mechanism such that in addition to requiring the attacker to break 
   each of the component algorithms, the difficulty of the attacks is 

   For example, consider the use of a Deterministic Random Number 
   Generator R(s) which returns a sequence of values R(s)_1, R(s)_2 from
   an initial seed s.

   Two major concerns in the design of such generators are the 
   possibility of bias and that the seed value be somehow leaked through
   a side channel. 

   Both concerns are mitigated if instead of using the output of one 
   generator directly, the value R1(s1) XOR R2(s2) is used where R1 and 
   R2 are independent random number generators and s1, s2 are distinct 

   The XOR function has the property of preserving randomness so that 
   the output is guaranteed to be at least as random as either of the 
   generators from which it is built (provided that there is not a 
   common failure mode). Further, recovery of either random seed is at 
   least as hard as using the corresponding generator on its own. Thus 
   the Hypothetical work factor for the combined system is improved to 
   at least the same extent as in the defense in depth case.

   But any attempt to break either generator must now face the 
   additional complexity introduced by the output being masked with the 
   unknown output of the other. An attacker cannot cryptanalyze the two 
   generator functions independently. If the two generators and the 
   seeds are genuinely independent, the combined hypothetical work 
   factor is the product of the hypothetical work factors from which it 
   is built.

   While implementing two independent generators and seeds represents a 
   significant increase in cost for the implementer, a similar 
   exponential leverage might be realized with negligible additional 
   complexity through use of a cryptographic digest of the generator 
   output to produce the masking value.

1.2.4. Safety in Numbers

   In a traditional security analysis the question of concern is whether
   a cryptanalytic attack is feasible or not. When considering an 
   indiscriminate intercept capability as in a PRISM-class attack, the 
   concern is not just whether an individual communication might be 
   compromised but the number of communications that may be compromised 

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   for a given amount of effort.

   'Perfect' Forward Secrecy is an optional feature supported in IPSec 
   and TLS. Current implementations of TLS offer a choice between:

      *  Direct key exchange with a work factor dependent on the 
         difficulty of breaking RSA 2048

      *  Direct key exchange followed by a perfect forward secrecy 
         exchange with a work factor dependent on the difficulty of 
         breaking RSA 2048 and DH 1024.

   Using the computational work factor alone suggests that the second 
   scheme has little advantage over the first since the computational 
   work factor of Diffie Hellman using the best known techniques 2^80 
   while the computational work factor for RSA 2048 is 2^112. Use of the
   perfect forward secrecy exchange has a significant impact on server 
   performance but does not increase the difficulty of cryptanalysis.

   Use of perfect forward secrecy with a combination of RSA and Diffie 
   Hellman does not provide a significant improvement in the 
   hypothetical work factor either if individual messages are 
   considered. The RSA and Diffie Hellman systems are closely related 
   and so an attacker that can break RSA 2048 can almost certainly break
   RSA 1024. Moreover computational work factor for DH 1024 is only 2^80
   and thus feasibly within the reach of a well funded and determined 

   Use of perfect forward secrecy does provide an important security 
   benefit when multiple messages are considered. While a sufficiently 
   funded and determined attacker could conceivably break tens, hundreds
   or even thousands of DH 1024 keys a year, it is rather less likely 
   that an attacker could break millions a year. The Comodo OCSP server 
   receives over 2 billion hits a day and this represents only a 
   fraction of the number of uses of SSL on the Internet. Use of perfect
   forward secrecy does not prevent an attacker from decrypting any 
   particular message but raises the cost of indiscriminate intercept 
   and decryption.

   There is security in numbers: If every communication is protected by 
   perfect forward secrecy the hypothetical work factor for decrypting 
   every communication is the hypothetical work factor of decrypting one
   communication times the number of communications. 

1.3. Cost Factor

   As previously discussed, cryptanalysis is not the only tool available
   to an attacker. Faced with a robust cryptographic defense, Internet 
   criminals have employed 'social engineering' instead. A PRISM-class 
   attacker may use any and every tool at their disposal including tools
   that are unique to government backed adversaries such as the threat 

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   of legal sanctions against trusted intermediaries.

   Although attackers can and will use every tool at their disposal, 
   each tool carries a cost and some tools require considerable advance 
   planning to use. It is conceivable that the AES standard published by
   NIST contains a backdoor that somehow escaped the extensive peer 
   review. But any such effort would have had to have begun well in 
   advance of 1998 when the Rijndael cipher was first published. 
   Subversion of cryptographic apparatus such as Hardware Security 
   Modules (HSMs) and SSL accelerators faces similar constraints. A HSM 
   may be compromised by an adversary but the compromise must have taken
   place before the device was manufactured or serviced.

