Network Working Group T. Freeman
Internet-Draft Microsoft Corp.
Intended status: Informational J. Schaad
Expires: April 22, 2012 Soaring Hawk Consulting
P. Patterson
Carillon Information Security Inc
October 20, 2011
Requirements for Message Access Control
draft-freeman-message-access-control-req-03
Abstract
There are many situations where organizations want to protect
information with robust access control, either for implementation of
intellectual property right protections, enforcement of information
contractual confidentiality agreements or because of externally
imposed legal regulations. The Enhanced Security Services (ESS) for
S/MIME defines an access control mechanism which is enforced by the
recipients client after decryption of the message. The ESS mechanism
therefore is dependent on the correct access policy configuration of
every recipients client. This mechanism also provides full access to
the data to all recipients prior to the access control check which is
considered to be inadequate for due to the difficulty in
demonstrating policy compliance.
This document lays out the deficiencies of the current ESS security
label, and presents requirements for new model for doing access
control to messages where the access check is performed prior to
message content decryption. This new model also does not require
policy configuration on the client to simplify deployment and
compliance verification.
The proposed model additionally provides a method where non-X.509
certificate credentials can be used for encryption/decryption of
S/MIME messages.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on January 20, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Access Control for Regulated Content in Email . . . . . . 4
1.2. Encrypted E-Mail Using Web-based Credentials . . . . . . . 5
1.3. Vocabulary . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. ESS Security Labels . . . . . . . . . . . . . . . . . . . 8
2.2. Access Control and the Web . . . . . . . . . . . . . . . . 10
2.3. Information Asset Protection . . . . . . . . . . . . . . . 11
2.4. Authentication Assurance Frameworks . . . . . . . . . . . 12
3. Use Case Scenarios . . . . . . . . . . . . . . . . . . . . . . 13
3.1. Business to Consumer Secure Email . . . . . . . . . . . . 13
3.2. Business to Business Ad-Hoc Email (add alternate
recipients as well) . . . . . . . . . . . . . . . . . . . 14
3.3. Business to Business Regulated Email . . . . . . . . . . . 15
3.4 Regulated Industry Email . . . . . . . . . . . . . . . . . . 18
3.5 Regulated Email Compliance Verification . . . . . . . . . . 19
3.5 Email Pipeline Inspection (add pre-auth and regular
access) . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.6. Related scenarios . . . . . . . . . . . . . . . . . . . . 20
4. General Data Model . . . . . . . . . . . . . . . . . . . . . . 23
4.1 Access Control Model . . . . . . . . . . . . . . . . . . . . 28
4.2 Content Creation Workflow . . . . . . . . . . . . . . . . . 31
4.3 Content Consumption Workflow . . . . . . . . . . . . . . . . 31
4.4 Policy Types . . . . . . . . . . . . . . . . . . . . . . . . 32
5. Message Protection Requirements . . . . . . . . . . . . . . . 33
5.1. General Requirements . . . . . . . . . . . . . . . . . . . 33
5.2. Basic Policy Requirements . . . . . . . . . . . . . . . . 35
5.3. Advanced Policy Requirements . . . . . . . . . . . . . . . 35
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
6. Security Considerations . . . . . . . . . . . . . . . . . . . 38
Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . . 39
Appendix A. References . . . . . . . . . . . . . . . . . . . . . 40
A.1. Normative References . . . . . . . . . . . . . . . . . . . 40
A.2. Informative References . . . . . . . . . . . . . . . . . . 40
Appendix B Authors' Addresses . . . . . . . . . . . . . . . . . . 41
Appendix C Document Change History . . . . . . . . . . . . . . . . 42
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1. Introduction
The S/MIME (Secure/Multipurpose Internet Mail Extensions) standard
[rfc5652] today provides digital signatures (for message integrity
and data origination) and encryption (for data confidentiality). The
Enhanced Security Services (ESS) for S/MIME [rfc2634] provides for
additional services including security labels (eSSSecurityLabel)
which represent the access control policy. These labels are placed as
a signed attribute in the signed data block of a message. The
recipient of the message is then responsible for checking that the
recipient has a legitimate right to see the message based on the
label on the message. This type of security labeling is similar to
that of stamping top-secret on the cover of a document. It relies on
the reader to not open and read the document when discovered.
The Cryptographic Message Syntax (CMS) [rfc5652] allows for a variety
of different lock boxes to be applied to an encrypted message. This
allows for a variety of different type of security mechanisms to be
used by the sender and the recipient to process the message. However
the S/MIME standard is currently solely based on X.509 certificates.
This means anyone without an X.509 certificate is unable to leverage
the S/MIME protocol for securing email. The vast majority of users
on the Internet have other forms of credentials (passwords, one time
passwords, GPG/PGP keys etc.).
1.1. Access Control for Regulated Content in Email
There are many situations where organizations want to include
information which is subject to regulatory or other complex access
control policy in email. Regulated information requires some form of
robust access control to protect the confidentiality of the
information. While ESS for S/MIME [rfc2634] defines an access
control mechanism for S/MIME (eSSSecurityLabel), this is an extremely
weak form of access control as the recipient is responsible for the
enforcement and is given access to the data even if they fail to meet
the criteria of the label.
An access control policy defines a set of criteria which is required
to be met in order to grant access to the information. These
criteria are defined in terms of attributes about the subject
requesting access. Examples of the types of attributes would include
what roles the subject is assigned to (Role Based Access Control) or
one or more attributes about the subject (Attribute Based Access
Control). While S/MIME provides a standardized representation for
the security label, it does not provide for any method of obtaining
the necessary attributes for enforcement of the policy. Standards
now exist to that enable for the transport of subject attributes
[SAML-overview]. Adoption of these subject attribute protocols would
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allow a rich set of access control polices to be supported by S/MIME
in line with other applications.
ESS Security labels are a signed attribute of a SignedData object
which indicates the access control policy for the message. The fact
that this is a signed attribute protects the integrity of the data
and the binding of the label to the message but does not protect the
confidentiality of the information i.e. at the point where you learn
the access control policy to the data you also have access to the
data. While the signature provides integrity for the label over the
clear text, it is susceptible to unauthorized removal who is able to
decrypt the message. I.e. if you only have SignedData message, any
Message Transport Agent (MTA) in the path can remove a signature
layer and therefore remove the access control data. Encrypting the
signed message protects the confidentiality of the data and protects
the SignedData from users unable to decrypt the message but this
hides the ESS security label.
