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Versions: (RFC 2510) 00 01 02 03 04 05 06 07
08 09 RFC 4210
Network Working Group C. Adams
Internet-Draft University of Ottawa
Obsoletes: 2510 (if approved) S. Farrell
Expires: August 12, 2004 Trinity College Dublin
T. Kause
SSH
T. Mononen
SafeNet
February 12, 2004
Internet X.509 Public Key Infrastructure -- Certificate Management
Protocol (CMP)
draft-ietf-pkix-rfc2510bis-09.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes the Internet X.509 Public Key Infrastructure
(PKI) Certificate Management Protocol (CMP). Protocol messages are
defined for X.509v3 certificate creation and management. CMP
provides online interactions between PKI components, including an
exchange between a Certification Authority (CA) and a client system.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . 5
2. Requirements . . . . . . . . . . . . . . . . . . . . . 6
3. PKI Management Overview . . . . . . . . . . . . . . . 7
3.1 PKI Management Model . . . . . . . . . . . . . . . . . 7
3.1.1 Definitions of PKI Entities . . . . . . . . . . . . . 7
3.1.1.1 Subjects and End Entities . . . . . . . . . . . . . . 7
3.1.1.2 Certification Authority . . . . . . . . . . . . . . . 8
3.1.1.3 Registration Authority . . . . . . . . . . . . . . . . 9
3.1.2 PKI Management Requirements . . . . . . . . . . . . . 9
3.1.3 PKI Management Operations . . . . . . . . . . . . . . 11
4. Assumptions and Restrictions . . . . . . . . . . . . . 16
4.1 End Entity Initialization . . . . . . . . . . . . . . 16
4.2 Initial Registration/Certification . . . . . . . . . . 16
4.2.1 Criteria Used . . . . . . . . . . . . . . . . . . . . 16
4.2.1.1 Initiation of Registration / Certification . . . . . . 16
4.2.1.2 End Entity Message Origin Authentication . . . . . . . 17
4.2.1.3 Location of Key Generation . . . . . . . . . . . . . . 17
4.2.1.4 Confirmation of Successful Certification . . . . . . . 17
4.2.2 Mandatory Schemes . . . . . . . . . . . . . . . . . . 18
4.2.2.1 Centralized Scheme . . . . . . . . . . . . . . . . . . 18
4.2.2.2 Basic Authenticated Scheme . . . . . . . . . . . . . . 18
4.3 Proof of Possession (POP) of Private Key . . . . . . . 19
4.3.1 Signature Keys . . . . . . . . . . . . . . . . . . . . 20
4.3.2 Encryption Keys . . . . . . . . . . . . . . . . . . . 20
4.3.3 Key Agreement Keys . . . . . . . . . . . . . . . . . . 20
4.4 Root CA Key Update . . . . . . . . . . . . . . . . . . 21
4.4.1 CA Operator Actions . . . . . . . . . . . . . . . . . 22
4.4.2 Verifying Certificates . . . . . . . . . . . . . . . . 22
4.4.2.1 Verification in Cases 1, 4, 5 and 8 . . . . . . . . . 23
4.4.2.2 Verification in Case 2 . . . . . . . . . . . . . . . . 24
4.4.2.3 Verification in Case 3 . . . . . . . . . . . . . . . . 24
4.4.2.4 Failure of Verification in Case 6 . . . . . . . . . . 24
4.4.2.5 Failure of Verification in Case 7 . . . . . . . . . . 25
4.4.3 Revocation - Change of CA Key . . . . . . . . . . . . 25
5. Data Structures . . . . . . . . . . . . . . . . . . . 26
5.1 Overall PKI Message . . . . . . . . . . . . . . . . . 26
5.1.1 PKI Message Header . . . . . . . . . . . . . . . . . . 26
5.1.1.1 ImplicitConfirm . . . . . . . . . . . . . . . . . . . 29
5.1.1.2 ConfirmWaitTime . . . . . . . . . . . . . . . . . . . 29
5.1.2 PKI Message Body . . . . . . . . . . . . . . . . . . . 29
5.1.3 PKI Message Protection . . . . . . . . . . . . . . . . 30
5.1.3.1 Shared Secret Information . . . . . . . . . . . . . . 31
5.1.3.2 DH Key Pairs . . . . . . . . . . . . . . . . . . . . . 32
