Securing Header Fields with S/MIME
draft-cailleux-secure-headers-03

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Network Working Group                                 L. Cailleux 
Internet-Draft                                             DGA MI 
Intended status: Experimental                          C. Bonatti 
Expires: 30 January 2014                                     IECA 
                                                     30 July 2013 
 
 
                 Securing Header Fields with S/MIME 
                  draft-cailleux-secure-headers-03 

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 
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  Drafts as reference material or to cite them other than as 
  "work in progress." 
   
  This Internet-Draft will expire on 30 January 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 
 
 
 
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  (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 
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  Provisions and are provided without warranty as described in 
  the Simplified BSD License. 
   
Table of Contents 

   1. Introduction..............................................2 
   2. Terminology and conventions used in this document.........3 
   3. Context...................................................4 
   4. Mechanisms to secure message header fields................6 
      4.1. ASN.1 syntax of secure header fields.................7 
      4.2. Secure header fields length and format...............8 
      4.3. Canonization algorithm...............................8 
      4.4. Header fields statuses...............................8 
      4.5. Signature Process....................................9 
         4.5.1. Signature Generation Process....................9 
         4.5.2. Signature verification process.................10 
      4.6. Encryption and Decryption Processes.................12 
         4.6.1. Encryption Process.............................12 
         4.6.2. Decryption Process.............................13 
   5. Case of triple wrapping..................................14 
   6. Security Gateways........................................14 
   7. Security Considerations..................................14 
   8. IANA Considerations......................................15 
   9. References...............................................15 
      9.1. Normative References................................15 
      9.2. Informative References..............................16 
   Appendix A. Formal syntax of Secure Header..................17 
   Appendix B. Secure Header Fields example....................18 
   Appendix C. Acknowledgements................................20 
    

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.  
 
 
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  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 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 
  "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. 

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

 
 
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  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 
  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). 

 
 
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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. 
   
  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 

 
 
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  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 } 
    
   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 { 
 
 
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      field-Name HeaderFieldName, 
      field-Value HeaderFieldValue, 
      field-Status HeaderFieldStatus DEFAULT duplicated } 
    
   HeaderFieldName ::= IA5String 
    
   HeaderFieldValue ::= IA5String 
    
   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. 

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, 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 required to provide a 
  confidentiality service toward message headers. Since this 
  mechanism is OPTIONAL, the status field is also OPTIONAL. The 
  three following statuses MUST be used: 

     - Duplicated (default). When this status is present or if no 
     status is specified, the signature process MUST embed the 
     header field in the signature. 
 
 
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     - Deleted. When this status is present, the signature 
     process MUST embed the header field in the signature. Then, 
     the encryption process MUST delete this field from the 
     message. This guarantees header confidentiality during the 
     message transfer. Mandatory header fields, as specified in 
     IMF MUST be kept in the message. 

     - Modified. When this status is present, the signature 
     process MUST embed the header field in the signature. Then, 
     the encryption process MUST modify the value of the header 
     field in the message. This guarantees header confidentiality 
     during the message transfer. Furthermore, modified values 
     MAY inform a receiver's non-compliant MUA that secure 
     headers are being used. The new value for each field is 
     configured by the sender (i.e., this header is secured, use 
     a compliant client). Mandatory header fields, as specified 
     in IMF MUST be kept well formed after the modification 
     process. For example, Date field MUST be compliant with the 
     IMF specification. 

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 
  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. 
   

 
 
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  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. 
   
     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. 
   
     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. 

 
 
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  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 
  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). 
   

 
 
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     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. 
   

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 
 
 
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  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. 
   
     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. 
   

 
 
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     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 
  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 

 
 
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  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 
  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 

  This document has no IANA actions. 

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. 

   [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. 
 
 
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   [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. 

   [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. 

 
 
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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 ::= IA5String 
    
   HeaderFieldValue ::= IA5String 
    
   HeaderFieldStatus ::= INTEGER { 
        duplicated(0), deleted(1), modified(2) } 
    

 
 
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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 
           :                     } 
  2422   28:                   SEQUENCE { 
  2424    7:                     IA5String 'subject' 
 
 
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  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. 

   

 
 
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Appendix C. Acknowledgements 

   
  The author would like to thank 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 Maitrise de l'information 
   BP 7 
   35998 Rennes Armees 
   France 
   Email: laurent.cailleux@dga.defense.gouv.fr 
    
   Chris Bonatti 
   IECA, Inc. 
   3057 Nutley Street, Suite 106 
   Fairfax, VA  22031 
   USA 
   Email: bonatti252@ieca.com 
    

 
 
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