From a regulatory enforcement perspective this is 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 the recipients email
client. Since the cryptographic access is granted before the access
checks, there is no cryptographic impediment for a recipient who is
unauthorized under the policy to access the data. A stronger
enforcement model is needed for regulatory control for email where
cryptographic access is only granted after the access check.
1.2. Encrypted E-Mail Using Web-based Credentials
There are many users on the Internet today who have some form of
authentication credential but the credentials are not X.509
certificates and who therefore cannot use S/MIME. There are now
available, standard based services (e.g. [SAML-overview]) which
abstract the specifics of a technology used to authenticate uses from
the application itself (S/MIME in this case). Adoption of this
abstraction model would enable a broader set of users who have other
types to authentication credentials to be able to use S/MIME to
secure email. It also allows for new authentication technology to be
deployed without impacting the core S/MIME protocol at the expense of
adding a third party to the transaction.
1.3. Vocabulary
A cryptographic lock box is a per recipient data structure which
holds the encrypted message encryption key.
Early Binding is the concept of creating the cryptographic lock
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box for a recipient at the time the message is sent. (Oppose to
Late Binding).
Late Binding is the concept of creating the cryptographic lock box
for a recipient when the recipient attempts to decrypt the
message. (Oppose to Early Binding)
Content Encryption Key (CEK) is a key used to encrypt protected end
user data.
Key Encryption Key (KEK) is a key used to encrypt a cryptographic
key.
Authenticated denotes:-
The sender is able to establish to a known level of confidence
the identity of the recipient or
The recipient is able to establish to a known level of
confidence the identity of the sender
Confidential denotes that a message has been protected to a known
level of confidence so that the contents are not decipherable by
unauthorized users.
Integrity protected denotes that a recipient of a message can
determine to a known level of confidence that a message has not
been modified between the time that it was created and it was
received by the recipient.
Front End Attribute Exchange is when subject attributes are relayed
from the issuer to the relying party by the subject
Bach End Attribute Exchange is when subject attributes are directly
send from the issuer to the the relying party
1.4. 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.
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2. Background
The S/MIME standard [rfc5751] provides a method to send and receive
secure MIME messages. While CMS allows for other types of security
credentials to be used, S/MIME exclusively uses X.509 certificates
[rfc5750] for the security credentials used for signing and
encryption operations. S/MIME uses an early binding mechanism for
encryption keys where the sender needs to discover the public key for
every recipient of encrypted messages before they can send the
encrypted message. 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
the 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. For example:
o The recipient may not have an X.509 encryption certificate
o The sender may not have received a signed email with the recipient
certificate
o The recipient may not have an available repository
o The sender may be unaware of the location of the recipients
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 implement the algorithms supported by the
recipient.
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 certificates from the message.
The use of secure mailing lists has the ability to provide some
relieve to the problem as the original sender does not need to know
the appropriate encryption information for all of the recipients on
the list. It can thus be thought of as a form of late-binding of
recipient information for 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 message will be send to. The
use of a mailing list also means that the originating sender has no
chance to perform any sender side filtering on who should receive an
email based on the recipients attributes as they do not know the full
list of the recipients.
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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 checks 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 the recipients since they don't have and frequently
should not have access to all the attributes about the recipients
because to do so may be a breach of the recipient's privacy. Indeed
to release the attributes to the sender may require that the sender's
attributes first be released to the recipient's attributes holder and
then recurse. It's a fundamental tenet of good security practice
that users must be in control of the release of data about
themselves.
2.1. ESS Security Labels
Security labels are an optional security service for S/MIME defined
in Enhanced Security Services for S/MIME [rfc2634]. 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 such as full time employees
only or US nationals only. ESS security labels are authenticated
attributes of a signer-info structure in a SignedData object. The
label when applied to signed clear text data provides the access
control decisions for the plain text. If applied to cipher text such
as with the outer layer of a triple wrapped S/MIME message the label
is used for course grained optimization such as routing.
2.1.1. Problems With ESS Security Labels
ESS Security Labels have been found to have a number of limitations.
1. If 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. Labels applied 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 Transit Agent). If the label is
protected by an encryption layer then it can be removed by any
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agent that has key access to the message (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 new access control
policy if the message contains a policy the client is unaware of.
This provides a stark 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 course
grained policy binding to express a limited set of choices such
as with sensitivity which typically a hierarchy of 3-5 choices.
Many data sets need to be subject to multiple access control
polices. For instance, a message may contain information that is
both propriety and export controlled. Trying to represent
combinations of polices via a single policy label would lead to
an exponential growth in the number of policy labels.
5. They do not provide for any auditing of who has been granted
accessed the message. All policy evaluation is local to the
recipient's machine, no centralized logging of access to the
message can be performed
6. Enforcement of the policy occurs on the recipients machine, this
means that compliance with the policy is dependent on the state
of the configuration of every receiving agent. This means that
the policy is enforce by whatever module is located on the user's
system. For cross cooperate 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 enforcement module.
Additionally any time that a new version of the policy module is
rolled out; there will be a time lag before every recipient
machine will have the updated module. This makes policy
compliance practically impossible anything but a small closed
environment.
7. Access to the message cannot be granted or removed after the
message has been sent, but before the recipient attempts to read
it.
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2.2. Access Control and the Web
A prerequisite for many web transactions is the disclosure of
attributes about the subject such as name, age, email address,
physical location, address, credit card number, social security
number etc. Some attributes lend themselves to easy verification but
many do not. An assertion of an email address can be verified by
sending an email to the address containing a secret ephemeral
challenge. Subsequent demonstration of knowledge of the ephemeral
challenge verifies the email address assertion. Other assertions
such as "this is my credit card account number" are not easily
verified. The fact it is a valid credit card number can be verified
but not the binding to the subject attempting to use it. Where a
claim is not easily verified it is often combined with other
assertions under the assumption that knowledge of this larger data
set verifies all the assertions in the data set. If you know the
account number, billing address, etc., of course you must be the
account holder. This is a very weak form of verification as is often
demonstrated by the growth of identity theft, much of this bigger
data set data set is often publicly available via social networks or
easily guessed e.g. the most popular professions for a parent is dead
or retired. Many of these assertions which are harder to verify are
based on government issued documents such as a birth certificates,
driver's license, identity card or passport. This requires an
exchange of the documents between the relying party and the subject.
