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Independent Submission L. Cailleux
Request for Comments: 7508 DGA MI
Category: Experimental C. Bonatti
ISSN: 2070-1721 IECA
April 2015
Securing Header Fields with S/MIME
Abstract
This document describes how the S/MIME protocol can be extended in
order to secure message header fields defined in RFC 5322. 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 document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. 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; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7508.
Copyright Notice
Copyright (c) 2015 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.
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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. Canonicalization Algorithm .................................8
4.4. Header Field 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 .......................11
4.6.1. Encryption Process .................................11
4.6.2. Decryption Process .................................12
5. Case of Triple Wrapping ........................................13
6. Security Gateways ..............................................13
7. Security Considerations ........................................13
8. IANA Considerations ............................................14
9. References .....................................................14
9.1. Normative References ......................................14
9.2. Informative References ....................................15
Appendix A. Formal Syntax of Secure Header ........................16
Appendix B. Example of Secure Header Fields .......................18
Acknowledgements ..................................................19
Authors' Addresses ................................................19
1. Introduction
The S/MIME [RFC5751] 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 message in a message/rfc822
wrapper in order to apply S/MIME security services to header fields".
However, the S/MIME solution doesn't provide any guidance regarding
what subset of message header fields to secure, procedures for
clients to reconcile the "inner" and "outer" headers, or procedures
for client interpretation or display of any failures.
Several other security specifications supplement S/MIME features but
fail to address the target requirement set of this document. Such
other security specifications include DomainKeys Identified Mail
(DKIM) [RFC6376], STARTTLS [RFC3207], TLS with IMAP [RFC2595], and an
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Internet-Draft referred to as "Protected Headers" [PRHDRS]. An
explanation of what these services accomplish and why they do not
solve this problem can be found in subsequent sections.
The goal of this document is to define end-to-end secure header field
mechanisms compliant with S/MIME standard. This technique is based
on the signed attribute fields of a Cryptographic Message Syntax
(CMS) [RFC5652] 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 [RFC2119].
The terms Message User Agent (MUA), Message Submission Agent (MSA),
and Message Transfer Agent (MTA) are defined in the email
architecture document [RFC5598].
The term Domain Confidentiality Authority (DCA) is defined in the
S/MIME Domain Security specification [RFC3183].
End-to-end Internet Mail exchanges are performed between message
originators and recipients.
The term message header fields is described in [RFC5322]. A header
field is composed of a name and a value.
Secure Headers technology uses header field 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 also
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
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noncompliant MUA that secure headers are being used. New values
for each field might be configured by the sender (i.e., "This
header is secured; use a compliant client.").
The term non-repudiation is used throughout this document in
deference to the usage in the S/MIME Message Specification [RFC5751].
It is recognized that this term carries with it much baggage, and
that there is some disagreement as to its proper meaning and usage.
However, in the context of this document, the term merely refers to
one set of possible security services that a conforming
implementation might be able to provide. This document specifies no
normative requirements for non-repudiation.
3. Context
Over the Internet, email use has grown and today represents a
fundamental service. Meanwhile, continually increasing threat levels
are motivating the implementation of security services.
Historically, SMTP [RFC5321] and the Internet Message Format (IMF)
[RFC5322] don't provide, by default, security services. The S/MIME
standard [RFC5751] was published in order to address 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.
In Section 3.1 of [RFC5751], S/MIME defines an encapsulation
mechanism:
[...] 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.
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Some partial header protection is provided by the S/MIME Certificate
Handling specification [RFC5750]:
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.
In some cases, this may provide assurance of the integrity of the
From or Sender header values. However, the solution in RFC 5750 only
provides a matching mechanism between email addresses and provides no
protection to other header fields.
Other security specifications (introduced below) exist such as DKIM,
STARTTLS and TLS with IMAP, but they meet other needs (signing
domain, secure channels, etc.).
STARTTLS and TLS with IMAP provide secure channels between components
of the email system (MUA, MSA, MTA, etc.), but end-to-end integrity
cannot be guaranteed.
DKIM defines a domain-level authentication framework for email.
While this permits integrity and origination checks on message header
fields and the message body, it does this for a domain actor (usually
the SMTP service or equivalent) and not for the entity that is
sending, and thus signing, the message. (Extensions to DKIM might be
able to solve this issue by authenticating the sender and making a
statement of this fact as part of the signed message headers.) DKIM
is also deficient for our purposes, as it does not provide a
confidentially service.
