Internet Draft                              Steve Dusse,
draft-dusse-smime-cert-04.txt               RSA Data Security
October 19, 1997                            Paul Hoffman,
Expires in six months                       Internet Mail Consortium
                                            Blake Ramsdell,
                                            Jeff Weinstein,

                 S/MIME Certificate Handling

Status of this memo

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
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1. Overview

S/MIME (Secure/Multipurpose Internet Mail Extensions), described in
[SMIME-MSG], provides a method to send and receive secure MIME
messages. In order to validate the keys of a message sent to it, an
S/MIME agent needs to certify that the key is valid. This draft
describes the mechanisms S/MIME uses to create and validate keys using

This specification is compatible with PKCS #7 in that it uses the data
types defined by PKCS #7. It also inherits all the varieties of
architectures for certificate-based key management supported by PKCS
#7. Note that the method S/MIME messages make certificate requests is
defined in [SMIME-MSG].

In order to handle S/MIME certificates, an agent has to follow
specifications in this draft, as well as some of the specifications
listed in the following documents:
 - "PKCS #1: RSA Encryption", [PKCS-1].
 - "PKCS #7: Cryptographic Message Syntax", [PKCS-7]
 - "PKCS #10: Certification Request Syntax", [PKCS-10].

1.1 Definitions

For the purposes of this draft, the following definitions apply.

ASN.1: Abstract Syntax Notation One, as defined in CCITT X.680-689.

BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.690.

Certificate: A type that binds an entity's distinguished name to a
public key with a digital signature. This type is defined in CCITT
X.509 [X.509]. This type also contains the distinguished name of the
certificate issuer (the signer), an issuer-specific serial number, the
issuer's signature algorithm identifier, and a validity period.

Certificate Revocation List (CRL): A type that contains information
about certificates whose validity an issuer has prematurely revoked.
The information consists of an issuer name, the time of issue, the
next scheduled time of issue, and a list of certificate serial numbers
and their associated revocation times. The CRL is signed by the
issuer. The type intended by this specification is the one defined in

DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT

1.2 Compatibility with Prior Practice of S/MIME

Appendix C contains important information about how S/MIME agents
following this specification should act in order to have the greatest
interoperability with earlier implementations of S/MIME.

1.3 Terminology

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 implementors achieve

1.4 Discussion of This Draft

This draft is being discussed on the "ietf-smime" mailing list.
To subscribe, send a message to:
with the single word
in the body of the message. There is a Web site for the mailing list
at <>.

2. PKCS #7 Options

The PKCS #7 message format allows for a wide variety of options in
content and algorithm support. This section puts forth a number of
support requirements and recommendations in order to achieve a base
level of interoperability among all S/MIME implementations. Most of
the PKCS #7 format for S/MIME messages is defined in [SMIME-MSG].

2.1 CertificateRevocationLists

Receiving agents MUST support for the Certificate Revocation List
(CRL) format defined in [KEYM]. If sending agents include CRLs in
outgoing messages, the CRL format defined in [KEYM] MUST be used.

All agents MUST validate CRLs and check certificates against CRLs, if
available, in accordance with [KEYM]. All agents SHOULD check the
nextUpdate field in the CRL against the current time. If the current
time is later than the nextUpdate time, the action that the agent
takes is a local decision. For instance, it could warn a human user,
it could  retrieve a new CRL if able, and so on.

Receiving agents MUST recognize CRLs in received S/MIME messages.

Clients MUST use revocation information included as a CRL in an S/MIME
message when verifying the signature and certificate path validity in
that message.  Clients SHOULD store CRLs received in messages for use
in processing later messages.

Clients MUST handle multiple valid Certificate Authority (CA)
certificates containing the same subject name and the same public keys
but with overlapping validity intervals.

2.2 ExtendedCertificateOrCertificate

Receiving agents MUST support X.509 v1 and X.509 v3 certificates. See
[KEYM] for details about the profile for certificate formats.
Certificates MUST include an Internet mail address, as described in
section 3.1.

2.2.1 Historical Note About PKCS #7 Certificates

The PKCS #7 message format supports a choice of certificate two
formats for public key content types: X.509 and PKCS #6 Extended
Certificates. The PKCS #6 format is not in widespread use. In
addition, proposed revisions of X.509 certificates address much of the
same functionality and flexibility as was intended in the PKCS #6.
Thus, sending and receiving agents MUST NOT use PKCS #6 extended

2.3 ExtendedCertificateAndCertificates

Receiving agents MUST be able to handle an arbitrary number of
certificates of arbitrary relationship to the message sender and to
each other in arbitrary order. In many cases, the certificates
included in a signed message may represent a chain of certification
from the sender to a particular root. There may be, however,
situations where the certificates in a signed message may be unrelated
and included for convenience.

