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Network Working Group                                            T. DeanRequest for Comments: 3183                                    W. OttawayCategory: Experimental                                           QinetiQ                                                            October 2001Domain Security Services using S/MIMEStatus of this Memo   This memo defines an Experimental Protocol for the Internet   community.  It does not specify an Internet standard of any kind.   Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2001).  All Rights Reserved.Abstract   This document describes how the S/MIME (Secure/Multipurpose Internet   Mail Extensions) protocol can be processed and generated by a number   of components of a communication system, such as message transfer   agents, guards and gateways to deliver security services.  These   services are collectively referred to as 'Domain Security Services'.Acknowledgements   Significant comments were made by Luis Barriga, Greg Colla, Trevor   Freeman, Russ Housley, Dave Kemp, Jim Schaad and Michael Zolotarev.1. Introduction   The S/MIME [1] series of standards define a data encapsulation format   for the provision of a number of security services including data   integrity, confidentiality, and 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, or when a single domain   wishes to communicate securely with one of its members residing on an   untrusted domain.  The scenarios covered by this document are   domain-to-domain, individual-to-domain and domain-to-individualDean & Ottaway                Experimental                      [Page 1]

RFC 3183         Domain Security Services using S/MIME      October 2001   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 (desktop-to-desktop) 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) 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.   2) Message screening and audit: Server-based mechanisms such as      searching for prohibited words or other content, virus scanning,      and audit, are incompatible with end-to-end encryption.   3) 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.   4) Heterogeneous message formats: One organization using X.400 series      protocols wishes to communicate with another using SMTP.  Message      reformatting at gateways makes end-to-end encryption and signature      validation impossible.   This document describes an approach to solving these problems by   providing message security services at the level of a domain or an   organization.  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.  For   example, a domain may decide to provide desktop-to-desktop signatures   but domain-to-domain encryption services.  Or it may allow desktop-   to-desktop 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.Dean & Ottaway                Experimental                      [Page 2]

RFC 3183         Domain Security Services using S/MIME      October 2001   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 here is a set of   tools that integrators can tailor in different ways to meet different   needs in different circumstances.   Message transfer agents (MTAs), guards, firewalls and protocol   translation gateways all provide domain security services.  As with   desktop 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.   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 [2].2. Overview of Domain Security Services   This section gives an informal overview of the security services that   are provided by S/MIME between different security domains.  These   services are provided by a combination of mechanisms in the sender's   and recipient's domains.   Later sections describe definitively how these services map onto   elements of the S/MIME protocol.   The following security mechanisms are specified in this document:   1. Domain signature   2. Review signature   3. Additional attributes signature   4. Domain encryption and decryption   The signature types defined in this document are referred to as   DOMSEC defined signatures.Dean & Ottaway                Experimental                      [Page 3]

RFC 3183         Domain Security Services using S/MIME      October 2001   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').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".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 [3].Dean & Ottaway                Experimental                      [Page 4]

RFC 3183         Domain Security Services using S/MIME      October 20012.4 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.3. Mapping of the Signature Services to the S/MIME Protocol   This section describes the S/MIME protocol elements that are used to   provide the security services described above.  ESS [3] 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 MAY 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   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 signersDean & Ottaway                Experimental                      [Page 5]

RFC 3183         Domain Security Services using S/MIME      October 2001      the eContentType, within the EncapsulatedContentInfo, MUST be id-      data as described in CMS [5].  However, the eContent field will      contain the unsigned message instead of being left empty as      suggested insection 5.2 in CMS [5].  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.3.1 Naming Conventions and Signature Types   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.   On receiving a message containing this attribute, the naming   convention checks are invoked.   Implementations conforming to this standard MUST support the naming   convention 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.Dean & Ottaway                Experimental                      [Page 6]