5.1.3.3 Signature . . . . . . . . . . . . . . . . . . . . . . 32
5.1.3.4 Multiple Protection . . . . . . . . . . . . . . . . . 33
5.2 Common Data Structures . . . . . . . . . . . . . . . . 33
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5.2.1 Requested Certificate Contents . . . . . . . . . . . . 33
5.2.2 Encrypted Values . . . . . . . . . . . . . . . . . . . 34
5.2.3 Status codes and Failure Information for PKI Messages 34
5.2.4 Certificate Identification . . . . . . . . . . . . . . 35
5.2.5 Out-of-band root CA Public Key . . . . . . . . . . . . 35
5.2.6 Archive Options . . . . . . . . . . . . . . . . . . . 36
5.2.7 Publication Information . . . . . . . . . . . . . . . 36
5.2.8 Proof-of-Possession Structures . . . . . . . . . . . . 37
5.2.8.1 Inclusion of the Private Key . . . . . . . . . . . . . 37
5.2.8.2 Indirect Method . . . . . . . . . . . . . . . . . . . 37
5.2.8.3 Challenge-Response Protocol . . . . . . . . . . . . . 38
5.2.8.4 Summary of PoP Options . . . . . . . . . . . . . . . . 39
5.3 Operation-Specific Data Structures . . . . . . . . . . 41
5.3.1 Initialization Request . . . . . . . . . . . . . . . . 41
5.3.2 Initialization Response . . . . . . . . . . . . . . . 41
5.3.3 Certification Request . . . . . . . . . . . . . . . . 41
5.3.4 Certification Response . . . . . . . . . . . . . . . . 41
5.3.5 Key Update Request Content . . . . . . . . . . . . . . 43
5.3.6 Key Update Response Content . . . . . . . . . . . . . 43
5.3.7 Key Recovery Request Content . . . . . . . . . . . . . 43
5.3.8 Key Recovery Response Content . . . . . . . . . . . . 43
5.3.9 Revocation Request Content . . . . . . . . . . . . . . 44
5.3.10 Revocation Response Content . . . . . . . . . . . . . 44
5.3.11 Cross Certification Request Content . . . . . . . . . 44
5.3.12 Cross Certification Response Content . . . . . . . . . 44
5.3.13 CA Key Update Announcement Content . . . . . . . . . . 45
5.3.14 Certificate Announcement . . . . . . . . . . . . . . . 45
5.3.15 Revocation Announcement . . . . . . . . . . . . . . . 45
5.3.16 CRL Announcement . . . . . . . . . . . . . . . . . . . 46
5.3.17 PKI Confirmation Content . . . . . . . . . . . . . . . 46
5.3.18 Certificate Confirmation Content . . . . . . . . . . . 46
5.3.19 PKI General Message Content . . . . . . . . . . . . . 47
5.3.19.1 CA Protocol Encryption Certificate . . . . . . . . . . 47
5.3.19.2 Signing Key Pair Types . . . . . . . . . . . . . . . . 47
5.3.19.3 Encryption/Key Agreement Key Pair Types . . . . . . . 47
5.3.19.4 Preferred Symmetric Algorithm . . . . . . . . . . . . 48
5.3.19.5 Updated CA Key Pair . . . . . . . . . . . . . . . . . 48
5.3.19.6 CRL . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.3.19.7 Unsupported Object Identifiers . . . . . . . . . . . . 48
5.3.19.8 Key Pair Parameters . . . . . . . . . . . . . . . . . 48
5.3.19.9 Revocation Passphrase . . . . . . . . . . . . . . . . 49
5.3.19.10 ImplicitConfirm . . . . . . . . . . . . . . . . . . . 49
5.3.19.11 ConfirmWaitTime . . . . . . . . . . . . . . . . . . . 49
5.3.19.12 Original PKIMessage . . . . . . . . . . . . . . . . . 49
5.3.19.13 Supported Language Tags . . . . . . . . . . . . . . . 49
5.3.20 PKI General Response Content . . . . . . . . . . . . . 49
5.3.21 Error Message Content . . . . . . . . . . . . . . . . 50
5.3.22 Polling Request and Response . . . . . . . . . . . . . 50
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6. Mandatory PKI Management Functions . . . . . . . . . . 53
6.1 Root CA Initialization . . . . . . . . . . . . . . . . 53
6.2 Root CA Key Update . . . . . . . . . . . . . . . . . . 53
