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Network Working Group                                         T. Freeman
Internet-Draft                                           Microsoft Corp.
Intended status: Informational                                 J. Schaad
Expires: August 17, 2014                         Soaring Hawk Consulting
                                                            P. Patterson
                                       Carillon Information Security Inc
                                                       February 13, 2014

                Requirements for Message Access Control
                  draft-freeman-plasma-requirements-09

Abstract

  S/MIME has a proven track record in delving confidentiality, integrity
  and data origination authentication for email. However, there are many
  situations where organizations want robust access control applied to
  information in messages. The Enhanced Security Services (ESS) RFC5035
  for S/MIME defines an access control mechanism for email, but the
  access check happens after the data is decrypted by the recipient
  which devalues the protection afforded by the cryptography and
  provides very week guarantees of policy compliance. Another major
  issues for S/MIME is its dependency on a single type of identity
  credential, an X.509 certificate. Many users on the Internet today do
  not have X.509 certificates and therefore cannot use S/MIME.
  Furthermore, the requirement to discover the X.509 certificate for
  every recipient of an encrypted message by the sender has proven to be
  an unreliable process for a number of reasons.

  This document presents requirements for an alternative model to ESS to
  address the identified issues with access control to deliver more
  robust compliance with S/MIME protected messages. This document
  describes an access control model which uses cryptographic keys to
  enforce access control policy decisions where the policy check is
  performed prior to the decryption of the message contents. The model
  also abstracts the specifics of the authentication technology thereby
  removing the dependency on X.509 certificate making it possible for
  other forms of credential to be used for S/MIME enabling much broader
  adoption. This model can be instantiated in many areas using existing
  standards, or with only minor updates to existing standards. This
  model in not intended to be a one off just for email and can also be
  applied to other data types. The model also removes the dependency on
  the need to discover encryption certificates at send time.

  The name Plasma was assigned to this effort as part of the IETF
  process. It is derived from PoLicy enhAnced Secure eMAil.

Status of this Memo




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  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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  Trust Legal Provisions and are provided without warranty as described
  in the Simplified BSD License.

















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Table of Contents

   1 Policy Based Management Vocabulary . . . . . . . . . . . . . . .  4
   2 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3. Access Control Models . . . . . . . . . . . . . . . . . . . . . 10
     3.1 Generic Access Control Model . . . . . . . . . . . . . . . . 11
   4 Use Case Scenarios . . . . . . . . . . . . . . . . . . . . . . . 14
     4.1 Consumer to Consumer Secure Email  . . . . . . . . . . . . . 14
     4.2 Business to Consumer Secure Email  . . . . . . . . . . . . . 15
     4.3 Business to Business Ad-Hoc Email  . . . . . . . . . . . . . 16
     4.4 Business to Business Regulated Email . . . . . . . . . . . . 17
     4.5 Delegation of Access to Email  . . . . . . . . . . . . . . . 22
     4.6 Email Compliance Verification  . . . . . . . . . . . . . . . 22
     4.7 Email Pipeline Inspection  . . . . . . . . . . . . . . . . . 23
     4.8 Distribution List Expansion  . . . . . . . . . . . . . . . . 24
     4.9 Scalable Decision Making . . . . . . . . . . . . . . . . . . 24
   5 Plasma Security Model  . . . . . . . . . . . . . . . . . . . . . 25
     5.1 Plasma Client/Server Key Exchange Level of Assurance . . . . 32
     5.2 Policy Data Binding  . . . . . . . . . . . . . . . . . . . . 32
     5.3 Content Creation Workflow  . . . . . . . . . . . . . . . . . 34
     5.4 Content Consumption Workflow . . . . . . . . . . . . . . . . 36
     5.5 Plasma Proxy Servers . . . . . . . . . . . . . . . . . . . . 37
     5.6 Policy Types . . . . . . . . . . . . . . . . . . . . . . . . 39
   6 Message Protection Requirements  . . . . . . . . . . . . . . . . 40
     6.1 General Requirements . . . . . . . . . . . . . . . . . . . . 40
     6.2 Basic Policy Requirements  . . . . . . . . . . . . . . . . . 42
     6.3 Advanced Policy Requirements . . . . . . . . . . . . . . . . 43
   7 IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 44
   8 Security Considerations  . . . . . . . . . . . . . . . . . . . . 44
   Appendix A.  References  . . . . . . . . . . . . . . . . . . . . . 45
     A.1.  Normative References . . . . . . . . . . . . . . . . . . . 45
     A.2.  Informative References . . . . . . . . . . . . . . . . . . 46
   Appendix B Authors' Addresses  . . . . . . . . . . . . . . . . . . 47


















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Keywords

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
  document are to be interpreted as described in RFC 2119.

1 Policy Based Management Vocabulary

  This document uses the established terminology for policy based
  management [RFC3198] where applicable. The following list supplements
  the terms defined in [RFC3198] as well as defining some new
  combinations of terms used in [RFC3198].

  Attribute Based     Where the access control policy is specified
  Access Control      by a set of attributes, their values, and any
  (ABAC)              relationship between attributes required to
                      authorize an action on a resource. These
                      attributes may be provided by the subject as part
                      of the decision request (Front End Attribute
                      Exchange) or discovered by the policy decision
                      service itself (Back End Attribute Exchange). The
                      policy, for example, may require attributes about
                      the subject, their device or environment, a
                      resource, or the intended use of the information.

  Back End Attribute  When subject attributes are directly sent from
  Exchange (BAE)      the Policy Information Point (PIP) to the Policy
                      Decision and Enforcement Point (PDEP) i.e. they
                      are not relayed via the Decision Requestor (DR).

  Capability Based    Where access control is via a communicable,
  Access Control      unforgeable token. A capability token is a
  (CBAC)              protected object which, by virtue of its
                      possession by a subject, grants that subject the
                      capability.

  Decision Requester  The service responsible for making policy
  (DR)                decision requests to the PDEP. In this model the
                      policy decision is enforced by the PDEP by its
                      control of cryptographic keys. The DR enforces any
                      obligations the PDEP may require such as signing
                      or encryption of the data, generating audit events
                      etc. A DR is distinct from a PEP in other models
                      such as XACML in that a DR is not by default
                      trusted with the clear text data. Policy
                      enforcement is performed by the PDEP. A DR may
                      establish trust by presentation of attributes
                      about itself and its environment to show it is



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                      trustworthy.

  Front End Attribute When subject attributes are relayed by the DR
  Exchange (FEE)      from the PIP to the PDEP i.e. they are not sent
                      directly.

  Level of Assurance  A quality grade assigned following the completion
  (LoA)               of a security evaluation. For example, it can be
                      used for an Identity where it provides the quality
                      of the identity of a subject. It can also be used
                      to represent the quality of a products or services
                      Common criteria evaluation.

  Metadata            Metadata is data about data. There are three kinds
                      of metadata.

                      (1) Content metadata is metadata about an instance
                      of data, the actual data content. An example of
                      content metadata would be "this data contains
                      Company Foo intellectual Property" or " this is a
                      patient record".
                      (2) Policy metadata is metadata about the policies
                      to apply to an instance of data. An example of
                      policy metadata would be "apply Company Foo XYZ
                      policy".
                      (3) Structural metadata is metadata about the
                      design and specification of the data. An example
                      of structural metadata would be "this is a patient
                      record table".

  Orthonym            The correct or legal name of a place, person or
                      thing. (See Pseudonym.)

  Policy              The system entity that creates, maintains, and
  Administration      publishes policies or policy collections. The
  Point (PAP)         policies define the rules, their conditions, and
                      actions associated with the policy.

  Policy Collection   A collection of one or more policies which is
                      associated with a role. The policy collection may
                      also define the logical relationship between the
                      policies. Each collection is identified by a name
                      known as a role name.

  Policy Decision     The system entity that evaluates the policy
  and Enforcement     criteria published by a PAP, using attributes
  Point (PDEP)        supplied by a PIP to render decisions on requests
                      made by DRs. The PDEP is able to enforce its



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                      decision via the use of cryptographic keys.

  Policy Identifier   The tag that is used to identify a policy. For the
                      purposes of this document the focus is on two
                      different types of policy identifiers.  Object
                      Identifiers (OIDs) are what are currently used in
                      many security policy systems and are the only
                      method of policy identification supported by ESS
                      security labels.  Additionally URIs are supported
                      as policy identifiers  as they provide a more
                      user-friendly method to uniquely identify a policy
                      and allow discovery of the policy.

  Policy Information  A service which issues assertions, for example
  Point (PIP)         about a subject, their device, or environment
                      e.g., a LDAP directory or SAML Security Token
                      Service.

  Policy Label         The data structure which holds one or more policy
                      identifiers and their logical relationship.

  Pseudonym           A name that a person or group assumes for a
                      particular purpose, which differs from their
                      original or true name. (See Orthonym.)

  Role Token          A token issued to a subject containing one or more
                      Policy Collections. The role token is used as part
                      of policy discovery and management in Plasma. It
                      is not used as part of access control decisions in
                      any way.


2 Introduction

  The S/MIME standard [RFC5751] provides a method to send and receive
  secure MIME messages. S/MIME uses CMS[RFC5652] as the means to protect
  the message.  While CMS allows for many types of key exchange
  mechanisms to be used, S/MIME [RFC5750] exclusively uses X.509
  certificates [RFC5280] for the security credentials for signing and
  encryption operations.  S/MIME also uses an early binding mechanism
  for encryption keys where the sender needs to discover the public key
  for every recipient of an encrypted message before it can be sent.
  This requires the sender to maintain a cache of all potential
  recipient certificates (e.g. in a personal address book) and/or have
  the ability to find an acceptable certificate for every recipient from
  a repository at message creation.  This key management model has
  limited the use of S/MIME for encryption for a variety of reasons and
  is a major factor on the lack of adoption for S/MIME. The S/MIME key



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  management model is fragile For example:

  o  The recipient may not have an X.509 encryption certificate
  o  The sender may not have previously received a signed email with the
     recipient's certificate
  o  The recipient may not have an available repository from which to
     publish their certificate for senders to discover
  o  The sender may be unaware of the location of the recipient's
     repository
  o  The recipient's repository may not be accessible to the sender,
     e.g., it's behind a firewall
  o  The sender may not have a valid certificate path to a trust anchor
     for the recipient's certificate

  If one or more recipient certificates are missing, then the sender is
  left with a stark choice: send the message unencrypted or remove the
  recipients without valid certificates from the message.

  The use of secure mailing lists has the ability to provide some relief
  to the problem. The original sender does not need to know the
  appropriate encryption information for all of the recipients of the
  mailing list, just for the mailing list itself.  It can thus be
  thought of as a form of late-binding of recipient information for the
  originating sender.  However it is still early-binding encryption for
  the mail list agent; as it needs to perform all of the gathering and
  processing of certificate information for every recipient that the
  agent will relay the message to.

