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RFC2632 - S/MIME Version 3 Certificate Handling

王朝other·作者佚名  2008-05-31
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Network Working Group B. Ramsdell, Editor

Request for Comments: 2632 Worldtalk

Category: Standards Track June 1999

S/MIME Version 3 Certificate Handling

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

1. Overview

S/MIME (Secure/Multipurpose Internet Mail Extensions), described in

[SMIME-MSG], provides a method to send and receive secure MIME

messages. Before using a public key to provide security services, the

S/MIME agent MUST certify that the public key is valid. S/MIME agents

MUST use PKIX certificates to validate public keys as described in

the Internet X.509 Public Key InfrastrUCture (PKIX) Certificate and

CRL Profile [KEYM]. S/MIME agents MUST meet the certificate

processing requirements documented in this document in addition to

those stated in [KEYM].

This specification is compatible with the Cryptographic Message

Syntax [CMS] in that it uses the data types defined by CMS. It also

inherits all the varieties of architectures for certificate-based key

management supported by CMS.

1.1 Definitions

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

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

Attribute Certificate (AC): An X.509 AC is a separate structure from

a subject's public key X.509 Certificate. A subject may have

multiple X.509 ACs associated with each of its public key X.509

Certificates. Each X.509 AC binds one or more Attributes with one of

the subject's public key X.509 Certificates. The X.509 AC syntax is

defined in [X.509]

BER: Basic Encoding Rules for ASN.1, as defined in ITU-T 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 the

Internet X.509 Public Key Infrastructure (PKIX) Certificate and CRL

Profile [KEYM]. This type also contains the distinguished name of the

certificate issuer (the signer), an issuer-specific serial number,

the issuer's signature algorithm identifier, a validity period, and

extensions also defined in that document.

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, a list of certificate serial numbers

and their associated revocation times, and extensions as defined in

[KEYM]. 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 ITU-T

X.690.

Receiving agent: software that interprets and processes S/MIME CMS

objects, MIME body parts that contain CMS objects, or both.

Sending agent: software that creates S/MIME CMS objects, MIME body

parts that contain CMS objects, or both.

S/MIME agent: user software that is a receiving agent, a sending

agent, or both.

1.2 Compatibility with Prior Practice of S/MIME

S/MIME version 3 agents should attempt to have the greatest

interoperability possible with S/MIME version 2 agents. S/MIME

version 2 is described in RFC2311 through RFC2315, inclusive. RFC

2311 also has historical information about the development of S/MIME.

1.3 Terminology

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in [MUSTSHOULD].

2. CMS Options

The CMS 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 CMS format for S/MIME messages is defined in [SMIME-MSG].

2.1 CertificateRevocationLists

Receiving agents MUST support 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 be capable of performing revocation checks using CRLs

as specified in [KEYM]. All agents MUST perform revocation status

checking in accordance with [KEYM]. Receiving agents MUST recognize

CRLs in received S/MIME messages.

Agents SHOULD store CRLs received in messages for use in processing

later messages.

Agents 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 CertificateChoices

Receiving agents MUST support PKIX v1 and PKIX v3 certificates. See

[KEYM] for details about the profile for certificate formats. End

entity certificates MAY include an Internet mail address, as

described in section 3.1.

Receiving agents SHOULD support X.509 attribute certificates.

2.2.1 Historical Note About CMS Certificates

The CMS message format supports a choice of certificate formats for

public key content types: PKIX, PKCS #6 Extended Certificates and

X.509 Attribute Certificates. The PKCS #6 format is not in widespread

use. In addition, PKIX certificate extensions 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 CertificateSet

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.

Agents 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. Also note that in

the case of DSA certificates the parameters may be located in the

root certificate. This would require that the recipient possess the

root certificate in order to perform a signature verification, and is

a valid example of a case where transmitting the root certificate may

be required.

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.

Receiving agents SHOULD support the decoding of X.509 attribute

certificates included in CMS objects. All other issues regarding the

generation and use of X.509 attribute certificates are outside of the

scope of this specification.

3. Using Distinguished Names for Internet Mail

End-entity certificates MAY 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. The email address SHOULD be in

the subjectAltName extension, and SHOULD NOT be in the subject

distinguished name.

Receiving agents MUST recognize email addresses in the subjectAltName

field. Receiving agents MUST recognize email addresses in the

Distinguished Name field in the PKCS #9 emailAddress attribute.

Sending agents SHOULD make the address in the From or Sender 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

or Sender header of a mail message matches an Internet mail address

in the signer's certificate, if mail addresses are present in the

certificate. A receiving agent SHOULD provide some eXPlicit alternate

processing of the message if this comparison fails, which may be to

display a message that shows the recipient the addresses in the

certificate or other certificate details.

All subject and issuer names MUST be populated (i.e. not an empty

SEQUENCE) in S/MIME-compliant PKIX certificates, except that the

subject DN in a user's (i.e. end-entity) certificate MAY be an empty

SEQUENCE in which case the subjectAltName extension will include the

subject's identifier and MUST be marked as critical.