   Just as computational attacks are limited by the cryptanalytic 
   techniques known to and the computational resources available to the 
   attacker, social attacks are limited by the cost of the attack and 
   the capacity of the attacker.

   The Cost Factor C(t) is an estimate of the cost of performing an 
   attack on or before a particular date in time (t). 

   For the sake of simplicity, currency units are used under the 
   assumption that all the resources required are fungible and that all 
   attackers face the same costs. But such assumptions may need to be 
   reconsidered when there is a range of attackers with very different 
   costs and capabilities. A hacktivist group could not conceivably 
   amass the computational and covert technical resources available to 
   the NSA but such a group could in certain circumstances conceivably 
   organize a protest with a million or more participants while the 
   number of NSA employees is believed to still be somewhat fewer.

   The computational and hypothetical work factors are compared against 
   estimates of the computational resources of the attacker. An attack 
   is considered to be infeasible if that available computational 
   resources do not allow the attack to be performed within a useful 
   period of time. 

   The cost factor is likewise compared against an incentive estimate, 
   I(t) which is also time based.

   An attack is considered to be productive for an attacker if there was
   a time t for which I(t) > C(t).

   An attack is considered to be unproductive if there is no time at 
   which it was productive for that attacker.

   Unlike Cost Factor for which a lower bound based on the lowest cost 
   and highest capacity may be usefully applied to all attackers, 
   differences in the incentive estimate between attackers are likely to
   be very significant. Almost every government has the means to perform
   financial fraud on a vast scale but only rarely does a government 

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   have the incentive and when governments do engage in activities such 
   as counterfeiting banknotes this has been done for motives beyond 
   mere peculation.

   While government actors do not respond to the same incentives as 
   Internet criminals, governments fund espionage activities in the 
   expectation of a return on their investment. A government agency 
   director who does not produce the desired returns is likely to be 

   For example, when the viability of SSL and the Web PKI for protecting
   Internet payments was considered in the mid 1990s, the key question 
   was whether the full cost of obtaining a fraudulently issued 
   certificate would exceed the expected financial return where the full
   cost is understood to include the cost of registering a bogus 
   corporation, submitting the documents and all the other activities 
   that would be required if a sustainable model for payments fraud was 
   to be established.

   For an attack to be attractive to an attacker it is not just 
   necessary for it to be productive, the time between the initial 
   investment and the reward and the likelihood of success are also 
   important factors. An attack that requires several years of advance 
   planning is much less attractive than an attack which returns an 
   immediate profit.

   An attack may be made less attractive by

      *  Increasing the cost

      *  Reducing the incentive

      *  Reducing the expected gain

      *  Reducing the probability that the incentive will be realized

      *  Increasing the time between the initial investment and the 

1.4. Social Work Factor

   In the cost factor analysis it is assumed that all costs are fungible
   and the attack capacity of the attacker is only limited by their 
   financial resources. Some costs are not fungible however, in 
   particular inducing a large number of people to accept a forgery 
   without the effort being noticed requires much more than a limitless 
   supply of funds.

   In a computational attack an operation will at worst fail to deliver 
   success. There is no penalty for failure beyond having failed to 
   succeed. When attempting to perpetuate a fraud on the general public,

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   every attempt carries a risk of exposure of the entire scheme. When 
   attempting to perform any covert activity, every additional person 
   who is indoctrinated into the conspiracy increases the chance of 

   The totalitarian state envisioned by George Orwell in 1984 is only 
   possible because each and every citizen is in effect a party to the 
   conspiracy. The erasure and replacement of the past is possible 
   because the risk of exposure is nil.

   In 2011 I expressed concern to a retired senior member of the NSA 
   staff that the number of contractors being hired to perform cyber-
   sabotage operations represented a security risk and might be creating
   a powerful constituency with an interest in the aggressive 
   militarization of cyberspace rather than preparing for its defense. 
   Subsequent disclosures by Robert Snowden have validated the 
   disclosure risk aspect of these concerns. 

   Empirically, the NSA, an organization charged with protecting the 
   secrecy of government documents, was unable to maintain the secrecy 
   of their most important secrets when the size of the conspiracy 
   reached a few ten thousand people.