RFC 3183         Domain Security Services using S/MIME      October 20013.1.1 Naming Conventions   The following naming conventions are specified for agents generating   signatures specified in this document:   *  For a domain signature, an agent generating this signature MUST be      named 'domain-signing-authority'   *  For a review signature, an agent generating this signature MUST be      named 'review-authority'.   *  For an additional attributes signature, an agent generating this      signature MUST be named 'attribute-authority'.   This name shall appear as the 'common name (CN)' component of the   subject field in the X.509 certificate.  There MUST be only one CN   component present.  Additionally, if the certificate contains anRFC822 address, this name shall appear in the end entity component of   the address - on the left-hand side of the '@' symbol.   In the case of a domain signature, an additional naming rule is   defined: the 'name mapping rule'.  The name mapping rule states that   for a domain signing authority, the domain part of its name MUST be   the same as, or an ascendant of, the domain name of the message   originator(s) that it is representing.  The domain part is defined as   follows:   *  In the case of an X.500 distinguished subject name of an X.509      certificate, the domain part is the country, organization,      organizational unit, state, and locality components of the      distinguished name.   *  In the case of anRFC 2247 distinguished name, the domain part is      the domain components of the distinguished name.   *  If the certificate contains anRFC 822 address, the domain part is      defined to be theRFC 822 address component on the right-hand side      of the '@' symbol.   For example, a domain signing authority acting on behalf of John Doe   of the Acme corporation, whose distinguished name is 'cn=John Doe,   ou=marketing,o=acme,c=us' and whose e-mail address is   John.Doe@marketing.acme.com, could have a certificate containing a   distinguished name of   'cn=domain-signing-authority,o=acme,c=us' and anRFC 822 address of   'domain-signing-authority@acme.com'.  If John Doe has anRFC 2247Dean & Ottaway                Experimental                      [Page 7]

RFC 3183         Domain Security Services using S/MIME      October 2001   defined address of 'cn=John Doe,dc=marketing,dc=acme,dc=us' then an   address of 'cn=domain-signing-authority,dc=acme,dc=us' could be used   to represent the domain signing authority.   When the X.500 distinguished subject name has consecutive   organizational units and/or localities it is important to understand   the ordering of these values in order to determine if the domain part   of the domain signature is an ascendant.  In this case, when parsing   the distinguished subject name from the most significant component   (i.e., country, locality or organization) the parsed organizational   unit or locality is deemed to be the ascendant of consecutive   (unparsed) organizational units or localities.   When parsing anRFC 2247 subject name from the most significant   component (i.e., the 'dc' entry that represents the country, locality   or organization) the parsed 'dc' entry is deemed to be the ascendant   of consecutive (unparsed) 'dc' entries.   For example, a domain signing authority acting on behalf of John Doe   of the Acme corporation, whose distinguished name is 'cn=John Doe,   ou=marketing,ou=defence,o=acme,c=us' and whose e-mail address is   John.Doe@marketing.defence.acme.com, could have a certificate   containing a distinguished name of 'cn=domain-signing-   authority,ou=defence,o=acme,c=us' and anRFC 822 address of 'domain-   signing-authority@defence.acme.com'.  If John Doe has anRFC 2247   defined address of 'cn=John   Doe,dc=marketing,dc=defense,dc=acme,dc=us' then the domain signing   authority could have a distinguished name of 'cn=domain-signing-   authority,dc=defence,dc=acme,dc=us'.   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.   This naming rule prevents agents from one organization masquerading   as domain signing authorities on behalf of another.  For the other   types of signature defined in this document, no such named mapping   rule is defined.   Implementations conforming to this standard MUST support this name   mapping 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.Dean & Ottaway                Experimental                      [Page 8]

RFC 3183         Domain Security Services using S/MIME      October 20013.1.2 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 the previous section.  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 ASN.1 [4] 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 [5].  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 [5].  They are distinguished by the presence of the   following values in the SignatureType signed attribute:      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 insection 3.Dean & Ottaway                Experimental                      [Page 9]

RFC 3183         Domain Security Services using S/MIME      October 2001   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 [5], 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.3.2 Domain Signature Generation and Verification   A 'domain signature' is a proxy signature generated on a user's   behalf in the user's domain.  The signature MUST adhere to the naming   conventions in 3.1.1, including the name mapping convention.  A   'domain signature' on a message authenticates the fact that the   message has been released from that domain.  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 is used, such as the physical address of   the originating client or an authenticated IP link.   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 insection 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 3.1.1.   If the originator's authenticity is not successfully verified or all   the signatures present are not valid, a 'domain signature' MUST NOT   be generated.Dean & Ottaway                Experimental                     [Page 10]

