Internet Draft                                                  Jim Schaad
draft-ietf-smime-certdist-01.txt                                Microsoft
May 28,
July 6, 1998
Expires in six months

           Certificate Distribution Specification

Status of this memo

  This document is an Internet-Draft. Internet-Drafts are working
  documents of the Internet Engineering Task Force (IETF), its areas,
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  Current methods of publishing certificates in directory services are
  restricted to just certificates.  This document provides a method of
  publishing certificates with secondary support information such as the
  SMimeCapabilities attribute (containing bulk algorithm support) in a
  way that is both authenticated and bound to a given certificate.

  This draft is being discussed on the "ietf-smime" mailing list.  To
  join the list, send a message to <ietf-smime-> <> with the
  single word "subscribe" in the body of the message.  Also, there is a
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1.   Introduction

  This document discusses a new method of publishing certificates in a
  directory to provide authenticated attributes as part of the
  certificate publishing process.  This allows for the addition of
  information such as the SMimeCapabilities attribute from [SMIME] which
  contains information about the bulk encryption algorithms supported by
  the End-Entity's cryptography module.

  Section 2 discusses the current set of publishing methods available
  for use, along with the benefits and restrictions of each method.
  Section 3 covers the definition and properties of a
  SMimeCertificatePublish object.

  Throughout this draft, the terms MUST, MUST NOT, SHOULD, and SHOULD
  NOT are used in capital letters. This conforms to the definitions in
  [MUSTSHOULD]. [MUSTSHOULD] defines the use of these key words to help
  make the intent of standards track documents as clear as possible. The
  same key words are used in this document to help implementers achieve

2.   Current Publishing Methods

  At the present time there

  There are several different ways to publish certificate information.
  These methods include the userCertificate property in LDAP
  directories, sending signed objects between users, and transport of
  certificate files (either bare or as CMS degenerate signed objects).
  Each of these methods has benefits and drawbacks.  Each of these
  methods will now be briefly discussed.

  A public directory may be used to distribute certificates.  LDAP
  currently has the userCertificate property defined just for that
  purpose.  The benefits of using a public directory are that a sender
  may create an encrypted object for a recipient without first receiving
  information (such as a signed message) from the recipient. Most public
  directories currently only contain leaf certificates for individuals
  in the directory entry for the individual.  While some directories,
  such as X.500 directories, provide for a directory entry to contain
  the CA certificate, this is not the case for all directories.  Outside
  of the structure of an X.500 directory the problems associated with
  chaining from the individual's certificate to the CA's directory entry
  in order to obtain it's certificate is difficult to impossible1. impossible.  This
  leads to two drawbacks: First, the set of bulk algorithms supported by
  the recipient is unknown.  Second, no additional certificates may be
  carried which would help in validating the recipient's certificates.

  Using certificate files for certificate distribution has the benefit
  of already being in wide spread use. (They are commonly used for
  certificate distribution from Certificate Authorities either as part
  of the enrollment protocol or from web based repositories.) In the The
  degenerate CMS signed object form, certificate files may carry a set
  of certificates to allow a sender to validate the recipients
  certificates.  However  However, they suffer from two drawbacks.  First, as
  with the public directory, the additional information is not available
  as part of the certificate file.  Second, the certificate is obtained
  from either the recipient one is encrypting for or a third party (not
  a directory).

  Using signed objects for certificate distribution has the benefit of
  allowing additional information such as the SMimeCapabilities
  attribute to be carried as part of the package.  It also allows for
  the inclusion of additional certificates to be used in verifying the
  encryption certificate used to build an encrypted object. However, it
  has the drawback that the initialization process is
  basically done via a one-on-one one-on-
  one process.

3.   SMimeEncryptionCerts

  When publishing one's own encryption certificates, it is often
  advisable to publish a wide selection of certificates to insure
  maximum interoperability.  This section describes an attribute that
  may be used to both identify the set of encryption certificates and
  establish the set of bulk encryption algorithms supported by each of
  the certificates.