Certificates MUST include an Internet mail address, as described in
section 3.1.

Sending agents SHOULD include any certificates for the user's public
key(s) and associated issuer certificates. This increases the
likelihood that the intended recipient can establish trust in the
originator's public key(s). This is especially important when sending
a message to recipients that may not have access to the sender's
public key through any other means or when sending a signed message to
a new recipient. The inclusion of certificates in outgoing messages
can be omitted if S/MIME objects are sent within a group of
correspondents that has established access to each other's
certificates by some other means such as a shared directory or manual
certificate distribution. Receiving S/MIME agents SHOULD be able to
handle messages without certificates using a database or directory
lookup scheme.

A sending agent SHOULD include at least one chain of certificates up
to, but not including, a Certificate Authority (CA) that it believes
that the recipient may trust as authoritative. A receiving agent
SHOULD be able to handle an arbitrarily large number of certificates
and chains.

Clients MAY send CA certificates, that is, certificates that are
self-signed and can be considered the "root" of other chains. Note
that receiving agents SHOULD NOT simply trust any self-signed
certificates as valid CAs, but SHOULD use some other mechanism to
determine if this is a CA that should be trusted.

Receiving agenst MUST support chaining based on the distinguished name
and SHOULD support chaining based on the subjectAltName field.

3. Distinguished Names in Certificates

3.1 Using Distinguished Names for Internet Mail

The format of an X.509 certificate includes fields for the subject
name and issuer name. The subject name identifies the owner of a
particular public key/private key pair while the issuer name is meant
to identify the entity that "certified" the subject (that is, who
signed the subject's certificate). The subject name and issuer name
are defined by X.509 as Distinguished Names.

Distinguished Names are defined by a CCITT standard X.501 [X.501]. A
Distinguished Name is broken into one or more Relative Distinguished
Names. Each Relative Distinguished Name is comprised of one or more
Attribute-Value Assertions. Each Attribute-Value Assertion consists of
a Attribute Identifier and its corresponding value information, such
as CountryName=US. Distinguished Names were intended to identify
entities in the X.500 directory tree [X.500]. Each Relative
Distinguished Name can be thought of as a node in the tree which is
described by some collection of Attribute-Value Assertions. The entire
Distinguished Name is some collection of nodes in the tree that
traverse a path from the root of the tree to some end node which
represents a particular entity.

The goal of the directory was to provide an infrastructure to uniquely
name every communications entity everywhere. However, adoption of a
global X.500 directory infrastructure has been slower than expected.
Consequently, there is no requirement for X.500 directory service
provision in the S/MIME environment, although such provision would
almost undoubtedly be of great value in facilitating key management
for S/MIME.

The use of Distinguished Names in accordance with the X.500 directory
is not very widespread. By contrast, Internet mail addresses, as
described in RFC 822 [RFC-822], are used almost exclusively in the
Internet environment to identify originators and recipients of
messages. However, Internet mail addresses bear no resemblance to
X.500 Distinguished Names (except, perhaps, that they are both
hierarchical in nature). Some method is needed to map Internet mail
addresses to entities that hold public keys. Some people have
suggested that the X.509 certificate format should be abandoned in
favor of other binding mechanisms. Instead, S/MIME keeps the X.509
certificate and Distinguished Name mechanisms while tailoring the
content of the naming information to suit the Internet mail

End-entity certificates MUST contain an Internet mail address as
described in [RFC-822]. The address must be an "addr-spec" as defined
in Section 6.1 of that specification.

Receiving agents MUST recognize email addresses in the subjectAltName
field. Receiving agents MUST recognize email addresses in the
Distinguished Name field.

Sending agents SHOULD make the address in the From header in a mail
message match an Internet mail address in the signer's certificate.
Receiving agents MUST check that the address in the From header of a
mail message matches an Internet mail address in the signer's
certificate. A receiving agent MUST provide some explicit alternate
processing of the message if this comparison fails, which may be to
reject the message.

3.2 Required Name Attributes

Receiving agents MUST support parsing of zero, one, or more instances
of each of the following set of name attributes within the
Distinguished Names in certificates.