6.3 Subordinate CA Initialization . . . . . . . . . . . . 53
6.4 CRL production . . . . . . . . . . . . . . . . . . . . 54
6.5 PKI Information Request . . . . . . . . . . . . . . . 54
6.6 Cross Certification . . . . . . . . . . . . . . . . . 54
6.6.1 One-Way Request-Response Scheme: . . . . . . . . . . . 54
6.7 End Entity Initialization . . . . . . . . . . . . . . 56
6.7.1 Acquisition of PKI Information . . . . . . . . . . . . 56
6.7.2 Out-of-Band Verification of Root-CA Key . . . . . . . 57
6.8 Certificate Request . . . . . . . . . . . . . . . . . 57
6.9 Key Update . . . . . . . . . . . . . . . . . . . . . . 57
7. Version Negotiation . . . . . . . . . . . . . . . . . 58
7.1 Supporting RFC 2510 Implementations . . . . . . . . . 58
7.1.1 Clients Talking to RFC 2510 Servers . . . . . . . . . 58
7.1.2 Servers Receiving Version cmp1999 PKIMessages . . . . 59
8. Security Considerations . . . . . . . . . . . . . . . 60
8.1 Proof-Of-Possesion with a Decryption Key . . . . . . . 60
8.2 Proof-Of-Possession by Exposing the Private Key . . . 60
8.3 Attack Against Diffie-Hellman Key Exchange . . . . . . 60
9. IANA considerations . . . . . . . . . . . . . . . . . 62
Normative References . . . . . . . . . . . . . . . . . 63
Informative References . . . . . . . . . . . . . . . . 64
Authors' Addresses . . . . . . . . . . . . . . . . . . 65
A. Reasons for the Presence of RAs . . . . . . . . . . . 66
B. The Use of Revocation Passphrase . . . . . . . . . . . 67
C. Request Message Behavioral Clarifications . . . . . . 69
D. PKI Management Message Profiles (REQUIRED). . . . . . 71
D.1 General Rules for Interpretation of These Profiles. . 71
D.2 Algorithm Use Profile . . . . . . . . . . . . . . . . 72
D.3 Proof of Possession Profile . . . . . . . . . . . . . 74
D.4 Initial Registration/Certification (Basic
Authenticated Scheme) . . . . . . . . . . . . . . . . 75
D.5 Certificate Request . . . . . . . . . . . . . . . . . 81
D.6 Key Update Request . . . . . . . . . . . . . . . . . . 82
E. PKI Management Message Profiles (OPTIONAL). . . . . . 83
E.1 General Rules for Interpretation of These Profiles. . 83
E.2 Algorithm Use Profile . . . . . . . . . . . . . . . . 83
E.3 Self-signed Certificates . . . . . . . . . . . . . . . 83
E.4 Root CA Key Update . . . . . . . . . . . . . . . . . . 84
E.5 PKI Information Request/Response . . . . . . . . . . . 84
E.6 Cross Certification Request/Response (1-way) . . . . . 86
E.7 In-band Initialization Using External Identity
Certificate . . . . . . . . . . . . . . . . . . . . . 89
F. Compilable ASN.1 Definitions . . . . . . . . . . . . . 91
G. Acknowledgements . . . . . . . . . . . . . . . . . . . 102
Intellectual Property and Copyright Statements . . . . 103
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1. Introduction
This document describes the Internet X.509 Public Key Infrastructure
(PKI) Certificate Management Protocol (CMP). Protocol messages are
defined for certificate creation and management. The term
"certificate" in this document refers to an X.509v3 Certificate as
defined in [X509].
This specification obsoletes RFC 2510. This specification differs
from RFC 2510 in the following areas:
The PKI management message profile section is split to two
appendices: the required profile and the optional profile. Some of
the formerly mandatory functionality is moved to the optional
profile.
The message confirmation mechanism has changed substantially.
A new polling mechanism is introduced, deprecating the old polling
method at the CMP transport level.
