[Docs] [txt|pdf] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 05 06 07 08 RFC 7508

Network Working Group                                 L. Cailleux
Internet-Draft                                             DGA MI
Intended status: Experimental                          C. Bonatti
Expires: 26 January 2015                                     IECA
                                                     26 July 2014




                 Securing Header Fields with S/MIME
                  draft-cailleux-secure-headers-06


Abstract

  This document describes how the S/MIME protocol can be
  extended in order to secure message header fields. This
  technology provides security services such as data integrity,
  non-repudiation and confidentiality. This extension is
  referred to as 'Secure Headers'.

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 working documents as Internet-Drafts.  The
  list of current Internet-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 26 January 2015.








Cailleux & Bonatti     Expires 26 January 2015          [Page 1]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




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

Table of Contents

   1. Introduction..............................................3
   2. Terminology and conventions used in this document.........3
   3. Context...................................................5
   4. Mechanisms to secure message header fields................7
      4.1. ASN.1 syntax of secure header fields.................8
      4.2. Secure header fields length and format...............9
      4.3. Canonization algorithm..............................10
      4.4. Header fields statuses..............................10
      4.5. Signature Process...................................10
         4.5.1. Signature Generation Process...................10
         4.5.2. Signature verification process.................12
      4.6. Encryption and Decryption Processes.................14
         4.6.1. Encryption Process.............................14
         4.6.2. Decryption Process.............................15
   5. Case of triple wrapping..................................15
   6. Security Gateways........................................16
   7. Security Considerations..................................16
   8. IANA Considerations......................................17
   9. References...............................................17
      9.1. Normative References................................17
      9.2. Informative References..............................18
   Appendix A. Formal syntax of Secure Header..................20


Cailleux & Bonatti     Expires 26 January 2015          [Page 2]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




   Appendix B. Secure Header Fields example....................21
   Appendix C. Acknowledgements................................23


1. Introduction

  S/MIME [RFC 5751] standard defines a data encapsulation format
  for the achievement of end to end security services such as
  integrity, authentication, non-repudiation and
  confidentiality. By default, S/MIME secures message body
  parts, at the exclusion of the message header fields.

  S/MIME provides an alternative solution to secure header
  fields. "The sending client MAY wrap a full MIME [RFC 2045]
  message in a message/rfc822 wrapper in order to apply S/MIME
  security services to header fields". However, the S/MIME
  solution doesn't allow selection of a subset of message header
  fields to secure. In addition, confidentiality service can not
  be implemented for message header fields. The solution
  described herein overcomes those limitations.

  Several security standards exist such as DKIM [RFC 6376],
  STARTTLS [RFC 3207] and TLS with IMAP [RFC 2595] but meet
  other needs (signing domain, secure channels). An internet
  draft referred to as PROTECTED HEADERS has been proposed, but
  doesn't address all the requirements. These different
  solutions are explained in the next chapters.

  The goal of this document is to define end to end secure
  header fields mechanisms compliant with S/MIME standard. This
  technique is based on the signed attribute fields of a
  Cryptographic Message Syntax (CMS) [RFC 5652] signature.

2. Terminology and conventions used in this document

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
  NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and



Cailleux & Bonatti     Expires 26 January 2015          [Page 3]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




  "OPTIONAL" in this document are to be interpreted as described
  in [RFC 2119].

  MUA, MSA and MTA terms are defined in Email architecture
  document [RFC 5598].

  DCA term is defined in the S/MIME Domain Security
  specification [RFC 3183].

  End-to-end Internet Mail exchanges are performed between
  message originators and recipients.

  Description of message header fields are described in [RFC
  5322]. A header field is composed of a name and a value.

  Secure Headers technology uses header fields statuses required
  to provide a confidentiality service toward message headers.
  The following three terms are used to describe the field
  statuses:

     - Duplicated (the default status). When this status is
     present or if no status is specified, the signature process
     embeds the header field value in the digital signature, but
     the value is will also be present in the message header
     fields.

     - Deleted. When this status is present, the signature
     process embeds the header field value in the digital
     signature, and the encryption process deletes this field
     from the message to preserve its confidentiality.