An Internet-Draft referred to as "Protected Headers" [PRHDRS] has
been proposed. Mechanisms described in that document are the
following:
[...] a digest value is computed over the canonicalized version of
some selected header fields. This technique resembles header
protection in [RFC4871]. Then the digest value is included in a
signed attribute field of a CMS signature.
(Note that RFC 4871 has been obsoleted by RFC 6376.)
That 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.
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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 the 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).
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. How this security policy is agreed upon
or communicated is beyond the scope of this document.
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 (SecureHeaderFields structure)
that is encapsulated in the signed attributes structure of the
SignerInfo object. There is only one value of HeaderFields encoded
into a single SignedAttribute in a signature. See Appendix A for an
example. For each header field present in the secure signature, a
status can be set. Then, as described in Section 5.4 of CMS
[RFC5652], the message digest calculation process computes a message
digest on the content together with the signed attributes. Details
of the signature generation process are in Section 4.5.1 of this
document.
Verification of secure header fields is based on the 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 in Section 4.5.2 of this document.
Non-conforming S/MIME clients will ignore the signed attribute
containing the SecureHeaderFields 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.
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For different reasons (e.g., usability, limits of IMAP [RFC3501]),
encryption and decryption processes are performed by a third party.
The third party that performs these processes is referred to in the
Domain Security specification as a Domain Confidentiality Authority
(DCA). Details of the encryption and decryption processes are in
Sections 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
The ASN.1 syntax [ASN1-88] of the SecureHeaderFields structure is as
follows:
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) secureHeaderFieldsIdentifier(55) }
Algorithm ::= ENUMERATED {
canonAlgorithmSimple(0),
canonAlgorithmRelaxed(1) }
HeaderFields ::= SEQUENCE SIZE (1..MAX) OF HeaderField
HeaderField ::= SEQUENCE {
field-Name HeaderFieldName,
field-Value HeaderFieldValue,
field-Status HeaderFieldStatus DEFAULT duplicated }
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HeaderFieldName ::= VisibleString (FROM (ALL EXCEPT (":")))
-- This description matches the description of
-- field name in Sections 2.2 and 3.6.8 of RFC 5322
HeaderFieldValue ::= UTF8String
-- This description matches the description of
-- field body in Section 2.2 of RFC 5322 as
-- extended by Section 3.1 of RFC 6532.
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. Canonicalization Algorithm
During a message transfer through a messaging system, some components
might modify headers (i.e., adding or deleting space, changing or
lowercase or uppercase). This might lead to a comparison mismatch of
header fields. This emphasizes the need of a conversion process in
order to transform data to their canonical form. This process is
named the canonicalization process.
Two canonicalization algorithms are considered here, according to
Section 3.4 of the DKIM specification [RFC6376]. The "simple"
algorithm doesn't allow any modification, whereas the "relaxed"
algorithm accepts slight modifications like space replacement or line
reformatting. Given the scope of this document, canonicalization
mechanisms only involve header fields.
Implementations SHOULD use the "relaxed" algorithm to promote
interoperability with non-conforming SMTP products.
4.4. Header Field Statuses
Header field statuses are necessary to provide a confidentiality
service for message headers. In this specification, the
confidentiality of header fields is provided by the DCA. This point
is described in Section 4. The DCA performs the message encryption
process and message decryption process; these processes are described
in detail in Sections 4.6.1 and 4.6.2. Although header field
statuses are embedded in the signature, the signature processes
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(generation and verification) ignore them. The header field status
defaults to "duplicated". If the header field is confidential, the
header field status MUST be either "deleted" or "modified".
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. The SecureHeaderFields structure MUST include a
canonicalization algorithm.
The sender's MUA MUST have a list of header fields to secure,
statuses, and a canonicalization algorithm, as defined by the
security policy.
Header fields (names and values) embedded in signed attributes MUST
be the same as those 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 the CMS and Multiple
Signer [RFC4853] specifications, the 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 header 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 canonicalization algorithm for each selected header
field.
3) Complete the following fields in the SecureHeaderFields
structure according to the initial message: HeaderFieldName,
HeaderFieldValue, and HeaderFieldStatus.
