Internet DRAFT - draft-zeilenga-email-seclabel
draft-zeilenga-email-seclabel
Network Working Group K. Zeilenga
Internet-Draft A. Melnikov
Intended status: Informational Isode Limited
Expires: April 20, 2015 October 17, 2014
Security Labels in Internet Email
draft-zeilenga-email-seclabel-09
Abstract
This document describes a header field, SIO-Label, for use in
Internet Mail to convey the sensitivity of the message. This header
field which may carry a textual representation (a display marking)
and/or a structural representation (a security label) of the
sensitivity of the message. This document also describes a header
field, SIO-Label-History, for recording changes in the message's
label.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 20, 2015.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Relationship to Inline Sensitivity Markings . . . . . . . 3
1.2. Relationship to preexisting Security Label Header Fields 4
1.3. Relationship to Enhanced Security Services for S/MIME . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. The SIO-Label header field . . . . . . . . . . . . . . . . . 6
5. The SIO-Label-History header field . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Security Considerations . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 15
1. Introduction
A security label, sometimes referred to as a confidentiality label,
is a structured representation of the sensitivity of a piece of
information. A security label can be used in conjunction with a
clearance, a structured representation of what information
sensitivities a person (or other entity) is authorized to access, and
a security policy to control access to each piece of information.
For instance, an email message could have a EXAMPLE CONFIDENTIAL
label, and hence requiring the sender and the receiver to have a
clearance granting access to EXAMPLE CONFIDENTIAL labeled
information. X.841 [X.841] provides a discussion of security labels,
clearances, and security policy.
A display marking is a textual representation of the sensitivity of a
piece of information. For instance, "EXAMPLE CONFIDENTIAL" is a
textual representation of the sensitivity. A security policy can be
used to generate display markings from security labels. Display
markings are generally expected to be prominently displayed whenever
the content is displayed.
Sensitivity-based authorization is used in networks which operate
under a set of information classification rules, such as in
government military agency networks. The standardized formats for
security labels, clearances, and security policy and associated
authorization models are generalized and can be used in non-
government deployments where appropriate.
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Security labels may also be used for purposes other than
authorization. In particular, they may be used simply to convey the
sensitivity of a piece information. The security label could be
used, for instance, to organize content in a content store.
This document describes a protocol for conveying the sensitivity of a
electronic mail message [RFC5322], as a whole. In particular, this
document describes a header field, SIO-Label, to carry a security
label, a display marking, and display colors. This document also
describes a header field, SIO-Label-History, to record changes in the
message's security label.
This protocol is based in part upon Security Labels in XMPP [XEP258]
protocol.
1.1. Relationship to Inline Sensitivity Markings
In environments requiring messages to be marked with an indication of
their sensitivity, it is common to place a textual representation of
the sensitivity, a display marking, within the body to the message
and/or in the Subject header field. For instance, the authors often
receives messages of the form:
To: author <author@example.com>;
From: Some One <someone@example.net>;
Subject: the subject (UNCLASSIFIED)
UNCLASSIFIED
Text of the message.
UNCLASSIFIED
Typically, when placed in the body of the message, the marking is
inserted into the content such that it appears as the first line(s)
of text of the body of the message. This is known as a FLOT (First
Line(s) of Text) marking. The marking may or may not be surrounded
by other text indicating the marking denotes the sensitivity of the
message. A FLOT may also accompanied by a LLOT (Last Line(s) of
Text) marking. The message above contains a two-line FLOT and a two-
line LLOT (in both cases, a line providing the marking and a empty
line between the marking and the original content).
Typically, when placed in the Subject of the message, the marking is
inserted before or after the original subject field contents
surrounded with by parentheses or the like, and/or separated from the
content by white space.
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The particulars syntax and semantics of inline sensitivity markings
is generally a local matter. This hinders interoperability within an
organization wanting to take actions based upon these markings, and
hinders interoperability between cooperating organizations wanting to
usefully share sensitivity information
The authors expect such markings to be continued to widely used,
especially in absence of ubiquitous support for a standardized header
field indicating the sensitivity of the message.
The authors hope that through the use of formally-specified header
field, interoperability within organizations and between
organizations can be improved.
1.2. Relationship to preexisting Security Label Header Fields
A number of non-standard header fields, such as the X-X411 field, are
used to carry a representation of the sensitivity of the message,
whether a structured representation or textual representation.
