Domain-based signing and encryption using S/MIME
draft-melnikov-smime-msa-to-mda-03

Network Working Group                                         W. Ottaway
Internet-Draft                                                   QinetiQ
Intended status: Standards Track                        A. Melnikov, Ed.
Expires: August 18, 2014                                       Isode Ltd
                                                       February 14, 2014

            Domain-based signing and encryption using S/MIME
                   draft-melnikov-smime-msa-to-mda-03

Abstract

   The S/MIME protocols Message Specification (RFC 5751), Cryptographic
   Message Syntax (RFC 5652), S/MIME Certificate Handling (RFC 5750) and
   Enhanced Security Services for S/MIME (RFC 2634) specify a consistent
   way to securely send and receive MIME messages providing end to end
   integrity, authentication, non-repudiation and confidentiality.  This
   document identifies a number of interoperability, technical,
   procedural and policy related issues that may result in end-to-end
   security services not being achievable.  To resolve such issues, this
   document profiles domain-based signing and encryption using S/MIME,
   such as specifying how S/MIME signing and encryption can be applied
   between a Message Submission Agent (MSA) and a Message Delivery Agent
   (MDA) or between 2 Message Transfer Agents (MTA).

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 18, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
     2.1.  Domain Signature  . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Review Signature  . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Additional Attributes Signature . . . . . . . . . . . . .   6
     2.4.  Domain Encryption and Decryption  . . . . . . . . . . . .   6
     2.5.  Signature Encapsulation . . . . . . . . . . . . . . . . .   6
     2.6.  Naming Conventions  . . . . . . . . . . . . . . . . . . .   7
   3.  Domain-Based S/MIME Signing . . . . . . . . . . . . . . . . .   8
     3.1.  General . . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Signature Type Attribute  . . . . . . . . . . . . . . . .   9
     3.3.  Domain Signature Generation and Verification  . . . . . .  10
     3.4.  Additional Attributes Signature Generation and
           Verification  . . . . . . . . . . . . . . . . . . . . . .  12
     3.5.  Review Signature Generation and Verification  . . . . . .  12
     3.6.  Originator Signature  . . . . . . . . . . . . . . . . . .  13
   4.  Domain-based S/MIME Encryption and Decryption . . . . . . . .  13
     4.1.  Key Management for DCA Encryption . . . . . . . . . . . .  14
     4.2.  Key Management for DCA Decryption . . . . . . . . . . . .  15
   5.  Applying a Domain Signature when Mail List Agents are Present  15
     5.1.  Examples of Rule Processing . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     6.1.  SMTP URI registration . . . . . . . . . . . . . . . . . .  20
     6.2.  SUBMIT URI registration . . . . . . . . . . . . . . . . .  21
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  23
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Appendix A.  Changes from RFC 3183  . . . . . . . . . . . . . . .  25
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   The S/MIME [RFC5750][RFC5751] series of standards define a data
   encapsulation format for the provision of a number of security
   services including data integrity, confidentiality, and

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   authentication.  S/MIME is designed for use by messaging clients to
   deliver security services to distributed messaging applications.

   The mechanisms described in this document are designed to solve a
   number of interoperability problems and technical limitations that
   arise when different security domains wish to communicate securely,
   for example when two domains use incompatible messaging technologies
   such as the X.400 series and SMTP/MIME [RFC5322], or when a single
   domain wishes to communicate securely with one of its members
   residing on an untrusted domain.  The main scenario covered by this
   document is domain-to-domain, although it is also applicable to
   individual-to-domain and domain-to-individual communications.  This
   document is also applicable to organizations and enterprises that
   have internal PKIs which are not accessible by the outside world, but
   wish to interoperate securely using the S/MIME protocol.

   There are many circumstances when it is not desirable or practical to
   provide end-to-end (MUA-to-MUA) security services, particularly
   between different security domains.  An organization that is
   considering providing end-to-end security services will typically
   have to deal with some if not all of the following issues:

   1.  Message screening and audit: Server-based mechanisms such as
       searching for prohibited words or other unauthorized content,
       virus scanning, and audit, are incompatible with end-to-end
       encryption.  It is generally not acceptable to allow content in/
       out of an organization without checking, so boundary decryption
       is vital.

   2.  PKI deployment issues: There may not be any certificate paths
       between two organizations.  Or an organization may be sensitive
       about aspects of its PKI and unwilling to expose them to outside
       access.  Also, full PKI deployment for all employees, may be
       expensive, not necessary or impractical for large organizations.
       For any of these reasons, direct end-to-end signature validation
       and encryption are impossible.

   3.  Heterogeneous message formats: One organization using X.400
       series protocols wishes to communicate with another using SMTP
       [RFC5321].  Message reformatting at gateways makes end-to-end
       encryption and signature validation impossible.

   4.  Heterogeneous message access methods: Users are accessing mail
       using mechanisms which re-format messages, such as using Web
       browsers.  Message reformatting in the Message Store makes end-
       to-end encryption and signature validation impossible.

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   5.  Problems deploying fully S/MIME capable email clients on some
       platforms.  Signature verification at a border MTA can be coupled
       with use of Authentication-Results header field [RFC7001] to
       convey results of verification.