  In many regulated environments end-to-end confidentiality between
  sender and recipients by itself is not enough.  The regulatory policy
  requires some form of access control check before access to the data
  should be granted.  In many inter-organization collaboration scenarios
  it's impossible for the sender to satisfy the access checks on behalf
  of all recipients since they don't have, and frequently should not
  have access to, all the recipient's attributes because to do so may be
  a breach of the recipients privacy. Indeed to release the attributes
  to the sender may require that the sender's attributes first be
  released to the recipient's attributes provider.  It's a fundamental
  tenet of good security practice that users should control the release
  of data about themselves.

  ESS Security labels are an optional security service for S/MIME.  The
  ESS security label allows classification of the sensitivity of the
  message contents using a hierarchical taxonomy in terms of the impact
  of unauthorized disclosure of the information [RFC3114].  The security
  label can also indicate access control policy.  ESS security labels
  are authenticated attributes of a CMS signer-info structure in a
  SignedData object.  The label when applied to signed clear text data



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  provides the access control decisions for the plain text.  If applied
  to cipher text such as the outer layer of a triple wrapped S/MIME
  message the label is used for coarse grained optimization such as
  routing.

  ESS Security Labels have been found to have a number of limitations.

  1.  When the label is on the innermost content, access to the plain
      text is provided to the recipient (in some form) independent of
      the label evaluation as it will be processed for the purpose of
      hash computation as part of signature validation.  Depending on
      how a triple wrapped message is processed by the recipient's CMS
      code, the inner content may be processed for signature validation
      even before the outer signature is validated.  This would happen
      for a stream based CMS processor which starts processing inner-
      layers immediately rather than finishing processing of each layer
      and caching the intermediate results.

  2.  While labels cannot altered, they can be removed in transit.  If a
      signed layer is seen then it can be removed by any agent that
      processes the message (such as a Message Transfer Agent).  If the
      label is protected by an encryption layer then it can only be
      removed by any agent that has a decryption key (Encryption Mail
      List agents or Spam Filtering software would be two such
      examples).

  3.  Policies are identified by Object Identifiers.  This makes for a
      small tight encoding, but it does not provide any mechanism for an
      email client to discover how to enforce a access control policy if
      the message contains a policy the client is unaware of. This
      provides an impossible choice: ignore the access control policy
      and grant access to the message or block access to the message.
      Object identifiers also do not provide a good display name for a
      user so that they could manually find and download a new policy.

  4.  The current ESS standard only allows for a single policy label in
      a message; no standardized method of composing multiple policy
      labels together has been defined.  This is adequate for coarse-
      grained policy binding to express a limited set of choices such as
      with information sensitivity which typically provides a hierarchy
      of 3-5 choices. Many data sets need to be subject to multiple
      access control policies.  For instance, a message may contain
      information that is both propriety and export controlled.  Trying
      to represent combinations of policies via a single policy label
      would lead to an exponential growth in the number of policy
      labels.

  5.  ESS Labels do not provide for any robust auditing of who has been



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      granted access to the message.  All policy evaluation is local to
      the recipient's machine; no centralized logging of access to the
      message can be performed

  6.  The biggest issue with ESS labels is enforcement of the policy
      occurs on the recipient's machine; the compliance with the policy
      is dependent on the state of the configuration of every receiving
      agent.  The policy is enforced by whatever module is located on
      the user's system. For cross corporate systems, this means that
      the policy provided by Company A must be installed on Company B
      machines, or Company B must install a policy that Company A will
      accept as being equivalent to their own policy. Additionally, any
      time that a new version of the policy module is rolled out, there
      will be a time lag before every recipients machine will have the
      updated module.  This makes policy compliance practically
      impossible in anything but a small, closed environment.

  From a regulatory enforcement perspective, ESS labels are an extremely
  weak form of access control because cryptographic access to the data
  is given before the access check.  The correct enforcement of the
  access check is dependent on the configuration of every recipient's
  email client.  Since the cryptographic access is granted before the
  access policy check, there is no cryptographic impediment for a
  recipient who is able to decrypt the data but is unauthorized under
  the policy, to ignore the policy and access the data. A stronger
  enforcement model is needed for regulatory control for email where
  cryptographic access is only granted after the access check is
  successful.

  S/MIME today can only use X.509 certificates to protect the
  confidentiality or the data origin authentication and integrity of the
  messages. There are many users on the Internet today who have other
  forms of authentication credentials. This means the many users without
  X.509 certificates cannot use S/MIME. There have been many
  developments in authentication technology and best practices since
  S/MIME was developed over a decade ago, and example of which is SAML
  [SAML-core]. The critical difference between SAML and X.509
  certificates is that SAML abstracts the details of the authentication
  protocol from the protocol. The PIP can use a broad range of
  authentication mechanisms such as passwords, one-time passwords,
  biometrics, X.509 certificates, etc., to authenticate the subject
  without impacting the protocol. Adopting the abstraction abstraction
  model for S/MIME would enable almost anybody with any kind of
  authentication credential registered with one of the many identity
  provider on the Internet today to use S/MIME making it possible that
  S/MIME use may become as pervasive as TLS is today.

  There are many other non-email use cases which would be subject to the



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  same access policy requirements.  Email allows users to create content
  and distribute it to a set of recipients.  Similar use cases can be
  performed with other data formats or applications such as documents
  and instant messages.  Policy is tied to the information not the data
  format or application therefore if an organization has a policy
  relating to a type of information, then that same policy would apply
  to the same information in any form; email, document, or instant
  message. While some aspects of this work will be specific to email,
  there will be many which would be reusable in other areas.

3. Access Control Models

  Access control is the process whereby systems are able to decide
  whether to grant a request to access a resource from a subject. There
  are a number of models the system can follow to make the decision.
  These are two types of models, those based on a subject attributes and
  those based on a subjects capabilities. For models based on subject
  attributes, the system obtains a set of attributes about the subject
  then applies a policy expression using the attributes as input to the
  policy to determine the result. For model based on subject
  capabilities, the subject has an unforgeable token or reference to a
  token attesting to an access to a resource.

  The simplest model based on subject attributes is Discretionary Access
  Control (DAC) where attributes are the subjects group membership and
  the policy is expressed as an Access Control List (ACL) which  is a
  list of groups and grants (or deny's) access to individual groups. The
  list is evaluated sequentially, and the first match is the result.
  Role Based Access Control (RBAC) is a refinement of DAC where the roll
  is an abstract subject which is granted a set of permissions. The role
  used to simplify management, in essence it is a collection of groups.
  Attribute Based Access Control (ABAC), where policies are defined in
  terms of arbitrary attributes of the subject, their device or
  environment, their intended action on or use of the information. ABAC
  requires the definition of the policy in a policy expression language
  e.g. eXtensible Access Control Markup Language [XACML-core].  ABAC
  also requires a secure way to exchange arbitrary attributes e.g. via
  the Security Assertion Markup Language [SAML-core] or via an LDAP
  directory.

  SAML [SAML-core] defines an XML framework for describing and
  exchanging assertion tokens containing attributes.  The entity issuing
  the assertion tokens is a Policy Information Point. The entity
  consuming the assertion with the attributes is known as the relying
  party (RP).  The well-known scenarios for using SAML are:

  o  Single Sign On across systems on different platform technology




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  o  Federated Identity between business partners

  o  Web Services and other standards, e.g.,  -SOAP based protocols

  SAML tokens can be either Bearer Tokens or Holder-of-Key tokens.
  Bearer tokens have no cryptographic key and their security is based on
  the time between when the token was issued and time it was presented
  to the relying party together with the token being issued for use with
  the RP. Low value transactions can use Bearer tokens where possession
  of the token alone is considered acceptable for the transaction risk.
  Holder-of-Key tokens contain a cryptographic key (either public or
  symmetric) and like X.509 identity certificates the subject proves
  their identity to the RP by demonstrating control over the key e.g.
  signature or HMAC over some data. The RP can therefor have a stronger
  proof of identity by the demonstration of proof of possession of
  cryptographic keys. SAML can also be used to express attributes about
  a subject to a RP where the subject has authenticated to the RP by
  some means.

3.1 Generic Access Control Model

  The terminology defined in [RFC3198] uses a generic information model
  for the actors and the way they relate to each other.




























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                                       ------------------
                                       |                |
                                       |     Policy     |
                                       | Administration |
                                       |     Point      |
                                       |                |
                                       ------------------
    -----------------                          |
    |               |                          |
    |   Policy      |                          |  Read
    |  Information  |                          |  Policy
    |   Point       |                          |
    |               |                          |
    -----------------                          v
         |  |                                  v
         |  |                          -----------------
         |  |                          |               |
         |  |   Back end Exchange      |     Policy    |
         |  ------------------------->>|    Decision   |
         |  Issue                      |     Point     |
         |  Attributes                 |               |
         |                             -----------------
         |                                     ^
         |  Front End                          ^  Decision
         |  Exchange                           |  Request +
         v                                     |  Attributes
         v                                     |
     -----------------                 -----------------
     |               |    Request +    |               |
     |  Subject      |    Attributes   |    Policy     |
     |  Decision     | -------------->>|  Enforcement  |
     |  Requestor    |                 |    Point      |
     |               |                 |               |
     -----------------                 -----------------

                 Figure 1 Generic Access Control Model

  o  Administrators manage and publish policies using the PAP. The
     published polices are then available to the PDP
  o  A decision requestor sends a request together with their attributes
     to the PEP
  o  The PEP sends a decision request to the PDP together with the
     subject attributes
  o  The PDP obtains the necessary policy from the PAP
  o  The PDP can request additional attributes from the PIP
  o  The PDP returns the decision request to the PEP
  o  The PEP enforces the decision request




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  This generic model assumes the PEP has control over the data i.e. when
  it gets the permit decision it releases the data to the subject.  This
  works well is client-server situations like access to a web site or
  database where there is a clear trust boundary between the subject and
  the PEP with the data. However it does not work well with applications
  like email were there the data is delivered to the subject prior to
  the access check. The model need to be extended to allow the data to
  be encrypted and the access check be performed prior to release of the
  decryption key.

  A dependency in the model is the reliability of the policy selection
  for the request by the PDP. The implementation of the policy selection
  process can make either a closed or open world assumption. Closed
  world assumes the policy set on the PDP is complete therefore there is
  a policy in the store for every request. Open world assumes it is the
  policy store is incomplete and there is a need to discover new polices
  as appropriate.  Closed world implementations work when there is
  reasonable control over the sets of data managed by the PEP and
  policies known to the PDP. However they result in unreliable results
  with mobile data i.e. if I receive data from a partner and try to
  process it via my PEP and PDP.  There is no linkage between the
  distribution of the data and the distribution of he polices in closed
  world models.