The authors hope the use of preexisting (non-standard) header fields
will be replaced, over time, with use of the header field described
in this document.
1.3. Relationship to Enhanced Security Services for S/MIME
Enhanced Security Services for S/MIME (ESS) [RFC2634] provides,
amongst other services, signature services "for content integrity,
non-repudiation with the proof of origin, and [securely] binding
attributes (such as a security label) to the original content.
While it may be possible to utilize the protocol described in this
document concurrently with ESS, this protocol should generally be
viewed as an alternative to ESS.
It is noted that in ESS, the security label applies to MIME [RFC2045]
content, where in this protocol the label applies to the message as a
whole.
It is also noted that in ESS, security labels are securely bound to
the MIME content through the use of digital signatures. This
protocol does not provide message signing services, and hence does
not provide securely binding the label to the message, or for content
integrity, or for non-repudiation of the proof of origin.
This protocol is designed for situations/environments where message
signing is not necessary to provide sufficient security.
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2. Conventions Used in This Document
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 [RFC2119].
The formal syntax specifications in this document use the Augmented
Backus-Naur Form (ABNF) as described in [RFC5234].
The term "base64 encoding" is used to refer to the Base 64 encoding
defined in Section 4 of [RFC4648]. The term "BER encoding" is used
to refer to encoding per the Basic Encoding Rules (BER) as defined in
[X.690].
3. Overview
A Mail User Agent (MUAs) originating a message can, if so configured,
offer the user with a menu of sensitivities to choose from and, upon
selection, insert the display marking, foreground and background
colors, and security label parameters associated with that selection
into the SIO-Label header field of the message.
Mail Submission Agents (MSAs), Mail Transfer Agents (MTAs), and Mail
Delivery Agents (MDAs) then can, if so configured, use the provided
(or lack thereof) sensitivity information in determining whether to
accept, forward, or otherwise act on the message as submitted. These
agents, here after referred to as Service Agents (SAs), can, if so
configured, modify the sensitivity information of the message, such
as replacing the security label and/or display marking with an
equivalent representations of the sensitivity of the message. SAs
which add or modify or delete the SIO-Label header field SHOULD add
an SIO-Label-History header.
Receiving MUAs which implement this extension SHALL, when displaying
the message, also prominently display the marking, if any, conveyed
in the SIO-Label header field or, if policy aware and configured to
display locally generated markings, a marking generated by the
conveyed label and the governing policy. It is also desirable to
display this marking in listings of messages. In the case the
conveyed marking is displayed, marking SHOULD be displayed using the
foreground and background colors conveyed in the header field. In
the case the marking was generated from conveyed label and the
governing policy, the marking SHOULD be displayed using the
foreground and background colors conveyed by the governing policy.
While MUAs are not expected to make authorization decisions based
upon values of the SIO-Label header field, MUAs can otherwise use the
provided (or lack thereof) sensitivity information in determining how
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to act on the message. For instance, the MUA may organize messages
in its store of messages based upon the content of this header field.
4. The SIO-Label header field
The header field name is "SIO-Label" and its content is a set of key/
value pairs, each referred to as a parameter.
Formal header field syntax:
sio-label = "SIO-Label:" [FWS] sio-label-parm-seq [FWS] CRLF
sio-label-parm-seq = sio-label-parm
[ [FWS] ";" [FWS] sio-label-parm-seq ]
sio-label-parm = parameter
where the parameter production is defined in [RFC2231], the FWS
production are defined in [RFC5322], and the CRLF production is
defined in [RFC5234]. It is noted that the RFC 2231 productions rely
on [RFC0822] ABNF which implicitly allows for white space in certain
cases. In particular, white space is implicitly allowed in the
parameter production immediately before and after the "=". It is
also noted that RFC 2231 allows for quoted-string values (of the
parameter production) of substantial length and for string characters
outside of US-ASCII, or other such cases. Implementors should
consult the referenced specifications for specifics.
The "marking" parameter is a display string for use by
implementations which are unable or unwilling to utilize the
governing security policy to generate display markings. The
"marking" parameter SHOULD generally be provided in SIO-Label header
fields. It ought only be absent where an SA relies on other SA to
generate the marking.
The "fgcolor" and "bgcolor" parameters are tokens restricted to color
production representing the foreground and background colors,
respectively, for use in colorizing the display marking string.
