One document matched: draft-ietf-smime-certdist-03.txt
Differences from draft-ietf-smime-certdist-02.txt
Internet Draft Jim Schaad
draft-ietf-smime-certdist-03.txt Microsoft
February 25, 1999
Expires in six months
Certificate Distribution Specification
Status of this memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026. 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
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Abstract
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-request@imc.org> with the
single word "subscribe" in the body of the message. Also, there is a
Web site for the mailing list at <http://www.imc.org/ietf-smime>.
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
interoperability.
2. Current Publishing Methods
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 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.) The
degenerate CMS signed object form, certificate files may carry a set
of certificates to allow a sender to validate the recipients
certificates. 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 done via a one-on-
one process.
3. SMimeEncryptCerts
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 SMimeEncryptCerts 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 SMimeEncryptCerts attribute are:
id-aa-smimeEncryptCerts OBJECT IDENTIFIER ::= { iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
smime(16) id-aa(2) 13 }
SMimeEncryptCert ::= SEQUENCE {
hash Hash,
capabilities SMIMECapabilities
}
SMimeEncryptCerts ::= SEQUENCE OF SmimeEncryptCert
Hash ::= OCTET STRING - SHA1 hash of the certificate
When a certificate appears in an SMimeEncryptCerts attribute, the
certificate MUST be included SignedData object. The order of
certificates in the SMimeEncryptCerts 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 SMimeEncryptCerts attribute.
If present, the SMimeEncryptCerts 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 SMimeEncryptCerts attribute. CMS
defines the ASN.1 syntax for the authenticated attributes to include
attrValues SET OF AttributeValue. A SMimeEncryptCerts 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 MAY contain 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)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) id-ct(1)
3)
When creating a SMimeCertificatePublish object, the eContent of the
Signed-Data object is omitted and the eContentType OID is set to 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]), 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
content.
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.
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, 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, proceed to step 4, else stop with failure.
4. Validate the referenced 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
found.
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 has been produced, it needs to be published
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'
SYNTAX `binary'
MULTI-VALUE
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".
A. ASN Module
SMimeCertDistributionSyntax
{ iso(1) member-body(2) us(840) rsadsi(113549)
pkcs(1) pkcs-9(9) smime(16) modules(0) <TBD> }
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.
IMPORTS
-- SMime Cryptographic Message Format
ContentInfo
FROM CryptographicMessageSyntax { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16)
modules(0) cms(1) }
-- SecureMimeMessageV3
SMIMECapabilities
FROM SecureMimeMessageV3 { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0)
smime(4)};
-- S/MIME Object Identifier Registry
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) }
-- Authenticated Attribute identifing Encryption Certificates
-- Value is a single SMimeEncryptCerts
id-aa-smimeEncryptCerts OBJECT IDENTIFIER ::= { id-smime id-aa(2) 13
}
SMimeEncryptCerts ::= SEQUENCE OF SMimeEncryptCert
SMimeEncryptCert ::= SEQUENCE {
hash Hash,
capabilities SMIMECapabilities
}
Hash ::= OCTET STRING -- SHA1 hash of the certificate
-- Content Type of Certificate publish message.
-- Signed content is detatched and empty
id-ct-publishCert OBJECT IDENTIFIER ::= { id-smime id-ct(1) 3 }
SMimeCertificatePublish ::= ContentInfo
END -- of SMimeCertDistributionSyntax
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
schema.
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. 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.
Microsoft has also produced an early version of the SMimeEncryptCerts
attribute. The syntax for this structure is
id-Microsoft-SMimeEncryptCert OBJECT IDENTIFIER ::= {1 3 6 1 4 1 311
16 4}
Microsoft-SMimeEncryptionert ::= IssuerAndSerialNumber
A description of IssuerAndSerialNumber can be find in [CMS].
C. Registration of MIME
To: ietf-types@iana.org
Subject: Registration of MIME media type application/pkcs7-mime-
publish
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 required
mechanisms are designed into the 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, jimsch@microsoft.com
Intended usage: COMMON
D. Open Issues
- Need Example Message
References
CMS "Cryptographic Message Syntax", Internet Draft ietf-draft-
smime-cms
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
SMIMECERT "S/MIME Version 3 Certificate Handling", Internet Draft
ietf-draft-smime-cert
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
Microsoft
One Microsoft Way
Redmond, WA 98052-6399
Jimsch@Microsoft.com
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