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Independent Submission                                       K. ZeilengaRequest for Comments: 7444                                   A. MelnikovCategory: Informational                                    Isode LimitedISSN: 2070-1721                                            February 2015Security Labels in Internet EmailAbstract   This document describes a header field, SIO-Label, for use in   Internet email to convey the sensitivity of the message.  This header   field 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 document is not an Internet Standards Track specification; it is   published for informational purposes.   This is a contribution to the RFC Series, independently of any other   RFC stream.  The RFC Editor has chosen to publish this document at   its discretion and makes no statement about its value for   implementation or deployment.  Documents approved for publication by   the RFC Editor are not a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7444.Copyright Notice   Copyright (c) 2015 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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.Zeilenga & Melnikov           Informational                     [Page 1]

RFC 7444            Security Labels in Internet Email      February 2015Table of Contents1. Introduction ....................................................21.1. Relationship to Inline Sensitivity Markings ................31.2. Relationship to Preexisting Security Label Header Fields ...41.3. Relationship to Enhanced Security Services for S/MIME ......42. Conventions Used in This Document ...............................53. Overview ........................................................54. The SIO-Label Header Field ......................................65. The SIO-Label-History Header Field ..............................96. IANA Considerations ............................................127. Security Considerations ........................................128. References .....................................................148.1. Normative References ......................................148.2. Informative References ....................................15   Acknowledgements ..................................................16   Authors' Addresses ................................................161.  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 sensitive information   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 an "EXAMPLE CONFIDENTIAL" label that   requires the sender and the receiver to have a clearance granting   access to information labeled "EXAMPLE CONFIDENTIAL".  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 that operate   under a set of information classification rules, such as in   government and military agency networks.  The standardized formats   for security labels, clearances, security policy, and associated   authorization models are generalized and can be used in non-   government deployments where appropriate.Zeilenga & Melnikov           Informational                     [Page 2]

RFC 7444            Security Labels in Internet Email      February 2015   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, that carries a security   label, a display marking, and display colors.  This document also   describes a header field, SIO-Label-History, that records changes in   the message's security label.   This protocol is based in part upon "XEP-0258: Security Labels in   XMPP" [XEP258].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 in 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 that the marking denotes the sensitivity of   the message.  A FLOT may also be 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 an   empty line between the marking and the original content appear).   Typically, when placed in the Subject of the message, the marking is   inserted before or after the contents of the original Subject field;   it is surrounded by parentheses or the like and/or separated from the   content by white space.Zeilenga & Melnikov           Informational                     [Page 3]

RFC 7444            Security Labels in Internet Email      February 2015   The particular syntax and semantics of inline sensitivity markings   are 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 that such markings will continue to be widely   used, especially in the absence of ubiquitous support for a   standardized header field indicating the sensitivity of the message.   The authors hope that through the use of a 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 that the use of preexisting (non-standard) header   fields will be replaced, over time, with the 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 secure binding the label to the message, content integrity,   or non-repudiation of the proof of origin.   This protocol is designed for situations/environments where message   signing is not necessary to provide sufficient security.Zeilenga & Melnikov           Informational                     [Page 4]

RFC 7444            Security Labels in Internet Email      February 20152.  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 inSection 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 (MUA) originating a message can, if so configured,   offer the user 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) can then, if so configured, use the provided   sensitivity information (or lack thereof) in determining whether to   accept, forward, or otherwise act on the message as submitted.  These   agents, hereafter 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   equivalent representations of the sensitivity of the message.  SAs   that add, modify, or delete the SIO-Label header field SHOULD add an   SIO-Label-History header.   Receiving MUAs that 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, the marking SHOULD be displayed using   the foreground and background colors conveyed in the header field.   In the case the marking was generated from a conveyed label and the   governing policy, the marking SHOULD be displayed using the   foreground and background colors conveyed by the governing policy.Zeilenga & Melnikov           Informational                     [Page 5]

RFC 7444            Security Labels in Internet Email      February 2015   While MUAs are not expected to make authorization decisions based   upon values of the SIO-Label header field, MUAs can otherwise use the   provided sensitivity information (or lack thereof) in determining how   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 is defined in [RFC5322], and the CRLF production is   defined in [RFC5234].  It is noted that the productions defined in   [RFC2231] rely on the ABNF in [RFC0822], 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 [RFC2231] allows for quoted-   string values (for parameter production) of substantial length, for   string characters outside of US-ASCII, or for other such cases.   Implementors should consult the referenced specifications for   details.   The "marking" parameter is a display string for use by   implementations that 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 SAs 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 Cascading Style Sheets (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 defaultZeilenga & Melnikov           Informational                     [Page 6]