   The community of commercial practitioners cryptographic information 
   security is small in size but encompases many nationalities. Many 
   members of the community are bound by ideological commitments to 
   protecting personal privacy as an unqualified moral objective.

   Introducing a backdoor into a HSM, application or operating system 
   platform requires that every person with access to the platform 
   source or who might be called in to audit the code be a party to the 
   conspiracy. Tapping the fiber optic cables that support the Internet 
   backbone requires only a small work crew and digging equipment. 
   Maintaining a covert backdoor in a major operating system platform 
   would require hundreds if not thousands of engineers to participate 
   in the conspiracy.

   The Social Work Factor WF_S(t) is a measure of the cost of 
   establishing a fraud in a conspiracy starting at date t. The cost is 
   measured in the number of actions that the party perpetrating the 
   fraud must perform that carry a risk of exposure.

   In general, the Social Work Factor will increase over time. 
   Perpetrating a fraud claiming that the Roman emperor Nero never 
   existed today would require that millions of printed histories be 
   erased and rewritten, every person who has ever taught or taken a 
   lesson in Roman history would have to participate in the fraud. The 
   Social Work Factor would be clearly prohibitive. 

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   The Social Work Factor of perpetrating such a fraud today is 
   prohibitive, the cost in the immediate aftermath of Nero's 
   assassination in 68 would have been considerably lower. While the 
   emperor Nero was obviously not erased from history there is a strong 
   consensus among Egyptian archeologists that something of the sort 
   happened to Tutankhamun before the discovery of his tomb by Howard 

2. The Problem of Evaluating Trust

   The Prism-Proof Email testbed attempts to facilitate the development 
   and deployment of a new email privacy protection infrastructure by 
   dividing the problem into the parts for which there are known, well 
   established (if not necessarily perfect) solutions and the parts for 
   which there are not with clearly defined interfaces between the two 

   The Trust Publication Web Service is a JSON/REST Web service that 
   supports the publication of all existing forms of trust assertion 
   (PKIX, OpenPGP, SAML). For the sake of future simplicity, a new ASN.1
   message format for OpenPGP-style key endorsement is proposed so that 
   all the forms of trust assertion that might be used in a PKI may be 
   expressed without recourse to multiple data encoding formats. The 
   Trust Publication Web Service need not be a trusted service since its
   role is essentially that of a proxy, routing messages such as 
   certificate requests to the appropriate destination(s).

   The Omnibroker Web Service is a trusted service that a mail user 
   agent or proxy can query to determine which security enhancements 
   (encryption, signature, etc.) should be added to an outbound message 
   (among other functions).

   Between the Trust Publication Web Service and the Omnibroker service 
   sits the hard research problem of how to make sense of and what value
   to place on the CA issued certificates, peer to peer key 
   endorsements, revocation information and other signed assertions that
   might exist.

   Robust implementation of public key cryptography allows the signature
   on a signed assertion to be verified as belonging to a holder of the 
   corresponding signature key with near certainty. But such an 
   assertion can only be considered trustworthy if the purported signer 
   is trustworthy and is the actual holder of the corresponding signing 
   key, claims that are in turn established by more signed assertions. 
   Expanding the scope of our search increases the number of documents 
   on which we are relying for trust rather than answering the question 
   we wish to answer.

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2.1. Probability and Risk

   Attempting to analyze the trustworthiness of a signed assertion in a 
   heterarchical topology such as Phil Zimmerman's Web of Trust leads to
   an infinite regression. Alice may see that Bob's key has been signed 
   by Carol, Doug and Edward but this should only give Alice more 
   confidence in the validity of Bon's key if she knows that Carol Doug 
   and Edward are distinct individuals.

   Probability is a model of events that are random. An attack is a 
   conscious act on the part of an attacker and is only random insofar 
   as the attacker's motive may not require a particular choice of 
   victim or method. In such cases the particular attack is 'random' 
   from the point of view of the victim but that an attack would take 
   place is due to the fact that the attacker had motive, means and 

   The motive for an attacker depends on the perceived rather than the 
   actual difficulty of breaking a system. It might be some time before 
   a competent attacker attempts to break an insecure system that is for
   some reason generally believed to be secure. But the rate of attack 
   is likely to increase rapidly once the vulnerability is widely known.
   The system has not become less trustworthy over time, rather the 
   system was always untrustworthy and it is only the consequences of 
   that fact that have changed. 