   This document describes an approach to solving these problems by
   providing message security services at the level of a domain or an
   organization.  Such domain-based or organization-based message
   security services are referred to as domain security services.  This
   document specifies how these 'domain security services' can be
   provided using the S/MIME protocol.  Domain security services may
   replace or complement mechanisms at the desktop/mobile device.  For
   example, a domain may decide to provide MUA-to-MUA signatures but
   domain-to-domain encryption services.  Or it may allow MUA-to-MUA
   services for intra-domain use, but enforce domain-based services for
   communication with other domains.

   Domain services can also be used by individual members of a
   corporation who are geographically remote and who wish to exchange
   encrypted and/or signed messages with their base.

   Whether or not a domain based service is inherently better or worse
   than desktop based solutions is an open question.  Some experts
   believe that only end-to-end solutions can be truly made secure,
   while others believe that the benefits offered by such things as
   content checking at domain boundaries offers considerable increase in
   practical security for many real systems.  The additional service of
   allowing signature checking at several points on a communications
   path is also an extra benefit in many situations.  This debate is
   outside the scope of this document.  What is offered is a
   specification for how domain-based S/MIME signing and encryption can
   be applied in different ways to meet different needs in different
   circumstances.

   Message Transfer Agents (MTAs), Message Submission Agents (MSAs),
   Message Delivery Agents (MDAs), guards, firewalls and protocol
   translation gateways can provide domain security services.  As with
   MUA based solutions, these components must be resilient against a
   wide variety of attacks intended to subvert the security services.
   Therefore, careful consideration should be given to security of these
   components, to make sure that their siting and configuration
   minimises the possibility of attack.

2.  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].

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   The signature types defined in this document are referred to as
   DOMSEC defined signatures.

   The term 'security domain' as used in this document is defined as a
   collection of hardware and personnel operating under a single
   security authority and performing a common business function.
   Members of a security domain will of necessity share a high degree of
   mutual trust, due to their shared aims and objectives.

   A security domain is typically protected from direct outside attack
   by physical measures and from indirect (electronic) attack by a
   combination of firewalls and guards at network boundaries.  The
   interface between two security domains is termed a 'security
   boundary'.  One example of a security domain is an organizational
   network ('Intranet').

   Domain-based Sending Agent - an MSA or sending domain MTA performing
   Domain-based service(s).

   Domain-based Receiving Agent - a receiving domain MTA or MDA
   performing Domain-based service(s).

   Message encryption may be performed by a third party on behalf of a
   set of originators in a domain.  This is referred to as domain
   encryption.  Message decryption may be performed by a third party on
   behalf of a set of recipients in a domain.  This is referred to as
   domain decryption.  The third party that performs these processes is
   referred to in this document as a "Domain Confidentiality Authority"
   (DCA).  As per above, a DCA can be a Domain-based Sending Agent or a
   Domain-based Receiving Agent.

2.1.  Domain Signature

   A domain signature is an S/MIME signature generated on behalf of a
   set of users in a domain.  A domain signature can be used to
   authenticate information sent between domains or between a certain
   domain and one of its individuals, for example, when two 'Intranets'
   are connected using the Internet, or when an Intranet is connected to
   a remote user over the Internet.  It can be used when two domains
   employ incompatible signature schemes internally or when there are no
   certification links between their PKIs.  In both cases messages from
   the originator's domain are signed over the original message and
   signature (if present) using an algorithm, key, and certificate which
   can be processed by the recipient(s) or the recipient(s) domain.  A
   domain signature is sometimes referred to as an "organizational
   signature".

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2.2.  Review Signature

   A third party may review messages before they are forwarded to the
   final recipient(s) who may be in the same or a different security
   domain.  Organizational policy and good security practice often
   require that messages be reviewed before they are released to
   external recipients.  Having reviewed a message, an S/MIME signature
   is added to it - a review signature.  An agent could check the review
   signature at the domain boundary, to ensure that only reviewed
   messages are released.

2.3.  Additional Attributes Signature

   A third party can add additional attributes to a signed message.  An
   S/MIME signature is used for this purpose - an additional attributes
   signature.  An example of an additional attribute is the 'Equivalent
   Label' attribute defined in ESS [RFC2634].

2.4.  Domain Encryption and Decryption

   Domain encryption is S/MIME encryption performed on behalf of a
   collection of users in a domain.  Domain encryption can be used to
   protect information between domains, for example, when two
   'Intranets' are connected using the Internet.  It can also be used
   when end users do not have PKI/encryption capabilities at the
   desktop, or when two domains employ incompatible encryption schemes
   internally.  In the latter case messages from the originator's domain
   are encrypted (or re-encrypted) using an algorithm, key, and
   certificate which can be decrypted by the recipient(s) or an entity
   in their domain.  This scheme also applies to protecting information
   between a single domain and one of its members when both are
   connected using an untrusted network, e.g., the Internet.

2.5.  Signature Encapsulation

   ESS [RFC2634] introduces the concept of triple-wrapped messages that
   are first signed, then encrypted, then signed again.  This document
   also uses this concept of triple-wrapping.  In addition, this
   document also uses the concept of 'signature encapsulation'.
   'Signature encapsulation' denotes a signed or unsigned message that
   is wrapped in a signature, this signature covering both the content
   and the first (inner) signature, if present.

   Signature encapsulation can be performed on the inner and/or the
   outer signature of a triple-wrapped message.