     - Modified. When this status is present, the signature
     process embeds the header field value in the digital
     signature, and the encryption process modifies the value of
     the header field in the message.  This preserves
     confidentiality and informs a receiver's non-compliant MUA
     that secure headers are being used.  New values for each


Cailleux & Bonatti     Expires 26 January 2015          [Page 4]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




     field might be configured by the sender (i.e., "This header
     is secured, use a compliant client").

3. Context

  Over the Internet, email usage has grown and today represents
  a fundamental service. Meanwhile, continually increasing
  threat levels are motivating the implementation of security
  services.

  Historically, SMTP [RFC 5321] and IMF [RFC 5322] don't
  provide, by default, security services. The S/MIME standard
  [RFC 5751] was published in order to encompass these needs.
  S/MIME defines a data encapsulation format for the provision
  of end to end security services such as integrity,
  authentication, non-repudiation and confidentiality. By
  default, S/MIME secures message body parts, at the exclusion
  of the message header fields. In order to protect message
  header fields (for instance, the "Subject", "To", "From" or
  customized fields), several solutions exist.

  S/MIME defines an encapsulation mechanism, chapter 3.1: "The
  sending client may wrap a full MIME message in a
  message/rfc822 wrapper in order to apply S/MIME security
  services to these header fields. It is up to the receiving
  client to decide how to present this inner header along with
  the unprotected outer header". However, some use cases are not
  addressed, especially in the case of message encryption. What
  happens when header fields are encrypted? How does the
  receiving client display these header fields? How can a subset
  of header fields be secured? S/MIME doesn't address these
  issues.

  An alternative solution is described in [RFC 5750]. "Receiving
  agents MUST check that the address in the From or Sender
  header of a mail message matches an Internet mail address, if
  present, in the signer's certificate, if mail addresses are


Cailleux & Bonatti     Expires 26 January 2015          [Page 5]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




  present in the certificate". However, this solution only
  provides a matching mechanism between email addresses, and
  provides no protection to other header fields.

  Other security standards (introduced below) exist such as
  DKIM, STARTTLS and TLS with IMAP but meet other needs (signing
  domain, secure channels...).

  STARTTLS and TLS with IMAP provide secure channels between
  components of email system (MUA, MSA, MTA...) but end to end
  integrity cannot be guaranteed.

  DKIM defines a domain-level authentication framework for email
  to permit verification of the source and contents of messages.
  It provides mechanisms to secure message header fields and
  message body but it doesn't guarantee non-repudiation and
  originator authentication. In addition, it doesn't provide
  confidentiality.

  An internet draft referred to as Protected Headers (PRHDRS)
  has been proposed. Mechanisms described in this draft are the
  following. "A digest value is computed over the canonicalized
  version of some selected header fields. This technique
  resembles header protection in DKIM. Then the digest value is
  included in a signed attribute field of a CMS signature". This
  specification doesn't address all conceivable requirements as
  noted below. If the protected header field has been altered,
  the original value cannot be determined by the recipient. In
  addition, the encryption service cannot provide
  confidentiality for fields that must remain present in the
  message header during transport.

  This document proposes a technology for securing message
  header fields. It's referred to as Secure Headers. It is based
  on S/MIME and CMS standards. It provides security services
  such as data integrity, confidentiality and non-repudiation of
  sender. Secure Headers is backward compatible with other


Cailleux & Bonatti     Expires 26 January 2015          [Page 6]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




  S/MIME clients. S/MIME clients who have not implemented Secure
  Headers technology need merely ignore specific signed
  attributes fields in a CMS signature (which is the default
  behavior).

4. Mechanisms to secure message header fields

  Secure Headers technology involves the description of a
  security policy. This policy MUST describe a secure message
  profile and list the header fields to secure.

  Secure headers are based on the signed attributes field as
  defined in CMS. The details are as follows. The message header
  fields to be secured are integrated in a structure (secure
  header structure) which is encapsulated in the signed
  attributes structure of the SignerInfo object. See Appendix A
  for an example. For each header field present in the secure
  signature, a status can be set. Then, as described in chapter
  5.4 of CMS, the message digest calculation process computes a
  message digest on the content together with the signed
  attributes. Details of the signature generation process are
  described in chapter 4.5.1 of this document.