Sending agents SHOULD include the Internet mail address during
Distinguished Name creation. Guidelines for the inclusion, omission,
and ordering of the remaining name attributes during the creation of a
distinguished name will most likely be dictated by the policies
associated with the certification service which will certify the
corresponding name and public key.


All attributes other than EmailAddress are described in X.520 [X.520].
EmailAddress is an IA5String that can have multiple attribute values.

4. Certificate Processing

A receiving agent needs to provide some certificate retrieval
mechanism in order to gain access to certificates for recipients of
digital envelopes. There are many ways to implement certificate
retrieval mechanisms. X.500 directory service is an excellent example
of a certificate retrieval-only mechanism that is compatible with
classic X.500 Distinguished Names. The PKIX Working Group is
investigating other mechanisms. Another method under consideration by
the IETF is to provide certificate retrieval services as part of the
existing Domain Name System (DNS). Until such mechanisms are widely
used, their utility may be limited by the small number of
correspondent's certificates that can be retrieved. At a minimum, for
initial S/MIME deployment, a user agent could automatically generate a
message to an intended recipient requesting that recipient's
certificate in a signed return message.

Receiving and sending agents SHOULD also provide a mechanism to allow
a user to "store and protect" certificates for correspondents in such
a way so as to guarantee their later retrieval. In many environments,
it may be desirable to link the certificate retrieval/storage
mechanisms together in some sort of certificate database. In its
simplest form, a certificate database would be local to a particular
user and would function in a similar way as a "address book" that
stores a user's frequent correspondents. In this way, the certificate
retrieval mechanism would be limited to the certificates that a user
has stored (presumably from incoming messages).  A comprehensive
certificate retrieval/storage solution may combine two or more
mechanisms to allow the greatest flexibility and utility to the user.
For instance, a secure Internet mail agent may resort to checking a
centralized certificate retrieval mechanism for a certificate if it
can not be found in a user's local certificate storage/retrieval

Receiving and sending agents SHOULD provide a mechanism for the import
and export of certificates, using a PKCS #7 certs-only message. This
allows for import and export of full certificate chains as opposed to
just a single certificate. This is described in [SMIME-MSG].

4.1 Certificate Revocation Lists

A receiving agent SHOULD have access to some certificate-revocation
list (CRL) retrieval mechanism in order to gain access to
certificate-revocation information when validating certificate chains.
A receiving or sending agent SHOULD also provide a mechanism to allow
a user to store incoming certificate-revocation information for
correspondents in such a way so as to guarantee its later retrieval.
However, it is always better to get the latest information from the CA
than to get information stored away from incoming messages.

Receiving and sending agents SHOULD retrieve and utilize CRL
information every time a certificate is verified as part of a
certificate chain validation even if the certificate was already
verified in the past. However, in many instances (such as off-line
verification) access to the latest CRL information may be difficult or
impossible. The use of CRL information, therefore, may be dictated
by the value of the information that is protected. The value of the
CRL information in a particular context is beyond the scope of this
draft but may be governed by the policies associated with particular
certificate hierarchies.

4.2 Certificate Chain Validation

In creating a user agent for secure messaging, certificate, CRL, and
certificate chain validation SHOULD be highly automated while still
acting in the best interests of the user. Certificate, CRL, and chain
validation MUST be performed when validating a correspondent's public
key. This is necessary when a) verifying a signature from a
correspondent and, b) creating a digital envelope with the
correspondent as the intended recipient.

Certificates and CRLs are made available to the chain validation
procedure in two ways: a) incoming messages, and b) certificate and
CRL retrieval mechanisms. Certificates and CRLs in incoming messages
are not required to be in any particular order nor are they required
to be in any way related to the sender or recipient of the message
(although in most cases they will be related to the sender). Incoming
certificates and CRLs SHOULD be cached for use in chain validation and
optionally stored for later use. This temporary certificate and CRL
cache SHOULD be used to augment any other certificate and CRL
retrieval mechanisms for chain validation on incoming signed messages.

4.3 Certificate and CRL Signing Algorithms

Certificates and Certificate-Revocation Lists (CRLs) are signed by the
certificate issuer. A receiving agent MUST be capable of verifying the
signatures on certificates and CRLs made with the
md2WithRSAEncryption, md5WithRSAEncryption and sha-1WithRSAEncryption
signature algorithms with key sizes from 512 bits to 2048 bits
described in [SMIME-MSG].