4) Complete the algorithm field according to the canonicalization
algorithm configured.
5) Embed the SecureHeaderFields structure in the signed attributes
of the SignerInfos object.
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6) Compute the signature generation process as described in
Section 5.5 of CMS [RFC5652].
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
canonicalization 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 header mechanisms, the signature verification
process MUST perform the following steps:
1) Execute the signature verification process as described Section
5.6 of CMS [RFC5652]. If the signature appears to be invalid,
the process ends. Otherwise, the process continues.
2) Read the type of canonicalization algorithm specified in the
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. Note that
any header fields encrypted as per Section 4.6 (i.e., status of
"deleted" or "modified") have been are already restored by the
DCA when the signature verification process is performed by the
MUA.
4) Compute the canonicalization algorithm for each header field
value in the message. If the "simple" algorithm is used, the
steps described in Section 3.4.1 of DKIM [RFC6376] are
performed. If the relaxed algorithm is used, the steps
described in Section 3.4.2 of DKIM [RFC6376] are performed.
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5) For each field, compare the value stored in the
SecureHeaderFields structure with the value returned by the
canonicalization algorithm. If the 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. This step is only performed if the sender and the
recipient share the same security policy.
7) Verify that each mandatory header in the security policy and
present in the message is 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 the properties of each secure header field
(name, value, and status) and the canonicalization algorithm used.
4.6. Encryption and Decryption Processes
Encryption and decryption operations are not performed by MUAs. This
is mainly justified by limitations of existing email delivery
protocols, for example, IMAP. The solution developed here relies on
concepts explained in Section 4 of the Domain Security specification
[RFC3183]. A fundamental component of the architecture is the Domain
Confidentiality Authority (DCA). Its purpose is to encrypt and
decrypt messages instead of that being performed by senders and
receivers (respectively).
4.6.1. Encryption Process
All the computations presented in this section MUST be performed only
if the following conditions are verified:
- The content to be encrypted MUST consist of a signed message
(application/pkcs7-mime with SignedData, or multipart/signed)
as shown in Section 3.4 of the S/MIME specification [RFC5751].
- A SecureHeaderFields structure MUST be included in the
signedAttrs field of the SignerInfo object of the signature.
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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. For extraction of the field status, 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 header field (name and value)
is removed from the message. Mandatory header fields specified
in [RFC5322] MUST be kept.
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 section 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 Section 3.4 of the S/MIME specification [RFC5751].
- 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.
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5. Case of Triple Wrapping
Secure Headers mechanisms MAY be used with triple wrapping, as
described in Enhanced Security Services (ESS) [RFC2634]. In this
case, a SecureHeaderFields structure MAY be present in the inner
signature, the outer signature, or both. In the last case, the two
SecureHeaderFields structures 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 purposes. The sender's MUA and
receiver's MUA must have a security policy for triple wrapping. This
security policy MUST be composed of two parts -- one for the inner
signature and the other for the outer signature.
6. Security Gateways
Some security gateways sign or verify messages that pass through
them. Compliant gateways MUST apply the process described in Section
4.5.
For noncompliant gateways, the presence of a SecureHeaderFields
structure does not change their behavior.
In some case, gateways MUST generate a 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 header integrity, non-repudiation, and
confidentiality. The signature and encryption processes are
complementary. However, according to the security policy, only the
signature mechanism is applicable. In this case, the signature
process is implemented between MUAs. The encryption process requires
signed messages with the 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. Messages sent and received by MUAs could be
transmitted as 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 Secure Headers
processing. Otherwise, there is a risk that headers are not obscured
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upon encryption or not restored upon decryption. 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 has registered value 65, mod-sMimeSecureHeadersV1, in the "SMI
Security for S/MIME Module Identifier (1.2.840.113549.1.9.16.0)"
registry.
IANA has also registered value 55,
id-aa-secureHeaderFieldsIdentifier, in the "SMI Security for S/MIME
Attributes (1.2.840.113549.1.9.16.2)" registry. This value will be
used to identify an authenticated attribute carried within a CMS
wrapper [RFC5652]. This attribute OID appears in Section 4.1 and
again in the reference definition in Appendix A.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999,
<http://www.rfc-editor.org/info/rfc2634>.