For a small number of high value transactions (e.g. opening a new
account) with relying parties that have widespread physical presence
(a bank) this is acceptable because the applicant can present
themselves with the required documentation in person. However with
web based relying parties they cannot perform an in person exchange
of documents to verify information on government issued documents.
The approach taken with such relying parties is to have trusted
assertion providers where the assertion provider can perform an in
person exchange of documents with the subject then vouch for the set
of assertions they have verified.
SAML [SAML-core] defines an XML framework for describing and
exchanging attributes about subjects. The entity making the
assertions about the subject is known as the assertion provider, the
entity consuming the assertions is known as the relying party. The
well-known scenarios for using SAML are:
o Single Sign On across systems on different platform technology
o Federated Identity between business partners
o Web Services and other standards e.g. SOAP based protocols
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The critical difference between SAML and pure authentication
protocols such as mutually authenticated TLS is that SAML is able to
exchange the rich and variable set of assertions which necessary for
authorizing transactions. X.509 certificate can exchange a limited
and fixed set of identity assertions such as an x.500 distinguished
name, email address, Kerberos principal name, etc. SAML is able to
do this as well as an extensible set of other assertions about the
subject such as: date of birth, business sign-off limits levels, etc.
SAML additionally defined a number of query/response style profiles
[SAML-QUERY] that allow for a relying party to specify the type of
attributes that are required to evaluate a policy.
SAML also abstracts the details of the authentication protocol from
the relying party. The assertion provider can use a broad range of
authentication mechanisms such as passwords, one time passwords,
biometrics, X.509 certificates, etc., without impacting the relying
party. The assertion provider can include the details of the
authentication mechanism or its strength using an established
strength scale such as NIST SP800-63-1 [sp800-63-1]. The relying
party can then inspect the claims about how or how strongly the
subject authenticated to the identity provider to determine if it
complies with its access policy. Low value transactions can use
simple short lived assertions where possession of the assertion alone
is considered acceptable for the transaction risk. These are known
as Bearer assertions. Higher value transaction can require proof of
possession keys (either symmetric or asymmetric cryptographic keys)
where the subject demonstrates knowledge of a cryptographic secret to
the relying party via a HMAC or digital signature. These are defined
by the SAML specification as Holder of Key assertions. The subject
has to demonstrate possession of the key to the relying party. Holder
of key assertions can be either symmetric or asymmetric keys.
2.3. Information Asset Protection
Information Asset Protection (IAP) is a concept developed by the
Transglobal Secure Collaboration Program (TSCP), a working group
comprised of the major players in the western Aerospace and Defense
industry. The industry is highly regulated and operates in an
environment with many policies governing the access to information
assets. These policies may be motivated by the desire to protect
intellectual property, the confidentiality of information, or are
imposed by government regulators such as the US International Traffic
in Arms Regulations (ITAR) from the US Department of State. They
apply to the information assets in whatever form the asset may take
and are independent of the application used to create the
information. These policies take many forms, e.g. verification the
recipient has demonstrated a need to know the information because
they are working on a specific project, that they have passed the
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appropriate background and nationality checks, or that they have
signed the appropriate non-disclosure agreement. What is needed is a
policy driven information centric protection where the applicable
policies either is manually or automatically attached to the
information and based on the policy the system understands what
access control and data protection is necessary.
Email is an application widely used in the Aerospace and Defense
industry. S/MIME is widely used today and provides sender to
recipient confidentiality. This protects the contents of the message
from discloser to unauthorized third parties e.g. while it is in
transit between MTA's or while at rest in a MTA message queue or
recipients mailbox. However it does not impose any finer grained
access control such as those required by many policies. S/MIME does
define an extension mechanism for access control via an ESS security
label [rfc2634] thou this mechanism has drawbacks (see above).
2.4. Authentication Assurance Frameworks
A number of organizations have created taxonomies to define the
possible levels of identity assurance for electronic authentication.
The objective of the framework is to provide a simple abstraction the
details of any specific combination of identity proofing, credential
technology, authentication technology from the authorization policy.
These frameworks have been drafted by industry organizations [lib-
iaf][kan-iaf] and governments [sp800-63-1]. While all of these
frameworks may not agree on every aspect, at a macro level they do
exhibit many similarities. A common theme in many is the adoption of
a small number levels of identity assurance, typically between 3-5. S
simplified description of the levels is:
Level 1 Negligible confidence in the asserted identity
Level 2 Some confidence in the asserted identity
Level 3 Significant confidence in the asserted identity
Level 4 High confidence in the asserted identity
The framework defines broad characteristics in the areas of identity
proofing, credential types and management, identity provider
authentication and relying party authentication.
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3. Use Case Scenarios
This section documents some use case scenarios the new protocol aims
to support.
3.1. Business to Consumer Secure Email
There are many examples of business to consumer secure email
scenarios where the email could potentially contain sensitive data.
This would include doctor, patient; bank, account holder; Medical
insurance, insured person. As an example, let's say that Alice is a
doctor and has received test results for her patient Bob. This
information is confidential, so any channel of communication she
selects must protect the Bob's privacy. Alice elects to use email to
reach Bob quickly with news of the results. Alice must ensure the
following:-
(a) Only Bob can read the email.
(b) When Bob authenticates to read the email, that there is at least
some confidence in his identity (i.e. level 2 or above).
(c) The Bob can verify the email is from Alice.
(d) The Bob can verify the email has not been modified after Alice
sent it.
The sequence of events would be as follows:
1. Alice composes the email to Bob. She includes some comments and
suggestions for Bob and attaches the test results.