Their values are RGB colors in hexadecimal format (e.g., "#ff0000"),
or one of the CSS color names (e.g., "red") given in named-color type
below (the 16 HTML4 colors + "orange") [CSS3-Color]. The default
foreground color is black. The default background is white. The
"fgcolor" and "bgcolor" parameters SHALL be absent if the marking
parameter is absent. The HEXDIG production below is defined in
[RFC5234]
Formal color syntax:
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color = hex-color / named-color
hex-color = "#" 6HEXDIG ; Hex encoded RGB
named-color =
"aqua" /
"black" /
"blue" /
"fuschia" /
"gray" /
"green" /
"lime" /
"maroon" /
"navy" /
"olive" /
"purple" /
"red" /
"silver" /
"teal" /
"white" /
"yellow" /
"orange" ; named colors
The "type" parameter is a quoted-string containing the string ":ess"
or the string ":x411" or the string ":xml" or a URI [RFC3986]
denoting the type and encoding of "label" parameter. The "label"
parameter value is a quoted string. The "type" parameter SHALL be
present if the "label" parameter is present. The "label" parameter
SHALL be present if the "type" parameter is present. The absence of
the "type" and "label" parameters indicates the message is handled,
where sensitivity-based authorization is performed, under default
handling rules (e.g., as if no SIO-Label was present).
The string ":ess" indicates the "label" parameter value is the base64
encoding of the BER encoding of an ESS security label [RFC2634].
ESS Label Example:
SIO-Label: marking="EXAMPLE CONFIDENTIAL";
fgcolor=black; bgcolor=red;
type=":ess"; label="MQYGASkCAQM="
The string ":x411" indicates the "label" parameter value is the
base64 encoding of the BER encoding of an X.411 security label
[X.411].
X.411 Label Example:
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SIO-Label: marking="EXAMPLE CONFIDENTIAL";
fgcolor=black; bgcolor=red;
type=":x411"; label="MQYGASkCAQM="
The string ":xml" indicates the "label" parameter value is the base64
encoding of a security label represented using [XML]. The XML prolog
SHOULD be absent unless specifically required (such as when the
character encoding is not UTF-8). The particular flavor of security
label representation is indicated by the root element name and its
name space.
XML Label Example:
SIO-Label: marking="EXAMPLE CONFIDENTIAL";
fgcolor=black; bgcolor=red;
type=":xml";
label*0="PFNlY0xhYmVsIHhtbG5zPSJodHRwOi8vZXhhbX";
label*1="BsZS5jb20vc2VjLWxhYmVsLzAiPjxQb2xpY3lJ";
label*2="ZGVudGlmaWVyIFVSST0idXJuOm9pZDoxLjEiLz";
label*3="48Q2xhc3NpZmljYXRpb24+MzwvQ2xhc3NpZmlj";
label*4="YXRpb24+PC9TZWNMYWJlbD4=";
where XML label, with new lines and white space added for
readability, is:
<SecLabel xmlns="http://example.com/sec-label/0">
<PolicyIdentifier URI="urn:oid:1.1"/>
<Classification>3</Classification>
</SecLabel>
The ":ess" and ":x411" formats SHOULD be used represent ESS or X.411
security labels, respectively, instead of any direct XML
representation of these formats.
The header field SHALL minimally contain a "marking" parameter or
contain both the "type" and "label" parameters.
This header field may be extended to include additional parameters by
future document formally updating (or replacing) this document.
Implementations SHOULD ignore additional parameters they do not
recognize. This recommendation is not a mandate so as to allow
agents to process a message with an SIO-header field with
unrecognized header fields differently than a message less those
unrecognized header fields.
Each message SHALL contain zero or one SIO-Label header field.
Extended Example:
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SIO-Label: marking*=us-ascii'en'EXAMPLE%20CONFIDENTIAL;
fgcolor = black ; bgcolor = red ;
type=":ess"; label*0="MQYG";
label*1="ASkCAQM="
The Extended Example is equivalent to the ESS Label Example above.
5. The SIO-Label-History header field
Any service agent MAY record label changes in an SIO-Label-History
header. This header field is intended to provide trace information
(and only trace information). For instance, it can be used to record
the label change when an SIO-Label header is added, modify, or
deleted by an service agent. This field use can be used in other
sitations as well. For instance, an X.400 to Internet messagging
gateway can use this header field to record labeling changes made
while translating a message.