The CMP transport protocol issues are handled in a separate
document [CMPtrans], thus the Transports section is removed.
A new implicit confirmation method is introduced to reduce the
number of protocol messages exchanged in a transaction.
The new specification contains some less prominent protocol
enhancements and improved explanatory text on several issues.
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2. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document (in uppercase,
as shown) are to be interpreted as described in [RFC2119].
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3. PKI Management Overview
The PKI must be structured to be consistent with the types of
individuals who must administer it. Providing such administrators
with unbounded choices not only complicates the software required but
also increases the chances that a subtle mistake by an administrator
or software developer will result in broader compromise. Similarly,
restricting administrators with cumbersome mechanisms will cause them
not to use the PKI.
Management protocols are REQUIRED to support on-line interactions
between Public Key Infrastructure (PKI) components. For example, a
management protocol might be used between a Certification Authority
(CA) and a client system with which a key pair is associated, or
between two CAs that issue cross-certificates for each other.
3.1 PKI Management Model
Before specifying particular message formats and procedures we first
define the entities involved in PKI management and their interactions
(in terms of the PKI management functions required). We then group
these functions in order to accommodate different identifiable types
of end entities.
3.1.1 Definitions of PKI Entities
The entities involved in PKI management include the end entity (i.e.,
the entity to whom the certificate is issued) and the certification
authority (i.e., the entity that issues the certificate). A
registration authority MAY also be involved in PKI management.
3.1.1.1 Subjects and End Entities
The term "subject" is used here to refer to the entity to whom the
certificate is issued, typically named in the subject or
subjectAltName field of a certificate. When we wish to distinguish
the tools and/or software used by the subject (e.g., a local
certificate management module) we will use the term "subject
equipment". In general, the term "end entity" (EE) rather than
subject is preferred in order to avoid confusion with the field name.
It is important to note that the end entities here will include not
only human users of applications, but also applications themselves
(e.g., for IP security). This factor influences the protocols which
the PKI management operations use; for example, application software
is far more likely to know exactly which certificate extensions are
required than are human users. PKI management entities are also end
entities in the sense that they are sometimes named in the subject or
subjectAltName field of a certificate or cross-certificate. Where
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appropriate, the term "end-entity" will be used to refer to end
entities who are not PKI management entities.
All end entities require secure local access to some information --
at a minimum, their own name and private key, the name of a CA which
is directly trusted by this entity and that CA's public key (or a
fingerprint of the public key where a self-certified version is
available elsewhere). Implementations MAY use secure local storage
for more than this minimum (e.g., the end entity's own certificate or
application-specific information). The form of storage will also vary
-- from files to tamper-resistant cryptographic tokens. The
information stored in such local trusted storage is referred to here
as the end entity's Personal Security Environment (PSE).
Though PSE formats are beyond the scope of this document (they are
very dependent on equipment, et cetera), a generic interchange format
for PSEs is defined here - a certification response message MAY be
used.
3.1.1.2 Certification Authority
The certification authority (CA) may or may not actually be a real
"third party" from the end entity's point of view. Quite often, the
CA will actually belong to the same organization as the end entities
it supports.
Again, we use the term CA to refer to the entity named in the issuer
field of a certificate; when it is necessary to distinguish the
software or hardware tools used by the CA we use the term "CA
equipment".
The CA equipment will often include both an "off-line" component and
an "on-line" component, with the CA private key only available to the
"off-line" component. This is, however, a matter for implementers
(though it is also relevant as a policy issue).
We use the term "root CA" to indicate a CA that is directly trusted
by an end entity; that is, securely acquiring the value of a root CA
public key requires some out-of-band step(s). This term is not meant
to imply that a root CA is necessarily at the top of any hierarchy,
simply that the CA in question is trusted directly.
A "subordinate CA" is one that is not a root CA for the end entity in
question. Often, a subordinate CA will not be a root CA for any
entity but this is not mandatory.
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3.1.1.3 Registration Authority
In addition to end-entities and CAs, many environments call for the
existence of a Registration Authority (RA) separate from the
Certification Authority. The functions which the registration
authority may carry out will vary from case to case but MAY include
personal authentication, token distribution, revocation reporting,
name assignment, key generation, archival of key pairs, et cetera.