2. Alice's email client allows her to classify the email. Alice
classifies the email as a Doctor-Patient communication. (a)
3. Alice's email client knows the protections to apply to Doctor-
Patient communication; it knows to encrypt and integrity-
protects the message and what level of assurance required for
the recipients 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 or at rest.(a)
5. Bob receives the email as sees it is a secure message from
Alice.(c) Bob can verify the message has not been
altered[Objective (d)].Bob attempts to opens the email. Bob
uses a Level 2 password with his cloud identity service to log
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onto his email system. Bob is able to use the same service to
prove his identity to open Alice's email. (b) After Bob has
proved his identity, he is able to read the email.
If Bob replies to the email from Alice, the new message inherits the
policy from the original message so doctor-patient messages and
replies to doctor-patient messages have the same policy. The doctor-
patient policy does not apply to messages forwarded by Bob because is
allowed to make his own choices for protecting of the contents when
sharing with others.
3.2. Business to Business Ad-Hoc Email (add alternate recipients as
well)
Early in the relationship between two companies, it is frequently
necessary to exchange sensitive information. This needs to occur
before the relationship has matured to the point that a formal
relationship is reflected through a legal agreement. Business owners
need the agility to interact with potential partners without having
to engage their respective IT staffs as a prerequisite of the
communication. As example, the IT staff might need to provide cross
certifications and exposure of certificate repositories.
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. When
Charlie sends the email to Dave with the sensitive content, he must
ensure the following objectives:
(a) Only Dave can read the email
(b) When Dave authenticates to read the email, that there is at
least some confidence in his identity (i.e. level 2 or above).
(c) The Dave can verify the email is from Charlie
(d) The 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 sensible
information relating to potential terms and conditions for the
new contract that Foo and Bar would sign to form a partnership
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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 recipients (Dave in
this case). The data is protected while in transit or at rest.
5. Dave receives the email as sees it is a secure message from
Charlie. (Charlie requires level 2, Dave uses a password) Dave
is able to prove his identity to the level of assurance
requested by Charlie so is able to read the email. The
organization Dave work for has an identity service which he uses
to prove his identity for Charlie's email. Dave opens the email.
If Dave 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.
3.3. Business to Business Regulated Email
As business relationship 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 legal 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 derives from their role or function.
Also many regulated environments need to be able to verify the
information for a extended period - well beyond the typical lifetime
of a users certificate. The set of policies applicable to an email
is potentially subject to change as the different users contribute
information to the email thread.
Company Foo has been awarded a contract to build some equipment
(Program X). The equipment is covered by export control. Company
Bar is a 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 export control requirements. Company Foo also
set up separate rules to cover the protection of its intellectual
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property contributed to Program A. Company Bar also sets up its own
polices to protect 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.
Frank is an employee of Company Foo. He has been assigned as a team
leader on Program X and an individual contributor onProgram Y. Frank
wants to send some mail 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 Program X. When Frank
sends the email with Program X regulated content he must ensure
compliance with the export control polices. When frank sends program
directions as team lead, recipients need to verify his authority and
for compliance the message need to be able to be verified for 10
years. If Frank includes Company Foo intellectual property, he must
also ensure compliance with his corporate IP protection policies.
When Frank sends a Program X email he must ensure the following
objectives:
(a) Only recipients who meet the Program X policy and or or Company
Foo's intellectual property protection policy can read the email
(b) To comply with policy as team lead, Frank must sign the message
with certificate to indicate the signature message is legally
binding e.g. it does not just indicate the identity of the sends
and protects the integrity of the message.
(c) The message is also signed to indicate originator signature
complies with the policy and the originator had presented the
necessary claims and the allows the message to be verified for
at least 10 years.
(d) The recipients authenticates with an acceptable level of
assurance (i.e. level 3 or above)
(e) Recipients present any other attributes about themselves
necessary to verify compliance with the applicable policies
(theirs program assignment, nationality, professional or
industry certifications)
(f) Recipients can verify the email is from Frank to the level of
assurance as defined by the message policy (i.e. level 3 or
above)
(g) Recipients can verify the email has not been tampered with the
level of assurance as defined by the message policy
(h) Recipients are made aware that the message is a Program X email
and the contents can only be shared with other Program X
workers.
The sequence of events Frank would use is as follows:
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(1) Frank composes the email and includes a Program X distribution
list as a recipients. He include some information relation to
Program X. Frank also includes some information which is Company
Foo's IP.
(2) Frank's email client allows him to select the Program Z team
lead business context which is appropriate for his work.
(3) Frank selects the Program X team lead and Company Foo IP polices
from the list of available policies.
The email client knows the protections to apply to the email; the
message needs to be encrypted and integrity-protect the message using
a certificate which is legally binding, it also has a signature which
allows the message to be verified for at least 10 years; the level of
assurance required for the recipients identity and what recipient
attributes are necessary to access the message.
(4) Frank clinks the send email button. The client signs the email
using Franks smart card and a certificate indicating the
signature is legally binding. The client obtains a second
signature on the message to indicate Franks authority to send
the message and allow it to be verified for at least 10 years.
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 recipients of the distribution list. Grace is on
the distribution list so receives the email from Frank.
(5) Grace receives the email as sees it is a secure message from
Frank. Grace's client provides the attributes necessary to
comply with the policy which includes her level 3 encryption
certificate to the PDP.
(6) Once Grace has shown she passes the policy requirements, the PDP
releases the message CEK to grace using her level 3 encryption
certificate.
(7) Grace uses her smart card to open the message. She sees the
message is 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 messages. Grace includes some
information which is Company Bar's IP so add her companies 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
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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 because it
contains information from Company Foo and Company Bar both companies
wants consistent policy enforcement on its information.
3.4 Regulated Industry Email
Some organizations work in area which are intrinsically subject to
policy such as regulatory policy e.g. healthcare. In such
environments the policies are often tied to the roles of the
participates, the institution they are working at and the subject of
the exchange.