The formal syntax of the SIO-Label-History header is the same as the
SIO-Label, but with parameters as discussed here
change - one of "add", "replace", "delete".
changed-by - contains a string identify the agent, commonly the
agent's fully qualified domain name.
changed-at - contains a date-time production, as specified in
[RFC5322] representing the date and time the header was rewritten.
changed-comment - contains a string containing a comment.
marking, fgcolor, bgcolor, type, label - records the message's label
information prior to add, modify, delete of SIO-Label, using same
parameter syntax used of SIO-Label. These parameters are absent when
the change action is add.
new-marking, new-fgcolor, new-bgcolor, new-type, new-label - records
the message's label information after add, modify, delete of SIO-
Label, using same parameter syntax used for corresponding SIO-Label
parameters. These parameters are absent when the change type is
delete.
The header field SHALL minimally contain the "change", "changed-by",
and "changed-at" parameters.
This header field can be extended to include additional parameters by
future document formally updating (or replacing) this document.
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Each message can contain zero or more SIO-Label-History header
fields. All SIO-Label-History header fields should immediately
follow the the SIO-Label header field, if any, and be grouped
together. Additional SIO-Label-History header fields should be added
immediately preceeding any existing SIO-Label-History header fields.
SIO Label History add, modify, delete example:
SIO-Label-History: marking="EXAMPLE CONFIDENTIAL";
fgcolor=black; bgcolor=red;
type=":xml";
label*0="PFNlY0xhYmVsIHhtbG5zPSJodHRwOi8vZXhhbX";
label*1="BsZS5jb20vc2VjLWxhYmVsLzAiPjxQb2xpY3lJ";
label*2="ZGVudGlmaWVyIFVSST0idXJuOm9pZDoxLjEiLz";
label*3="48Q2xhc3NpZmljYXRpb24+MzwvQ2xhc3NpZmlj";
label*4="YXRpb24+PC9TZWNMYWJlbD4=";
change=delete;
changed-by="delete.example.com";
changed-at="18 Feb 2013 9:24 PDT";
changed-comment="delete"
SIO-Label-History: marking="EXAMPLE CONFIDENTIAL";
fgcolor=black; bgcolor=red;
type=":ess"; label="MQYGASkCAQM=";
new-marking="EXAMPLE CONFIDENTIAL";
new-fgcolor=black; new-bgcolor=red;
new-type=":xml";
new-label*0="PFNlY0xhYmVsIHhtbG5zPSJodHRwOi8vZXhhbX";
new-label*1="BsZS5jb20vc2VjLWxhYmVsLzAiPjxQb2xpY3lJ";
new-label*2="ZGVudGlmaWVyIFVSST0idXJuOm9pZDoxLjEiLz";
new-label*3="48Q2xhc3NpZmljYXRpb24+MzwvQ2xhc3NpZmlj";
new-label*4="YXRpb24+PC9TZWNMYWJlbD4=";
change=replace;
changed-by="modify.example.net";
changed-at="18 Feb 2013 8:24 PDT";
changed-comment="replaced with XML variant"
SIO-Label-History: new-marking="EXAMPLE CONFIDENTIAL";
new-fgcolor=black; new-bgcolor=red;
new-type=":ess"; new-label="MQYGASkCAQM=";
change=add;
changed-by="add.example.net";
changed-at="18 Feb 2013 7:24 PDT";
changed-comment="added label"
6. IANA Considerations
Registration of the the SIO-Label and SIO-Label-History header fields
in the "Provisional Message Header Field Registry" is requested in
accordance with [RFC3864].
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Header field name: SIO-Label
Applicable protocol: mail [RFC5322]
Status: provisional
Author/change controller: Kurt Zeilenga (kurt.zeilenga@isode.com)
Specification document(s): this document
Header field name: SIO-Label-History
Applicable protocol: mail [RFC5322]
Status: provisional
Author/change controller: Kurt Zeilenga (kurt.zeilenga@isode.com)
Specification document(s): this document
7. Security Considerations
Sensitive information should be appropriately protected (whether
labeled or not). For email messages, it is generally appropriate for
the sending entity to authenticate the receiving entity and to
establish transport level security, including both data integrity and
data confidential protective services. Where a receiving entity to
make authorization decisions based upon assertions of the sending
entity, including assertions of identity, it is generally appropriate
for the receiving entity to authenticate the sending entity.