   Analyzing the trustworthiness of a Web of Trust using an estimate of 
   the probability that an assertion might be fraudulent is 
   unsatisfactory because it requires us to provide as an input to our 
   calculations the very quantity we are trying to arrive at as an 

   Attempting to estimate the probability of default for each assertion 
   in a Web of trust leads us to an infinite recursion as Alice trusts 
   Bob trusts Carol trusts Alice. We can define the inductive step but 
   have no base case to ground it with. 

2.2. Reputation

   Another frequently proposed metric for analyzing the trustworthiness 
   of assertions is 'reputation'. Reputation is a measure of risk based 
   on reports of past behavior.

   Reputation has proved somewhat effective in online restaurant 
   reviews. A restaurant that receives a high number of good reviews is 
   likely to be worth visiting but ratings based on a small number of 
   reviews can be wildly inaccurate. A glowing review may have been 
   written by a satisfied customer or by an unscrupulous proprietor. A 
   series of negative reviews may be written by unsatisfied customers or
   a jealous competitor.

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   Restaurant reviews work because there is a widely shared 
   understanding of what makes a good or a bad restaurant. There is no 
   similar shared understanding of the quality of a public key 
   validation process except among specialists in the field.

2.3. Curated Spaces

   'Reputation based' systems have proved highly effective at 
   controlling email abuse but these systems use a large quantity of 
   empirical data including from honeypot email servers, expert analysis
   of abuse traffic and content analysis to arrive at reputation scores.
   It is the action of the curator that turns the raw data into a useful
   measure of risk rather than the mechanical application of a clever 

   There is a good argument to be made for introducing a curator into a 
   PKI trust model but that is an argument about who should perform the 
   analysis rather than how the analysis is to be performed. 

2.4. Trustworthy Time

   The problem of grounding the Web of Trust is solved if there is 
   available a notary authority whose trustworthiness is beyond 
   reasonable dispute. Once a trust assertion has been notarized by such
   a notary authority, the cost of forgery becomes the cost of suborning
   the notary authority. 

   As is demonstrated later, use of linked timestamps and cross-
   notarization amongst notaries makes it possible to establish a 
   timestamp notary infrastructure such that perpetrating a forgery 
   requires each and every notary in the infrastructure is compromised.

   The analysis of the set of trust assertions begins by asserting a 
   Social Work Factor to the earliest assertion prior to the time at 
   which it was notarized. This provides the base case of the induction 
   from which the rest of the analysis proceeds.

3. Maximizing Social Work Factor to Maximize Trust

   As previously described, the purpose of the Social Work Factor is to 
   support the design process by allowing the consequences of different 
   design approaches to be considered.

   When designing a trust infrastructure, there are two different 
   attacks that need to be considered. First there is the attack where a
   credential is issued for an entirely fictitious persona, secondly 
   there is the attack where a credential is issued to an impostor 
   impersonating a real persona.

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   The consequences of the two attacks are very different, particularly 
   where a confidentiality infrastructure is concerned. Indeed it might 
   be considered desirable to encourage participants to create and use 
   fictitious personas to provide anonymity for their actions in certain
   circumstances. An impostor who gains a credential for a real person 
   can use it to persuade relying parties that their communications are 
   confidential when they are in fact compromised and can steal the use 
   of the target's reputation.

   The context of an attack is also important. The confidentiality of 
   the private communications of an individual is an issue for that 
   individual and their correspondents alone. The confidentiality of the
   communications of an individual acting for their employer is much 
   more complex. In addition to the employer having an interest in 
   protecting the confidentiality of the communication, there may be a 
   legitimate employer interest in being able to view the contents. For 
   example, it is now generally accepted in many countries that most 
   government employees do not have a right of privacy from the people 
   who they ultimately work for unless their job function falls into a 
   narrowly scoped exception.

3.1. Trust Specifiers

   A trust specifier is a mechanism that identifies a public key either 
   directly (e.g. a self-signed certificate) or indirectly (e.g. a Key 

   Trust specifiers are not trust assertions but may be used to create 
   trust assertions. For example, an OpenPGP fingerprint does not make 
   any statement about the owner of a public key but an OpenPGP 
   fingerprint printed on a business card is an explicit claim that the 
   specified public key may be used to send encrypted email to the 
   individual named on the card.

3.1.1. Key Identifiers

   PGP introduced the use of key fingerprints as the basis for key 
   exchange. A cryptographic digest value is computed from the user's 
   public key and used as the basis for key endorsement (called key 
   signing in the PGP terminology).