Hanna is a primary care physician working for FooBar Healthcare. She
has a patient which she is referring to a specialist Ida for further
investigations. Ida works at the Bar Hospital. Hanna needs to send
the relevant patient notes, test results and comments to Ida. Hanna
knows she needs to comply with the confidentiality regulations and
needs to respect her patients consent decree for the privacy of their
Healthcare information. When Hanna sends the referral message she
must ensure:
(a) Only recipients who meet the healthcare regulatory policy and
the patients consent decree can read the email
(b) The message has the appropriate level of integrity and data
origination as required by the policies
(c) The recipients authenticate with an acceptable level of
assurance (i.e. level 3 or above)
(d) Recipients present attributes about themselves necessary to
verify compliance with the policies (e.g. their professional
qualification, professional registration, affiliated healthcare
facility and department)
(e) Recipients can verify the email is from the sender (Hanna) to
the level of assurance as defined by the message policy (i.e.
level 3 or above)
(f) Recipients can verify the email has not been tampered with the
level of assurance as defined by the message policy
(g) Recipients are made aware that the message is a Patient referral
and contains sensitive patient data. workers.
The sequence of events Frank would use is as follows:
(1) Hanna composes the email and includes Ida as a recipients. SHe
includes the patient information, test results and comments in
the email
(2) Hanna's email client allows her to select a business context
which is appropriate for her work. Hanna selects a Patient
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Consultation context.
(3) Hanna selects the Patient Referral and Patient Consent decree
polices from the list of available policies.
The email client knows the protections to apply to the email; to
encrypt and integrity-protect the message, the level of assurance
required for the recipients identity and what recipient attributes
are necessary to access the message.
(4) Hanna clinks the send email button. The client signs the email
using Hanna smart card. The Client then encrypted the message
and sends it to his email server.
The email is able to flow securely and seamlessly through existing
email infrastructure recipients of the distribution list. Grace is on
the distribution list so receives the email from Frank.
(5) Ida receives the email as sees it is a secure message from
Hanna. Ida's client provides the attributes necessary to comply
with the policy which includes her level 3 encryption
certificate to the PDP.
(6) Once Ida has shown she passes the policy requirements, the PDP
releases the message CEK to Ida using her level 3 encryption
certificate.
(7) Ida uses her smart card to open the message. She sees the
message is marked with both the Patient Referral and Sensitive
Patient Data policies
3.5 Regulated Email Compliance Verification
TBD
3.5 Email Pipeline Inspection (add pre-auth and regular access)
Unsolicited bulk email (aka spam) is a problem for
organizations. Email can also contain malicious content such as
viruses or attempts at phishing. To combat these threats,
server side scanning of the email messages before they reach the
recipient is an effective countermeasure. Authorized agents
must be capable of getting access to the message.
Company Foo receives email from the Internet. Company Foo has a
policy to scan inbound email with a view to remove inappropriate
or malicious content. The scanning of inbound email for Company
Foo can happen on their own servers or it may be outsourced to a
third party. The fact that Company Foo scans inbound email is a
policy published to the intent to inform senders and allow them
to take appropriate action. Also the public keys for email
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hygiene servers for the Foo domain are also published to on the
Internet to enable senders to pre-approve the Company Foo
hygiene servers access to the messages. The server has the
ability to identify itself as a subject or delegate for Company
Foo who is authorized to scan email on behalf of Foo recipients
if the server has not been pre-approved for a message. If the
hygiene server has been pre-approved it therefore has immediate
access to the message on receipt, otherwise it can request
access to encrypted email. The email hygiene server or service
has a local policy on what content to remove. It also has policy
if a sending organization denies the server access to the
content. It can:-
o Allow the email to proceed
o Reject email so the sender is aware the message was rejected
o Drop the email so the sender is unaware of the actions the
server took
The server is able to establish the origin of the message without
decrypting so it is possible to set up exceptions for messages from
domain which may contain sensitive information. Dropping the message
is the simplest and is appropriate where the sending domain may be a
spammer.
3.6. Related scenarios
There are other scenarios which are related to the email cases
because they would be subject to the same policy requirements. Email
allows users to create content and transport it to a set of
recipients. You can perform similar actions with other formats such
as documents and instant messages. Policy is agnostic to the
underlying technology therefore if an organization has a policy
relating to a type of information, then that policy would apply to
the same content in an email, a document an instant message, etc.
3.6.1. Document Protection
This scenario is very similar to 4.2 and 4.3 above. The difference
is that the information being generated is in the form of a document
not an email. It could be as part of an ad-hoc sharing or a
regulated sharing or information.
Frank is an employee of Company Foo. He has been assigned to Program
X. Grace is an employee of Company Bar. She has been assigned to
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Program X. Frank creates a document for the program. He also
includes some Company Foo IP in the document. When Frank creates the
document he must ensure compliance with export control regulations
and his corporate IP protection policies. Fran must ensure:
1. Only users who meet the Program X policy or Company Foo's
intellectual property protection policy can open the document
2. Users authenticates with an acceptable level of assurance as
defined by the set of policies applied to the document
3. Users present any other attributes about themselves necessary to
verify compliance with the applicable policies.
4. Users can verify who the author was to an acceptable level of
assurance as defined by the document policy
5. Users can verify the document has not been tampered with to an
acceptable level of assurance as defined by the document policy
6. They can also tell it is a Program X document and the contents
can only be shared with other Program X workers.
Frank creates a document for Program X. He include some information
relation to Program X. Frank also includes some information which is
Company Foo's IP.
Franks word processor client allows him to classify the document.
Frank classifies the document as Program X and Company Foo
proprietary information.
The word processor client knows the protections to apply to the
document; to encrypt and integrity-protect the document, the level of
assurance required for the users identity and what user attributes
are necessary to access the document.
The document is able to be published on a cloud based Web portal. The
document is protected while in transit to the portal or at rest on
the portal. The document is also protected on any backup or replica
of the portal data. Frank does not to worry about where on the
portal he publishes the document. He can make the most appropriate
choose based on the project and the document content.
Grace sees the document on the portal and tries to open the document.
Grace is able to prove her identity to the level requested by Frank
and provides the requested attributes about herself to satisfy both
the Program X export control and the Company Foo IP protection
policies. Grace opens the document.
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If Grace edits the document and includes some information which is
Company Bar's IP so adds her companies IP protection policy
requirements to the document. Grace saves the updated document to
the same location on the portal.
Frank sees that Grace has updated the document on the portal. Frank
is able to prove his identity to the level requested by both the
Company Foo and company Bar policies 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 document.
Need XMPP scenario . Draft something and send to Leif and Peter for
review.