   For example, the originator signs a message which is then
   encapsulated with an 'additional attributes' signature.  This is then

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   encrypted.  A reviewer then signs this encrypted data, which is then
   encapsulated by a domain signature.

   There is a possibility that some policies will require signatures to
   be added in a specific order.  By only allowing signatures to be
   added by encapsulation it is possible to determine the order in which
   the signatures have been added.

   A DOMSEC defined signature MAY encapsulate a message in one of the
   following ways:

   1.  An unsigned message has an empty signature layer added to it
       (i.e., the message is wrapped in a signedData that has a
       signerInfos which contains no elements).  This is to enable
       backward compatibility with S/MIME software that does not have a
       DOMSEC capability.  Since the signerInfos will contain no signers
       the eContentType, within the EncapsulatedContentInfo, MUST be id-
       data as described in CMS [RFC5652].  However, the eContent field
       will contain the unsigned message instead of being left empty as
       suggested in section 5.2 in CMS [RFC5652].  This is so that when
       the DOMSEC defined signature is added, as defined in method 2)
       below, the signature will cover the unsigned message.

   2.  Signature Encapsulation is used to wrap the original signed
       message with a DOMSEC defined signature.  This is so that the
       DOMSEC defined signature covers the message and all the
       previously added signatures.  Also, it is possible to determine
       that the DOMSEC defined signature was added after the signatures
       that are already there.

2.6.  Naming Conventions

   The subject name of the Domain-based sending agent's X.509
   certificate is not restricted as specified in RFC 3183 [RFC3183].  In
   order for a verifier to recognize a signing/encrypting certificate as
   the Domain-based sending agent's certificate, it MUST contain
   uniformResourceIdentifier GeneralName of the format "<protocol
   >://<fully-qualified-domain>" and/or dNSName of the format <fully-
   qualified-domain> in its SubjectAltName [RFC5280].  (Here <fully-
   qualified-domain> is the domain that is being served by the signing/
   encrypting MSA/MTA. <protocol> is "submit" for MSAs and "smtp" for
   MTAs.)

   Any message received where the domain part of the domain signing
   agent's name does not match, or is not an ascendant of, the
   originator's domain name MUST be flagged to the user.

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   This naming rule prevents agents from one organization masquerading
   as domain signing or encryption authorities on behalf of another.
   For the other types of signature defined in future documents, no such
   namin rule is defined.

   Implementations conforming to this standard MUST support this naming
   convention as a minimum.  Implementations MAY choose to supplement
   this convention with other locally defined conventions.  However,
   these MUST be agreed between sender and recipient domains prior to
   secure exchange of messages.

   On verifying the signature, a receiving agent MUST ensure that the
   naming convention has been adhered to.  Any message that violates the
   convention MUST be flagged to the user.

   Note that a X.509 certificate of a signing Domain-based sending agent
   can be distinguished from a certificate of encrypting domain-based
   sending agent by checking for keyUsage as specified in [RFC5280]
   Section 4.2.1.3.

3.  Domain-Based S/MIME Signing

3.1.  General

   An entity receiving an S/MIME signed message would normally expect
   the signature to be that of the originator of the message.  However,
   the message security services defined in this document require the
   recipient to be able to accept messages signed by other entities and/
   or the originator.  When other entities sign the message the name in
   the certificate will not match the message sender's name.  An S/MIME
   compliant implementation would normally flag a warning if there were
   a mismatch between the name in the certificate and the message
   sender's name.  (This check prevents a number of types of masquerade
   attack.)

   In the case of domain security services, this warning condition
   SHOULD be suppressed under certain circumstances.  These
   circumstances are defined by a naming convention that specifies the
   form that the signers name SHOULD adhere to.  Adherence to this
   naming convention avoids the problems of uncontrolled naming and the
   possible masquerade attacks that this would produce.

   As an assistance to implementation, a signed attribute is defined to
   be included in the S/MIME signature - the 'signature type' attribute
   Section 3.2.  On receiving a message containing this attribute, the
   naming convention (see Section 2.6) checks are invoked.

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   Implementations conforming to this standard MUST support the naming
   convention specified in Section 2.6 for signature generation and
   verification.  Implementations conforming to this standard MUST
   recognize the signature type attribute for signature verification.
   Implementations conforming to this standard MUST support the
   signature type attribute for signature generation.

3.2.  Signature Type Attribute

   An S/MIME signed attribute is used to indicate the type of signature.
   This should be used in conjunction with the naming conventions
   specified in Section 2.6.  When an S/MIME signed message containing
   the signature type attribute is received it triggers the software to
   verify that the correct naming convention has been used.

   The following object identifier identifies the SignatureType
   attribute:

   id-aa-signatureType OBJECT IDENTIFIER ::= { iso(1)
             member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) 28 }

   The ASN.1 [ASN.1] notation of this attribute is: -

   SignatureType ::= SEQUENCE OF OBJECT IDENTIFIER

   id-sti  OBJECT IDENTIFIER ::= {iso(1) member-body(2) us(840)
             rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) 9 }
             -- signature type identifier

   If present, the SignatureType attribute MUST be a signed attribute,
   as defined in [RFC5652].  If the SignatureType attribute is absent
   and there are no further encapsulated signatures the recipient SHOULD
   assume that the signature is that of the message originator.

