Network Working Group A. Melnikov
Internet-Draft K. Zeilenga
Intended status: Standards Track Isode Limited
Expires: February 9, 2012 August 8, 2011
Security Labels in Internet Email
draft-zeilenga-email-seclabel-01
Abstract
This document describes a header field for use in Internet Mail to
convey a security label and associated data.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Relationship to Enhanced Security Services for S/MIME . . . . . 3
3. Conventions Used in This Document . . . . . . . . . . . . . . . 4
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. The SIO-Label header field . . . . . . . . . . . . . . . . . . 4
6. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 9
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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 is 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 SECRET" label,
and hence requiring the sender and the receiver to have a clearance
granting access to "EXAMPLE SECRET" 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 SECRET" is a textual
representation of the sensitivity. A security policy can be used to
generate display markings from security labels.
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 are generalized and
do have application in non-government networks.
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.
2. Relationship to Enhanced Security Services for S/MIME
While it may be possible to utilize the protocol described in this
document concurrently with Enhanced Security Services for S/MIME
(ESS) [RFC2634], this protocol is generally viewed as an alternative
to ESS.
It is noted that in ESS, the 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, labels are securely bound to the MIME
content through the use of digital signatures, where, in this
protocol, labels are not security bound to the message. This
protocol is designed for use where securely binding the label to the
labeled information, at least through sender signing, is not
necessary but may well be inappropriate, such in deployment where
labels are replaced when crossing between security domains.
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3. 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].
4. 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. MTAs
can, if so configured, modify the sensitivity information of the
message, such as replacing the security label with an equivalent
security label of a different policy.
Receiving MUAs which implement this extension SHALL prominently
display the display marking conveyed in the header field. The
display marking SHOULD be displayed using the foreground and
background colors conveyed in the header field.
MUAs are not expected to make authorization decisions based upon
values of the SIO-Label header field.
5. 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.
The marking parameter contains a display string for use by
implementations which are unable or unwilling to utilize the
governing security policy to generate display markings.
The fgcolor and bgcolor parameters contain 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.
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The type parameter is either the string ":ess" or the string ":x411"
or a URI [RFC3986] denoting the type of label and its encoding as
held contained in the label parameter. The type parameter SHALL be
present if the label parameter is present.
The string ":ess" indicates the label parameter value is the base64
[RFC4648] encoding of the BER [X.690] encoding of an ESS security
label [RFC2634]. The label parameter SHALL be present if the type
parameter is present.
ESS Label Example:
SIO-Label: marking="EXAMPLE SECRET";
fgcolor=black; bgcolor=red;
type=":ess" label="MQYCAQIGASk="
The URI ":x411" indicates the label parameter value is the base64
[RFC4648] encoding of the BER [X.690] encoding of an X.411 security
label [X.411]. The label parameter SHALL be present if the type
parameter is present.
X.411 Label Example:
SIO-Label: marking="EXAMPLE SECRET";
fgcolor=black; bgcolor=red;
type=":x411" label="MQYCAQIGASk="
6. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) as described in [RFC5234]. Terms not defined here are
taken from [RFC5322].
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sio-label = "SIO-Label:" [FWS] sio-label-parm-seq CRLF
sio-label-parm-seq = sio-label-parm [ [FWS] ";" [FWS] sio-label-parm-seq ]
sio-label-parm = marking-parm /
bgcolor-parm /
fgcolor-parm /
type-parm /
label-parm
marking-parm = "marking" "=" quoted-string
bgcolor-parm = "bgcolor" "=" color
; default value "white" is assumed
fgcolor-parm = "fgcolor" "=" color
; default value "black" is assumed
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
type-parm = quoted-string ; ":ess" or ":x411" or URI
label-parm = quoted-string ; encoded as indicated by type-parm URI
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7. IANA Considerations
IANA is requested to add, as detailed below, the SIO-Label header
field to the "Permanent Message Header Field Registry", defined by
Registration Procedures for Message Header Fields [RFC3864].
Header field name: SIO-Label
Applicable protocol: mail [RFC5322]
Status: standard
Author/change controller: IETF (iesg@ietf.org)
Specification document(s): [[anchor7: this document]]
8. Security Considerations
Security labels and display markings are used to express the
sensitivity of a piece of information. This document describes how
to convey the security label and display marking expressing the
sensitivity of an email message, as a whole, including its headers
and body. This extension 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 extension is intended to be used in environments/situations
where it not necessary for the security label and other sensitivity
information to be securely bound to message through use of digital
signatures, and hence this document does not detail how security
labels digital signatures could be used to securely bind the security
label to the message.
In environments/situations where security labels are used to make
authorization decisions, MTAs making those authorization decisions
are to ensure appropriate data confidential and integrity protection
services are in place, such as requiring use of TLS [RFC5246] in
email protocols such as SMTP [RFC5321] and IMAP [RFC3501].
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2634] Hoffman, P., "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,
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RFC 3864, September 2004.
[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.
[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>.
9.2. Informative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet
Mail Extensions (MIME) Part One: Format of Internet
Message Bodies", RFC 2045, November 1996.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
[X.841] International Telephone and Telegraph Consultative
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Committee, "Security information objects for access
control", CCITT Recommendation X.841, October 2000.
Appendix A. Acknowledgements
The authors appreciate the review, comment, and text provided by
community members, including Dave Cridland, Brad Hards, Steve Kille,
Alan Ross, and David Wilson.
Authors' Addresses
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: Alexey.Melnikov@isode.com
Kurt Zeilenga
Isode Limited
EMail: Kurt.Zeilenga@isode.com
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