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4. General Data Model
This work is modeled on a well established set of Actors for policy
enforcement [rfc3198] [XACML-core].
Policy Administration Point (PAP): A service that creates policy
rules
Policy Repository: The location where a PAP publishes policy rules
Policy Decision Point (PDP): A service that is able to interpret
the policy rules published by a PAP to make decisions to allow or
deny requests.
Policy Information Point (PIP): A service with issues assertions
about subjects e.g. .a SAML Security Token Service. This model
supports both front end and back end exchange of assertions.
Attributes can be distributed directly between the PIP and the PDP
(Backbend Attribute Exchange;BAE). There are two types of PIP based
on the types of attribute the PIP would assert about the subject. A
Identity Provider (IdP) PIP will issue authentication attributes
e.g. information about how and when the subject authenticated to
the IdP. An IdP may also issue attributes about the subject
themselves e.g. their name, age or citizenship. An attribute
provider (AtP)PIP only issues attributes about the subject.
Policy Enforcement Point (PEP): The service responsible for making
policy decision requests to the PDP.
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------------------
| |
| Policy |
| Administration |
| Point |
| |
------------------
|
| Publish
v Policy
v
-----------------
| |
| Policy |
----------------- | Repository | -----------------
| | | | | |
| Policy | | | | Policy |
| Information | ----------------- | Information |
| Point | | | Point |
| | | Read | |
----------------- v Policy -----------------
| | v | |
| |Issue ----------------- Issue | |
| |Attributes | | Attributes| |
| |(BAE) | | (BAE) | |
| -------------->>| Policy |<<--------------- |
| | Decision | |
| | Point | |
| -------------->>| |<<----------- |
| |Protect | | Consume | |
| |Content ----------------- Content | |
| |Request+ Request+ | |
| |Attributes Attributes| |
| |(FEE) (FEE) | |
v | v v
v | v v
----------------- -----------------
| | | |
| Content | Distribute | Content |
| Creation | Content | Consumption |
| Policy | ---------------------------->>| Policy |
| Enforcement | | Enforcement |
| Point | | Point |
| | | |
----------------- -----------------
Figure 1 General Scheme for Publishing and Consuming Protected Content
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The model is applicable to any data e.g. email, documents, databases
etc. Another objective is to not require the PEP to have access to
the plain text content in order to be able to make decision requests
to the PDP. Policy process is complex so the PEP in this model just
uses policy pointers or policy labels to policy. The model allows
the content creation PEP to discover the set of policies a PDP would
allow the user to assert based on a role assignment based. The
Content consuming PEP dynamic may discover the PDP's who are
authoritative for the protected content in question.
The PDP makes its decisions based on the requested action from the
PEP, the policy requirements from the PAP and the information from
the PIP about the subject and the subjects environment. The
information abut the subject may be exchanged direly between the PIP
and the PDP (Back end Attribute Exchange) or indirectly via the PEP
(Front end Attribute Exchange) or both.
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--------------- --------------- ---------------
| | | | | |
| Policy | | Policy | | Policy |
| Decision | | Decision | | Decision |
| Point | | Point | | Point |
| | | | | |
--------------- --------------- ---------------
| | |
| T | T |
| TTTTTTT|TTTTTTT |
V V V
V V V
--------------- --------------- ---------------
| | | | | |
| Policy | | Policy | | Policy |
| Enforcement | | Enforcement | | Enforcement |
| Point | | Point | | Point |
| | | | | |
--------------- --------------- ---------------
| | |
T | T | |
TTTTTTT|TTTTTT | |
V V V
V V V
--------------- --------------- ---------------
| | | | | |
| End | | End | | End |
| User | | User | | User |
| Application | | Application | | Application |
| | | | | |
--------------- --------------- ---------------
(a) (b) (c)
Figure 2 Options Full Trust With Clear Text Data.
Drawing the 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 PEP. This is the model for current standard access control e.g.
XACML [XACML-core]. In 2a, the PEP has full access to the clear text
data. It makes decision requests to the PDP and if the decision is
allow the PEP releases the data to the application. To use fig 2a for
secure email would require every MTA to act a a PEP so to be fully
trusted with clear text data which is impossible.
Figure 2b shows the full trust line between the PDP and the PEP. In
2b, the PEP only has cipher text data. THe data is encrypted with a
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content encryption key (CEK) and the PDP has the CEK. THe PDP
releases the CEK to the end user application when access is granted.
This mode is viable for secure email as either the MTA or the MUA can
act as a PEP.
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 PDP and the PDP
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.
4.1 Access Control Model
This model is a hybrid of many access control models.
o This model uses name based binding between the resource and the
policy. When information is created, it is encrypted and a list of
policies that must be enforce by the PDP is bound to the protected
information
o The model is fundamentally an Attribute-Based Access Control
(ABAC) model. Access is granted to information based on attributes
of the subject. Any subject that can prove they possess the
necessary attributes to meet all the necessary policies is granted
access to the information.
o Access does not require the subject provide their orthonym.
Subjects could be anonymous or use pseudonymous.
o The subject is required to bind the attributes to the channel
with the relying party to a level of assurance as required by the
relying party. If the PDP only requires low assurance, bearer token
over SSL would be suitable. If the PDP requires higher assurance,
then holder of key tokens over SSL would be suitable.
o This model also supports Token-Based Access Control (TBAC) where
security tokens represent a capability to meet a policy. Once a
subject has proven compliance with a policy, they can be issued a
token to that effect. The client can subsequently present this
token in lieu of a token with the set of subject attributes. The
net result is the model can transition to a Capability Based Access
Control (CBAC) because the policy token is an unforgeable token of
compliance with a policy. The token can be used with any resource
tagged with the same policy.