  Verification of secure header fields is based on signature
  verification process described in CMS. At the end of this
  process, a comparison between the secure header fields and the
  corresponding message header fields is performed. If they
  match, the signature is valid. Otherwise, the signature is
  invalid. Details of the signature verification process are
  described in chapter 4.5.2 of this document.

  Non-conforming S/MIME clients will ignore the signed attribute
  containing the secure headers structure, and only perform the
  verification process described in CMS. This guarantees
  backward compatibility.




Cailleux & Bonatti     Expires 26 January 2015          [Page 7]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




  Secure headers provide security services such as data
  integrity, non-repudiation and confidentiality.

  For different reasons (e.g., usability, limits of IMAP [RFC
  3501]), encryption and decryption processes are performed by a
  third party. The third party that performs these processes is
  referred to in Domain Security specification as a "Domain
  Confidentiality Authority" (DCA). Details of the encryption
  and decryption processes are described in chapters 4.6.1 and
  4.6.2 of this document.

  The architecture of Secure Headers is presented below. The MUA
  performs the signature generation process (C) and signature
  verification process (F). The DCA performs the message
  encryption process (D) and message decryption process (E). The
  encryption and decryption processes are optional.

          A Domain                             B Domain
  +----------------------+             +----------------------+

  +-----+          +-----+             +-----+          +-----+
  | MUA | -------> | DCA | ----------> | DCA |--------> | MUA |
  |  C  |          |  D  |             |  E  |          |  F  |
  +-----+          +-----+             +-----+          +-----+
          SignedMsg        EncryptedMsg        SignedMsg

               Figure 1: Architecture of Secure Headers

4.1. ASN.1 syntax of secure header fields

  ASN.1 notation [X.680] of secure header structure is the
  follow:

   SecureHeaderFields ::= SET {
      canonAlgorithm Algorithm,
      secHeaderFields HeaderFields }




Cailleux & Bonatti     Expires 26 January 2015          [Page 8]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




   id-aa-secureHeaderFieldsIdentifier OBJECT IDENTIFIER ::=
      {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
      pkcs-9(9) smime(16) id-aa(2) secure-headers (to be
      defined) }

   Algorithm ::= ENUMERATED {
      canonAlgorithmSimple(0),
      canonAlgorithmRelaxed(1) }

   HeaderFields ::= SET SIZE (1..max-header-fields) OF
      HeaderField

   max-header-fields INTEGER ::= MAX

   HeaderField ::= SEQUENCE {
      field-Name HeaderFieldName,
      field-Value HeaderFieldValue,
      field-Status HeaderFieldStatus DEFAULT duplicated }

   HeaderFieldName ::= VisibleString (FROM (ALL EXCEPT (":")))
        -- This description matches with the description of
        -- field name in the chapters 2.2 and 3.6.8 of RFC 5322

   HeaderFieldValue ::= IA5String (FROM (ALL EXCEPT (
      { 0, 0 }..{ 0, 9 } | { 0, 11 } | { 0, 12 } |
      { 0, 14 }..{ 1, 15 } | { 7, 15 })))
        -- This description matches with the description of
        -- field body in the chapter 2.2 of RFC 5322

   HeaderFieldStatus ::= INTEGER {
      duplicated(0), deleted(1), modified(2) }

4.2. Secure header fields length and format

  This specification requires MUA security capabilities in order
  to process well formed headers, as specified in IMF. Notice
  that it includes long header fields and folded header fields.






Cailleux & Bonatti     Expires 26 January 2015          [Page 9]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




4.3. Canonization algorithm

  During a message transfer through a messaging system, some
  components might modify headers (i.e., space adding or
  deletion, lowercase/uppercase rewriting...). This might lead
  to header fields comparison mismatch. This emphasizes the need
  of a conversion process in order to transform data to their
  canonical form. This process is named canonization process.

  Two canonization algorithms are considered here, according to
  DKIM specification [RFC 6376], chapter 3.4. The simple
  algorithm doesn't allow any modification whereas the relaxed
  algorithm accepts slight modifications like spaces replacement
  or line reformatting. Given the scope of this document,
  canonization mechanisms only involve header fields.