   All of the signatures defined here are generated and processed as
   described in [RFC5652].  They are distinguished by the presence of
   the following values in the SignatureType signed attribute:

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   id-sti-domainSig OBJECT IDENTIFIER ::= { id-sti 2 }
             -- domain signature.

   id-sti-addAttribSig OBJECT IDENTIFIER ::= { id-sti 3 }
             -- additional attributes signature.

   id-sti-reviewSig OBJECT IDENTIFIER ::= { id-sti 4 }
             -- review signature.

   For completeness, an attribute type is also specified for an
   originator signature.  However, this signature type is optional.  It
   is defined as follows:

   id-sti-originatorSig OBJECT IDENTIFIER ::= { id-sti 1 }
             -- originator's signature.

   All signature types, except the originator type, MUST encapsulate
   other signatures.  Note a DOMSEC defined signature could be
   encapsulating an empty signature as defined in Section 2.5.

   A SignerInfo MUST NOT include multiple instances of SignatureType.  A
   signed attribute representing a SignatureType MAY include multiple
   instances of different SignatureType values as an AttributeValue of
   attrValues [RFC5652], as long as the SignatureType 'additional
   attributes' is not present.

   If there is more than one SignerInfo in a signerInfos (i.e., when
   different algorithms are used) then the SignatureType attribute in
   all the SignerInfos MUST contain the same content.

   The following sections describe the conditions under which each of
   these types of signature may be generated, and how they are
   processed.

3.3.  Domain Signature Generation and Verification

   A 'domain signature' is a signature generated on a behalf of a set of
   users in the domain the users are a member of.  The signature MUST
   adhere to the naming conventions in Section 2.6.  A 'domain
   signature' on a message authenticates the fact that the message has
   been released from that domain.  (It also provides integrity and non-
   repudiation of message between Domain-based Sending Agents and
   Receiving Agents.)  Before signing, a process generating a 'domain
   signature' MUST first satisfy itself of the authenticity of the
   message originator.  This is achieved by one of two methods.  Either
   the 'originator's signature' is checked, if S/MIME signatures are
   used inside a domain.  Or if not, some mechanism external to S/MIME

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   is used, such as the physical address of the originating client or an
   authenticated IP link, SMTP authentication credentials, etc.

   If the originator's authenticity is successfully verified by one of
   the above methods and all other signatures present are valid,
   including those that have been encrypted, a 'domain signature' can be
   added to a message.

   If a 'domain signature' is added and the message is received by a
   Mail List Agent (MLA) there is a possibility that the 'domain
   signature' will be removed.  To stop the 'domain signature' from
   being removed the steps in Section 5 MUST be followed.

   An entity generating a domain signature MUST do so using a
   certificate containing a subject name that follows the naming
   convention specified in Section 2.6.

   If the originator's authenticity is not successfully verified or all
   the signatures present are not valid, a 'domain signature' MUST NOT
   be generated.

   On reception, the 'domain signature' SHOULD be used to verify the
   authenticity of a message.  A check MUST be made to ensure that the
   naming convention has been used as specified in Section 2.6.

   A recipient can assume that successful verification of the domain
   signature also authenticates the message originator.

   If there is an originator signature present, the name in that
   certificate SHOULD be used to identify the originator.  This
   information can then be displayed to the recipient.

   If there is no originator signature present, the only assumption that
   can be made is the domain the message originated from.

   A domain signer can be assumed to have verified any signatures that
   it encapsulates.  Therefore, it is not necessary to verify these
   signatures before treating the message as authentic.  However, this
   standard does not preclude a recipient from attempting to verify any
   other signatures that are present.

   The 'domain signature' is indicated by the presence of the value id-
   sti-domainSig in a 'signature type' signed attribute.

   There MAY be one or more 'domain signature' signatures in an S/MIME
   encoding.

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3.4.  Additional Attributes Signature Generation and Verification

   The 'additional attributes' signature type indicates that the
   SignerInfo contains additional attributes that are associated with
   the message.

   All attributes in the applicable SignerInfo MUST be treated as
   additional attributes.  Successful verification of an 'additional
   attributes' signature means only that the attributes are
   authentically bound to the message.  A recipient MUST NOT assume that
   its successful verification also authenticates the message
   originator.

   An entity generating an 'additional attributes' signature MUST do so
   using a certificate that follows the naming convention specified in
   Section 2.6.  On reception, a check MUST be made to ensure that the
   naming convention has been used.

   A signer MAY include any of the attributes listed in [RFC2634] or in
   this document when generating an 'additional attributes' signature.
   The following attributes have a special meaning, when present in an
   'additional attributes' signature:

   1.  Equivalent Label: label values in this attribute are to be
       treated as equivalent to the security label contained in an
       encapsulated SignerInfo, if present.

   2.  Security Label: the label value indicates the aggregate
       sensitivity of the inner message content plus any encapsulated
       signedData and envelopedData containers.  The label on the
       original data is indicated by the value in the originator's
       signature, if present.

   An 'additional attributes' signature is indicated by the presence of
   the value id-sti-addAttribSig in a 'signature type' signed attribute.
   Other Object Identifiers MUST NOT be included in the sequence of OIDs
   if this value is present.

   There can be multiple 'additional attributes' signatures in an S/MIME
   encoding.

3.5.  Review Signature Generation and Verification

   The review signature indicates that the signer has reviewed the
   message.  Successful verification of a review signature means only
   that the signer has approved the message for onward transmission to
   the recipient(s).  When the recipient is in another domain, an agent
   on a domain boundary such as a Mail Guard or firewall may be

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   configured to check review signatures.  A recipient MUST NOT assume
   that its successful verification also authenticates the message
   originator.