  Access control models based on subject attributes depend availability
  of assertions with attributes about subjects. The model has PIP
  issuing attributes about subjects and RP consuming attributes about
  subjects. A subject can be a human, a device or service. The subject
  must have a relationship with the PIP since they have been through
  some form of registration process with the PIP. There is no
  requirement to have a relationship between the PIP and a RP. The RP
  must trust the PIP, but not vice versa. This is the same model as
  exists with X.509. The subject must have a relationship with the CA,
  the RP must trust the certificates issued by the CA, but there is no
  requirement for the CA to have any form of relationship or trust with
  the RP. Release of subject attributes to a RP must be under a policy
  due to the sensitivity of the data. The subjects themselves can
  request and give approval for the release of attributes from the PIP
  and relay them to the RP (Front End Attribute Exchange). If the
  subject has given prior consent, the RP may receive attributes
  directly from the PIP(Back end Attribute Exchange).  Subject
  attributes are potentially sensitive data as are similarly subject to
  access control. SAML has the capability to encrypt sensitive data in
  the token. The PIP would also develop policy to regulate the set of
  data it would release to a RP.

  The challengers for S/MIME is therefore




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  o   How to apply this generic access control model to the email
      scenarios so there is convergence with other applications i.e.
      email access control is not a one off vertical solution

  o   How to ensure the access check is possible prior to the recipient
      having access to the clear text so the access check is robust for
      regulators

  o   How to abstract the authentication credential technology use from
      the S/MIME protocol to enable use of the many forms of
      authentication in widespread use today on the Internet.

4 Use Case Scenarios

      This section documents some email based use cases that the new
      protocol aims to support. Also included are some related scenarios
      where the same underlying theme of consistent policy enforcement
      equally applies.

4.1 Consumer to Consumer Secure Email

      One of the issues that is stopping the use of secure email in
      personal mail is the fact that consumers find X.509 certificates
      difficult and expensive to obtain and then use - especially across
      a set of devices (phone, tablet, workstation). One of the possible
      use cases of Plasma is to try and deal with this by removing the
      dependency on X.509 certificates.  The details of the use case are
      therefore: Alice wants to send an email message to Bob that
      contains sensitive, personal data so she is concerted about
      ensuring only Bob can read it. Bob has a strong credential he can
      use to identity himself, but it's not an X.509 certificate.  Alice
      needs to ensure the following:

  (a)  Only Bob can read the email.
  (b)  Bob has the ability to verify the email is from Alice.
  (c)  Bob has the ability to verify the email message has not been
       modified since Alice sent it.

  The sequence of events could be as follows:

  1.   Alice composes the email to Bob.
  2.   Alice's email client allows here to classify the email.  Alice
       classifies the email as Personal Communication which is a policy
       provided by her ISP.
  3.   Alice's email client knows the protections to apply to a Personal
       Communication; it knows to encrypt and sign the message.
  4.   The protected email is able to flow securely and seamlessly
       through existing email infrastructure to Bob. The data is



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       protected while in transit and rest.
  5.   Bob receives the email and sees that it is a secure message.  Bob
       can verify that the secure message has not been altered. Bob
       attempts to open and decrypt the email.  If Bob is on the same
       ISP as Alice, then the same username/password as he uses to get
       his Email is used to obtain the needed keys.  If Bob is on an ISP
       that is federated with Alice's ISP then an infrastructure such as
       SAML, OpenID, OAUTH or ABFAB could be used to validate Bob's
       identity and allow the needed decryption keys to be released.

4.2 Business to Consumer Secure Email

  There are many examples of business-to-consumer secure email scenarios
  where the email could potentially contain sensitive medical or
  financial data. This would include doctor-patient; bank-account
  holder; Medical insurance-insured person and mortgage broker-customer
  communications. This example is illustrative of the many use cases for
  business to consumer email.

  A bank (The Bank of Foo) has determined that it will be using email to
  distribute statements to its customers (Bob).  The information is
  confidential, so any channel of communication the bank selects must
  protect Bob's privacy.  The bank needs to ensure the following:

  (a)  Only Bob (or additional owners of the account) can read the email
  (b)  Bob authenticates with a sufficient level of identity assurance.
       The same identity assurance authentication level used to do on-
       line banking would be considered sufficient
  (c)  Bob can verify the statement is from his bank
  (d)  Bob can verify the statement has not been modified since his bank
       sent it.

  The sequence of events would be as follows:

  1.   As part of routine end-of-the-month processing, the bank composes
       an email to Bob. They include the statement of balances and
       activity either as an attachment or as the body of the message.
  2.   The statement mailer for the Bank of Foo has been configured to
       apply a specific policy to the email.
  3.   The statement mailer for the Bank of Foo knows the protections to
       apply based on the policy; it knows to encrypt and integrity
       protect the message and what level of assurance is required for
       the recipient's identity.
  4.   The protected email is able to flow securely and seamlessly
       through existing email infrastructure to Bob. The data is
       protected while in transit and at rest.
  5.   Bob receives the email and sees it is a secure message from the
       Bank of Foo. Bob can verify the message has not been altered as



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       it is signed by his bank.  Bob uses the same credential as he
       would for on-line banking to prove his identity to the email
       system and obtain the keys necessary to decrypt the message.

  The same process could be used for any messages sent between the
  business or organization and and its customers.  Thus, messages
  dealing with loan applications and changes in bank policies can be
  sent out in the same manner, potentially using different policies.  In
  some of these cases it might be in the bank's interests to record in
  an audit trail if and when the keys were handed out on certain emails.
  For a statement, the bank would not expect a reply to occur, however,
  for other types of messages it should be possible for Bob to reply
  under the same level of protection.  Bob is able to use the same
  credential when sending or replying to a message from the bank, as he
  uses for accessing the bank's Web site then the bank has the same
  assurance of Bob's identity for all transactions.

4.3 Business to Business Ad-Hoc Email

  Early in the relationship between two companies, it is frequently
  necessary to exchange sensitive information as a preliminary to a more
  formal business relationship e.g., contract negotiations. This is
  similar guarantees to the security afforded by mail, i.e., you enclose
  a letter in a envelop which provides a level of security to the
  contents while in transit, there is a level of expectation that only
  the recipients or their delegate would open the envelope, once the the
  recipient has the letter, you trust them to treat the contents
  appropriately.

  As an example, Charlie works for Company Foo. He has just met Dave
  from Company Bar to discuss the prospect of a potential new business
  opportunity.  Following the meeting, Charlie wants to send Dave some
  sensitive information relating to the new business opportunity.
  Charlie trusts Dave to treat the information appropriately.  When
  Charlie sends the email to Dave with the sensitive content, he must
  ensure the following objectives:

  (a)  Only Dave or his delegate can read the email
  (b)  Dave or his delegate is required to authenticate with an identity
       assurance level 2 or above
  (c)  That Dave can verify the email is from Charlie
  (d)  That Dave can verify the email has not been tampered with
  (e)  Charlie may also need to keep a record of the fact that Dave
       accessed the message and when it was done.

  The sequence of events Charlie would use is as follows:

  1.   Charlie composes the email to Dave.  He include some sensitive



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       information relating to potential terms and conditions for the
       new contract that Foo and Bar would sign to form a partnership
       for the business opportunity.
  2.   Charlie's email client allows him to classify the email.  He
       classifies the email as an ad-hoc pre-contractual communication.
  3.   Charlie's client knows the protections to apply to ad-hoc pre-
       contractual communication; it knows to encrypt and integrity-
       protect the message and the level of assurance required for the
       recipients identity.
  4.   The protected email is able to flow securely and seamlessly
       through existing email infrastructure to the recipient (Dave in
       this case).  The data is protected while in transit and at rest.
  5.   Dave receives the email and sees it is a secure message from
       Charlie. (Charlie policy requires level 2 authentication for
       which, Dave uses a password). Dave is able to prove his identity
       to the level of assurance requested by Charlie so he is able to
       read the email. The organization Dave works for has an identity
       service which he uses to prove his identity for Charlie's email.
       Dave opens the email.

  If Dave or his delegate replies to the email from Charlie, the new
  message inherits the policy from the original messages so the entire
  message thread has the same policy.  The policy also applies to
  messages forwarded by Dave because it contains information from
  Charlie and Company Foo wants consistent policy enforcement on its
  information.

4.4 Business to Business Regulated Email

  As business relationships mature they often result in a formal
  contractual agreement to work together. Contractual agreements would
  define a number of work areas and deliverables. These deliverables may
  be subject to multiple corporate and/or regulatory policies for access
  control, authentication and integrity. Some classes of email may have
  information which is legally binding or the sender needs to
  demonstrate authorization to send some types of message where
  authority to send the message is derived from their role or function.
  Also many regulated environments need to be able to verify the
  information for an extended period - well beyond the typical lifetime
  of a user's certificate.  The set of policies applicable to an email
  is potentially subject to change as the different user's contribute
  information to the email thread.

4.4.1 Regulated Email Requiring a Confidentiality Policy

  Company Foo has been awarded a contract to build some equipment
  (Program X).  The equipment is covered by export control which
  requires information only be released to authorized recipients under



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  the terms of the export control license.  Company Bar is a foreign
  subcontractor to Company Foo working on Program X. Company Foo sets up
  some business rules for access to program X data to ensure compliance
  with the  export control license requirements.  Company Foo also set
  up separate rules to cover the confidentiality of its intellectual
  property contributed to Program X. Company Bar also sets up its own
  policies to protect the confidentiality of its own intellectual
  property it contributes to Program X. As part of the agreement between
  Foo and Bar, they have agreed to mutually respect each other's
  policies.

  Confidentiality policies can change over time. It is important to be
  able to implement the changes without the need to update the data
  itself to reflect the change as finding all instances of the data in
  an intrinsically impossible problem to solve.

  Frank is an employee of Company Foo. He has been assigned as a design
  team leader on Program X and as an individual contributor on Program X
  integration. Frank wants to send some email as a team leader to
  colleagues working on Program X in both Companies Foo and Bar.

  Grace is an employee of Company Bar. She has also been assigned to the
  design team of Program X.

  When Frank sends the email with Program X regulated content he must
  ensure compliance with the export control policies. When Frank sends a
  Program X email he must ensure recipients are authorized to read the
  contents to ensure Company Foo remains in compliance with its export
  control license.

  If Frank also includes Company Foo intellectual property in an email,
  he must also ensure recipients are authorized to read the
  intellectual property contents.