RFC 3183         Domain Security Services using S/MIME      October 2001   On reception, the 'domain signature' SHOULD be used to verify the   authenticity of a message.  A check MUST be made to ensure that both   the naming convention and the name mapping convention have been used   as specified in this standard.   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.3.3 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 containing a subject name that follows the naming   convention specified in 3.1.1.  On reception, a check MUST be made to   ensure that the naming convention has been used.Dean & Ottaway                Experimental                     [Page 11]

RFC 3183         Domain Security Services using S/MIME      October 2001   A signer MAY include any of the attributes listed in [3] 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 MAY be multiple 'additional attributes' signatures in an S/MIME   encoding.3.4 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, a device   on a domain boundary such as a Mail Guard or firewall may be   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 containing a subject name that follows the naming   convention specified in 3.1.1.  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 MAY be multiple review signatures in an S/MIME encoding.Dean & Ottaway                Experimental                     [Page 12]

RFC 3183         Domain Security Services using S/MIME      October 20013.5 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. Encryption and Decryption   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 section as a "Domain Confidentiality Authority"   (DCA).  Both of these processes are described in this section.   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   processes 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 [5].  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.   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.Dean & Ottaway                Experimental                     [Page 13]

RFC 3183         Domain Security Services using S/MIME      October 2001   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 [5].   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 and name mapping convention are 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 [5].  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 Domain Confidentiality Naming Conventions   A DCA MUST be named 'domain-confidentiality-authority'.  This name   MUST appear in the 'common name(CN)' component of the subject field   in the X.509 certificate.  Additionally, if the certificate contains   anRFC 822 address, this name MUST appear in the end entity part of   the address, i.e., on the left-hand side of the '@' symbol.   Along with this naming convention, an additional naming rule is   defined:  the 'name mapping rule'.  The name mapping rule states that   for a DCA, the domain part of its name MUST be the same as, or an   ascendant of (as defined insection 3.1.1), the domain name of the   set of entities that it represents.  The domain part is defined as   follows:   *  In the case of an X.500 distinguished name of an X.509      certificate, the domain part is the country, organization,      organizational unit, state, and locality components of the      distinguished name.   * In the case of anRFC 2247 distinguished name, the domain part is      the domain components of the distinguished name.   * If the certificate contains anRFC 822 address, the domain part is      defined to be theRFC 822 address part on the right-hand side of      the '@' symbol.Dean & Ottaway                Experimental                     [Page 14]

RFC 3183         Domain Security Services using S/MIME      October 2001   For example, a DCA acting on behalf of John Doe of the Acme   corporation, whose distinguished name is 'cn=John Doe,ou=marketing,   o=acme,c=us' and whose e-mail address is John.Doe@marketing.acme.com,   could have a certificate containing a distinguished name of   'cn=domain-confidentiality-authority,o=acme,c=us' and an e-mail   address of 'domain-confidentiality-authority@acme.com'.  If John Doe   has anRFC 2247 defined address of 'cn=John Doe,dc=marketing,   dc=defense,dc=acme,dc=us' then the domain signing authority could   have a distinguished name of   'cn=domain-signing-authority,dc=defence,dc=acme,dc=us'.  The key   associated with this certificate would be used for encrypting   messages for John Doe.   Any message received where the domain part of the domain encrypting   agents name does not match, or is not an ascendant of, the domain   name of the entities it represents MUST be flagged.   This naming rule prevents messages being encrypted for the wrong   domain decryption agent.   Implementations conforming to this standard MUST support this name   mapping 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 sending any messages.4.2 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 [5].  A DCA   encryption agent MUST be named according to the naming convention   specified insection 4.1.  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 directory may be used in   one of the following ways:Dean & Ottaway                Experimental                     [Page 15]

RFC 3183         Domain Security Services using S/MIME      October 2001   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 4.1.  The user certificate attribute      associated with this directory entry is then obtained.   This document does not mandate either of these processes.  Whichever   one is used, the name mapping 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 [5].   DCA encryption may be performed for decryption by the end recipient   and/or by a DCA.  End recipient decryption is described in CMS [5].   DCA decryption is described insection 4.3.4.3 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 [5]; in the second case, the encryption process is   described in 4.2.5. 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 [3]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.Dean & Ottaway                Experimental                     [Page 16]