   An entity generating a signed review signature MUST do so using a
   certificate that follows the naming convention specified in
   Section 2.6.  On reception, a check MUST be made to ensure that the
   naming convention has been used.

   A review signature is indicated by the presence of the value id-sti-
   reviewSig in a 'signature type' signed attribute.

   There can be multiple review signatures in an S/MIME encoding.

3.6.  Originator Signature

   The 'originator signature' is used to indicate that the signer is the
   originator of the message and its contents.  It is included in this
   document for completeness only.  An originator signature is indicated
   either by the absence of the signature type attribute, or by the
   presence of the value id-sti-originatorSig in a 'signature type'
   signed attribute.

4.  Domain-based S/MIME Encryption and Decryption

   Messages may be encrypted for decryption by the final recipient and/
   or by a DCA in the recipient's domain.  The message may also be
   encrypted for decryption by a DCA in the originator's domain (e.g.,
   for content analysis, audit, key word scanning, etc.).  The choice of
   which of these is actually performed is a system specific issue that
   depends on system security policy.  It is therefore outside the scope
   of this document.  These processes of encryption and decryption are
   shown in the following table.

   +-----------------------+----------------------+-------------------+
   |                       | Recipient Decryption | Domain Decryption |
   +-----------------------+----------------------+-------------------+
   | Originator Encryption |       Case(a)        |      Case(b)      |
   |                       |                      |                   |
   | Domain Encryption     |       Case(c)        |      Case(d)      |
   +-----------------------+----------------------+-------------------+

   Case (a), encryption of messages by the originator for decryption by
   the final recipient(s), is described in CMS [RFC5652].  In cases (c)
   and (d), encryption is performed not by the originator but by the DCA
   in the originator's domain.  In cases (b) and (d), decryption is
   performed not by the recipient(s) but by the DCA in the recipient's
   domain.

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   A client implementation that conforms to this standard MUST support
   case (b) for transmission, case (c) for reception and case (a) for
   transmission and reception.

   A DCA implementation that conforms to this standard MUST support
   cases (c) and (d), for transmission, and cases (b) and (d) for
   reception.  In cases (c) and (d) the 'domain signature' SHOULD be
   applied before the encryption.  In cases (b) and (d) the message
   SHOULD be decrypted before the originators 'domain signature' is
   obtained and verified.

   The process of encryption and decryption is documented in CMS
   [RFC5652].  The only additional requirement introduced by domain
   encryption and decryption is for greater flexibility in the
   management of keys, as described in the following subsections.  As
   with signatures, a naming convention is used to locate the correct
   public key.

   The mechanisms described below are applicable both to key agreement
   and key transport systems, as documented in CMS [RFC5652].  The
   phrase 'encryption key' is used as a collective term to cover the key
   management keys used by both techniques.

   The mechanisms below are also applicable to individual roving users
   who wish to encrypt messages that are sent back to base.

4.1.  Key Management for DCA Encryption

   At the sender's domain, DCA encryption is achieved using the
   recipient DCA's certificate or the end recipient's certificate.  For
   this, the encrypting process must be able to correctly locate the
   certificate for the corresponding DCA in the recipient's domain or
   the one corresponding to the end recipient.  Having located the
   correct certificate, the encryption process is then performed and
   additional information required for decryption is conveyed to the
   recipient in the recipientInfo field as specified in CMS [RFC5652].
   A DCA encryption agent MUST be named according to the naming
   convention specified in Section 2.6.  This is so that the
   corresponding certificate can be found.

   No specific method for locating the certificate to the corresponding
   DCA in the recipient's domain or the one corresponding to the end
   recipient is mandated in this document.  An implementation may choose
   to access a local certificate store to locate the correct
   certificate.  Alternatively, a X.500 or LDAP [RFC4510] directory may
   be used in one of the following ways:

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   1.  The directory may store the DCA certificate in the recipient's
       directory entry.  When the user certificate attribute is
       requested, this certificate is returned.

   2.  The encrypting agent maps the recipient's name to the DCA name in
       the manner specified in Section 2.6.  The user certificate
       attribute associated with the DCA's directory entry is then
       obtained.

   This document does not mandate either of these processes.  Whichever
   one is used, the naming conventions must be adhered to, in order to
   maintain confidentiality.

   Having located the correct certificate, the encryption process is
   then performed.  A recipientInfo for the DCA or end recipient is then
   generated, as described in CMS [RFC5652].

   DCA encryption may be performed for decryption by the end recipient
   and/or by a DCA.  End recipient decryption is described in CMS
   [RFC5652].  DCA decryption is described in Section 4.2.

4.2.  Key Management for DCA Decryption

   DCA decryption uses a private-key belonging to the DCA and the
   necessary information conveyed in the DCA's recipientInfo field.

   It should be noted that domain decryption can be performed on
   messages encrypted by the originator and/or by a DCA in the
   originator's domain.  In the first case, the encryption process is
   described in CMS [RFC5652]; in the second case, the encryption
   process is described in Section 4.1.