RFC 7444            Security Labels in Internet Email      February 2015   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:   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",   the string ":x411", the string ":xml", or a URI [RFC3986] denoting   the type and encoding of the "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.  When   sensitivity-based authorization is performed, the absence of the   "type" and "label" parameters indicates that the message is handled   under default handling rules (e.g., as if no SIO-Label was present).   The string ":ess" indicates that the "label" parameter value is the   base64 encoding of the BER encoding of an ESS security label   [RFC2634].Zeilenga & Melnikov           Informational                     [Page 7]

RFC 7444            Security Labels in Internet Email      February 2015   ESS Label Example:   SIO-Label: marking="EXAMPLE CONFIDENTIAL";       fgcolor=black; bgcolor=red;       type=":ess"; label="MQYGASkCAQM="   The string ":x411" indicates that the "label" parameter value is the   base64 encoding of the BER encoding of an X.411 security label   [X.411].   X.411 Label Example:   SIO-Label: marking="EXAMPLE CONFIDENTIAL";       fgcolor=black; bgcolor=red;       type=":x411"; label="MQYGASkCAQM="   The string ":xml" indicates that 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 the 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 to 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.Zeilenga & Melnikov           Informational                     [Page 8]

RFC 7444            Security Labels in Internet Email      February 2015   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-Label header field with   unrecognized parameters differently than a message with an SIO-Label   header field without the unrecognized parameters.   Each message SHALL contain zero or one SIO-Label header field.   Extended Example:   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, modified, or   deleted by a service agent.  This field can be used in other   situations as well.  For instance, a gateway that translates X.400   messages toRFC 5322 mail can use this header field to record   labeling changes made while translating a message.   The SIO-Label-History header field is considered to be a trace field   as defined inSection 3.6.7 of [RFC5322].   The formal syntax of the SIO-Label-History header is the same as the   SIO-Label, but with the following parameters:   o  change - one of "add", "replace", "delete".   o  changed-by - contains a string identifying the agent, commonly the      agent's fully qualified domain name.   o  changed-at - contains a date-time production, as specified in      [RFC5322], representing the date and time the header was      rewritten.   o  changed-comment - contains a string containing a comment.Zeilenga & Melnikov           Informational                     [Page 9]

RFC 7444            Security Labels in Internet Email      February 2015   o  marking, fgcolor, bgcolor, type, label - records the message's      label information prior to adding, modifying, or deleting SIO-      Label, using the same parameter syntax used for SIO-Label.  These      parameters are absent when the change action is "add".   o  new-marking, new-fgcolor, new-bgcolor, new-type, new-label -      records the message's label information after adding, modifying,      or deleting SIO-Label, using the 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 documents formally updating (or replacing) this document.   Each message can contain zero or more SIO-Label-History header   fields.  All SIO-Label-History header fields should immediately   follow the SIO-Label header field, if any, and be grouped together.   Additional SIO-Label-History header fields should be added   immediately preceding any existing SIO-Label-History header fields.Zeilenga & Melnikov           Informational                    [Page 10]

RFC 7444            Security Labels in Internet Email      February 2015   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"Zeilenga & Melnikov           Informational                    [Page 11]

RFC 7444            Security Labels in Internet Email      February 20156.  IANA Considerations   The SIO-Label and SIO-Label-History header fields have been   registered in the "Provisional Message Header Field Registry" in   accordance with [RFC3864].   Header field name: SIO-Label   Applicable protocol: mail [RFC5322]   Status: provisional   Author/change controller: Kurt Zeilenga (kurt.zeilenga@isode.com)   Specification document(s):RFC 7444   Header field name: SIO-Label-History   Applicable protocol: mail [RFC5322]   Status: provisional   Author/change controller: Kurt Zeilenga (kurt.zeilenga@isode.com)   Specification document(s):RFC 74447.  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 protective services for   both data integrity and data confidentiality.  When a receiving   entity makes 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   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.  The approach used in this   document favors simplicity and ease of use (i.e., a single expression   of sensitivity) over the complexity and difficulty of marking and   labeling portions of a message.Zeilenga & Melnikov           Informational                    [Page 12]