   The term 'fingerprint' has no formal definition in the PKIX 
   specifications but the term is widely used to refer to a message 
   digest of the entire contents of a certificate. Since this use is 
   incompatible with the PGP usage, the term Key Identifier is prefered 
   as this is unambiguous in both contexts.

   In PKIX, the key identifier value is a value chosen by the issuer to 
   uniquely identify a public key. The Key Identifier value of the 
   issuing public key is specified using the authorityKeyInfo extension 
   and the Key Identifier value of the subject is specified in the 

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   subjectKeyIdentifier extension.

   While a PKIX Key Identifier is not required to have the strong 
   binding to the corresponding public key that an OpenPGP identifier 
   does, a profile could require that certificates specify 'strong' Key 
   Identifiers formed using a cryptographic message digest of the public
   key parameters.

   Strong Key Identifiers are not trust assertions but they may be used 
   to facilitate the creation of trust assertions through key signing, a
   form of the endorsement mechanism discussed below.

   Strong Key Identifiers may also be used to publish informal key 
   assertions by adding them to a business card or a Web Page. Such uses
   might be facilitated through definition of appropriate URI and QR 
   code formats. 

3.1.2. Self Signed Certificates

   In the context of PKI, the term 'certificate' is generally understood
   to refer to an X.509 public key certificate that binds a name and/or 
   an Internet address to a public key.ublic key.

   A certificate may be either a self signed certificate or a CA issued 
   certificate. Since the work factor of creating a self-signed 
   certificate is negligible, such certificates demonstrate little in 
   themselves but present the subject's public key data in a format that
   is compatible with many existing applications.

   As with Key Identifiers, a self signed certificate is not a useful 
   trust assertion in its own right but may be used to facilitate the 
   creation of trust assertions through notarization or endorsement. A 
   public key certificate may also be used as the basis for making a 
   Certificate Signing Request to a Certificate Authority.

3.2. Trust Assertions

   In a PKI, a trust assertion makes a statement about the holder of a 
   public key. In the PKIX model as currently deployed and used, the 
   only forms of trust assertion are Key Signing Certificates and End 
   Entity Certificates. In the OpenPGP model every user is also a trust 
   provider and trust assertions are created in peer-to-peer fashion to 
   create a Web of Trust.

3.2.1. Certificate Authority Issued Certificates

   A Certificate Authority (CA) is a trusted third party that issues 
   digital certificates. A private CA issues certificates for a closed 
   community of relying parties, a public CA issues certificates without
   restriction on the relying parties.

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   In the Web PKI, providers of Web browsers and platform providers 
   embed the trust anchors of selected public Certificate Authorities 
   into the application as default trust providers.

   Operation of a Certificate Authority involves two types of 
   certificate. A certificate is either a certificate signing 
   certificate or an end entity certificate. While it is possible for a 
   Certificate Authority to lose control of a signing key used to issue 
   certificates, the Social Work Factor for such attacks can be made 
   prohibitively high.

   A CA Certificate Policy defines (among other things) the validation 
   criteria that the CA applies before deciding to issue a certificate. 
   A certificate policy is designed to provide a balance between the 
   social work factor presented to an attacker and the cost to the CA 
   and the subject. The CA-Browser forum Extended Validation practices 
   are designed to present a very high social work factor to attackers 
   while Domain Validation presents a significantly lower cost to both 
   subjects and attackers.

   The EV guidelines in particular are designed to present a social work
   factor that increases each time an attacker attempts an attack. 
   Registering one corporation is relatively straightforward. 
   Registering a corporation in a way that prevents ownership being 
   traced back to the owners is rather more difficult. Registering 
   hundreds of false front corporations is considerably harder as any 
   common link between the corporations means that if one corporation is
   discovered to be a front for fraudulent purposes, the rest will come 
   under close scrutiny. scrutiny.

   A major advantage of the CA certificate issue approach is that they 
   allow a high degree of trust to be established very quickly while it 
   takes a considerable time to establish trust in a pure Web of Trust 

   The main drawback to the CA issue approach is in providing trust to 
   individuals for personal use rather than corporate or government 
   entities or to employees working for such entities. Corporations and 
   Government entities invest in obtaining EV validated certificates as 
   a cost of doing business that has an established return. There is no 
   obvious return on obtaining a similar high assurance credential for 
   personal use but validating an individual's credentials is just as 
   complicated as validating the credentials of a corporation. The 
   history of the UK National Identity card suggests that there is no 
   reason to expect that the cost of provisioning credentials would be 
   significantly reduced at scale. 