This document views the RA as an OPTIONAL component - when it is not
present the CA is assumed to be able to carry out the RA's functions
so that the PKI management protocols are the same from the end-
entity's point of view.
Again, we distinguish, where necessary, between the RA and the tools
used (the "RA equipment").
Note that an RA is itself an end entity. We further assume that all
RAs are in fact certified end entities and that RAs have private keys
that are usable for signing. How a particular CA equipment identifies
some end entities as RAs is an implementation issue (i.e., this
document specifies no special RA certification operation). We do not
mandate that the RA is certified by the CA with which it is
interacting at the moment (so one RA may work with more than one CA
whilst only being certified once).
In some circumstances end entities will communicate directly with a
CA even where an RA is present. For example, for initial registration
and/or certification the subject may use its RA, but communicate
directly with the CA in order to refresh its certificate.
3.1.2 PKI Management Requirements
The protocols given here meet the following requirements on PKI
management
1. PKI management must conform to the ISO/IEC 9594-8 / ITU-T X.509
standards.
2. It must be possible to regularly update any key pair without
affecting any other key pair.
3. The use of confidentiality in PKI management protocols must be
kept to a minimum in order to ease acceptance in environments
where strong confidentiality might cause regulatory problems.
4. PKI management protocols must allow the use of different
industry-standard cryptographic algorithms, (specifically
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including RSA, DSA, MD5, SHA-1) -- this means that any given CA,
RA, or end entity may, in principle, use whichever algorithms
suit it for its own key pair(s).
5. PKI management protocols must not preclude the generation of key
pairs by the end-entity concerned, by an RA, or by a CA -- key
generation may also occur elsewhere, but for the purposes of PKI
management we can regard key generation as occurring wherever
the key is first present at an end entity, RA, or CA.
6. PKI management protocols must support the publication of
certificates by the end-entity concerned, by an RA, or by a CA.
Different implementations and different environments may choose
any of the above approaches.
7. PKI management protocols must support the production of
Certificate Revocation Lists (CRLs) by allowing certified end
entities to make requests for the revocation of certificates -
this must be done in such a way that the denial-of-service
attacks which are possible are not made simpler.
8. PKI management protocols must be usable over a variety of
"transport" mechanisms, specifically including mail, http, TCP/
IP and ftp.
9. Final authority for certification creation rests with the CA; no
RA or end-entity equipment can assume that any certificate
issued by a CA will contain what was requested -- a CA may alter
certificate field values or may add, delete or alter extensions
according to its operating policy. In other words, all PKI
entities (end-entities, RAs, and CAs) must be capable of
handling responses to requests for certificates in which the
actual certificate issued is different from that requested (for
example, a CA may shorten the validity period requested). Note
that policy may dictate that the CA must not publish or
otherwise distribute the certificate until the requesting entity
has reviewed and accepted the newly-created certificate
(typically through use of the certConf message).
10. A graceful, scheduled change-over from one non-compromised CA
key pair to the next (CA key update) must be supported (note
that if the CA key is compromised, re-initialization must be
performed for all entities in the domain of that CA). An end
entity whose PSE contains the new CA public key (following a CA
key update) must also be able to verify certificates verifiable
using the old public key. End entities who directly trust the
old CA key pair must also be able to verify certificates signed
using the new CA private key. (Required for situations where
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the old CA public key is "hardwired" into the end entity's
cryptographic equipment).
11. The functions of an RA may, in some implementations or
environments, be carried out by the CA itself. The protocols
must be designed so that end entities will use the same protocol
regardless of whether the communication is with an RA or CA.
Naturally the end entity must use the correct RA of CA public
key to protect the communication.
12. Where an end entity requests a certificate containing a given
public key value, the end entity must be ready to demonstrate
possession of the corresponding private key value. This may be
accomplished in various ways, depending on the type of
certification request. See Section 4.3 for details of the
in-band methods defined for the PKIX-CMP (i.e., Certificate
Management Protocol) messages.
3.1.3 PKI Management Operations
The following diagram shows the relationship between the entities
defined above in terms of the PKI management operations. The letters
in the diagram indicate "protocols" in the sense that a defined set
of PKI management messages can be sent along each of the lettered
lines.