5.  Applying a Domain Signature when Mail List Agents are Present

   It is possible that a message leaving a DOMSEC domain may encounter a
   Mail List Agent (MLA) before it reaches the final recipient.  There
   is a possibility that this would result in the 'domain signature'
   being stripped off the message.  We do not want a MLA to remove the
   'domain signature'.  Therefore, the 'domain signature' must be
   applied to the message in such a way that will prevent a MLA from
   removing it.

   A MLA will search a message for the "outer" signedData layer, as
   defined in ESS [RFC2634] section 4.2, and strip off all signedData
   layers that encapsulate this "outer" signedData layer.  Where this
   "outer" signedData layer is found will depend on whether the message
   contains a mlExpansionHistory attribute or an envelopedData layer.

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   There is a possibility that a message leaving a DOMSEC domain has
   already been processed by a MLA, in which case a 'mlExpansionHistory'
   attribute will be present within the message.

   There is a possibility that the message will contain an envelopedData
   layer.  This will be the case when the message has been encrypted
   within the domain for the domain's "Domain Confidentiality Authority"
   (see Section 4), and, possibly, the final recipient.

   How the 'domain signature' is applied will depend on what is already
   present within the message.  Before the 'domain signature' can be
   applied the message MUST be searched for the "outer" signedData
   layer, this search is complete when one of the following is found:

   o  The "outer" signedData layer that includes an mlExpansionHistory
      attribute or encapsulates an envelopedData object.

   o  An envelopedData layer.

   o  The original content (that is, a layer that is neither
      envelopedData nor signedData).

   If a signedData layer containing a mlExpansionHistory attribute has
   been found then:

   1.  Strip off the signedData layer (after remembering the included
       signedAttributes).

   2.  Search the rest of the message until an envelopedData layer or
       the original content is found.

   3.

       A.  If an envelopedData layer has been found then:

           +  Strip off all the signedData layers down to the
              envelopedData layer.

           +  Locate the RecipientInfo for the local DCA and use the
              information it contains to obtain the message key.

           +  Decrypt the encryptedContent using the message key.

           +  Encapsulate the decrypted message with a 'domain
              signature'.

           +  If local policy requires the message to be encrypted using
              S/MIME encryption before leaving the domain then

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              encapsulate the 'domain signature' with an envelopedData
              layer containing RecipientInfo structures for each of the
              recipients and an originatorInfo value built from
              information describing this DCA.

              If local policy does not require the message to be
              encrypted using S/MIME encryption but there is an
              envelopedData at a lower level within the message then the
              'domain signature' MUST be encapsulated by an
              envelopedData as described above.

              An example when it may not be local policy to require S/
              MIME encryption is when there is a link crypto present.

       B.  If an envelopedData layer has not been found then:

           - Encapsulate the new message with a 'domain signature'.

   4.  Encapsulate the new message in a signedData layer, adding the
       signedAttributes from the signedData layer that contained the
       mlExpansionHistory attribute.

   If no signedData layer containing a mlExpansionHistory attribute has
   been found but an envelopedData has been found then: -

   1.  Strip off all the signedData layers down to the envelopedData
       layer.

   2.  Locate the RecipientInfo for the local DCA and use the
       information it contains to obtain the message key.

   3.  Decrypt the encryptedContent using the message key.

   4.  Encapsulate the decrypted message with a 'domain signature'.

   5.  If local policy requires the message to be encrypted before
       leaving the domain then encapsulate the 'domain signature' with
       an envelopedData layer containing RecipientInfo structures for
       each of the recipients and an originatorInfo value built from
       information describing this DCA.

   6.  If local policy does not require the message to be encrypted
       using S/MIME encryption but there is an envelopedData at a lower
       level within the message then the 'domain signature' MUST be
       encapsulated by an envelopedData as described above.

   If no signedData layer containing a mlExpansionHistory attribute has
   been found and no envelopedData has been found then: -

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   1.  Strip off all the signedData layers down to the envelopedData
       Encapsulate the message in a 'domain signature'.

5.1.  Examples of Rule Processing

   The following examples help explain the above rules.  All of the
   signedData objects are valid and none of them are a domain signature.
   If a signedData object was a domain signature then it would not be
   necessary to validate any further signedData objects.

   1.  A message (S1 (Original Content)) (where S = signedData) in which
       the signedData does not include an mlExpansionHistory attribute
       is to have a 'domain signature' applied.  The signedData, S1, is
       verified.  No "outer" signedData is found, after searching for
       one as defined above, since the original content is found, nor is
       an envelopedData or a mlExpansionHistory attribute found.  A new
       signedData layer, S2, is created that contains a 'domain
       signature', resulting in the following message sent out of the
       domain (S2 (S1 (Original Content))).

   2.  A message (S3 (S2 (S1 (Original Content))) in which none of the
       signedData layers includes an mlExpansionHistory attribute is to
       have a 'domain signature' applied.  The signedData objects S1, S2
       and S3 are verified.  There is not an original, "outer"
       signedData layer since the original content is found, nor is an
       envelopedData or a mlExpansionHistory attribute found.  A new
       signedData layer, S4, is created that contains a 'domain
       signature', resulting in the following message sent out of the
       domain (S4 (S3 (S2 (S1 (Original Content))).