  The SMimeEncryptionCerts attribute is used to identify one's own
  encryption certificates to the other party.  This attribute is a
  sequence so that more than one encryption certificate can be
  identified in a single SignerInfo object.  Each certificate is then
  given a set of capabilities so senders can identify the correct
  certificate to use for specific capabilities.

  The structure and OID for the SMimeEncryptionCerts attribute are:

       id-aa-smimeEncryptionCerts OBJECT IDENTIFIER ::= { iso(1)
            member-body(2) us(840)
            rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-aa(2) <TBD> }

       SMimeEncryptionCert ::= SEQUENCE {
            hash           Hash,
            capabilities   SMIMECapabilities

       SMimeEncryptionCerts ::= SEQUENCE OF SmimeEncryptionCert

       Hash ::= OCTET STRING - SHA1 hash of the certificate

  When a certificate appears in an SMimeEncryptionCerts attribute, the
  certificate MUST be included SignedData object.  The order of
  certificates in the SMimeEncryptionCerts attribute is the preferred
  order of use by the sender.  It is expected that the preferred
  certificate in the SMIMEEncrpytionKeyPreference would be the first
  certificate in the SMimeEncryptionCerts attribute.

  If present, the SMimeEncryptionCerts attribute MUST be an
  authenticated attribute; it MUST NOT be an unauthenticated attribute.
  CMS defines authenticatedAttributes as a SET OF AuthAttribute.  A
  SignerInfo MUST NOT include multiple instances of the
  SMimeEncryptionCerts attribute. CMS defines the ASN.1 syntax for the
  authenticated attributes to include attrValues SET OF AttributeValue.
  A SMimeEncryptionCerts attribute MUST only include a single instance
  of AttributeValue. There MUST NOT be zero or multiple instances of
  AttributeValue present in the attrValues SET OF AttributeValue.

4.   SMimeCertificatePublish Object

  The structure of the SMimeCertificatePublish object is defined in this
  section.  This object has the benefit that it is published into a
  directory service (and thus is available to all parties) and it
  contains a signed object that allows it to carry the additional
  information desired to increase interoperability.

  This section describes the LDAP directory schema, the body content and
  additional restrictions on the attribute and signers of the SignedData
  object used in publishing the user's certificate.

  The ASN definition of a SMimeCertificatePublish object is the same a
  CMS signed object.

  SMimeCertificatePublish ::= ContentInfo

  Where the contentType is id-signed-data and the content is a
  SignedData content.

  A SMimeCertificatePublish object MUST MAY contain exactly multiple SignerInfo
  objects.  Each SignerInfo object is independent.  This document
  imposes no restrictions on attributes that appear in more that one
  SignerInfo object.


4.1  Signed Content

  The SMimeCertificatePublish object is explicitly designed to carry no
  body content.  All information is carried in the signed attribute
  section of the SignerInfo.

  The following object identifier is used to distinguish the content of
  a SMimeCertificatePublish:

  id-ct-publishCert OBJECT IDENTIFIER ::=  { iso(1) member-
  body(2) member-body(2)
     us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-ct(1)

  When creating a SMimeCertificatePublish object, the eContent of the
  Signed-Data object is omitted and the eContentType oid OID is set to id-ct-publishCert. id-ct-
  publishCert.  Note this is different from an empty content, which
  would be represented as an octet string containing zero bytes.  The
  hash of the body (used in the id-message-digest attribute) is set to
  the initialization value of the hash function.  (This is expected to
  provide the same result as if you had hashed a body containing exactly
  0 bytes.)


4.2  Signed Attributes

  The signed attributes section MUST be present in the SignerInfo
  object, and the following signed attributes MUST be present: The
  signing-time attribute (from [CMS])
  and [CMS]), the SMimeCapabilities and
  SMIMEEncryptionKeyPreference (from [SMIME]).