   The CA issue model has been successfully applied to issue of 
   credentials to employees for use within an organization and such 
   credentials are occasionally used for external purposes such as 
   S/MIME email. But such certificates are not intended for personal use

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   and are typically revoked when employment ends.

3.2.2. Key Signingey Signing

   In the OpenPGP model every key holder is a trust provider and every 
   key may be used to provide trust through 'key signing'. The trust 
   value of key signing depends on how close the relying party is to the
   signer. A key that I have signed myself is far more trustworthy than 
   a key signed by a friend of a friend of a friend.

   The Social Work Factor of forging an individual key signing is low 
   but could be fixed in time through use of a notary. A recent key 
   signing purporting to be for the public key of Barack Obama would 
   have negligible evidentiary value unless produced by someone very 
   close to me or endorsed through other means. A key signing that had 
   been notarized during his time as a Harvard student would be 
   considerably more trustworthy.

   As the separation between the relying party and the signer becomes 
   larger it becomes increasingly desirable to require multiple 
   independent key signing paths. The Social Work Factor combined with 
   an absolutely reliable notary service provide a firm basis for 
   evaluating the trust value of such Webs of trust: The trustworthiness
   of a key is dependent on whether the incentive to create a forgery 
   ever exceeded the Social Work Factor of performing a forgery.

   Use of peer-to-peer key signing does provide a viable model for 
   establishing high assurance credentials for personal use but 
   establishing a high degree of assurance is like making the best 
   quality Scotch whisky: the product takes an inordinately long time to
   reach maturity. It is hard to see how key-signing could be made a 
   viable model for commercial use. It is an especially poor fit for 
   issue of credentials to employees as the employer has no control over
   the issue or use of the credential and no ability to revoke the 
   credential when an employee is terminated.

   One advantage frequently cited for OpenPGP over PKIX is that there is
   no CA role and the infrastructure is therefore 'free'. Neither the 
   premise, nor the conclusion holds, Many profitable businesses have 
   been built on the basis of open source software and a Web of Trust 
   that reached critical mass would offer abundant opportunities to 
   companies with PKI expertise. 

   Nor is the lack of business stakeholders in an infrastructure 
   necessarily an advantage. One of the reasons that the Web PKI has 
   been successful is that there are many businesses that promote the 
   use of SSL certificates. 

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3.2.3. Adding Key Endorsement to PKIX

   Considering the Social Work Factor presented by trust assertions in 
   the PKIX and OpenPGP model establishes clear benefits to both 
   approaches. Each model best suits a different community. 

   One approach to a next generation PKI therefore would be to simply 
   enable the use of either scheme in any application. This is not the 
   best approach however as it leaves the email security market in a 
   BetaMax/VHS standards war that has been stalemated for almost two 
   decades. Further, attempting to maintain the use of two different 
   PKIs leads to an unnecessary increase in complexity.

   A PKI that combines the PKIX and OpenPGP approaches offers clear 
   advantages over both. For many practical reasons that will only be 
   summarised here, it is better to extend the existing PKIX 
   infrastructure to support Key Signing rather than start from the 
   OpenPGP message formats or attempt to design a system based on a new 
   encoding format. These include the deployed base of S/MIME email 
   clients, the use of PKIX to support SSL and the widespread support 
   for PKIX in common code platforms.

   Rather than attempting to force peer-to-peer Key Signing model into 
   the existing PKIX model, a new structure, the Key Endorsement is 
   proposed instead. While it is technically possible to use PKIX cross 
   certificates as a substitute for PGP Key Signing, the legacy PKIX 
   infrastructure is designed on the assumption that a cross certificate
   is either completely trustworthy or completely untrustworthy. It is 
   better to introduce a new data format to represent new semantics than
   to attempt to retrofit new semantics into a legacy format with a 
   deployed infrastructure.

   I propose the introduction of a Key Endorsement, similar to a PKIX 
   certificate except that:

      *  A Key Endorsement is signed by a PKIX end entity certificate, 
         not a Key Signing Certificate.

      *  Instead of a validity interval, there is an issue time.

      *  The signer and subject key identifiers are required elements of
         the structure rather than optional extensions.

      *  Key endorsements are not intended for direct use by relying 

   Adding Key Endorsements to the PKIX model allows the use of both 
   trust models in combination.