This document provides a facility for expressing the sensitivity of
an email message. The mere expression of actual sensitivity of a
generally does not elevate the sensitivity of the message, however
expressions of sensitivities can themselves be regarded as sensitive
information. For instance, a marking of "BLACK PROJECT RESTRICTED"
could disclose the existence of a sensitivity project.
The SIO-Label header field expresses the sensitivity of the whole
message, including the header and body. This document does not
provide a means to express the sensitivity of portions of an email
message, such as the possibly different sensitivities of various MIME
parts that the message may be composed of. This approach used in
this favors simplicity and ease of use of a single expression of
sensitivity over the complexity and difficultly of use of portion
marking and labeling.
The expressed sensitivity can be used in determining how to handle a
message. For instance, the value of the SIO-Label header (or lack
thereof) field can be used to determine if it appropriate to be
forwarded to a particular entity and, if so, what the minimum
security services are that which ought to be used in the forwarding
exchange. The mechanism for determining how to handle a message
based expressed sensitivity is beyond the scope of this document.
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The actual content may be more or less sensitivity than indicated by
the security label. Agents should avoid lowering security
requirements for message exchange with a particular entity based upon
conveyed sensitivity.
This protocol does not itself provide message signing services, such
a used in providing message integrity protection, non-repudiation,
and binding of attributes, such the security label to the message.
While it possible that this protocol could be used with a general
message signing service, this document does not detail such use.
While security label and display marking parameters are expected to
express the same sensitivity, nothing in this specification ensures
that the security label and display marking values express the same
sensitivity. For instance, an MUA could submit a message which
contains security label which expresses one sensitivity and a display
marking a different sensitivity, and by doing so, possibly cause an
SA to inappropriately handle the message. It is generally
appropriate for each SA making use of the SIO-Label values to
determine if the security label and display marking values express
the same sensitivity and, if not, take appropriate action (such as
rejecting the message).
This document also provides a facility for expressing changes to the
label of a message. This is intended to be used for trace purposes
only. It is noted that this SIO-Label-History header field can
include sensitive information and, as such, can be removed from the
message where its inclusion would result in an inapprorpriate
information disclosure.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and Encoded
Word Extensions:
Character Sets, Languages, and Continuations", RFC 2231,
November 1997.
[RFC2634] Hoffman, P., "Enhanced Security Services for S/MIME", RFC
2634, June 1999.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
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[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[XML] Paoli, J., Maler, E., Sperberg-McQueen, C., Yergeau, F.,
and T. Bray, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation REC-
xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126>.
[X.411] International Telephone and Telegraph Consultative
Committee, "Message Handling Systems (MHS) - Message
Transfer System: Abstract Service Definition and
Procedures", CCITT Recommendation X.411, June 1999.
[X.690] International Telephone and Telegraph Consultative
Committee, "ASN.1 encoding rules: Specification of basic
encoding Rules (BER), Canonical encoding rules (CER) and
Distinguished encoding rules (DER)", CCITT Recommendation
X.690, July 2002.
[CSS3-Color]
Celik, T. and C. Lilley, "CSS3 Color Module", World Wide
Web Consortium CR CR-css3-color-20030514, May 2003,
<http://www.w3.org/TR/2003/CR-css3-color-20030514>.
8.2. Informative References
[RFC0822] Crocker, D., "Standard for the format of ARPA Internet
text messages", STD 11, RFC 822, August 1982.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[X.841] International Telephone and Telegraph Consultative
Committee, "Security information objects for access
control", CCITT Recommendation X.841, October 2000.
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[XEP258] Zeilenga, K., "XEP-0258: Security Labels in XMPP", XEP
XMPP Extension Protocols, August 2011.
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Appendix A. Acknowledgements
The authors appreciate the review, comment, and text provided by
community members, including Dave Cridland, Brad Hards, Russ Housley,
Steve Kille, Graeme Lunt, Alan Ross, Jim Schaad, and David Wilson.
Authors' Addresses
Kurt Zeilenga
Isode Limited
EMail: Kurt.Zeilenga@isode.com
Alexey Melnikov
Isode Limited
14 Castle Mews
Hampton, Middlesex TW12 2NP
UK
EMail: Alexey.Melnikov@isode.com
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