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+---+ cert. publish +------------+ j
| | <--------------------- | End Entity | <-------
| C | g +------------+ "out-of-band"
| e | | ^ loading
| r | | | initial
| t | a | | b registration/
| | | | certification
| / | | | key pair recovery
| | | | key pair update
| C | | | certificate update
| R | PKI "USERS" V | revocation request
| L | -------------------+-+-----+-+------+-+-------------------
| | PKI MANAGEMENT | ^ | ^
| | ENTITIES a | | b a | | b
| R | V | | |
| e | g +------+ d | |
| p | <------------ | RA | <-----+ | |
| o | cert. | | ----+ | | |
| s | publish +------+ c | | | |
| i | | | | |
| t | V | V |
| o | g +------------+ i
| r | <------------------------| CA |------->
| y | h +------------+ "out-of-band"
| | cert. publish | ^ publication
| | CRL publish | |
+---+ | | cross-certification
e | | f cross-certificate
| | update
| |
V |
+------+
| CA-2 |
+------+
Figure 1 - PKI Entities
At a high level the set of operations for which management messages
are defined can be grouped as follows.
1. CA establishment: When establishing a new CA, certain steps are
required (e.g., production of initial CRLs, export of CA public
key).
2. End entity initialization: this includes importing a root CA
public key and requesting information about the options supported
by a PKI management entity.
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3. Certification: various operations result in the creation of new
certificates:
1. initial registration/certification: This is the process
whereby an end entity first makes itself known to a CA or RA,
prior to the CA issuing a certificate or certificates for
that end entity. The end result of this process (when it is
successful) is that a CA issues a certificate for an end
entity's public key, and returns that certificate to the end
entity and/or posts that certificate in a public repository.
This process may, and typically will, involve multiple
"steps", possibly including an initialization of the end
entity's equipment. For example, the end entity's equipment
must be securely initialized with the public key of a CA, to
be used in validating certificate paths. Furthermore, an end
entity typically needs to be initialized with its own key
pair(s).
2. key pair update: Every key pair needs to be updated regularly
(i.e., replaced with a new key pair), and a new certificate
needs to be issued.
3. certificate update: As certificates expire they may be
"refreshed" if nothing relevant in the environment has
changed.
4. CA key pair update: As with end entities, CA key pairs need
to be updated regularly; however, different mechanisms are
required.
5. cross-certification request: One CA requests issuance of a
cross-certificate from another CA. For the purposes of this
standard, the following terms are defined. A "cross-
certificate" is a certificate in which the subject CA and the
issuer CA are distinct and SubjectPublicKeyInfo contains a
verification key (i.e., the certificate has been issued for
the subject CA's signing key pair). When it is necessary to
distinguish more finely, the following terms may be used: a
cross-certificate is called an "inter-domain
cross-certificate" if the subject and issuer CAs belong to
different administrative domains; it is called an "intra-
domain cross-certificate" otherwise.
1. Note 1. The above definition of "cross-certificate"
aligns with the defined term "CA-certificate" in X.509.
Note that this term is not to be confused with the X.500
"cACertificate" attribute type, which is unrelated.
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2. Note 2. In many environments the term
"cross-certificate", unless further qualified, will be
understood to be synonymous with "inter-domain
cross-certificate" as defined above.
3. Note 3. Issuance of cross-certificates may be, but is not
necessarily, mutual; that is, two CAs may issue
cross-certificates for each other.
6. cross-certificate update: Similar to a normal certificate
update but involving a cross-certificate.
4. Certificate/CRL discovery operations: some PKI management
operations result in the publication of certificates or CRLs:
1. certificate publication: Having gone to the trouble of
producing a certificate, some means for publishing it is
needed. The "means" defined in PKIX MAY involve the messages
specified in Section Section 5.3.13 - Section 5.3.16, or MAY
involve other methods (LDAP, for example) as described in
[RFC2559], [RFC2585] (the "Operational Protocols" documents
of the PKIX series of specifications).
2. 4.2 CRL publication: As for certificate publication.
5. Recovery operations: some PKI management operations are used when
an end entity has "lost" its PSE:
1. key pair recovery: As an option, user client key materials
(e.g., a user's private key used for decryption purposes) MAY
be backed up by a CA, an RA, or a key backup system
associated with a CA or RA. If an entity needs to recover
these backed up key materials (e.g., as a result of a
forgotten password or a lost key chain file), a protocol
exchange may be needed to support such recovery.