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   Key endorsement may also be used to endorse keys of peers. While the 
   ability of the 16 year old Alice to accurately validate the public 
   keys of her peers is questionable, the value of a notarized 
   endorsement increases over time.

   Relying on peer endorsement alone requires that the relying party be 
   'close' to the signer. Using a combination of CA issued certificates 
   and Key Endorsements allows a high social work factor to be 
   established even for a remote population. It is quite feasible for an
   attacker to generate a network of one, a hundred or even a million 
   key signing events but generating a fraudulent network that contains 
   a mixture of CA validated certificates and key endorsements has a 
   much higher social work factor. Self Endorsement

   Self endorsement is a special form of endorsement as a person is 
   least likely to make false statements that might harm their own 
   security. Although this is possible in the case that coercion or 
   undue psychological pressure is applied.

   A user may be expected to have multiple public keys issued over the 
   course of their life, for use at school, university, at different 
   employers and for personal use. In the existing PKIX model each of 
   these keys are independent. Key endorsement allows a user to 
   countersign new keys with older keys, establishing a personal web of 
   trust that develops over time so that the social work factor is 
   preserved and increased over time rather than starting from scratch 
   each time a certificate is issued.

   For example, Alice is issued a certificate at school which she uses 
   to sign a key endorsement for a personal key she creates. When Alice 
   goes to University, she endorses her university key with her personal
   key and vice versa. On graduating, Alice becomes a journalist. 
   Sources can send her encrypted messages containing tips confident in 
   the knowledge that Alice is the same Alice who attended the high 
   school and university listed in her official biography. Key Endorsement Parties

   A PGP Key Signing party is an event held to facilitate exchange of 
   PGP fingerprints. Like the use of hashtags and many other important 
   constructs in social media, key signing parties are a practice that 
   have arisen out of use rather than being part of the original model.

   Recognizing a Key Endorsement Party as being a special form of peer 
   endorsement enables special consideration when making a trust 
   evaluation. It is not practical for a thousand attendees at an 
   international conference to perform mutual key endorsements with 

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   every other participant (a half million pairs!) but it is entirely 
   practical to establish a scheme in which anyone who gets their key 
   endorsed by some number (e.g. five) of qualified Key Endorsers will 
   have their key endorsed by the Key Endorsement Party key at the end.

3.3. Trust Meta Assertions

   Trust Meta Assertions are trust assertions that make statements about
   other trust assertions 

3.3.1. Revocation and Status Checkings Checking

   Real users and real administrators make mistakes from time to time. 
   Private keys are lost or stolen or misused. Any system that does not 
   provide a mechanism for forgiving mistakes is unlikely to be 
   practical in the real world.

   Revocation checking limits the incentive for attack by allowing the 
   time window of vulnerability to be limited in the case of a 
   fraudulent certificate issue or that a private key is discovered to 
   be lost or stolen. This does not increase the social work factor but 
   decreases the value of an attack.

3.3.2. Notarization

   Notarization or a Trust Assertion of Key Identifier by a trustworthy 
   notary allows the social work factor of forging the trust assertion 
   or key identifier to be raised to an infeasible level for dates after
   the notarization took place.

   While a traditional notary might be suborned relatively easily, a 
   digital notary can be constructed in such a fashion that post dating 
   a notary assertion by more than a few hours or days is infeasible.

3.3.3. Transparency

   Certificate Transparency is a proposed infrastructure to (Laurie et. 
   al.). [[Describe]

   Publishing issued certificates increases the probability that an 
   attempted fraud will be discovered and thus increases the Social Work
   Factor and reduces the incentive.

3.4. Other Approaches

   The X.509/PKIX and OpenPGP infrastructures are not the only 
   infrastructures that have been proposed by they are the only large 
   scale infrastructures which large numbers of users rely on today. The
   Social Work Factor over time may be used to evaluate alternative PKI 
   options that have not (yet) achieved widespread use.

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3.4.1. DNSSEC

   Like PKIX, DNSSEC is based on a hierarchical trust model. Unlike 
   PKIX, DNSSEC is only capable of making statements about DNS names.

   The Social Work Factor is not a useful tool to analyze DNSSEC 
   assertions because there is no historical dimension to the DNSSEC 
   infrastructure. The trustworthiness of a DNSSEC assertion depends on 
   the trustworthiness of the root operator, the TLD registry and the 
   party that signed the corresponding DNS zone. If these are 
   trustworthy then so is the assertion, there is no alternative if not.