4.4. Header fields statuses

  Header fields statuses are necessary to provide a
  confidentiality service toward message headers. In this
  specification, the confidentiality of header fields is
  provided by the DCA. This point is described in chapter 4. The
  DCA performs the message encryption process and message
  decryption process and these processes are described in
  details in the chapters 4.6.1 and 4.6.2. Although header
  fields statuses are embedded in the signature, the signature
  processes (generation and verification) ignore them. Since the
  confidentiality service is OPTIONAL, the status field is also
  OPTIONAL.

4.5. Signature Process


4.5.1. Signature Generation Process

  During the signature generation process, the sender's MUA MUST
  embed the SecureHeaderFields structure in the signed



Cailleux & Bonatti     Expires 26 January 2015         [Page 10]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




  attributes, as described in CMS. SecureHeaderFields structure
  MUST include a canonization algorithm.

  The sender's MUA MUST have a list of header fields to secure,
  statuses and a canonization algorithm, as defined by the
  security policy.

  Header fields (names and values) embedded in signed attributes
  MUST be the same as the ones included in the initial message.

  If different headers share the same name, all instances MUST
  be included in the SecureHeaderFields structure.

  If multiple signatures are used, as explained in CMS and
  MULTISIGN [RFC 4853] specifications, SecureHeaderFields
  structure MUST be the same in each SignerInfos object.

  If a header field is present and its value is empty,
  HeaderFieldValue MUST have a zero-length field-value.

  Considering secure headers mechanisms, the signature
  generation process MUST perform the following steps:

     1) Select the relevant header fields to secure. This subset
     of headers is defined according the security policy.

     2) Apply the canonization algorithm for each selected header
     field. header field.

     3) Complete the following fields in SecureHeaderFields
     structure according to the initial message: HeaderFieldName,
     HeaderFieldValue, HeaderFieldStatus (OPTIONAL).

     4) Complete the algorithm field according to the
     canonization algorithm configured.




Cailleux & Bonatti     Expires 26 January 2015         [Page 11]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




     5) Embed the SecureHeaderFields structure in the signed
     attributes of the SignerInfos object.

     6) Compute the signature generation process as described in
     CMS, chapter 5.5

4.5.2. Signature verification process

  During the signature verification process, the receiver's MUA
  compares header fields embedded in the SecureHeaderFields
  structure with those present in the message. For this purpose,
  it uses the canonization algorithm identified in the signed
  attributes. If a mismatch appears during the comparison
  process, the receiver's MUA MUST invalidate the signature. The
  MUA MUST display information on the validity of each header
  field. It MUST also display the values embedded in the
  signature.

  The receiver's MUA MUST know the list of mandatory header
  fields in order to verify their presence in the message. If a
  header field defined in a message is in the secure header
  list, it MUST be included in the SecureHeaderFields structure.
  Otherwise, the receiver's MUA MUST warn the user that a non-
  secure header is present.

  Considering secure headers mechanisms, the signature
  verification process MUST perform the following steps:

     1) Execute the signature verification process as described
     in CMS, chapter 5.6. If the signature appears to be invalid,
     the process ends. Otherwise, the process continues.

     2) Read the type of canonization algorithm specified in
     SecureHeaderFields structure.

     3) For each field present in the signature, find the
     matching header in the message. If there is no matching


Cailleux & Bonatti     Expires 26 January 2015         [Page 12]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




     header, the verification process MUST warn the user,
     specifying the missing header name. The signature is tagged
     as invalid.

     4) Compute the canonization algorithm for each header field
     value in the message. If the simple algorithm is used, the
     steps described in DKIM, chapter 3.4.1, are performed. If
     the relaxed algorithm is used, the steps described in DKIM,
     chapter 3.4.2, are performed.

     5) For each field, compare the value stored in the
     SecureHeaderFields structure with the value returned by the
     canonization algorithm. If values don't match, the
     verification process MUST warn the user. This warning MUST
     mention mismatching fields. The signature is tagged as
     invalid. If all the comparisons succeed, the verification
     process MUST also notify the user (i.e., using an
     appropriate icon).