[RFC4853] Housley, R., "Cryptographic Message Syntax (CMS) Multiple
Signer Clarification", RFC 4853, April 2007,
<http://www.rfc-editor.org/info/rfc4853>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008, <http://www.rfc-editor.org/info/rfc5322>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009,
<http://www.rfc-editor.org/info/rfc5652>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, September 2011,
<http://www.rfc-editor.org/info/rfc6376>.
[ASN1-88] CCITT, Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1), 1988.
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9.2. Informative References
[PRHDRS] Liao, L. and J. Schwenk, "Header Protection for S/MIME",
draft-liao-smimeheaderprotect-05, Work in Progress, June
2009.
[RFC2595] Newman, C., "Using TLS with IMAP, POP3 and ACAP", RFC
2595, June 1999, <http://www.rfc-editor.org/info/rfc2595>.
[RFC3183] Dean, T. and W. Ottaway, "Domain Security Services using
S/MIME", RFC 3183, October 2001,
<http://www.rfc-editor.org/info/rfc3183>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, February 2002,
<http://www.rfc-editor.org/info/rfc3207>.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008, <http://www.rfc-editor.org/info/rfc5321>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598, July
2009, <http://www.rfc-editor.org/info/rfc5598>.
[RFC5750] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Certificate
Handling", RFC 5750, January 2010,
<http://www.rfc-editor.org/info/rfc5750>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010,
<http://www.rfc-editor.org/info/rfc5751>.
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Appendix A. Formal Syntax of Secure Header
Note: The ASN.1 module contained herein uses the 1988 version of
ASN.1 notation [ASN1-88] for the purposes of alignment with the
existing S/MIME specifications. The SecureHeaderFields structure is
defined as follows:
mod-SMimeSecureHeadersV1
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-9(9) smime(16) modules(0) secure-headers-v1(65) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
id-aa
FROM SecureMimeMessageV3dot1
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0)
msg-v3dot1(21) };
-- id-aa is the arc with all new authenticated and
-- unauthenticated attributes produced by the S/MIME
-- Working Group
id-aa-secureHeaderFieldsIdentifier OBJECT IDENTIFIER ::= {
id-aa secure-headers(55) }
SecureHeaderFields ::= SET {
canonAlgorithm Algorithm,
secHeaderFields HeaderFields }
Algorithm ::= ENUMERATED {
canonAlgorithmSimple(0),
canonAlgorithmRelaxed(1) }
HeaderFields ::= SEQUENCE SIZE (1..MAX) OF HeaderField
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 Sections 2.2 and 3.6.8 of RFC 5322
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HeaderFieldValue ::= UTF8String
-- This description matches with the description of
-- field body in the Section 2.2 of RFC 5322 as
-- extended by Section 3.1 of RFC 6532.
HeaderFieldStatus ::= INTEGER {
duplicated(0), deleted(1), modified(2) }
END
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Appendix B. Example of Secure Header Fields
In the following example, the header fields subject,
x-ximf-primary-precedence, and x-ximf-correspondance-type are secured
and integrated in a SecureHeaderFields structure. This example
should produce a valid signature.
Extract from the message header fields:
From: John Doe <jdoe@example.com>
To: Mary Smith <mary@example.com>
subject: This is a test of Ext.
x-ximf-primary-precedence: priority
x-ximf-correspondance-type: official
The SecureHeaderFields structure extracted from the signature:
2286 150: SEQUENCE {
2289 11: OBJECT IDENTIFIER '1 2 840 113549 1 9 16 2 80'
2302 134: SET {
2305 131: SET {
2308 4: ENUMERATED 1
2314 123: SEQUENCE {
2316 40: SEQUENCE {
2318 25: VisibleString 'x-ximf-primary-precedence'
2345 8: UTF8String 'priority'
2355 1: INTEGER 0
: }
2358 41: SEQUENCE {
2360 26: VisibleString 'x-ximf-correspondance-type'
2388 8: UTF8String 'official'
2398 1: INTEGER 0
: }
2401 36: SEQUENCE {
2403 7: VisibleString 'subject'
2412 22: UTF8String 'This is a test of Ext.'
2436 1: INTEGER 0
: }
: }
: }
: }
: }
The example is displayed as an output of Peter Gutmann's "dumpasn1"
program.
OID used in this example is nonofficial.
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Acknowledgements
The authors would like to thank Jim Schaad, 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
United States
EMail: bonatti252@ieca.com
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