   While the root and registry operators have strong commercial 
   incentives not to default, a default may be coerced through 
   government action and relying parties have no independent means to 
   determine if a default has taken place. 

3.4.2. SPKI / SDSI

   The chief distinguishing characteristic of SPKI is that SPKI names 
   are not universal. While this has the interesting effect of 
   simplifying the evaluation of trust within the SPKI naming 
   infrastructure, this effect is lost when attempting to send an email 
   because the Internet email system is based on the assumption that the
   namespace is universal. It does not make sense to talk about 'Alice's' (although UUCP email did use a scheme of that type.

3.4.3. Identity Based Cryptography

   Identity based cryptography is a frequently touted 'alternative' to 
   conventional public key cryptography in which the public key of each 
   subject is a deterministic function of their name and a master public
   key which is known to everyone. Each user obtains their private key 
   from the party that created the master public key pair using the 
   master private key.

   While many benefits have been claimed for the Identity based 
   approach, applying to the holder of the master private key for a 
   private key offers no benefits to key holders over applying to a 
   certificate from a CA.

   Identity based cryptography does offer relying parties the ability to
   obtain the public key for a counterparty without the need to 
   communicate with another party, but this is hardly much of an 
   advantage unless there is no means for certificates to be passed in-
   ban and there is no advantage at all if an external source has to be 
   queried to obtain the status of a public key to determine that the 
   private key has not been reported lost or compromised.

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   The simplicity of certificate chain validation and status checking in
   Identity Based Cryptography is the result of the technology being 
   unable to support these features rather than the features being 

4. Maximizing Social Work Factor in a Notary Infrastructure

   The ability to determine that a trust event occurred before a certain
   point in time increases the social work factor for forging the event 
   after that point in time. If suborning the notary is infeasible, the 
   Social Work Factor is raised to an infeasible level.

   Harber and Stornetta proposed a notary that produced a 'catenate 
   certificate' in which each notary output is fed as an input into the 
   next. It is thus impossible to insert notary events between two prior
   notary events without breaking the cryptographic algorithm used for 

   A chain of notary events may be fixed in time by notarizing events 
   that are unpredictable in advance but known with a high degree of 
   certainty afterward. For example the weekly lottery numbers or the 
   closing prices on various equity markets.

   The principal drawback to grounding the notary chains using external 
   events is that the ability of a party to verify the trustworthiness 
   of the notary is bounded by the trustworthiness of the reports of the
   external events used for verification.

   A better approach is to have establish a large number of 
   independently operated notaries and a procedure for cross 
   notification that ensures that no notary can default unless every 
   other notary defaults. The Social Work Factor of suborning any notary
   then becomes the Social Work Factor of suborning every notary and 
   erasing all histories for their notary events.

   For ease of explanation, a two tier notary infrastructure is 
   envisaged in which a notary is either a local notary or a meta 

   Local notaries are notaries that produce a catenate log of 
   notarization requests submitted by its users. The current value of 
   the notary chain is presented to one or more meta-notaries at regular
   time intervals (e.g. an hour) to prevent backdating of notary claims 
   and notarizes the output of one or more meta notaries at regular 
   intervals to prevent predating.

   Meta notaries are notaries that only notarize the requests submitted 
   by local notaries and by other meta notaries. Before accepting a 
   notarization request, a meta notary audits the actions of the notary 
   making the request to ensure that the time-stamp values etc. are 
   consistent and correct. 

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   The schedule of peer to peer notification among meta notaries is set 
   such that a notary request made to any of the local notaries served 
   will affect the output of every meta notary within a predetermined 
   period of time. 

   To make use of such a notary infrastructure, a relying party chooses 
   at least one meta notary and obtains and maintains a record of the 
   catenate certificate chain over time. The trust chain of the chosen 
   meta notary may then be used to verify any notary assertion presented
   by any other meta notary which may in turn be used to verify any 
   notary assertion from a local notary that participates in the scheme.

   A significant benefit of this approach is that the ability to verify 
   notary assertions is assured even if the Local Notary that originally
   produced it ceases functioning. All that is necessary for the 
   continued operation of the system to be assured is for the pool of 
   meta notaries to be sufficiently large to render suborning all of 
   them infeasible.

5. Conclusions and Related Work

   It has not escaped the notice of the author that the social work 
   factor might be applied as a general metric for assessing the 
   viability of a political conspiracy hypothesis. 

Author's Address

   Phillip Hallam-Baker
   Comodo Group Inc.

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