     6) Verify that no secure header has been added to the
     message header, given the initial fields. If an extra header
     field has been added, the verification process MUST warn the
     user. This warning MUST mention extra fields. The signature
     is tagged as invalid.

     7) Verify that every mandatory headers in the security
     policy and present in the message are also embedded in the
     SecureHeaderFields structure. If such headers are missing,
     the verification process MUST warn the user and indicate the
     names of the missing headers.

  The MUA MUST display features for each secure header field
  (name, value and status) and canonization algorithm used.






Cailleux & Bonatti     Expires 26 January 2015         [Page 13]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




4.6. Encryption and Decryption Processes

   Encryption and decryption operations are not performed by
   MUAs. This is mainly justified by IMAP limitations. The
   solution developed here relies on concepts explained in Domain
   Security specification, chapter 4. A fundamental component of
   the architecture is the Domain Confidentiality Authority
   (DCA). Its purpose is to encrypt and decrypt messages instead
   of (respectively) senders and receivers.


4.6.1. Encryption Process

  All the computations presented in this chapter MUST be
  performed only if the following conditions are verified:

     - The content to be encrypted MUST consist of a signature
     object or a multipart object, where one part is a detached
     signature, as shown in S/MIME specification, chapter 3.4.

     - A SecureHeaderFields structure MUST be included in the
     signedAttrs field of the SignerInfo object of the signature.

  All the mechanisms described below MUST start at the beginning
  of the encryption process, as explained in CMS. They are
  performed by the sender's DCA. The following steps MUST be
  performed for each field included in the SecureHeaderFields
  structure:

  1. Extraction of the field status;

     1.1 If the status is Duplicated, the field is left at its
     existing value.

     1.2 If the status is Deleted, the header field (name and
     value) is removed from the message. Mandatory header fields
     specified in [RFC 5322] MUST be kept.



Cailleux & Bonatti     Expires 26 January 2015         [Page 14]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




     1.3 If the status is Modified, the header value is replaced
     by a new value, as configured in the DCA.


4.6.2. Decryption Process

  All the computations presented in this chapter MUST be
  performed only if the following conditions are verified:

     - The decrypted content MUST consist of a signature object
     or a multipart object, where one part is a detached
     signature, as shown in S/MIME specification, chapter 3.4.

     - A SecureHeaderFields structure MUST be included in the
     SignerInfo object of the signature.

  All the mechanisms described below MUST start at the end of
  the decryption process, as explained in CMS. They are executed
  by the receiver's DCA. The following steps MUST be performed
  for each field included in the SecureHeaderFields structure:

     1. If the status is Duplicated, the field is left at its
     existing value.

     2. If the status is Deleted, the DCA MUST write a header
     field (name and value) in the message. This header MUST be
     compliant with the information embedded in the signature.

     3. If the status is Modified, the DCA MUST rewrite a header
     field in the message. This header MUST be compliant with the
     SecureHeaderFields structure.

5. Case of triple wrapping

  Secure Headers mechanisms MAY be used with triple wrapping, as
  described in ESS [RFC 2634]. In this case, a
  SecureHeaderFields structure MAY be present in the inner



Cailleux & Bonatti     Expires 26 January 2015         [Page 15]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




  signature, in the outer signature, or both. In the last case,
  the two structure SecureHeaderFields MAY differ. One MAY
  consider the encapsulation of a header field in the inner
  signature in order to satisfy confidentiality needs. On the
  contrary, an outer signature encapsulation MAY help for
  delivery purpose. Header fields processing, given the
  signature type (inner or outer), is out of the scope of this
  document.

6. Security Gateways

  Some security gateways sign or verify messages that pass
  through them. Compliant gateways MUST apply the process
  described in chapter 4.5.

  For non-compliant gateways, the presence of SecureHeaderFields
  structure do not change their behavior.

  In some case, gateways MUST generate new signature or insert
  signerInfos into the signedData block. The format of
  signatures generated by gateways is outside the scope of this
  document.