4.4 X.509 Version 3 Certificate Extensions

The X.509 v3 standard describes an extensible framework in which the
basic certificate information can be extended and how such extensions
can be used to control the process of issuing and validating
certificates. The PKIX Working Group has ongoing efforts to identify
and create extensions which have value in particular certification
environments. As such, there is still a fair amount of profiling work
to be done before there is widespread agreement on which v3 extensions
will be used. Further, there are active efforts underway to issue
X.509 v3 certificates for business purposes. This draft identifies the
minumum required set of certificate extensions which have the greatest value
in the S/MIME environment. The basicConstraints, keyUsage, and
certificatePolicies extensions are defined in [X.509].

Sending and receiving agents MUST correctly handle the v3 Basic
Constraints Certificate Extension, the Key Usage Certificate
Extension, authorityKeyID, subjectKeyID, and the subjectAltNames when
they appear in end-user certificates. Some mechanism SHOULD exist to
handle the defined v3 certificate extensions when they appear in
intermediate or CA certificates.

Certificates issued for the S/MIME environment SHOULD NOT contain any
critical extensions other than those listed here. These extensions SHOULD
be marked as non-critical unless the proper handling of the extension is
deemed critical to the correct interpretation of the associated certificate.
Other extensions may be included, but those extensions SHOULD NOT be marked as

4.4.1 Basic Constraints Certificate Extension

The basic constraints extension serves to delimit the role and
position of an issuing authority or end-user certificate plays in a
chain of certificates.

For example, certificates issued to CAs and subordinate CAs contain a
basic constraint extension that identifies them as issuing authority
certificates. End-user subscriber certificates contain an extension
that constrains the certificate from being an issuing authority

Certificates SHOULD contain a basicContstraints extension.

4.4.2 Key Usage Certificate Extension

The key usage extension serves to limit the technical purposes for
which a public key listed in a valid certificate may be used. Issuing
authority certificates may contain a key usage extension that
restricts the key to signing certificates, certificate revocation
lists and other data.

For example, a certification authority may create subordinate issuer
certificates which contain a keyUsage extension which specifies that
the corresponding public key can be used to sign end user certs and
sign CRLs.

5. Generating Keys and Certification Requests

5.1 Binding Names and Keys

An S/MIME agent or some related administrative utility or function
MUST be capable of generating a certification request given a user's
public key and associated name information. In most cases, the user's
public key/private key pair will be generated simultaneously. However,
there are cases where the keying information may be generated by an
external process (such as when a key pair is generated on a
cryptographic token or by a "key recovery" service).

There SHOULD NOT be multiple valid (that is, non-expired and
non-revoked) certificates for the same key pair bound to different
Distinguished Names. Otherwise, a security flaw exists where an
attacker can substitute one valid certificate for another in such a
way that can not be detected by a message recipient. If a users wishes
to change their name (or create an alternate name), the user agent
SHOULD generate a new key pair. If the user wishes to reuse an
existing key pair with a new or alternate name, the user SHOULD first
have any valid certificates for the existing public key revoked.

In general, it is possible for a user to request certification for the
same name and different public key from the same or different
certification authorities.  This is acceptable both for end-entity and
issuer certificates and can be useful in supporting a change of issuer
keys in a smooth fashion.

CAs that re-use their own name with distinct keys MUST include the
AuthorityKeyIdentifier extension in certificates that they issue, and
MUST have the SubjectKeyIdentifier extension in their own certificate.
CAs SHOULD use these extensions uniformly.

Clients MUST handle multiple valid CA certificates that certify
different public keys but contain the same subject name (in this case,
that CA's name).

When selecting an appropriate issuer's certificate to use to verify a
given certificate, clients SHOULD process the AuthorityKeyIdentifier
and SubjectKeyIdentifier extensions.

5.2 Using PKCS #10 for Certification Requests

PKCS #10 is a flexible and extensible message format for representing
the results of cryptographic operations on some data. The choice of
naming information is largely dictated by the policies and procedures
associated with the intended certification service.

In addition to key and naming information, the PKCS #10 format
supports the inclusion of optional attributes, signed by the entity
requesting certification. This allows for information to be conveyed
in a certification request which may be useful to the request process,
but not necessarily part of the Distinguished Name being certified.

Receiving agents MUST support the identification of an RSA key with
the rsa defined in X.509 and the rsaEncryption OID. Certification
authorities MUST also support the verification of signatures on
certificate requests made with sha-1WithRSAEncryption,
md5WithRSAEncryption, and MD2WithRSAEncryption signature algorithms
described in [SMIME-MSG].