  When Grace receives a Program X email, she must provide attributes
  about herself to prove compliance with the export control policy. If
  the email also contains Company Foo intellectual property, she must
  also provide attributes to show she is authorized to read the
  information under the agreement between Company Foo and Company Bar.
  Grace would not know the complete set of attributes, so would start
  with a basic set to identify herself. The PDEP may be able to discover
  more attributes about Grace, and if it is still missing some, it can
  request those from Grace.

  If Grace sends an email with Company Bar intellectual property, she
  must ensure recipients are authorized to read the contents under the
  agreement between Company Bar and Company Foo.




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  When Frank sends a Program X email he must ensure the following
  objectives:

  (a)  Only recipients who meet the Program X export control policy
       and/or Company Foo's intellectual property protection policy can
       read the email.
  (b)  Recipients authenticate with a identity assurance level 3 or
       above.
  (c)  Recipients present all other attributes about themselves
       necessary to verify compliance with the applicable policies
       (their program assignment, nationality, professional or industry
       certifications, etc.).
  (d)  Recipients can verify the email is from Frank to the level of
       identity assurance as defined by the message policy (i.e., level
       3 or above).

  (e)  Recipients can verify the email has not been tampered with to the
       level of identity assurance as defined by the message policy.
  (f)  Recipients are made aware that the message is a Program X email
       (and the contents can only be shared with other Program X
       workers) and/or the message contains Company Foo's intellectual
       property.

  The sequence of events Frank would use is as follows:

  (1)  Frank composes the email and includes a Program X distribution
       list as a recipient. He include some information related to
       Program X. Frank also includes some information which is Company
       Foo's Intellectual Property.
  (2)  Frank's email client allows him to select the Program X role. The
       client then allows Frank to select from a set of policies
       appropriate for Program X.
  (3)  Frank selects the Program X content and Company Foo IP policies
       from the list of available policies.
  (4)  The email client knows to encrypt the message, the key size and
       algorithm to use. It also knows that the message needs to be
       signed with a level 3 or above private key.
  (5)  Frank clicks the "send email" button. The client signs the email
       using his smart card private key and includes the certificate
       with the appropriate public key for verification of the signature
       by recipients. The Client then encrypts the message and obtains
       data from a server that will enforce the access control
       requirements for Frank, and sends it to his email server.

  The email is able to flow securely and seamlessly through existing
  email infrastructure to recipients of the distribution list. Grace is
  on the distribution list so she receives the email from Frank.




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  (6)  Grace receives the email. Grace's client provides the attributes
       necessary to comply with the policy which includes her level 3
       encryption certificate to the PDEP.
  (7)  Once Grace has shown she passes the policy requirements, the PDEP
       releases the message CEK to Grace using her level 3 encryption
       certificate.
  (8)  Grace uses her smart card to open the message. She sees the
       message is signed by Frank and marked with both the Program X and
       Company Foo IP policies

  If Grace replies to the email from Frank, the new message inherits the
  policy from the original message.  If Grace includes some information
  which is Company Bar's IP she also adds her company's IP protection
  policy requirements to the message.

  Frank receives the reply from Grace.  Frank is able to prove his
  identity to the level requested by Grace and provides the requested
  attributes about himself to satisfy both the Program X export control,
  the Company Foo IP protection policies, as well as the Company Bar IP
  protection policies.  Frank opens the email.

  The policy also applies to messages forwarded by Frank and Grace
  because they contain information from Company Foo and Company Bar and
  both companies wants consistent policy enforcement on their
  information.

  After some time, Company Bar fails an audit to show they are complying
  which all the requirements for Program X. As a result, Company Foo
  updates its policies for Program X to remove Company Bar as an entity
  approved to access Program X data. Grace will no longer be able to
  receive CEKs for Program X email as she can no longer satisfy the
  Program X policy requirements.

4.4.2 Regulated Email Requiring an Integrity Policy

  Company Foo has been awarded a contract to build some equipment
  (Program X). This equipment is regulated by the National Aviation
  Authority (NAA) that has oversight of Company Foo.  The NAA requires
  strict procedures at a number of significant events for Program X such
  as in the design and maintenance of the Program X (e.g., when a design
  is complete and released to manufacturing). The sign-off process
  requires personal be suitability qualified and that the documentation
  needs to be maintained for the service life of the project (25 years
  for Program X).

  Company Foo has instigated an email-based sign off procedure to
  simplify sign-off and reduce costs. It also has authored a policy for
  compliance with the NAA requirements. At the appropriate time, signoff



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  email is sent to the designated program members. Recipients apply the
  NAA policy  when they reply to the sign-off request message.

  Frank is the lead on the Program X design team. They have a design
  which they believe can be released to the integration team. Frank
  initiates the sign-off process for the design.

  Grace is one of the sign-off design team members for Program X. She
  receives the sign-off email. Grace responds and applies the sign-off
  signature policy to the email. The policy requires Grace to
  authenticate with the required level of assurance,  present attributes
  about herself, her work effort assignments and professional
  qualifications to demonstrate compliance with the policy to send the
  message. The message is signed to indicate Grace met the policy. It is
  a matter of the LoA of the sign off process if Grace signs fist,
  followed by the policy compliance signature or just the policy
  compliance signature which attests that Grace initiated the process.

  When Frank initiates a Program X sign-off email the system must ensure
  the following objectives:

  (a)  Frank was authenticated to the level of identity assurance
       required under the policy to initiate the sign-off process.
  (b)  Frank possessed the necessary attributes as required by policy to
       initiate the sign-off process.
  (c)  The contents of the email are accurate to the level of integrity
       assurance required by the policy.
  (d)  Frank was fully aware and intended to initiate the sign-off
       process.
  (e)  The state of Frank's system was known to the level of assurance
       required under the policy to be free from agents which might
       interfere with the sign off process.
  (f)  Recipients can easily confirm over the lifetime of the design as
       required by the policy that the sign-off process met the policy
       without having to know the specifics of what the policy
       entailed.

  The sequence of events Grace would use is as follows:

  (1)  Grace receives the sign-off request email.
  (2)  Grace replies to the email and completes the form data in the
       email to show she is approving the sign-off.
  (3)  Grace clicks the send button to send the email.
  (4)  Grace receives a sign-off confirmation dialogue before the email
       is sent where she is able to confirm her intent is to approve the
       sign-off of the component.

  Grace's system submits the decision request to send the sign-off



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  email. Her system is asked to provide attributes about Grace, the
  state of her system and the data being authenticated as part of the
  decision request. Grace would not know the complete set of attributes
  required to submit and would start with a basic set to identify
  herself. The PDEP may be able to discover additional attributes about
  Grace, and if is still missing some, can request those from Grace. If
  Grace's request meets the policy, her system receives a signed
  statement that the message meets the policy which is attached to the
  email and the message sent.

4.5 Delegation of Access to Email
  There are a number of times when others are given access to a
  recipient's mailbox or email is forwarded to other recipients based on
  the original recipient's rules. This may be a long-standing
  relationship such as when an assistant is given access to an
  executive's mailbox. Alternatively, it may be a temporary relationship
  due to short-term needs (e.g., to cover for a  vacation).  There are
  also organizational role mailboxes where the recipient is a role and
  one or more users are assigned to the role.

  Grace is going on vacation. While Grace is away, Brian will act as a
  delegate for Grace. Grace configures a mailbox rule to forward Program
  X email to Brian for the duration of her vacation. Brian is able to
  satisfy the policy requirements for the Program X email as outlined
  above and is therefore able to open the protected email sent to Grace.
  Frank does not need to take any actions to allow Brian to access the
  email.

4.6 Email Compliance Verification
  Verification is an essential part of compliance. Verification may be
  conducted by internal staff or external auditors. The verification
  need to confirm that the policy rules are being enforced.  Auditing
  relies on the generation of artifacts to capture information about
  events. Typically, this is done via some form of logging. A challenge
  here is that for distributed system, the set of logs which completely
  describes the transaction are scattered across many systems so
  consistency of the audit settings and correlating all the audit data
  is problematic. Another consideration is accurately capturing only the
  set of desired data, i.e., accurately targeting the set of events that
  needs to be logged

  Jerry is the compliance officer for Company Foo. He has a procedure
  for ensuring compliance for Program X. The procedure defines what to
  log and when to audit access to Program X data. Jerry has tools to
  collect the audit data and run an analysis to verify the polices are
  being followed.

  The sequence of events Jerry would use is as follows:



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  (1)  Jerry configures an audit obligation for access to Program X
       data. The obligation defines the set of attributes to capture
       when Program X data is accessed. The obligation is part of the
       Program X policy. Part of the Program X policy is the set of
       PDEPs which can process policy decisions on Program X data.
  (2)  Jerry configures his audit log collection to download Program X
       audit log entries from the designated PDEPs.
  (3)  Jerry also has an audit confirmation tool which "pings" the PDEPs
       for access to Program X data. Jerry's audit log analysis tool
       looks for these pings to confirm that auditing is taking place as
       expected.

4.7 Email Pipeline Inspection

  Organizations have a huge incentive to inspect emails entering or
  leaving the organization.  Such inspection is desired for many
  different reasons.  Inspection of mail leaving an organization is
  targeted towards making sure that it does not leak confidential
  information. It also behooves organizations to check that they are not
  a source of malicious content or spam.  Inbound mail is checked
  primarily for malicious content and phishing attempts as well as spam.
  For domains with a high volume of messages there is a strong need to
  process email with minimal overhead. Such domains may mandate that
  they be pre-authorized to process an email due to the overhead a per-
  message request to an external service would add to message
  processing.

  Company Foo has a policy to scan all inbound and outbound email to
  ensure it is free from malware. Company Foo also wants to ensure email
  is not spam. Company Foo can own their scanning servers or such checks
  may be outsourced to a third party service.  Company Foo wants to
  ensure that its policy of scanning message contents also applies to
  encrypted email.

  The ability to decrypt and check the message content for malicious
  content is highly desirable. There are a number of methods that can
  accomplish this:

  1.   When a Company Foo client requests to send a Plasma email, the
       PDEP is able to check to see if the policy allows email content
       inspection by MTA for this policy, and if it does, that Company
       Foo has an outbound email scanning, and that the scanning servers
       meet the policy requirements. It is able to pre-authorize the
       Company Foo email scanning servers to access the email.
  2.   The scanning MTA authenticates to the PDEP as an entity doing
       virus and malware scanning on a protected message.  If the PDEP
       has specific policy that allows for access to such a scanning MTA
       service, the appropriate decryption keys will be released and the



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       server will scan the mail and take appropriate action.
  3.   The policy server is configured with information about various
       gateways (both internal and external) and has certificates for
       the known gateways.  The policy server can then return a normal
       X.509 recipient info structure (cryptographic lockbox) to the
       sender of the message for direct inclusion in the recipient info
       list of the message.  This allows normal S/MIME processing by the
       scanning MTA without the necessity to query the PDEP server for
       keys for specific messages.
  4.   If the scanning MTA server cannot gain access to the decrypted
       content using one of the two proceeding methods, it either passes
       the encrypted mail on to the recipient(s) without scanning it or
       it rejects the mail.  This decision is based on local policy of
       the scanning MTA.  If the message is passed to the recipient(s),
       then the necessary scanning either will not be done, done by a
       downstream MTA,  or done on the recipient's system after the
       message has been decrypted.