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 such as directory servers. 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 CMS 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

In general, it is always better to get the latest CRL information

from a CA than to get information stored away from incoming messages.

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.

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

memo but may be governed by the policies associated with particular

certificate hierarchies.

All agents MUST be capable of performing revocation checks using CRLs

as specified in [KEYM]. All agents MUST perform revocation status

checking in accordance with [KEYM]. Receiving agents MUST recognize

CRLs in received S/MIME messages.

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 as per [KEYM] when validating a

correspondent's public key. This is necessary before using a public

key to provide security services such as: verifying a signature;

encrypting a content-encryption key (ex: RSA); or forming a pairwise

symmetric key (ex: Diffie-Hellman) to be used to encrypt or decrypt a

content-encryption key.

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 id-dsa-

with-sha1 [DSS].

A receiving agent SHOULD be capable of verifying the signatures on

certificates and CRLs made with md2WithRSAEncryption,

md5WithRSAEncryption and sha-1WithRSAEncryption signature algorithms

with key sizes from 512 bits to 2048 bits described in [PKCS#1V2].

4.4 PKIX Certificate Extensions

PKIX 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. Further,

there are active efforts underway to issue PKIX certificates for

business purposes. This document identifies the minumum required set

of certificate extensions which have the greatest value in the S/MIME

environment. The syntax and semantics of all the identified

extensions are defined in [KEYM].

Sending and receiving agents MUST correctly handle the 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 certificate extensions when they appear in

intermediate or CA certificates.

Certificates issued for the S/MIME environment SHOULD NOT contain any

critical extensions (extensions that have the critical field set to

TRUE) 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.

Interpretation and syntax for all extensions MUST follow [KEYM],

unless otherwise specified here.

4.4.1 Basic Constraints Certificate Extension

The basic constraints extension serves to delimit the role and

position of an issuing authority or end-entity 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-entity certificates contain an extension that

constrains the certificate from being an issuing authority

certificate.

Certificates SHOULD contain a basicConstraints extension in CA

certificates, and SHOULD NOT contain that extension in end entity

certificates.

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.

If a key usage extension is included in a PKIX certificate, then it

MUST be marked as critical.

4.4.2.1 Key Usage in Diffie-Hellman Key Exchange Certificates

For Diffie-Hellman key exchange certificates (certificates in which

the subject public key algorithm is dhpublicnumber), if the keyUsage

keyAgreement bit is set to 1 AND if the public key is to be used to

form a pairwise key to decrypt data, then the S/MIME agent MUST only

use the public key if the keyUsage encipherOnly bit is set to 0. If

the keyUsage keyAgreement bit is set to 1 AND if the key is to be

used to form a pairwise key to encrypt data, then the S/MIME agent

MUST only use the public key if the keyUsage decipherOnly bit is set

to 0.

4.4.3 Subject Alternative Name Extension

The subject alternative name extension is used in S/MIME as the

preferred means to convey the RFC-822 email address(es) that

correspond to the entity for this certificate. Any RFC-822 email

addresses present MUST be encoded using the rfc822Name CHOICE of the

GeneralName type. Since the SubjectAltName type is a SEQUENCE OF

GeneralName, multiple RFC-822 email addresses MAY be present.

5. 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. References

[CERTV2] Dusse, S., Hoffman, P. and B. Ramsdell,"S/MIME Version 2

Certificate Handling", RFC2312, March 1998.

[CMS] Housley, R., "Cryptographic Message Syntax", RFC2630,

June 1999.

[DSS] NIST FIPS PUB 186, "Digital Signature Standard", 18 May

1994.

[KEYM] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet

X.509 Public Key Infrastructure Certificate and CRL

Profile", RFC2459, January 1999.

[MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[PKCS#1V2] Kaliski, B., "PKCS #1: RSA Cryptography Specifications

Version 2.0", RFC2437, October 1998.

[RFC-822] Crocker, D., "Standard For The Format Of ARPA Internet

Text Messages", STD 11, RFC822, August 1982.

[SMIME-MSG] Ramsdell, B., Editor, "S/MIME Version 3 Message

Specification", RFC2633, June 1999.

[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.

B. Acknowledgements

Many thanks go out to the other authors of the S/MIME v2 RFC: Steve

Dusse, Paul Hoffman and Jeff Weinstein. Without v2, there wouldn't be

a v3.

A number of the members of the S/MIME Working Group have also worked

very hard and contributed to this document. Any list of people is

doomed to omission and for that I apologize. In alphabetical order,

the following people stand out in my mind due to the fact that they

made direct contributions to this document.

Bill Flanigan Elliott Ginsburg Paul Hoffman Russ Housley Michael

Myers John Pawling Denis Pinkas Jim Schaad

Editor's Address

Blake Ramsdell

Worldtalk

17720 NE 65th St Ste 201

Redmond, WA 98052

Phone: +1 425 376 0225

EMail: blaker@deming.com

Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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