RFC 7444            Security Labels in Internet Email      February 2015   The expressed sensitivity can be used in determining how to handle a   message.  For instance, the value of the SIO-Label header field (or   lack thereof) can be used to determine if it is appropriate to be   forwarded to a particular entity and, if so, what minimum security   services 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.   The actual content may have 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   as used in providing message integrity protection, non-repudiation,   and binding of attributes (such as 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 that   contains a security label that expresses one sensitivity and a   display marking with a different sensitivity, and by doing so,   possibly cause an SA to inappropriately handle the message.  It is   generally appropriate for each SA using 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 the SIO-Label-History header field can   include sensitive information and, as such, can be removed from the   message when its inclusion would result in disclosure of   inappropriate information.Zeilenga & Melnikov           Informational                    [Page 13]

RFC 7444            Security Labels in Internet Email      February 20158.  References8.1.  Normative References   [CSS3-Color] Celik, T. and C. Lilley, "CSS3 Color Module",                W3C Candidate Recommendation                CR-css3-color-20030514, May 2003,                <http://www.w3.org/TR/2003/CR-css3-color-20030514>.   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate                Requirement Levels",BCP 14,RFC 2119, March 1997,                <http://www.rfc-editor.org/info/rfc2119>.   [RFC2231]    Freed, N. and K. Moore, "MIME Parameter Value and                Encoded Word Extensions: Character Sets, Languages, and                Continuations",RFC 2231, November 1997,                <http://www.rfc-editor.org/info/rfc2231>.   [RFC2634]    Hoffman, P., Ed., "Enhanced Security Services for                S/MIME",RFC 2634, June 1999,                <http://www.rfc-editor.org/info/rfc2634>.   [RFC3864]    Klyne, G., Nottingham, M., and J. Mogul, "Registration                Procedures for Message Header Fields",BCP 90,RFC 3864,                September 2004,                <http://www.rfc-editor.org/info/rfc3864>.   [RFC3986]    Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform                Resource Identifier (URI): Generic Syntax", STD 66,RFC3986, January 2005,                <http://www.rfc-editor.org/info/rfc3986>.   [RFC4648]    Josefsson, S., "The Base16, Base32, and Base64 Data                Encodings",RFC 4648, October 2006,                <http://www.rfc-editor.org/info/rfc4648>.   [RFC5234]    Crocker, D., Ed. and P. Overell, "Augmented BNF for                Syntax Specifications: ABNF", STD 68,RFC 5234, January                2008, <http://www.rfc-editor.org/info/rfc5234>.   [RFC5322]    Resnick, P., Ed., "Internet Message Format",RFC 5322,                October 2008, <http://www.rfc-editor.org/info/rfc5322>.   [X.411]      ITU-T, "Message Handling Systems (MHS) - Message                Transfer System: Abstract Service Definition and                Procedures", ITU-T Recommendation X.411, June 1999.Zeilenga & Melnikov           Informational                    [Page 14]

RFC 7444            Security Labels in Internet Email      February 2015   [X.690]      ITU-T, "ASN.1 encoding rules: Specification of Basic                Encoding Rules (BER), Canonical Encoding Rules (CER) and                Distinguished Encoding Rules (DER)", ITU-T                Recommendation X.690, November 2008.   [XML]        Bray, T., Paoli, J., Sperberg-McQueen, C., Maler, E.,                and F. Yergeau, "Extensible Markup Language (XML) 1.0                (Fifth Edition)", W3C Recommendation REC-xml-20081126,                November 2008,                <http://www.w3.org/TR/2008/REC-xml-20081126>.8.2.  Informative References   [RFC0822]    Crocker, D., "STANDARD FOR THE FORMAT OF ARPA INTERNET                TEXT MESSAGES", STD 11,RFC 822, August 1982,                <http://www.rfc-editor.org/info/rfc822>.   [RFC2045]    Freed, N. and N. Borenstein, "Multipurpose Internet Mail                Extensions (MIME) Part One: Format of Internet Message                Bodies",RFC 2045, November 1996,                <http://www.rfc-editor.org/info/rfc2045>.   [X.841]      ITU-T, "Security information objects for access                control", ITU-T Recommendation X.841, October 2000.   [XEP258]     Zeilenga, K., "XEP-0258: Security Labels in XMPP", XEP                XMPP Extension Protocols, April 2013.Zeilenga & Melnikov           Informational                    [Page 15]

RFC 7444            Security Labels in Internet Email      February 2015Acknowledgements   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   United Kingdom   EMail: Alexey.Melnikov@isode.comZeilenga & Melnikov           Informational                    [Page 16]