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o When a subject tries to access the information they must present
tokens to the PDP to prove they meet the required policies
o When a subject creates information, this model uses business
contexts (BCs) as a means to mange the set of policies a subject is
allowed to assert on information they create. The PAP publish a set
of policies associated with each business context. This is the
Business Context Policy Collection(BCPC). Multiple PAP can publish
policies for a BCPC. The PIP would provide information on the set
of BCs a subject belongs to. The PDP issues a BCPC token to subject
for each BC they belong to based on the information provided by the
PIP. Each BCPC token contains the aggregation of all the policies
from all the PAPs for each BCPC. Each BCPC list is a hint to the
subject on the specific list of policies to pick from when creating
information for each BC. The act of aggregation by the PDP allows
the BCPC token to contain project wide policies used by all
subjects across a collaborative project as well as organization
specific policies applicable to the BC.
o The BCPC token is used by the subject to authorize the creation
of content with specific polices. The PDP will check the requested
list of policies for the information is a subset of the policies in
the BCPC token. If the set of policies are a subset of the policies
in the BCPC, then it will issue the metadata token to be attached
to the protected information.
4.1.1 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 a
Access Control List for the file or directory or where a rights
management agent has embedded a copy of the policy expressed in a
policy expression language in the rights protects data. When an
access request is made to the data, the PDP compares the access
request to the policy on the data itself.
o By name. 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.
For example with S/MIME the ESS label identifies the applicable
policy by an OID. When an access 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 description. This is where the policy has a target description
in terms of characteristics of the sets of data resources the
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policy applies to. When an access request is made, the set of
polices are evaluated at run time to determine the set of policies
to apply. For example when you author a XACML policy, you also
define a target for the policy. When an access request is made to
the data, the PDP finds the policy using the set of attributes of
the resource looking for any polices that match the target
description associated with the policy. It then compares the access
request to the identified policy.
The chief strength of binding policy by value is its simplicity. The
policy is local to the data can can easily and quickly be read. The
chief weakness in binding policy by value is maintaining policy over
time. Many polices have a multi-year life span and during the course
of that time there is a very high probability that the policy would
need to be updated. Given the high number of copies, it has proven to
be an 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 which has to be updated.
The chief strength of binding by names is once bound to the data the
association with the policy travels with the data. The chief weakness
in binding by name is it requires the reference to be strongly bound
to the data. This is possible using cryptography but then process of
persisting the binding impacts the storage format. This can break
backwards compatibility.
The chief strength of binding by description is it can be applied to
data without impacting the storage format. The chief weakness in
binding by description is the reliability of the matching. Any
matching process must have a false positive and falsie negative rate.
This rate has to be evaluated on a case by case basis over time as it
can change making compliance expensive. The set of available
attributes also varies with different data types e.g. structured
database information has a rich set of attributes whereas documents
and files have a poor set of attributes. This inconsistently over
available attributes impacts matching reliability. The resultant set
of polices for a policy target is also dependent on the correctness
of the set of policies evaluated. Its also impossible to detect if a
policy is missing from the policy store which again would mean
incorrect policy enforcement
This model is choosing to use binding by name because we need to
encrypt the data which means we will impacting the storage format
anyway which negates the main weakness of binding my name. We get the
reliability of policy enforcement which is independent of location
and we get low maintenance since we are only storing a reference to
the policy and not the policy wit the data..
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4.2 Content Creation Workflow
The Content Creation PEP bootstraps itself via the following sequence
of events:
(1) The content creation PEP is configured with the set PIP's and
PDP's it trusts.
(2) The content creation PEP summits a request to all the trusted
PDPs for the set of BCs it allows for the user. THe subject is
authenticated via a token from a PIP. The token may contain
attributes abbot the subject or the PDP may exchange information
directly with the PIP about the subject.
(3) The content creation PEP receives a list of the BCs the PDP can
configured for the user
(4) The PEP submits a request for the policy collection for each BC.
Additional attributes may be required from the PIP to authorize
the release of the BCPC token.
Now the PEP is bootstrapped with a list of BCs and for each BC a list
of policies associated with each BC. Now the PEP is ready to create
content. When the user wants to release protected content, they use
the following sequence of events
(i) The user creates the new content
(ii) The user select the appropriate business context for the
content, then selects one or more policies applicable to the
content
(iii) The PEP encrypts the content with one or more locally
generated CEKs
(iv) The PEP submits the CEK, the set of requires policies to be
applied and the hash of the encrypted content to the PDP. THe
CEK can be a raw key or a CEK key encrypted by a KEK if the
user does not want the PDP to have the ability to access the
plain text data.
(v) The PDP generates the encrypted metadata which contains the
list of policies and the CEKs. The metadata is encrypted by
the PDP for itself. The PDP includes a URL for itself and the
hash of the protected content as authenticated attributes then
signs the encrypted metadata.
(vi) The PDP returns the metadata to the PEP
(vii) The PEP attaches the PDP metadata to the protected content and
distributes the content.
4.3 Content Consumption Workflow
When a user want to open some protected content they would follow the
following workflow.
(A) The PEP verifies the certificate in the signed metadata then
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determines via local policy if it want to process the
protected information based on the identity of the PDP
(B) The PEP verifies the signature on the metadata token and the
binding to the encrypted data by hashing the encrypted
information and comparing it to the authenticated attribute in
the metadata
(C) The PEP forwards the signed metadata and requests a read token
from the PDP using the location in the authenticated attribute
in the metadata
(D) The PDP decrypts the metadata, de-references the policy
pointers and determines the set of access rules based on the
policy published by the PAP. The PDP then determines the set
of subject attributes it needs to evaluate the access rules.
The PDP can the use PIP is has relationships with to query
attributers about the subject. The list of attributes the PDP
is missing is then returned to the PEP
(E) The PEP obtains the missing attributes requested by the PDP
and sends them to the PDP
(F) Once the PDP has a complete set of attributes, and the
attribute values match those required under the access policy,
the PDP releases the CEK to the PEP along with a TTL which
defines how long the PEP can use the CEK before it must
discard the CEK and reapply for access.
(G) Once the PEP has the CEK it decrypts the information. It
caches the CEK until the TTL expires.
4.4 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 would issue to subjects.
This is likely constrained by the physical procedures a PIP would
support to capture and verify the information about the subject. To
manage this range of requirements, this model uses type types of
policy.
4.4.1 Basic Policy
Basic policy is intended to be universally usable by using a small
fixed set of attributes. For example, basic policy is intended to be
equivalent to sending encrypted email with S/MIME today. It is a
simple policy that authenticated recipients of the email get access
to the message. Its intended target is simple scenarios involving
consumers and small businesses who are using public PIP which issue a
limited set of attributes. It is expected that all Plasma clients and
commercial IdP would be capable of supporting basic policy due to
their simplicity and basic attribute set.