   3.  A message (E1 (S1 (Original Content))) (where E = envelopedData)
       in which S1 does not include a mlExpansionHistory attribute is to
       have a 'domain signature' applied.  There is not an original,
       received "outer" signedData layer since the envelopedData, E1, is
       found at the outer layer.  The encryptedContent is decrypted.
       The signedData, S1, is verified.  The decrypted content is
       wrapped in a new signedData layer, S2, which contains a 'domain
       signature'.  If local policy requires the message to be
       encrypted, using S/MIME encryption, before it leaves the domain
       then this new message is wrapped in an envelopedData layer, E2,
       resulting in the following message sent out of the domain (E2 (S2
       (S1 (Original Content)))), else the message is not wrapped in an
       envelopedData layer resulting in the following message (S2 (S1
       (Original Content))) being sent.

   4.  A message (S2 (E1 (S1 (Original Content)))) in which S2 includes
       a mlExpansionHistory attribute is to have a 'domain signature'
       applied.  The signedData object S2 is verified.  The

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       mlExpansionHistory attribute is found in S2, so S2 is the "outer"
       signedData.  The signed attributes in S2 are remembered for later
       inclusion in the new outer signedData that is applied to the
       message.  S2 is stripped off and the message is decrypted.  The
       signedData object S1 is verified.  The decrypted message is
       wrapped in a signedData layer, S3, which contains a 'domain
       signature'.  If local policy requires the message to be
       encrypted, using S/MIME encryption, before it leaves the domain
       then this new message is wrapped in an envelopedData layer, E2.
       A new signedData layer, S4, is then wrapped around the
       envelopedData, E2, resulting in the following message sent out of
       the domain (S4 (E2 (S3 (S1 (Original Content))))).  If local
       policy does not require the message to be encrypted, using S/MIME
       encryption, before it leaves the domain then the message is not
       wrapped in an envelopedData layer but is wrapped in a new
       signedData layer, S4, resulting in the following message sent out
       of the domain (S4 (S3 (S1 (Original Content).  The signedData S4,
       in both cases, contains the signed attributes from S2.

   5.  A message (S3 (S2 (E1 (S1 (Original Content))))) in which none of
       the signedData layers include a mlExpansionHistory attribute is
       to have a 'domain signature' applied.  The signedData objects S3
       and S2 are verified.  When the envelopedData E1 is found the
       signedData objects S3 and S2 are stripped off.  The
       encryptedContent is decrypted.  The signedData object S1 is
       verified.  The decrypted content is wrapped in a new signedData
       layer, S4, which contains a 'domain signature'.  If local policy
       requires the message to be encrypted, using S/MIME encryption,
       before it leaves the domain then this new message is wrapped in
       an envelopedData layer, E2, resulting in the following message
       sent out of the domain (E2 (S4 (S1 (Original Content)))), else
       the message is not wrapped in an envelopedData layer resulting in
       the following message (S4 (S1 (Original Content))) being sent.

   6.  A message (S3 (S2 (E1 (S1 (Original Content))))) in which S3
       includes a mlExpansionHistory attribute is to have a 'domain
       signature' applied.  The signedData objects S3 and S2 are
       verified.  The mlExpansionHistory attribute is found in S3, so S3
       is the "outer" signedData.  The signed attributes in S3 are
       remembered for later inclusion in the new outer signedData that
       is applied to the message.  The signedData object S3 is stripped
       off.  When the envelopedData layer, E1, is found the signedData
       object S2 is stripped off.  The encryptedContent is decrypted.
       The signedData object S1 is verified.  The decrypted content is
       wrapped in a new signedData layer, S4, which contains a 'domain
       signature'.  If local policy requires the message to be
       encrypted, using S/MIME encryption, before it leaves the domain
       then this new message is wrapped in an envelopedData layer, E2.

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       A new signedData layer, S5, is then wrapped around the
       envelopedData, E2, resulting in the following message sent out of
       the domain (S5 (E2 (S4 (S1 (Original Content))))).  If local
       policy does not require the message to be encrypted, using S/MIME
       encryption, before it leaves the domain then the message is not
       wrapped in an envelopedData layer but is wrapped in a new
       signedData layer, S5, resulting in the following message sent out
       of the domain (S5 (S4 (S1 (Original Content).  The signedData S5,
       in both cases, contains the signed attributes from S3.

   7.  A message (S3 (E2 (S2 (E1 (S1 (Original Content)))))) in which S3
       does not include a mlExpansionHistory attribute is to have a
       'domain signature' applied.  The signedData object S3 is
       verified.  When the envelopedData E2 is found the signedData
       object S3 is stripped off.  The encryptedContent is decrypted.
       The signedData object S2 is verified, the envelopedData E1 is
       decrypted and the signedData object S1 is verified.  The
       signedData object S2 is wrapped in a new signedData layer S4,
       which contains a 'domain signature'.  Since there is an
       envelopedData E1 lower down in the message, the new message is
       wrapped in an envelopedData layer, E3, resulting in the following
       message sent out of the domain (E3 (S4 (S2 (E1 (S1 (Original
       Content)))))).

6.  IANA Considerations

   This document registers 2 URI schemes, described in subsections of
   this section.  IANA is requested to add them to the list of Permanent
   URI schemes.

6.1.  SMTP URI registration

      URI scheme name: smtp

      Status: permanent

      URI scheme syntax:
      smtpuri = "smtp://" authority ["/" [ "?" query ]]
      authority = <defined in RFC 3986>
      query = <defined in RFC 3986>
      If :<port> is omitted from authority, the port defaults to 25.
      The query component is reserved for future extensions.