4.8 Distribution List Expansion

  A distribution list (DL) is a function of an MTA that allows a user to
  send an email to a group of recipients without having to address all
  the recipients individually. The membership of the DL may be
  confidential so the sender may not know all the recipients. The DL may
  be maintained by an external organization. Since a DL is identified by
  an email address, the user may be unaware they are sending to a DL.

  Plasma polices may have the list of recipients as a parameter, thus
  the fact that the message is being process by the distribution list
  means the MTA processing the message needs to update the policy to
  allow the new recipients to access the message. Organizations may also
  require inbound scanning of email and have thus published keys to
  enable pre-authentication of the MTA by the sender to expedite
  processing. For both scenarios the DL MTA has to notify the Plasma
  server that it is adding recipients to the message and supply the list
  of new recipients. The Plasma server can then take appropriate action
  on the message token and return an updated token if required.

4.9 Scalable Decision Making

  Collaboration involves working with external organizations; e.g.,
  partners and suppliers. These collaborations may be short or long-
  lived, with a small or very large number of participants.
  Organizations therefore need flexibility in deployment and scaling.
  Organizations do not want to be forced into having to provide capacity
  themselves for all decision making over their data. Senders would be
  happy to delegate decisions where appropriate to partners or external
  services provided those decisions use the rules they define for their



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  data. Likewise, recipients might be happy to leverage their local
  decision capacity providing they don't have to duplicate the rules of
  the partners, and can simply and easily use policies published by
  their partners. An organization may also want to use cloud based PDEPs
  where appropriate as a cost effective way to add capacity and to be
  able to respond to transient capacity fluctuations.

  See section 3.4.1 for a description of the scenario.

  The Program Managers for Program X at Companies Foo and Bar agree to a
  series of roles which are used to manage personnel and their assigned
  policy groups. The policy administrators for Company Foo and Bar
  respectively publish the roles and a policy collection for each role.
  There are rules associated with the policy collection, for example
  every roles uses the Program X policies published by Company Foo.
  Employees from Company Foo also get the Company Foo Intellectual
  Property polices for those roles, whereas employees from Company Bar
  get the Company Bar intellectual property polices for Program X.
  Company Foo has also decided to allow enforcement of Program X
  policies by decision engines in both Company Foo and Company Bar.
  Company Foo has also decided to use a cloud-based decision engine for
  Program X to allow lower cost capacity and scaling. Company Foo is
  able to add new instances of the cloud-based decision services as the
  program scales up and more uses start working on the program. Each
  decision engine dynamically discovers the policies it needs from the
  set published by Company Foo and Company Bar. Both Company Foo and
  Company Bar can add new polices to the policy collections at any time
  and they are dynamically discovered by all the policy decision
  engines.

5 Plasma Security Model

  A common theme from these scenarios is the need to closely tie the
  information asset to the set of technical controls via the data
  owner's policies in such a way so it is possible to consistently apply
  the technical controls across a broad set of applications (not just
  email), for a broad set of users (not just those within an
  organization), and in a broad set of environments. Assumptions based
  on closed-world, enterprise security models are increasingly breaking
  down. Perimeter security continues to diminish in relevance and focus
  needs to be shifted to self-protecting data as opposed to protecting
  the machines that stores such data. The binding between the data and
  the applicable polices needs to happen as close to the data creation
  time as possible so ad-hoc trust decisions are not required.

  The delivery of the documented use cases will require the integration
  of many existing and some new protocols. In order to ensure the right
  overall direction for Plasma as each part of the work proceeds, a high



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  level data model is documented here to  act as a guide. While this is
  technically informative to the developments of each individual
  component, it is normative to the work overall.

  This Data Centric Security model is based on a well-established set of
  actors for policy enforcement used elsewhere [RFC3198] [XACML-core].

  Figure 2 shows the relationship between the actors.

                            ------------------
                            |                |
                            |     Policy     |
                            | Administration |
                            |     Point      |
                            |                |
                            ------------------
                                     |
    -----------------                |               -----------------
    |               |                |               |               |
    |   Policy      |                |  Read         |   Policy      |
    |  Information  |                |  Policy       |  Information  |
    |   Point       |                |               |   Point       |
    |               |                |               |               |
    -----------------                v               -----------------
         |  |                        v                       |  |
         |  |Issue          -----------------      Issue     |  |
         |  |Attributes     |               |      Attributes|  |
         |  |(BAE)          |     Policy    |      (BAE)     |  |
         |  -------------->>|    Decision   |<<---------------  |
         |                  |      and      |                   |
         |                  |  Enforcement  |                   |
         |  -------------->>|     Point     |<<-----------      |
         |  |Protect        |               |  Consume   |      |
         |  |Content        -----------------  Content   |      |
         |  |Request+                          Request+  |      |
         |  |Attributes                        Attributes|      |
         |  |(FAE)                             (FAE)     |      |
         v  |                                            v      v
         v  |                                            v      v
     -----------------                               -----------------
     |               |                               |               |
     |   Content     |           Distribute          |   Content     |
     |  Creation     |           Content             |  Consumption  |
     |  Decision     | ---------------------------->>|  Decision     |
     |  Requestor    |                               |  Requestor    |
     |               |                               |               |
     -----------------                               -----------------
 Figure 2 General Scheme for Publishing and Consuming Protected Content



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  The Plasma model is applicable to any type data (email, documents,
  databases, IM, VoIP, etc.). This is to facilitate consistent policy
  enforcement for data across multiple applications.  Another objective
  is to not require the data holder to have access to the plain text
  data in order to be able to make decision requests to the PDEP. The
  policy decision is complex so the content creation DR in Plasma just
  uses policy pointers or labels to indicate the set of policies
  applicable to content. The content consuming DR dynamically discovers
  the PDEP's that are authoritative for the decisions on protected
  content in question. The PDEP's dynamically discover the specifics of
  a policy from a PAP using the policy references. The specifics of
  policy authoring and policy decision logic modules are matters beyond
  the scope of this document. It is important to note that the actors in
  this model are logical entities and as such can be combined physically
  in different configurations.

  o    The Plasma model uses references to bind the data and the policy.
       When information is created, it is encrypted and a list of
       policies that must be enforce by the PDEP is bound to the
       protected data.
  O    The Plasma model includes policy discovery capability for
       subjects. This enables subjects to interact with one or more PDEP
       to discover the set of polices the set of polices each PDEP would
       permit the subject to use to protect new content. ?The PDEP
       issues a role token to subject which contains one or more policy
       collections. Each policy collection is identified by a role name.
       Subjects can pick any combination of polices from a policy
       collection, but cannot mix polices from different policy
       collections. The token issued to subjects containing the policy
       collections is known as a role token.
  o    The Plasma model is an Attribute-Based Access Control (ABAC)
       model where the ABAC policy is specified in terms of a set of
       attributes, their values, and their relationships. The policy may
       specify attributes about the subject, their device, or their
       environment, or attributes about a resource.
  o    The ABAC policy does not require the subject provide their
       orthonym.  Subjects could be anonymous or pseudonymous. What is
       required is the presentation of a set of attributes that
       satisfies the policy.
  o    The subject can be required to bind the supplied attributes to
       the channel with the PDEP to a level of assurance as required by
       the PDEP. If the PDEP only requires low assurance, bearer tokens
       over TLS would be suitable. If the PDEP requires higher
       assurance, then the holder of key tokens over TLS would be
       required where the token key is bound to the TLS channel.
  o    This model also supports Capability-Based Access Control (CBAC)
       where security tokens represent a capability to meet a policy.
       Once a subject has proven compliance with a policy, they can be



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       issued a capability token. The client can subsequently  present
       this capability token in lieu of a token or tokens with the set
       of subject attributes.  The net result is that the model can
       transition to a Capability-Based Access Control because the
       capability token is an un-forgeable token of compliance with a
       policy. The token can be used with any resource tagged with the
       same policy.
  o    Plasma has a baseline of a secure transport between the DR and
       the PDEP. One of the decisions the PDEP has to make is the level
       of assurance on the release of the CEK to the subject. For
       example, the PDEP can release a clear text CEK over the secure
       transport to the DR. Alternatively, the PDEP could require the
       production of a high-assurance X.509 encryption certificate as a
       subject attribute to generate an encrypted CEK.

  For the purpose of the Plasma work, it is desirable that the DR and
  PDEP be clearly defined as separate services which may be on separate
  systems.  This allows for a generalization of the model and makes it
  less dependent on any specific deployment model, policy representation
  or implementation method. It also allows for a greater degree of
  control of the PDEP by an organization such that it is possible to
  keep all of the PDEP resources directly under it's control and
  independent of the data storage location.

  The base set of information for a Plasma client is as follows:

  o  The address of one or more IdP(s) able to issue identity attributes
     to the subject
  o  A means to authenticate to the IdP(s)and issue attributes to the
     subject
  o  The address of zero or more AtP(s) able to issue additional
     attributes to the subject
  o  The address of one or more Plasma PDEPs able to issue role tokens
     to the subject to initiate Plasma policy discovery.

  From this base set of data, the subject is able to authenticate to
  each Plasma PDEP in turn using the identity token from the IdP and
  discover the set of assigned roles. Each role has a set of policies
  which can be applied to data. A subject may be assigned to multiple
  roles and therefore has the ability to select the most appropriate
  role for the content being created. Once a role is selected, the
  subject is able to choose one or more policies from the policy
  collection for that role. Role assignment is dynamic so the role
  discovery needs to be done on a regular (but not frequent) basis.
  Policy selection during content creation can be either manual or
  automatic. A DR may have sufficient context to be able to select the
  role and policies for the subject or have some rules that facilitate
  policy selection.



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  The model allows the content creation DR to discover the role
  assignments from multiple PDEP which would allow the subject to access
  policies based on roles from within their organization and from any
  partner organization due to cross-organization collaboration. The
  PDEP's that are authoritative for the role assignment for a subject
  may be different from the PDEP that are authoritative for enforcement
  of a policy collection in question. The DR uses the role token to
  authenticate the content creation request. The PDEP will check that
  the requested list of policies for the information is a subset of the
  policies in the role token. If the set of policies is a subset of the
  policies in the role token, then it will issue the policy metadata
  token to be attached to the protected data.