7. Security Considerations

  This specification describes an extension of the S/MIME
  standard. It provides message headers integrity, non-
  repudiation and confidentiality. The signature and encryption
  processes are complementary. However, according to the
  security policy, only the signature mechanism MAY be
  prescribed. In this case, the signature process is implemented
  between MUAs. The encryption process requires signed messages
  with Secure Headers extension. If required, the encryption
  process is implemented by DCAs.

  This specification doesn't address end-to-end confidentiality
  for message header fields. Sent and received messages by MUAs



Cailleux & Bonatti     Expires 26 January 2015         [Page 16]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




  MAY appear in plaintext. In order to avoid interception, the
  use of TLS is recommended between MUAs and DCAs (uplink and
  downlink). Another solution might be the use of S/MIME between
  MUAs and DCAs in the same domain.

  For the header field confidentiality mechanism to be effective
  all DCAs supporting confidentiality must support SH
  processing. Otherwise, there is a risk in the case where
  headers are not obscured upon encryption, or not restored upon
  decryption process. In the former case confidentiality of the
  header fields is compromised. In the latter case the integrity
  of the headers will appear to be compromised.

8. IANA Considerations

  IANA must register a suitable Object Identifier (OID) value
  for the identifier id-aa-secureHeaderFieldsIdentifier.  This
  value will be used to identify an authenticated attribute
  carried within a CMS [RFC 5652] wrapper.  This attribute OID
  appears in Section 4.1, and again in the reference definition
  in Appendix A.  An appropriate registry arc is suggested in
  those instances of the draft text.

9. References


9.1. Normative References

   [RFC 2045]  Borenstein, N., "Multipurpose Internet Mail
               Extensions Part One", RFC 2045, November 1996.

   [RFC 2119]  Bradner, S., "Key words for use in RFCs to
               indicate requirement levels", RFC 2119, March
               1997.

   [RFC 2634]  Hoffman, P., "Enhanced Security Services for
               S/MIME", RFC 2634, June 1999.




Cailleux & Bonatti     Expires 26 January 2015         [Page 17]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




   [RFC 4853]  Housley, R., "Cryptographic Message Syntax (CMS),
               Multiple Signer Clarification", RFC 4853, April
               2007.

   [RFC 5322]  Resnick, P., "Internet Message Format", RFC 5322,
               October 2008.

   [RFC 5652]  Housley, R., "Cryptographic Message Syntax (CMS)",
               RFC 5652, September 2009.

   [RFC 6376]  Crocker, D., Hansen, T., Kucherawy, M., DomainKeys
               Identified Mail (DKIM) Signatures", RFC 6376,
               September 2011.

   [X.680]     ITU-T. Recommendation X.680 : Abstract Syntax
               Notation One (ASN.1): Specification of basic
               notation, November 2008.


9.2. Informative References

   [RFC 2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
               RFC 2595, June 1999.

   [RFC 3183]  Dean, T., Ottaway, W., "Domain security services
               using S/MIME", RFC 3183, October 2001.

   [RFC 3207]  Hoffman, P., "SMTP Service Extension for secure
               SMTP over Transport Layer Security", RFC 3207,
               February 2002.

   [RFC 3501]  Crispin, M., "Internet Message Access Protocol,
               version 4rev1", RFC 3501, March 2003.

   [RFC 5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC
               5321, October 2008.

   [RFC 5598]  Crocker, D., "Internet Mail Architecture", RFC
               5598, July 2009.





Cailleux & Bonatti     Expires 26 January 2015         [Page 18]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




   [RFC 5750]  Ramsdell, B., Turner, S., "Secure/Multipurpose
               Internet Mail Extensions (S/MIME) Version 3.2
               Certificate Handling", RFC 5750, January 2010.

   [RFC 5751]  Ramsdell, B., Turner, S., "Secure/Multipurpose
               Internet Mail Extensions (S/MIME) Version 3.2,
               Message Specification", RFC 5751, January 2010.





