      URI scheme semantics:
      The smtp: URI scheme is used to designate SMTP servers (e.g.
      listener endpoints), SMTP accounts.
      There is no MIME type associated with this URI.

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      Encoding considerations:
      SMTP user names are UTF-8 strings and MUST be percent-encoded as
      required by the URI specification [RFC3986], Section 2.1.

      Applications/protocols that use this URI scheme name:
      The smtp: URI is intended to be used by applications that might
      need access to an SMTP server (for example email clients or MTAs)
      or for SMTP servers to describe their listener endpoints.

      Interoperability considerations:
      Several implementations are already using smtp: URIs for server
      configuration.

      Security considerations: Clients resolving smtp: URIs that wish to
      achieve data confidentiality and/or integrity SHOULD use the
      STARTTLS command (if supported by the server) before starting
      authentication, or use a SASL mechanism, such as GSSAPI, that
      provides a confidentiality security layer.

      Contact: Alexey Melnikov <alexey.melnikov@isode.com>

      Author/Change controller: IESG

      References: [[This document]] and [RFC5321].

6.2.  SUBMIT URI registration

      URI scheme name: submit

      Status: permanent

      URI scheme syntax:
      submituri = "submit://" authority ["/" [ "?" query ]]
      authority = <defined in RFC 3986>
      query = <defined in RFC 3986>
      If :<port> is omitted from authority, the port defaults to 587.
      The query component is reserved for future extensions.

      URI scheme semantics:
      The submit: URI scheme is used to designate SMTP Submission
      servers (e.g. listener endpoints), SMTP accounts.
      There is no MIME type associated with this URI.

      Encoding considerations:
      SMTP user names are UTF-8 strings and MUST be percent-encoded as
      required by the URI specification [RFC3986], Section 2.1.

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      Applications/protocols that use this URI scheme name:
      The submit: URI is intended to be used by applications that might
      need access to an SMTP Submission server (for example email
      clients) or for SMTP Submission servers to describe their listener
      endpoints.

      Interoperability considerations:
      None.

      Security considerations: Clients resolving submit: URIs that wish
      to achieve data confidentiality and/or integrity SHOULD use the
      STARTTLS command (if supported by the server) before starting
      authentication, or use a SASL mechanism, such as GSSAPI, that
      provides a confidentiality security layer.

      Contact: Alexey Melnikov <alexey.melnikov@isode.com>

      Author/Change controller: IESG

      References: [[This document]] and [RFC6409].

7.  Security Considerations

   Implementations MUST protect all private keys.  Compromise of the
   signer's private key permits masquerade.

   Similarly, compromise of the content-encryption key may result in
   disclosure of the encrypted content.

   Compromise of key material is regarded as an even more serious issue
   for domain security services than for an S/MIME client.  This is
   because compromise of the private key may in turn compromise the
   security of a whole domain.  Therefore, great care should be used
   when considering its protection.

   Domain encryption alone is not secure and should be used in
   conjunction with a domain signature to avoid a masquerade attack,
   where an attacker that has obtained a DCA certificate can fake a
   message to that domain pretending to be another domain.

   When an encrypted DOMSEC message is sent to an end user in such a way
   that the message is decrypted by the end users DCA the message will
   be in plain text and therefore confidentiality could be compromised.
   If the recipient's DCA is compromised then the recipient can not
   guarantee the integrity of the message.  Furthermore, even if the
   recipient's DCA correctly verifies a message's signatures, then a

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   message could be undetectably modified, when there are no signatures
   on a message that the recipient can verify.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2634]  Hoffman, P., "Enhanced Security Services for S/MIME", RFC
              2634, June 1999.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, September 2009.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5750]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Certificate
              Handling", RFC 5750, January 2010.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, January 2010.

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC6409]  Gellens, R. and J. Klensin, "Message Submission for Mail",
              STD 72, RFC 6409, November 2011.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [ASN.1]    International Telecommunications Union, , "Open systems
              interconnection: specification of Abstract Syntax Notation
              (ASN.1)", CCITT Recommendation X.208, 1989.

8.2.  Informative References

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

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   [RFC3183]  Dean, T. and W. Ottaway, "Domain Security Services using S
              /MIME", RFC 3183, October 2001.

   [RFC4510]  Zeilenga, K., "Lightweight Directory Access Protocol
              (LDAP): Technical Specification Road Map", RFC 4510, June
              2006.

   [RFC7001]  Kucherawy, M., "Message Header Field for Indicating
              Message Authentication Status", RFC 7001, September 2013.

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Appendix A.  Changes from RFC 3183

   Unlike RFC 3183, subject names of domain signing/encrypting X.509
   certificates don't have to have a specific form.  But Subject
   Alternative Names need to include URIs for domain being protected.

   Incorporated erratum 3757 resolution.

   Updated references and some minor editorial corrections.

Appendix B.  Acknowledgements

   This document contains lots of text from RFC 3183.

   Editors would like to thank Steve Kille, David Wilson and Alan Ross
   for comments and corrections.

Authors' Addresses

   William Ottaway
   QinetiQ
   St. Andrews Road
   Malvern, Worcs  WR14 3PS
   UK

   EMail: wjottaway@QinetiQ.com

   Alexey Melnikov (editor)
   Isode Ltd
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   EMail: Alexey.Melnikov@isode.com

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