S/MIME Version 2 Certificate Handling
draft-dusse-smime-cert-05

Internet Draft                                   Steve Dusse,
draft-dusse-smime-cert-05.txt                    RSA Data Security
November 08, 1997                                Paul Hoffman,
Expires in six months                            Internet Mail Consortium
                                                 Blake Ramsdell,
                                                 Worldtalk
                                                 Jeff Weinstein,
                                                 Netscape

                 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.

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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 certificates.

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 [KEYM].

DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT X.690.

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 interoperability.

1.4 Discussion of This Draft

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

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. End entity
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 certificates.

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.

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 agents MUST support chaining based on the distinguished name
fields. Other methods of building certificate chains may be supported but
are not currently recommended.

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 environment.

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 MUST 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.

CountryName
StateOrProvinceName
Locality
CommonName
Title
Organization
OrganizationalUnit
StreetAddress
PostalCode
PhoneNumber
EmailAddress

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
database.

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 andCRLs made with md5WithRSAEncryption and
sha-1WithRSAEncryption signature algorithms with key sizes from 512 bits to
2048 bits described in [SMIME-MSG].  A receiving agent SHOULD be capable of
verifying the signatures on certificates and CRLs made with the
md2WithRSAEncryption signature algorithm with key sizes from 512 bits to
2048 bits.

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,
and keyUsage 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 critical.

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 certificate.

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 SHOULD 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
support sha-1WithRSAEncryption and md5WithRSAEncryption and SHOULD support
MD2WithRSAEncryption for verification of signatures on certificate requests
as 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-requests MUST
NOT be signed with the md2WithRSAEncryption signature algorithm.

Certification requests MUST include a valid Internet mail address, either
as part of the certificate (as described in 3.2) or as part of the PKCS #10
attribute list. Certification authorities MUST check that the address in
the "From:" header matches either of these addresses. CAs SHOULD allow the
CA operator to configure processing of messages whose addresses do not
match.

Certification authorities SHOULD support parsing of zero or one instance of
each of the following set of certification-request attributes on incoming
messages. Attributes that a particular implementation does not support may
generate a warning message to the requestor, or may be silently ignored.
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
challengePassword
unstructuredAddress

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:

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 here.

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 fails.

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}

CountryName OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 6}

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

locality OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 7}

CommonName OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 3}

Title OBJECT IDENTIFIER ::=
     {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}

StreetAddress OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 9}

Postal Code OBJECT IDENTIFIER ::=
     {joint-iso-ccitt(2) ds(5) attributeType(4) 17}

Phone Number OBJECT IDENTIFIER ::=
     {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 }

keyUsage OBJECT IDENTIFIER ::=
     {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
status

[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
draft-dusse-smime-msg-xx.

[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:
Models

[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 -04 and -05 revisions of this
draft:

There was general agreement that this draft should reflect reality of
S/MIME v2 implementations and not what "should be", which is left to S/MIME
v3. The changes listed here are to bring this draft back to what is being
deployed today.

Changed the end of 2.3 to only require DN chaining.

Changed the MUST to SHOULD for MD2 hashing in 4.3 and 5.2.

Made the "certificates" in 2.2 "end entity" certificates.

Removed certificatePolicies from 4.4.

In 5.1, changed "Clients MUST handle multiple valid CA certificates" to
"Clients SHOULD...".

In 5.2, changed the MUST to SHOULD for the certification-request
attributes.

In 3.2, changed the SHOULD to MUS for including the email address during DN
creation. Added text in 5.2 to support this.

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
spock@rsa.com

Paul Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA  95060
(408) 426-9827
phoffman@imc.org

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

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