Cailleux & Bonatti     Expires 26 January 2015         [Page 19]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




Appendix A. Formal syntax of Secure Header

  ASN.1 notation [X.680] of secure header structure is the
  follow:

   SecureHeaderFields ::= SET {
        canonAlgorithm Algorithm,
        secHeaderFields HeaderFields }

   id-aa-secureHeaderFieldsIdentifier OBJECT IDENTIFIER ::=
      iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-
      9(9) smime(16) id-aa(2) secure-headers (to be defined) }

   Algorithm ::= ENUMERATED {
        canonAlgorithmSimple(0),
        canonAlgorithmRelaxed(1) }

   HeaderFields ::= SET SIZE (1..max-header-fields) OF
      HeaderField

   max-header-fields INTEGER ::= MAX

   HeaderField ::= SEQUENCE {
        field-Name HeaderFieldName,
        field-Value HeaderFieldValue,
        field-Status HeaderFieldStatus DEFAULT duplicated }

   HeaderFieldName ::= VisibleString (FROM (ALL EXCEPT (":")))
        -- This description matches with the description of
        -- field name in the chapters 2.2 and 3.6.8 of RFC 5322

   HeaderFieldValue ::= IA5String (FROM (ALL EXCEPT (
      { 0, 0 }..{ 0, 9 } | { 0, 11 } | { 0, 12 } |
      { 0, 14 }..{ 1, 15 } | { 7, 15 })))
        -- This description matches with the description of
        -- field body in the chapter 2.2 of RFC 5322

   HeaderFieldStatus ::= INTEGER {
        duplicated(0), deleted(1), modified(2) }




Cailleux & Bonatti     Expires 26 January 2015         [Page 20]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




Appendix B. Secure Header Fields example

   In the following example, header fields subject, from, to and
   x-ximf-primary-precedence are secured and integrated in a
   SecureHeaders structure.

   Extract of message header fields

       From: John Doe <jdoe@example.com>
       To: Mary Smith <mary@example.com>
       Subject: This is a test
       X-ximf-primary-precedence: priority

SecureHeaders structure extracted from signature:

  2286  163:         SEQUENCE {
  2289   11:           OBJECT IDENTIFIER
                            '1 2 840 113549 1 9 16 2 80'
  2302  147:           SET {
  2305  144:             SET {
  2308    4:               ENUMERATED 1
  2314  135:                 SET {
  2317   40:                   SEQUENCE {
  2319   25:                     IA5String 'x-ximf-primary-
                                            precedence'
  2346    8:                     IA5String 'priority'
  2356    1:                     INTEGER 0
           :                     }
  2359   25:                   SEQUENCE {
  2361    2:                     IA5String 'to'
  2365   16:                     IA5String 'mary@example.com'
  2383    1:                     INTEGER 0
           :                     }
  2386   34:                   SEQUENCE {
  2388    4:                     IA5String 'from'
  2394   23:                     IA5String 'jdoe
                                           <jdoe@example.com>'
  2419    1:                     INTEGER 0


Cailleux & Bonatti     Expires 26 January 2015         [Page 21]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




           :                     }
  2422   28:                   SEQUENCE {
  2424    7:                     IA5String 'subject'
  2433   14:                     IA5String 'This is a test'
  2449    1:                     INTEGER 0
           :                     }
           :                   }
           :                 }
           :              }
           :           }


   Example is displayed as an output of Peter Gutmann's
   "dumpasn1" program.

   OID used in this example is non-official.


























Cailleux & Bonatti     Expires 26 January 2015         [Page 22]


Internet-Draft    Securing Header Fields with S/MIME     Jul 2014




Appendix C. Acknowledgements

  The authors would like to thank Alexey Melnikov, Damien Roque,
  Thibault Cassan, William Ottaway, and Sean Turner who kindly
  provided reviews of the document and/or suggestions for
  improvement. As always, all errors and omissions are the
  responsibility of the authors.

Authors' Addresses

  Laurent CAILLEUX
  DGA MI
  BP 7
  35998 RENNES CEDEX 9
  France
  Email: laurent.cailleux@intradef.gouv.fr

  Chris Bonatti
  IECA, Inc.
  3057 Nutley Street, Suite 106
  Fairfax, VA  22031
  USA
  Email: bonatti252@ieca.com


















Cailleux & Bonatti     Expires 26 January 2015         [Page 23]


Html markup produced by rfcmarkup 1.129b, available from https://tools.ietf.org/tools/rfcmarkup/