  The policy metadata token is an signed data structure created by the
  PDEP which is bound to the protected data. It contains public policy
  metadata attributes which are used by the DR. An example of a public
  policy metadata attribute is a list of one or more URLs which
  represent the PDEPs that can make policy decisions using the policy
  metadata token. The DR can submit the decision request to any PDEP in
  the list. The policy metadata token also has a confidential payload
  containing private policy metadata attributes used by the PDEP to make
  policy decisions. An examples of a confidential policy metadata
  attribute is the list of CEKs for the protected data which would be
  released to the DR if it passes the policy checks.

  Policy rule processing and distribution is complex, so the Plasma
  model does not require policy rules to be distributed to the DR. The
  DR submits the policy metadata token as part of the decision request.
  The confidential portion of the policy metadata token contains a logic
  tree of policy references. The PDEP uses the policy references to
  discover the policy rules to apply to the request.  The logic tree
  defines the relationship between the polices. The tree has a series of
  nodes where each node represents a set of polices and the relationship
  for the polices at the node e.g., are they combined via and AND clause
  or an OR clause. The pinnacle of the tree represents the decision from
  all the polices in the tree. The use of policy references minimizes
  any policy maintenance issues relating to the protected data due to
  policy updates. The policy rues can be updated and the new rules
  discovered on subsequent decision requests.

  The DR and PDEP are required to carry out obligations of the policy
  such as specific encryption requirements, e.g., key size or algorithm,
  data integrity requirements, time-to-live of the CEK, or  audit record
  creation requirements. It is a matter for the policy on how to
  determine if the DR or PDEP is trusted to carry out the obligations.
  This could be achieved by devise type and state attributes.

  The PDEP makes its decisions based on the requested action from the



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  DR, the policy requirements from the PAP(s), and the information from
  the PIP(s) about the subject, the subject's device, and the subject's
  environment. The information about the subject may be exchanged
  directly between the PIP(s) and the PDEP (Back End Attribute Exchange)
  or indirectly via the DR (Front End Attribute Exchange) or both. The
  content creator can also include attributes in the policy metadata.

  There is no guarantee that identity and attribute providers will
  consistently use the same name to identity a specific attribute or
  attribute data. For example they may use different schemas to identify
  an email address or use localized names to describe job functions or
  roles. These kinds of values may be standardized within communities of
  interest, but not globally across all identity and attribute
  providers. Therefore it is necessary to canonicalize the attribute
  names and values before processing by the policy. The attribute name
  and value mapping is part of the policy data set, i.e., it is in
  addition to the policy processing rules.


































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    ---------------          -----------------         -----------------
    |             |          |               |         |               |
    |             |          |   Policy      |         |  Policy       |
    |  Policy     |          |  Decision and |         |  Decision and |
    | Decision    |          |  Enforcement  |         |  Enforcement  |
    |  Point      |          |   Point       |         |  Point        |
    |             |          |               |         |               |
    ---------------          -----------------         -----------------
           |                         |                         |
           |                  T      |      T                  |
           |                  TTTTTTT|TTTTTTT                  |
           V                         V                         V
           V                         V                         V
    ---------------           ---------------           ---------------
    |             |           |             |           |             |
    |  Policy     |           | Decision    |           | Decision    |
    | Enforcement |           | Requestor   |           | Requestor   |
    |  Point      |           |             |           |             |
    |             |           |             |           |             |
    ---------------           ---------------           ---------------
           |                         |                         |
    T      |     T                   |                         |
    TTTTTTT|TTTTTT                   |                         |
           V                         V                         V
           V                         V                         V
    ---------------           ---------------           ---------------
    |             |           |             |           |             |
    |  End        |           |  End        |           |  End        |
    |  User       |           |  User       |           |  User       |
    | Application |           | Application |           | Application |
    |             |           |             |           |             |
    ---------------           ---------------           ---------------
          (a)                        (b)                      (c)

             Figure 3 Options For Trusted Actors with Data.

  When drawing a line where the actors in the model are full trusted
  with the clear text data there are three possibilities (see figure 2).

  Figure 2a shows the full trust line between the user application and
  the Policy Enforcement Point(PEP). This is the model for current
  standard access control mechanism, e.g., XACML [XACML-core]. In 2a,
  the PEP has full access to the plain text data. It makes decision
  requests to the PDP and if the decision is affirmative, allows the PEP
  to release the data to the application. To use figure 2a for secure
  email would require every MTA and MUA to be fully trusted with plain
  text data which is impossible.




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  Figure 2b shows the full trust line between the PDEP and the DR. In
  2b, the DR only has cipher text data. The data is encrypted with a
  content encryption key (CEK) and the PDEP has access to the CEK. The
  PDEP releases the CEK to the end-user application when access is
  granted so the application can recover the plain text. This mode is
  viable for secure email as it does not require the MTA to be trusted
  with the plain text data and either the MTA or MUA can act as a DR.

  In figure 2c, no actor is given full trust. When the data is
  encrypted, the CEK is encrypted for each recipient just as S/MIME does
  today. The encrypted CEKs are given to the PDEP and the PDEP releases
  the encrypted CEK when access is granted. This mode is also viable for
  secure email as the sender can use either conventional public key
  cryptography or Identity-Based Encryption[RFC5408] to protect the CEK
  for each recipient.

5.1 Plasma Client/Server Key Exchange Level of Assurance

  There are a number of mechanisms by which a client and servers can
  exchange CEKs. As a baseline, Plasma is establishing a secure
  transport between the client and server via TLS. However the client
  may be a proxy acting on behalf of the subject, therefore transporting
  a clear text CEK over the TLS transport would expose the key to the
  proxy. There also may be a proxy at the server which is terminating
  the TLS transports and forwarding the requests to another server which
  would mean a clear text CEK sent over the transport would be exposed
  to the server proxy. Policies may require a higher level of assurance
  that the CEK is not exposed to unauthorized principals. This requires
  encrypting the CEK for the subject before transport. This would
  further require the client or the server to provide a public key to
  the other party to be used to protect the CEK before sending it over
  the secure transport.

5.2 Policy Data Binding

  There are three ways to bind policy to data:-

  o  By value. This is where a copy of the machine-readable rule set is
     directly associated with the data, e.g., where a file system has an
     Access Control List for the file or directory, or where a rights
     management agent embeds a copy of the policy expressed in a policy
     expression language in the rights-protected data. When an access
     request is made to the data, the PDEP compares the access request
     to the policy on the data itself.

  o  By reference. This is where a reference to the policy is directly
     associated with the data, e.g., a URI or a URN which identifies the
     policy to be enforced or points to where the policy is published.



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     For example with S/MIME, the ESS label identifies the applicable
     policy by an OID. When an decision request is made to the data, the
     PDP finds the policy based on the identifier and then compares the
     access request to the referenced policy.

  o  By inference. This is where the policy has a target description in
     terms of resource attributes the policy applies to. When a decision
     request is made, a set of attributes describing the resource which
     is the subject of the decision request is included in the request
     by a PEP. The PDP then compares the resource attributes to the set
     of target descriptions of the policies in its policy store to
     determine the set of policies to apply to the request. For example
     when an XACML policy is authored, a target description in terms of
     the attributes of the resource for the policy is also defined. When
     an XACML decision request is made, the PDP finds the policy set to
     apply to the request by matching the set of attributes in the
     request against the target description associated with the policies
     in its store. It then processes the decision request using the
     identified policy set.

  The chief strength of binding policy by value is its simplicity. The
  policy, being local to the data, can easily and quickly be read by the
  PDP. The chief weakness in binding policy by value is maintaining
  policy over time as binding by value results in the policy being
  replicated for every instance of data the policy is applied to. Many
  policies have a multi-year life span and over the course of time,
  there is a very high probability that the policy would need to be
  updated. Given the high number of copies, updating a value-bound
  policy has proven to be a very costly and imperfect process both from
  an enforcement and audit perspective. This process is complicated by
  the fact that because only the result is stored and not an identifier,
  it is hard to identify the policy that has to be updated.

  The chief strength of binding by names is that once the policies are
  bound to the data, the same policies continue to be applied regardless
  of PDEP configuration or state. These policies may change their rules
  over time, but there is no doubt which policies would be enforced on
  the data. Another strength of binding policy by reference is it has a
  clear result as to the set of policies the PDEP has to apply. It the
  PDP does not have a policy, the reference allows the PDEP to discover
  the missing policy. If the PDEP is unable to access a policy for
  whatever reason, it knows to fail the decision request with a
  different error, i.e., "don't know", which means the DR can reasonably
  try other PDEPs. The chief weakness in binding by name is adding or
  removing policies requires updating the policy metadata. Adding or
  removing policies has the same difficulties as maintaining policies by
  value.




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  The chief strength of binding by inference is it can often be applied
  to data without impacting the storage format providing the data
  already has rich and well defined set of metadata such as the
  structural metadata of an SQL table. It also allows new policies to be
  applied to the data without updating the metadata.  Unstructured data
  such as documents have the ability to store metadata but the challenge
  here is what metadata to capture. The nature of the metadata is also
  context specific, e.g., the policy target description required to
  match structural metadata from an SQL query would be different from
  the policy target description for matching content metadata for a
  document. The chief weakness in binding by inference is the
  reliability of the matching of the metadata to the policy target
  description. There are a number of factors which affects the policy
  matching process:

  *   The set of available metadata varies with different data types
      which makes the policy target definition more complex, e.g.,
      structured data such as SQL databases have structural metadata
      whereas unstructured data such as documents have content metadata.
  *   There is a relationship between the metadata you need to capture
      and the policies you need to enforce. It's therefore hard to
      generalize the rules for what metadata is necessary independent of
      knowing what metadata policies require.
  *   The resultant set of policies to enforce for a decision request is
      dependent on the PDP having a complete the set of policies. It is
      impossible,  however, to detect missing policies based on the
      request. Likewise, it is also impossible to detect if erroneous
      policies have been selected based on the request.  If data moves
      from store to store and thereby uses a different PDPs, it's
      impossible to determine the correctness of the result of the
      policy matching process by the new PDP.

      The Plasma model is choosing to use binding by name for two
      reasons:

  1   The overarching need to consistently enforce the policies selected
      at creation time over the lifetime of the data. The typical use
      case is that the set of policies to be enforced on the data may
      change their rules over time but it is the same set of policies
      that are enforced over the lifetime of the data.
  2   Data in many cases is mobile and travels between users and
      organizations. Any dependency on consistency of the decision
      making entity would be difficult to enforce or verify.