RFC 3183         Domain Security Services using S/MIME      October 2001   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", seesection 4.0, 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: -      -  The "outer" signedData layer that includes an         mlExpansionHistory attribute or encapsulates an envelopedData         object.      -  An envelopedData layer.      -  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               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.Dean & Ottaway                Experimental                     [Page 17]

RFC 3183         Domain Security Services using S/MIME      October 2001               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.         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: -      1) 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.Dean & Ottaway                Experimental                     [Page 18]

RFC 3183         Domain Security Services using S/MIME      October 2001   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      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,Dean & Ottaway                Experimental                     [Page 19]

RFC 3183         Domain Security Services using S/MIME      October 2001      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.  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.Dean & Ottaway                Experimental                     [Page 20]

RFC 3183         Domain Security Services using S/MIME      October 2001   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. Security Considerations   This specification relies on the existence of several well known   names, such as domain-confidentiality-authority.  Organizations must   take care with these names, even if they do not support DOMSEC, so   that certificates issued in these names are only issued to legitimate   entities.  If this is not true then an individual could get a   certificate associated with domain-confidentiality-authority@acme.com   and as a result might be able to read messages the a DOMSEC client   intended for others.   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.Dean & Ottaway                Experimental                     [Page 21]

RFC 3183         Domain Security Services using S/MIME      October 2001   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   message could be undetectably modified, when there are no signatures   on a message that the recipient can verify.7. DOMSEC ASN.1 Module   DOMSECSyntax    { iso(1) member-body(2) us(840) rsadsi(113549)          pkcs(1) pkcs-9(9) smime(16) modules(0) domsec(10) }    DEFINITIONS IMPLICIT TAGS ::=    BEGIN    -- EXPORTS All    -- The types and values defined in this module are exported for    -- use in the other ASN.1 modules.  Other applications may use    -- them for their own purposes.    SignatureType ::= SEQUENCE OF OBJECT IDENTIFIER    id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)             us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 16 }    id-sti  OBJECT IDENTIFIER ::= { id-smime 9 }   -- signature type    identifier    -- Signature Type Identifiers    id-sti-originatorSig       OBJECT IDENTIFIER ::= { id-sti 1 }    id-sti-domainSig           OBJECT IDENTIFIER ::= { id-sti 2 }    id-sti-addAttribSig        OBJECT IDENTIFIER ::= { id-sti 3 }    id-sti-reviewSig           OBJECT IDENTIFIER ::= { id-sti 4 }    END -- of DOMSECSyntaxDean & Ottaway                Experimental                     [Page 22]

RFC 3183         Domain Security Services using S/MIME      October 20018. References   [1] Ramsdell, B., "S/MIME Version 3 Message Specification",RFC 2633,       June 1999.   [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement       Levels",BCP 14,RFC 2119, March 1997.   [3] Hoffman, P., "Enhanced Security Services for S/MIME",RFC 2634,       June 1999.   [4] International Telecommunications Union, Recommendation X.208,       "Open systems interconnection: specification of Abstract Syntax       Notation (ASN.1)", CCITT Blue Book, 1989.   [5] Housley, R., "Cryptographic Message Syntax",RFC 2630, June 1999.9. Authors' Addresses   Tim Dean   QinetiQ   St. Andrews Road   Malvern   Worcs   WR14 3PS   Phone: +44 (0) 1684 894239   Fax:   +44 (0) 1684 896660   EMail: tbdean@QinetiQ.com   William Ottaway   QinetiQ   St. Andrews Road   Malvern   Worcs   WR14 3PS   Phone: +44 (0) 1684 894079   Fax:   +44 (0) 1684 896660   EMail: wjottaway@QinetiQ.comDean & Ottaway                Experimental                     [Page 23]

RFC 3183         Domain Security Services using S/MIME      October 200110.  Full Copyright Statement   Copyright (C) The Internet Society (2001).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Dean & Ottaway                Experimental                     [Page 24]