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4.4.2 Advanced Policy
Advanced policy is intended to be used where one or more arbitrary
policies are required on the content . It is intended to target more
complex scenarios such as content with regulated information or
content subject to other organization and contractual policies. The
input set of attributes is defined by the policies and can be either
primordial or derived attributes or both. Multiple policies have a
logical relationship e.g. they can be AND or ORed together. It is not
expected that all Plasma clients support advances policy.
5. Message Protection Requirements
5.1. General Requirements
Protected content MUST be where the content is confidential,
integrity protected AND provides data origination.
Every authentication has a level of assurance associated with it
depending on attributes such as the identity checks made about the
subject and the authentication technology used. The authentication
of content creator and content consumers MUST support the multiple
levels of identity assurance. (see scenarios 3.1, 3.2, 3.3 and 3.4)
It is not possible for the PDP to know the specifics of every
possible authentication mechanism or every detail about how the
subject was screened. The specifics of how the identity of the
content creator or the content consumer is established and what
technical means they use to authenticate and level of identity
assurance resulting from the process as a whole MUST be abstracted
from the PDP by use of a simple numeric scale ( e,g, 0-4, or 1-6)
liked to an identity assurance framework which defines the specifics
of how to derive the LoA.(See section 3.1, 3.2, 3.3 and 3.4)
Access to the plain text of the content MUST only be provided to the
content consumer after either the consumer obtained suitable valid
attributes from a PIP and provide them to the PDP or the PDP was able
to find attributes about the content consume from a PIP to satisfy
the policy as defined by the content creator (See section 2.1.1)
The content creator MUST be provided with a list of policies
applicable to content they create and scoped to their current
business context i.e. .what tasks they are currently assigned to
deliver(see scenarios 3.1, 3.2, 3.3).
The specifics of the access control policy used by the PDP MUST be
abstracted from both the content creator and content consumer i.e.
the PEP MUST NOT make the access control decision or need specifics
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of the policy(see scenarios 3.1, 3.2, 3.3 and 3.4).
Content consumers PEP 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
the PEP can confirm who created the content and that the content has
not been altered (see section 3.1, 3.2, 3.3 and 3.4)
The key exchange between content creator and content consumer and the
PDP 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 SSL. For high
assurance situations recipient MAY be required to provide a suitable
key exchange key such as an X.509 certificate to encrypt the CEK.
(See scenarios 3.3 and 3.4)
The level of key exchange assurance requited MUST be selected by the
sender and enforced by the PDP (See section 3.1, 3.2, 3.3 and 3.4).
If the content consumers is unable to initially comply with the
content creators policy, they MUST be able resolve any issues by
getting the suitable credentials or attributes and gain access to the
content without intervention from the content creator.
A time-to-live MUST be provided to content consumers when access is
granted by the PDP to define when the PEP MUST discard the message
CEK and submit a new access request to the PDP. The TTL value MUST be
based on the message policy and optional attributes about the content
consumer and their environment.
The PDP 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 PDP
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 PDP MUST be capable of generating audit events associated with
access to protected content.
5.1.1 Email Specific General Requirements
It MUST be possible for PDP to pre-authorize MTA of recipient domains
access to protected email at send time so for email scanning i.e. the
MTA does not need to contact the PDP because the PDP as already
included an encrypted CEK for the MTA in the protected message (see
section 3.5).
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It MUST be possible for MTAs to request access to protected messages
which have not been preauthorized by the sender (see section 3.5).
5.2. Basic Policy Requirements
When using Basic Policy, the sending agent MUST define which basic
policy and the list of recipients.
Basic policy MUST support multiple levels of identity assurance. The
levels of identity assurance MUST map to an existing identity
authentication assurance framework e.g. to NIST 800-63-1 or
equivalent. need rewording to multiple basic policies
A sender using Basic policy MUST be able to send protected messages
without discovering any recipient's encryption key.
Using basic policy MUST NOT require bilateral agreements between
sender and recipients a priori to sending the message. reword to a
must
5.2.1 Email Specific Basic Policy Requirements
The use of Basic Policy MUST be backwards compatible with existing
S/MIME.
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 recipients without certificates.
5.3. Advanced Policy Requirements
It MUST be possible to apply one or more Advanced Policies to a
protected content. Where 2 or more policies are applied to protected
content, the logical relationship between the policies MUST also be
expressed i.e. 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 is attempting to access the
protected content from
Advances policy MUST support an extensible list of obligations on the
content creator where use of the policy requires some specific action
on the part of the content creator e.g. sign content with 2 factor
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smart card and/or that the signature is legally binding, or the
message needs to be verified for an extended period(see scenarios 3.3
and 3.4).
Advanced policies must support the ability to verify the content for
an extended period (10 or more years)
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5. IANA Considerations
This document describes the requirements for message access control.
As such no action by IANA is necessary for this document
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6. Security Considerations
Authentication by itself is not a good trust indicator for users.
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. Domain should have sufficient granularity of policy to
handle situations where their email pipeline agents have not been
authorized 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 Senders and Recipients 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.
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Editorial Comments
[anchor21] JLS: Are these really the terms that we want to be using?
I normally use data origination rather than
authenticated. It would be assumed that the data
origination is being attested to by a middle man for a
sender w/o signature capability rather than
authentication being a correct term.
[anchor22] JLS: We need to talk about what operation you are getting
this level of assurance for. and who you are
authenticating to.
[anchor23] JLS: Same text should apply for senders?
[anchor24] JLS: What does assurance level mean here? Are we talking
about security levels or authentication levels or
something else? Are levels required to define a set of
requirements. I.e. An assumance level defines:
Authentication requirements, confidentiality requirements
(other).
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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.
[rfc2634] Hoffman, P. Ed., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.
[rfc3198] Westerinen et. al., "Terminology for Policy Based
Management", November 2001.
[rfc5280] Cooper, D, et al, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 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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Appendix C Document Change History
Added general data model (section 4)
Added regulated industry email scenario (section 3.4
Split requirements into general requirements and email specific
requirements
Cleaned up scenarios to differentiate requirements and workflow
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