4.3  CertificateSet

  This draft imposes additional restrictions on the set of certificates
  to be included in the SignedData object beyond those specified in
  [CMS] and [SMIMECERT].  A chain of certificate from the end-entity
  certificate(s) to the root certificate(s) MUST be included in the
  CertificateSet. Unlike in S/MIME messages the root certificate MUST be
  included in the CertificateSet. The root certificate is included so
  that end-entities have a better chance of finding and independently
  verifying the trustworthiness of the root certificate based on its

  User agents MUST NOT automatically trust any root certificate found in
  a SMimeCertificatePublish object.


4.4  Signing Certificate

  The SMimeCertificatePublish object MUST be signed by a signing
  certificate associated with the end-entity, or a signing certificate
  of a CA in the validation path of the encryption certificate.

  Part of the process of extracting certificates involves comparing the
  certificate found to the address matching the directory look-up.  The
  validation SHOULD match the address used to look up the certificate
  with one of the names found in the certificate.  Thus if an RFC822
  name was used to do the directory look-up, the RFC822 name would be in
  the SubjectAltName extension on the certificate.

  Thus the

  The steps for extracting the encryption certificate from a
  SMimeCertificatePublish object are as follows:

  1. Verify that the SMimeCertificatePublish object contains a valid
    signature and the certificate used to sign the message can be validated.
  2. Does the certificate used to sign the SMimeCertificatePublish
    object "match" the intended recipient of the encryption object.  If so so,
    proceed to step 6 else step 3.
  3. Does the certificate referenced in the SMIMEEncryptionKeyPreference
    attribute "match" the intended recipient of the encryption object?  If so
    so, proceed to step 4, else stop.
  4. Validate the reference encryption certificate.
  5. Compare the signing certificate to the set of certificates used to
    verify the encryption certificate.  Is the signing certificate in the
    set of verification certificates?  If yes then the encryption
    certificate has been located.  If no, no encryption certificate was
  6. Locate the encryption certificate using the
    SMIMEEncryptionKeyPreference attribute in the signed attributes of the
    SMimeCertificatePublish object.


  In all cases, once an encryption certificate has been obtained, the
  standard methods of validating signatures on the certificate and
  checking for revocation MUST be followed.

4.5  LDAP Schema


  After a SignedData object produced as described in section 3.?
  Needs has been produced, it needs to be published in a directory for usage by others.
  into one or more directories. This section describes the LDAP schema
  used to support this.

  A new LDAP attribute userSMimeCertificate is defined by this document.
  The attribute is defined according to the syntax provided in [LDAPV3].
  The definition of this attribute is:

  ( 1 2 840 113549 1 9 16 <TBD>
    NAME    'userSMimeCertificate'    `userSMimeCertificate'
    SYNTAX  'binary'  `binary'
    USAGE userApplications

  If the CA is the only entity that can write to the directory, it may
  wish to provide some mechanism for updating the attributes such as the
  smimeUserCapabilities in the published object.


4.6  MIME Encoding

  The application/pkcs7-mime-publish type is used to carry
  SMimeCertificatePublish objects as mime objects.  The optional "name"
  parameter SHOULD be emitted as part of the Content-Type field.  The
  file extension for the file name SHOULD be ".p7p". ". p7p".

A.   ASN Module
  To be supplied

B.   Backwards Compatibility

  The SMimeCertificatePublish object is based on work previously done at
  both Microsoft and Netscape.

  Both of these companies have implemented a version of
  userSMimeCertificate in their mail LDAP directory structures.
  Microsoft has also put the property into its MAPI based directory

  Both companies use a ContentInfo object containing a SignedData object
  with one SignerInfo object.  In both cases however the eContent is
  tagged with id-data not id-
  ct-publishCert. id-ct-publishCert.  The actual content is
  omitted from the SMimeCertificatePublish object.

  In the case of both companies, clients who implement this feature
  require that the end-entity is the signer of the object, the CA is not
  permitted to sign and publish the object.