For the creation and submission of certification-requests, RSA keys
SHOULD be identified with the rsaEncryption OID and signed with the
sha-1WithRSAEncryption signature algorithm.

Certification authorities MUST support parsing of zero or one instance
of each of the following set of certification-request attributes on
incoming messages. Inclusion of the following attributes during the
creation and submission of a certification-request will most likely be
dictated by the policies associated with the certification service
which will certify the corresponding name and public key.


postalAddress is described in [X.520].

5.2.1 Challenge Password

The challenge-password attribute type specifies a password by which an
entity may request certificate revocation. The interpretation of the
password is intended to be specified by the issuer of the certificate;
no particular interpretation is required. The challenge-password
attribute type is intended for PKCS #10 certification requests.

Challenge-password attribute values have ASN.1 type ChallengePassword:

ChallengePassword ::= CHOICE {
  PrintableString, T61String }

A challenge-password attribute must have a single attribute value.

It is expected that if UCS becomes an ASN.1 type (e.g., UNIVERSAL
STRING), ChallengePassword will become a CHOICE type:

ChallengePassword ::= CHOICE {
    PrintableString, T61String, UNIVERSAL STRING }

5.2.2 Unstructured Address

The unstructured-address attribute type specifies the address or
addresses of the subject of a certificate as an unstructured ASCII or
T.61 string. The interpretation of the addresses is intended to be
specified by the issuer of the certificate; no particular
interpretation is required. A likely interpretation is as an
alternative to the X.520 postalAddress attribute type. The
unstructured-address attribute type is intended for PKCS #10
certification requests.

Unstructured-address attribute values have ASN.1 type

UnstructuredAddress ::= CHOICE {
  PrintableString, T61String }

An unstructured-address attribute can have multiple attribute values.

Note: T.61's newline character (hexadecimal code 0d) is recommended as
a line separator in multi-line addresses.

It is expected that if UCS becomes an ASN.1 type (e.g., UNIVERSAL
STRING), UnstructuredAddress will become a CHOICE type:

UnstructuredAddress ::= CHOICE {
    PrintableString, T61String, UNIVERSAL STRING }

5.3 Fulfilling a Certification Request

Certification authorities SHOULD use the sha-1WithRSAEncryption
signature algorithms when signing certificates.

5.4 Using PKCS #7 for Fulfilled Certificate Response

[PKCS-7] supports a degenerate case of the SignedData content type
where there are no signers on the content (and hence, the content
value is "irrelevant"). This degenerate case is used to convey
certificate and CRL information. Certification authorities MUST use
this format for returning certificate information resulting from the
successful fulfillment of a certification request. At a minimum, the
fulfilled certificate response MUST include the actual subject
certificate (corresponding to the information in the certification
request). The response SHOULD include other certificates which link
the issuer to higher level certification authorities and corresponding
certificate-revocation lists. Unrelated certificates and revocation
information is also acceptable.

Receiving agents MUST parse this degenerate PKCS #7 message type and
handle the certificates and CRLs according to the requirements and
recommendations in Section 4.

6. Security Considerations

All of the security issues faced by any cryptographic application must
be faced by a S/MIME agent. Among these issues are protecting the
user's private key, preventing various attacks, and helping the user
avoid mistakes such as inadvertently encrypting a message for the
wrong recipient. The entire list of security considerations is beyond
the scope of this document, but some significant concerns are listed

When processing certificates, there are many situations where the
processing might fail. Because the processing may be done by a user
agent, a security gateway, or other program, there is no single way to
handle such failures. Just because the methods to handle the failures
has not been listed, however, the reader should not assume that they
are not important. The opposite is true: if a certificate is not
provably valid and associated with the message, the processing
software should take immediate and noticable steps to inform the end
user about it.

Some of the many places where signature and certificate checking might
fail include:
- no Internet mail addresses in a certificate match the sender of
  a message
- no certificate chain leads to a trusted CA
- no ability to check the CRL for a certificate
- an invalid CRL was received
- the CRL being checked is expired
- the certificate is expired
- the certificate has been revoked
There are certainly other instances where a certificate may be
invalid, and it is the responsibility of the processing software to
check them all thoroughly, and to decide what to do if the check

A. Object Identifiers and Syntax

Sections A.1 through A.4 are adopted from [SMIME-MSG].