5.3 Content Creation Workflow

      The content creation DR bootstraps itself via the following



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      sequence of events:

  (1)  The content creation DR is configured with the set PIPs and PDEPs
       it trusts.
  (2)  The content creation DR submits a request for a role token to all
       the trusted PDEPs. The role token defines the set of roles the
       PDEP allows for the subject. The subject is authenticated and the
       contents of the role token authorized by the PDEP via attributes
       from the PIP(s). The PIP attributes can be obtained by the PDEP
       either via front-end (relayed to the PDEP from the PIP via the
       subject) or back-end (direct exchange between the PDEP and the
       PIP) processing.
  (3)  The content creation DR receives zero or more roles tokens from
       each of the PDEP. Each role token has a one or more policy
       collections defining the set of allowed policies for that role
       when creating new content.

  the DR is now initialized with a list of roles and role tokens. It is
  now ready to create content and request protection of that content
  from PDEPs. This role token request process would typically be
  performed as part of the application initialization process. Role
  tokens can be cached to reduce the number of times the application has
  to invoke the role token request process. When the user wants to
  create new content, they use the following sequence of events:

  (i)   The user creates the new content
  (ii)  The user selects the appropriate role for the content, then
        selects one or more policies from the policy collection that are
        applicable to the content. When the content creation process is
        complete, the DR:
  (iii) Encrypts the content with one or more locally-generated CEKs
  (iv)  Submits a policy metadata token request to the PDEP together
        with the CEK(s), the set of required policies to be applied, the
        role token from the PDEP, and the hash of the encrypted content.
        The CEK(s) in the request can be either raw key(s) or CEK(s)
        encrypted by a KEK if the policy does not allow the PDEP to have
        the ability to access the plain text data.
  (v)   The PDEP verifies the set of requested policies is a subset of
        the policy set in the role token.  In addition to the role
        token, the PDEP may also require  any other attributes from the
        subject as defined by policy to process the creation request.

  If the request satisfies the policy requirements, the PDEP generates
  the encrypted policy metadata which contains the list of policies and
  the CEKs. The metadata is encrypted by the PDEP for all the PDEPs
  allowed to make decision requests for the data (the content creation
  PDEP does not have to be in the set of PDEPS allowed to make access
  control decisions). The PDEP includes a list of URLs for all of the



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  PDEPs allowed to process decision requests and the hash of the
  protected content as signed authenticated attributes in the policy
  metadata token, then it signs the encrypted metadata.

  (vi)  The PDEP returns the policy metadata token to the DR
  (vii) The DR attaches the policy metadata token to the protected
        content and distributes the content.

5.4 Content Consumption Workflow

  When a user wants to open some protected content they would use the
  following workflow.

  (a)   The DR verifies the certificate in the signed policy metadata
        then determines via local policy if it wants to process the
        protected information based on the identity of the PDEP.
  (b)   The DR verifies the signature on the policy metadata token and
        the binding to the encrypted data by hashing the encrypted
        information and comparing it to the authenticated attribute in
        the policy metadata
  (c)   The DR creates read token request. The request contains the
        signed metadata from the content together with one or more
        authentication tokens issued by a PIP. The request may also
        contain attributes about the request such as the purpose of use
        of the data.
  (d)   The DR sends the read token request to one of the of the URL's
        of the PDEPs in the authenticated attributes of the signed
        metadata
  (e)   The PDEP decrypts the policy metadata, de-references the policy
        pointers, and determines the set of rules to apply to the
        request based on the policy published by the PAP. The PDEP then
        determines the set of attributes it needs to evaluate the policy
        rules. The PDEP can use PIPs is has direct relationships with to
        query attributes about the subject. If the PDEP is missing
        attributes it need to process the policy, it returns a list of
        the missing attributes to the DR.
  (f)   If the DR receives a list of missing attributes from the PDEP,
        it obtains the missing attributes requested by the PDEP from a
        PIP and sends them to the PDEP in a new read token request.
  (g)   Once the PDEP has a complete set of attributes, and the
        attribute values match those required under the access policy,
        the PDEP releases the CEK to the DR along with a TTL which
        defines how long the DR can use the CEK before it must discard
        the CEK and reapply for access.
  (h)   Once the DR has the CEK it decrypts the information. It caches
        the CEK until the TTL expires.





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5.5 Plasma Proxy Servers

  There are two separate use cases for proxy servers in Plasma. The
  forward proxy use case where a DR client needs to connect to a PDEP
  outside of its organization and the reverse proxy use case where a DR
  client outside an organization, needs to connect to a PDEP.

  A recipient has no control over senders creating Plasma email (or any
  other type of Plasma protected content) and sending it to them.
  Malicious senders can craft harmful payloads and protect it in a
  Plasma envelope. Therefore, Plasma recipients need a policy to
  determine the set of Plasma DPEP services they are willing to interact
  with. This can be a local policy i.e., a policy for the allowed set of
  PDEPs a DR client can interact with. This policy would need to be
  distributed to every DR client. An alternate approach is to have a
  forward proxy manage the policy on behalf of the DR client. A forward
  proxy would eliminate the need to distribute policy by mediating the
  connection requests from the DR clients to the PDEP services. The
  forward proxy could be a server belonging to the DR client
  organization or a cloud service.

  In the no-proxy use case the DR client would connect via TLS directly
  to the URL contained in the policy metadata. The DR would thus need
  local policy to determine whether to connect to the PDEP URL. If a
  forward proxy is preset, the DR client would attempt to connect via
  TLS to the forward proxy. The forward proxy would then connect to the
  PDEP if its policy allowed.
























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          Internet |         DMZ             |       Intranet
                   |                         |
                   |                         |
                   |                         |      ---------------
                   |                         |      |             |
             TLS   |                         | TLS  |  DR         |
         ----------|<------------------------|------|  Client     |
                   |                         |      |             |
            (a)    |                         |      ---------------
          no proxy |                         |
                   |                         |
                   |                         |
                   |      ---------------    |      ---------------
                   |      |             |    |      |             |
             TLS   |      |  Plasma     |    | TLS  |  DR         |
         ----------|<-----|  Forward    |<---|------|  Client     |
                   |      |  Proxy      |    |      |             |
             (b)   |      |             |    |      ---------------
           Forward |      ---------------    |
           Proxy   |                         |

                     Figure 4 Forward Plasma Proxy

  Since the Plasma service has sensitive cryptographic keys used to
  protect the data CEKs, it would be unwise to host those servers
  directly connected to the Internet. However, PDEPs will need to be
  Internet addressable for requests from DR clients outside the
  organization.  The simplest possible configuration would be to have a
  passive reverse  proxy in front of the Plasma server. Since Plasma is
  using TLS, a passive proxy cannot inspect the data inside the TLS
  session. The passive proxy has therefore a limited function and would
  be only able to filter based on session characteristics e.g., source
  IP addresses.  The Plasma protocol is a series of request-response
  messages, so an active reverse proxy can be implemented like other
  store-and-forward message based services (e.g., SMTP). The Internet-
  facing proxy server would terminate the TLS connections from the
  external DRs. The active proxy can then scan submitted requests to
  ensure they are not malformed and are free from malicious content
  before relaying messages to a full PDEP server further inside the
  network for processing of the request.











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          Internet |         DMZ             |       Intranet
                   |                         |
                   |                         |
                   |      ---------------    |      ---------------
                   |      |             |    |      |             |
             TLS   |      |  Passive    |    | TLS  |  Full       |
         ----------|------|-------------|----|----->|  PDEP       |
                   |      |  Reverse    |    |      |  Server     |
                   |      |  Proxy      |    |      |             |
            (a)    |      |             |    |      |             |
                   |      ---------------    |      |  TLS Keys,  |
                   |                         |      |  Message    |
                   |                         |      |  Encryption |
                   |                         |      |  Keys       |
                   |                         |      |             |
                   |                         |      ---------------
                   |                         |
                   |      ---------------    |      ---------------
                   |      |             |    |      |             |
             TLS   |      |  Active     |    | TLS  |  Full       |
         ----------|----->|  Reverse    |----|----->|  PDEP       |
                   |      |  Proxy      |    |      |  Server     |
             (b)   |      |             |    |      |             |
                   |      |  TLS keys   |    |      |  TLS Keys,  |
                   |      |             |    |      |  Message    |
                   |      ---------------    |      |  Encryption |
                   |                         |      |  Keys       |
                   |                         |      |             |
                   |                         |      ---------------
                   |                         |
                   |                         |
                     Figure 5 Reverse Plasma Proxy

5.6 Policy Types

  Policies range from very simple to very complex. Policies have
  dependencies not only on the technical implementation of the software
  but on the range of attributes a PIP would issue to subjects. This is
  likely constrained by the physical procedures a PIP could support to
  capture and verify the information about the subject. To manage this
  range of requirements, this model uses two type types of policy.

5.6.1 Basic Policies

  Basic policies are intended to be universally usable by employing a
  small, fixed set of attributes that are available from all PIPs. For
  example, basic policies are intended to be equivalent to sending
  encrypted email with S/MIME today i.e., authenticated recipients of



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  the email get access to the message.  Basic policies target scenarios
  involving consumers and small businesses who are using public PIPs
  which issue a limited set of attributes. It is expected that all
  Plasma clients and commercial IdPs would be capable of supporting
  basic policies due to the finite set of attribute set required which
  will simplifies development, testing and deployment. Later standards
  may expand the set of attributes supported by basic policies and hence
  define richer basic policies.

5.6.2 Advanced Policies

  Advanced policies are intended to be used where one or more policies
  are required on the content that require an expanded set of attributes
  from an IdP. They are intended to target more complex policy
  requirements such as content with regulated information or content
  subject to organizational and contractual policies. The input set of
  attributes are defined by the policies. These attributes are, in
  theory, unbounded and can be either primordial such as date of birth,
  or derived attributes such as age, or both. In practice, advanced
  policies are constrained by the set of attributes  available under the
  IdP Trust Framework for the subjects. A data object may require
  multiple policies and any instance of multiple policies requires a
  logical relationships between the policies, e.g., they can be AND-ed
  or OR-ed together. It is not expected that all Plasma clients will
  support the rich set of attributes necessary for advanced policies.

6 Message Protection Requirements

6.1 General Requirements

  Confidentiality policy protected data MUST be protected from
  unauthorized disclosure, protected from unauthorized alteration and
  provide data origin authentication.

  Integrity policy protected data MUST be integrity protected from
  unauthorized alteration and provide data origin authentication.

  Every authentication has a level of identity assurance associated with
  it depending on attributes such as the identity checks made about the
  subject and the authentication technology used by the subject.  The
  authentication of content creators and content consumers MUST support
  the multiple levels of identity assurance framework(see sections 3.1,
  3.2, 3.3, and 3.4.)

  The specifics of every possible authentication mechanism or every
  detail about how the subject's identity was proofed by the IdP cannot
  be known to the DR and PDEP, therefore the specifics of how the sender
  or recipient achieves the required level of identity assurance MUST be



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  abstracted from the PDEP and DR by use of a simple numeric scale,
  e,g., 0-n where n is linked to an identity assurance framework that
  defines the specifics of how to derive the LoA(See sections 3.1, 3.2,
  3.3, and 3.4.)