C.   Registration of MIME

  Subject: Registration of MIME media type application/pkcs7-
  mime-publish application/pkcs7-mime-

  MIME media type name: application

  MIME subtype name: pkcs7-mime-publish

  Required parameters: none
  Optional parameters: name, filename

  Encoding considerations: Will be binary data, therefore should use
  base-64 encoding

  Security considerations: There is no requirement for additional
  security mechanisms to be applied at this level. The rqeuired required
  mechanisms are designed into the
  SmimeCertificatePublish SMimeCertificatePublish content.

  Interoperability considerations: -

  Published specification: this document

  Applications that use this media type: Secure Internet mail and other
  secure data transports.

  Additional information:
    File extension (s): p7p
    Macintosh File Type Code (s): -

  Person and email address to contact for further information:
  Jim Schaad,

  Intended usage: COMMON

D.   Open Issues

  -    At some level an argument can be made that more than
    one SignerInfo object should be allowed in a
    SMimeCertificatePublish object.  My initial reason for
    making this restriction is that I generally expect signing
    and encryption certificates to come in pairs and thus would
    be matched in a single object.  If we allow for multiple
    SignerInfos then we must define consistency rules about the
    attributes that can appear in the signatures.
  - Currently SMimeCertificatePublish objects contain no content.  One
    could make a case that some content, such as a vCard should be allowed.
    I don't see a big win for this as we are talking about publishing in
    directories and the additional information could be carried in the
    directory itself.
  - I would like to allow RAs to publish SMimeCertificatePublish
    objects into a directory as well.  I don't however see a way (short of
    adding an extension to a certificate) which allows one to distinguish
    between the case of an RA signing and publishing the
    SMimeCertificatePublish object and an arbitrary agent doing so.
  -    The current draft is setup to allow for the publishing
    of a single encryption certificate as part of a directory
    entry.  If one had 2 different encryption certificates then
    both would need to be published in independent
    SMimeCertificatePublish objects.  Does it make sense

E.   Changes

  Add SMimeEncryptionCerts attribute to
    allow for the publishing of identify multiple encryption
    within a single object?  If this is allowed how is this
    represented? With a sequence attribute listing all of the
    certificates or by using name matching rules.
          JSP:  I believe that a single
          SMimeCertificatePublish object should be capable
          of including multiple encryption (a.k.a. key
          management) certs.  This strategy can be used to
          minimize the number of objects that need to be
          verified.  I believe that your strategy should
          also allow for and their capabilities.
  Allow multiple SMimeCertificatePublish objects SignerInfos to be
          stored in the userSMimeCertificate attribute so
          that various entity can store
          SMimeCertificatePublish objects in the user's
          userSMimeCertificate attribute.

          Suggestions on how to accomplish this are welcome.
          I think it is going to require a new attribute,
          should we define a new attribute or expand the
          present one?

  -    If multiple certificates are allowed to occur in a single SMimeCertificatePublish object, do we need a way to
    represent the fact SignedData object when
  publishing certificates.
  Clarify that different encryption certificates will probably
    need different sets of capabilities? must still be validated after one
  is found.

  CMS     "Cryptographic Message Syntax", Internet Draft
          ietf-draft-smime-cms ietf-draft-
  MUSTSHOULD "Key words for use in RFCs to Indicate Requirement Levels",
          RFC 2119
  LDAPV3  "Lightweight Directory Access Protocol (v3): Attribute Syntax
          Definitions", RFC 2252
  SMIME   "S/MIME Version 3 Message Specification", Internet Draft ietf-draft-smime-msg ietf-
  SMIMECERT    "S/MIME Version 3 Certificate Handling", Internet Draft

Security Considerations
  Something goes here about making sure that you have the correct
  certificate and that no substitutions are done when getting
  certificates and information from the directory service.

Author Address
  Jim Schaad
  One Microsoft Way
  Redmond, WA 98052-6399