A.5 Name Attributes

emailAddress OBJECT IDENTIFIER ::=

     {iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1) pkcs-9(9) 1}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 6}

StateOrProvinceName OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 8}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 7}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 3}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 12}

Organization OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 10}

OrganizationalUnit OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 11}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 9}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 17}

     {joint-iso-ccitt(2) ds(5) attributeType(4) 20}

A.6 Certification Request Attributes

postalAddress OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 16}

challengePassword OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1) pkcs-9(9) 7}

unstructuredAddress OBJECT IDENTIFIER ::=
     {iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1) pkcs-9(9) 8}

A.7 X.509 V3 Certificate Extensions

basicConstraints OBJECT IDENTIFIER ::=

     {joint-iso-ccitt(2) ds(5) 29 19 }

The ASN.1 definition of basicConstraints certificate extension is:

basicConstraints basicConstraints EXTENSION ::= {
     SYNTAX  BasicConstraintsSyntax
     IDENTIFIED BY { id-ce 19 } }

BasicConstraintsSyntax ::= SEQUENCE {
     cA                 BOOLEAN DEFAULT FALSE,
     pathLenConstraint  INTEGER (0..MAX) OPTIONAL }

     {joint-iso-ccitt(2) ds(5) 29 15 }

The ASN.1 definition of keyUsage certificate extension is:

keyUsage EXTENSION ::= {
     SYNTAX  KeyUsage
     IDENTIFIED BY { id-ce 15 }}

KeyUsage ::= BIT STRING {
     digitalSignature      (0),
     nonRepudiation        (1),
     keyEncipherment       (2),
     dataEncipherment      (3),
     keyAgreement          (4),
     keyCertSign           (5),
     cRLSign               (6)}

B. References

[KEYM] PKIX Part 1. At the time of this writing, PKIX is in Internet
Draft stage, but it is expected that there will be standards-track
RFCs at some point in the future.

[MUSTSHOULD] "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119

[PKCS-1], "PKCS #1: RSA Encryption", draft has been submitted for RFC

[PKCS-7], "PKCS #7: Cryptographic Message Syntax", draft has been
submitted for RFC status

[PKCS-10], "PKCS #10: Certification Request Syntax", draft has been
submitted for RFC status

[RFC-822], "Standard For The Format Of ARPA Internet Text Messages",
RFC 822.

[SMIME-MSG] "S/MIME Message Specification", Internet Draft

[X.500] ITU-T Recommendation X.500 (1997) | ISO/IEC 9594-1:1997,
Information technology - Open Systems Interconnection - The Directory:
Overview of concepts, models and services

[X.501] ITU-T Recommendation X.501 (1997) | ISO/IEC 9594-2:1997,
Information technology - Open Systems Interconnection - The Directory:

[X.509] ITU-T Recommendation X.509 (1997) | ISO/IEC 9594-8:1997,
Information technology - Open Systems Interconnection - The Directory:
Authentication framework

[X.520] ITU-T Recommendation X.520 (1997) | ISO/IEC 9594-6:1997,
Information technology - Open Systems Interconnection - The Directory:
Selected attribute types.

C. Compatibility with Prior Practice in S/MIME

S/MIME was originally developed by RSA Data Security, Inc. Many
developers implemented S/MIME agents before this document was
published. All S/MIME receiving agents SHOULD make every attempt to
interoperate with these earlier implementations of S/MIME.

D. Revision History

The following changes were made between the -03 and -04 revisions of
this draft:

In 2.1, changed "revocation time" to "nextUpdate time".

In 2.1, removed the somewhat ambiguous sentence about agents retrieving
CRLs in any fashion they are provided.

In 2.3, added "Clients MUST support chaining based on the
distinguished name and SHOULD support chaining based on the
subjectAltName field."

In 4.4, replaced last paragraph with explanation of what should and
shouldn't be marked as critical.

In 4.4.1, added "Certificates SHOULD contain a basicContstraints

Open issue: what is the OID being referred to in 5.2?

E. Acknowledgements

Significant contributions to the content of this draft were made by
many people, including David Solo, Anil Gangolli, Jeff Thompson, and
Lisa Repka.

F. Authors' addresses

Steve Dusse
RSA Data Security, Inc.
100 Marine Parkway, #500
Redwood City, CA  94065  USA
(415) 595-8782

Paul Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA  95060
(408) 426-9827

Blake Ramsdell
13122 NE 20th St., Suite C
Bellevue, WA 98005
(425) 882-8861

Jeff Weinstein
Netscape Communications Corporation
501 East Middlefield Road
Mountain View, CA  94043
(415) 254-1900