  Access policies are complex and subject to change over time.  For this
  reason, policies MUST be identified by reference rather than inclusion
  of the actual policy with the data so the policy change can be
  implemented without updating the data.(See section 3.4.1.)

  Access to the plaintext of the content MUST only be provided after the
  recipient has either provided suitable valid attributes to the PDEP or
  the PDEP finds attributes about recipient directly from a PIP, thus
  satisfying the policy as defined by the sender.(See sections 3.1 3.2,
  3.3, 3.4.1, and 3.5.)

  The sender MUST be provided with a list of policies applicable to
  content they create and scoped to their current role, i.e., what tasks
  they are currently assigned to deliver.(see sections 3.1, 3.2, and
  3.3.)

  The specifics of the access control policy used by the PDEP MUST be
  abstracted from both the sender's and the recipient's DR, i.e., the DR
  MUST NOT make the access control decision or need specifics of the
  access policy requirements.(See sections 3.1, 3.2, 3.3, and 3.4.)

  A content consumer DR MUST receive authenticated attributes of the
  identity of the creator, the level of identity assurance of the
  creator, and the cryptographic fingerprint of the original content so
  that the DR can confirm who created the content and that the content
  has not been altered.(see sections 3.1, 3.2, 3.3, and 3.4)

  The key exchange between content creator and content consumer and the
  PDEP MUST support multiple levels of assurance so an appropriate
  strength of mechanism can be selected based on the level of assurance
  required. For example, for low assurance situations this could be via
  a plan text CEK over a secure transport such as TLS.  For high
  assurance situations, the recipient MAY be required to provide a
  suitable key exchange key such as an X.509 certificate to encrypt the
  CEK.(see sections 3.3 and 3.4)

  The level of key exchange assurance required MUST be selected by the
  sender's policy and enforced by the PDEP. (See sections 3.1, 3.2, 3.3,
  and 3.4.)

  If the recipient is unable to initially comply with the sender's
  policy, then if they are subsequently able to get the required
  credentials or attributes  it MUST be possible  for to retry access to



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  the content without intervention from the content creator.

  A time-to-live (TTL) MUST be provided to content consumers when access
  is granted by the PDEP to define when the DR MUST discard the message
  CEK and submit a new access request to the PDEP. The TTL value MUST be
  based on the message policy and optional attributes about the content
  consumer and its environment.

  The PDEP MUST be stateless for processing policy requests from content
  creators and consumers with respect to any instance of protected
  content. It MUST be possible to have multiple instances of a PDEP
  service and load balance requests across all instances of the service
  transparently to the client and not require synchronization of state
  about requests between instances of the service.

  A PDEP MUST be capable of generating audit events associated with
  access to protected content using policy defined by the PAP.

6.1.1 Email Specific General Requirements

  It MUST be possible for domains to publish keys and attributes about
  the boundary inspection agents.  This allows senders to pre-authorize
  the inspection agents of recipients for access to messages.

  It MUST be possible for MTAs to request access to protected messages
  for which they have not been authorized by the sender (See section
  3.8).

  Is should be possible for an MTA to pre-authorize another to access a
  protected message (See section 3.8).

6.2 Basic Policy Requirements

  The use of a Basic policy MUST be backwards compatible with existing
  S/MIME.

  A sender's agent MAY discover some recipients' encryption certificates
   and create recipient info structures using the existing S/MIME
  standard (unless specifically forbidden by the selected policy).

  A sender's agent MAY elect to use a Basic Policy mechanism for
  recipients for whom encryption certificates cannot be discovered.

  Four Basic policies are to be defined by this work.  These Basic
  policies MUST map to the LoA of NIST 800-63-1.  This does not preclude
  other Basic policies to be defined by other groups, trust frameworks,
  or even within the context of the IETF.




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  When using a Basic policy defined by this work, the sending agent MUST
  define which Basic policy is required and the list of rfc5322[RFC5322]
  recipients.

  A sender using  Basic policy MUST be able to send protected messages
  without discovering a recipient's encryption key.

  A sender using Basic policy MUST NOT require a bilateral agreement
  between sender and recipients as a prerequisite to sending the
  message.

6.2.1 Email Specific Basic Policy Requirements

  The use of Basic Policy MUST be backwards compatible with existing
  S/MIME encryption.

  A sender's agent MAY discover some recipient's certificates and create
  recipient info structures as per the existing S/MIME standard and
  elect to use the new mechanism for recipients it cannot discover keys
  for rather than remove the recipient's without certificates.

6.3 Advanced Policy Requirements

  A Basic policy MAY be combined with Advanced policies

  It MUST be possible to apply one or more Advanced policies to content.


  Where two or more policies are applied to content, the logical
  relationship between the policies MUST also be expressed e.g., are the
  policies a logical AND or a logical OR. (See section 3.3)

  An advanced policy MAY require attributes about:

  o  The content consumer
  o  The device the content consumer is using
  o  The environment of the device that is attempting to access the
        protected content
  o The content being accessed

  Advanced policy MUST support an extensible list of obligations on the
  DR or PDEP such as use of the policy requires some specific action on
  the part of the content creator, e.g., signing content with two-factor
  smart card and/or that the signature complies with the legal
  requirements for the transaction, or the signature needs to be able to
  be verified for an extended period.(See sections 3.3 and 3.4.)

  Advanced policies MUST support the ability to verify the content for



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  an extended period as required by policy. For example policy may
  require signatures to be verifiable for a period of 10 years.

  Advanced policies MUST support the ability to resign the data to
  support the verification over the extended period.

7 IANA Considerations

  This document describes the requirements for message access control.
  As such, no action by IANA is necessary for this document

8 Security Considerations

  Authentication by itself is not a good trust indicator. Authentication
  raises the level of assurance the identity is correct but does not
  address whether the identity is trustworthy or noteworthy to the
  recipient.  Authentication should be coupled with some form of
  reputation e.g. the domain is on a white list or is not or a black
  list.  Malicious actors may attempt to "legitimize" a message if an
  indication of authentication is not coupled with some form of
  reputation.

  Malicious actors could attempt to use encrypted email as a way to
  bypass existing message pipeline controls or to mine information from
  a domain.  Domains should have sufficient granularity of policy to
  handle situations where their email pipeline agents are not able to
  inspect the contents.

  It must be possible for a third party to, upon correctly presenting a
  legitimate legal justification, to recover the content of a message.
  This includes the Sender's and Recipient's companies for business
  continuity purposes, as well as Law Enforcement.  If the entity
  requesting the information and the entity controlling the access are
  in different jurisdictions, then the process would be subject to some
  form of rendition.

  The use of a security label type that requires the recipient of a
  message to query a PDEP in order to obtain the contents of a message
  opens an additional method for adversaries to confirm that an email
  address does or does not exist.

  Additionally it allows for a new channel for materials to be delivered
  to the recipient's mail processor that is not checked for malware or
  viruses by the standard mail scanning methods in place.

  Email is frequently used as part of a password reset ceremony by an
  identity provider. This is problematic when combined with access to
  sensitive email. This could be part of an escalation attack e.g.



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  compromise low value email account password, initiate password reset
  via email for higher value account. This would then give access to any
  email protected using the higher value identity.

  Providing differential access to different parts of a message based on
  different policies should only be done via use of different encryption
  keys. All data protected by the same key is the same access control
  policy.

  It would be desirable to be able to indicate the times and other data
  like request location when a user has asked for access (successful or
  otherwise) to some content as a means to show malicious activity to
  the user.

  Part of the policy are obligations on how to protect the data e.g.
  algorithms and parameters required. This can change over time,
  therefore a client may become obligated to re-encrypt or re-digest the
  data  if it encounters data which does not meet the current mandate.

  The act of requesting access to messages is a potential privacy issue
  as it allows the sender to gather data about the recipient. For
  business to business transaction, disclose of employee information is
  handled by the organization. For consumers, there is a need to be able
  to consent to the privacy obligations associated with disclosure of
  information. This would include information the consumer releases to
  the PDPE as well as information the PEDP is able to gather such as
  time and location of access requests.

  The fact the PDEP is able to grant access to the data could be used by
  law enforcement to access information. One of the parameters the
  sender needs to be aware of is the jurisdiction the PDPE is under so
  they can make an informed choice.



Appendix A.  References

A.1.  Normative References

 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3198]     Westerinen et. al., "Terminology for Policy Based
               Management", November 2001.
 [RFC5035]     Schaad, J., "Enhanced Security Services (ESS) Update",
               August 2007.
 [RFC5280]     Cooper, D, et al, "Internet X.509 Public Key
               Infrastructure Certificate and Certificate Revocation
               List (CRL) Profile", RFC5280, May 2008



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 [RFC5322]     Resnick, P., "Internet Message Format", RFC5322, October
               2008.
 [RFC5652]     Housley, R., "Cryptographic Message Syntax (CMS)", RFC
               5652, September 2009.
 [RFC5750]     Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
               Mail Extensions (S/MIME) Version 3.2 Certificate
               Handling", RFC 5750, January 2010.
 [RFC5751]     Ramsdell B., Turner S., "Secure/Multipurpose Internet
               Mail Extensions (S/MIME) Version 3.2 Message
               Specification", January 2010
 [SAML-core]   OASIS, Assertions and Protocols for the Security
               Assertion Markup Language (SAML) Version 2.0, March 2005
 [sp800-63-1]  NIST SP 800-63-1 "Electronic Authentication Guideline",
               December 2008

A.2.  Informative References

 [bc-iaf]      Province of British Columbia; Electronic Credential And
               Authentication Standard, version 1.0
 [kan-iaf]     Kantara Initiative; Identity Assurance Framework: 4
               Assurance Levels, version 2.0
 [lib- iaf]    Liberty Alliance; Liberty Identity Assurance Framework,
               version 1.1
 [RFC3114]     Nicolls, W., "Implementing Company Classification Policy
               with the S/MIME Security Label", RFC 3114, May 2002.
 [RFC5408]     Appenzeller, G., "Identity-Based Encryption Architecture
               and Supporting Data Structures", RFC5408, January 2009.

 [SAML-over]   OASIS, Security Assertion Markup Language (SAML) Version
               2.0 Technical Overview
 [XACML-core]  OASIS, eXtensible Access Control Markup Language (XACML)
               Version 3.0 Core Specification



















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Appendix B Authors' Addresses

   Trevor Freeman

          Microsoft Corp.

          Email: trevorf@microsoft.com

   Jim Schaad

          Soaring Hawk Consulting

          Email: ietf@augustcellars.com

   Patrick Patterson

          Carillon Information Security Inc

          Email: ppatterson@carillon.ca
































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