6. Revocation operations: some PKI operations result in the creation
of new CRL entries and/or new CRLs:
1. revocation request: An authorized person advises a CA of an
abnormal situation requiring certificate revocation.
7. PSE operations: whilst the definition of PSE operations (e.g.,
moving a PSE, changing a PIN, etc.) are beyond the scope of this
specification, we do define a PKIMessage (CertRepMessage) which
can form the basis of such operations.
Note that on-line protocols are not the only way of implementing the
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above operations. For all operations there are off-line methods of
achieving the same result, and this specification does not mandate
use of on-line protocols. For example, when hardware tokens are
used, many of the operations MAY be achieved as part of the physical
token delivery.
Later sections define a set of standard messages supporting the above
operations. Transport protocols for conveying these exchanges in
different environments (file based, on-line, E-mail, and WWW) are
beyond the scope of this document and are specified separately.
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4. Assumptions and Restrictions
4.1 End Entity Initialization
The first step for an end entity in dealing with PKI management
entities is to request information about the PKI functions supported
and to securely acquire a copy of the relevant root CA public key(s).
4.2 Initial Registration/Certification
There are many schemes that can be used to achieve initial
registration and certification of end entities. No one method is
suitable for all situations due to the range of policies which a CA
may implement and the variation in the types of end entity which can
occur.
We can however, classify the initial registration / certification
schemes that are supported by this specification. Note that the word
"initial", above, is crucial - we are dealing with the situation
where the end entity in question has had no previous contact with the
PKI. Where the end entity already possesses certified keys then some
simplifications/alternatives are possible.
Having classified the schemes that are supported by this
specification we can then specify some as mandatory and some as
optional. The goal is that the mandatory schemes cover a sufficient
number of the cases which will arise in real use, whilst the optional
schemes are available for special cases which arise less frequently.
In this way we achieve a balance between flexibility and ease of
implementation.
We will now describe the classification of initial registration /
certification schemes.
4.2.1 Criteria Used
4.2.1.1 Initiation of Registration / Certification
In terms of the PKI messages which are produced we can regard the
initiation of the initial registration / certification exchanges as
occurring wherever the first PKI message relating to the end entity
is produced. Note that the real-world initiation of the registration
/ certification procedure may occur elsewhere (e.g., a personnel
department may telephone an RA operator).
The possible locations are at the end entity, an RA, or a CA.
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4.2.1.2 End Entity Message Origin Authentication
The on-line messages produced by the end entity that requires a
certificate may be authenticated or not. The requirement here is to
authenticate the origin of any messages from the end entity to the
PKI (CA/RA).
In this specification, such authentication is achieved by the PKI
(CA/RA) issuing the end entity with a secret value (initial
authentication key) and reference value (used to identify the secret
value) via some out-of-band means. The initial authentication key can
then be used to protect relevant PKI messages.
We can thus classify the initial registration/certification scheme
according to whether or not the on-line end entity -> PKI messages
are authenticated or not.
Note 1: We do not discuss the authentication of the PKI -> end entity
messages here as this is always REQUIRED. In any case, it can be
achieved simply once the root-CA public key has been installed at the
end entity's equipment or it can be based on the initial
authentication key.
Note 2: An initial registration / certification procedure can be
secure where the messages from the end entity are authenticated via
some out- of-band means (e.g., a subsequent visit).
4.2.1.3 Location of Key Generation
In this specification, "key generation" is regarded as occurring
wherever either the public or private component of a key pair first
occurs in a PKIMessage. Note that this does not preclude a
centralized key generation service - the actual key pair MAY have
been generated elsewhere and transported to the end entity, RA, or CA
using a (proprietary or standardized) key generation request/response
protocol (outside the scope of this specification).
There are thus three possibilities for the location of "key
generation": the end entity, an RA, or a CA.
4.2.1.4 Confirmation of Successful Certification
Following the creation of an initial certificate for an end entity,
additional assurance can be gained by having the end entity
explicitly confirm successful receipt of the message containing (or
indicating the creation of) the certificate. Naturally, this
confirmation message must be protected (based on the initial
authentication key or other means).
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This gives two further possibilities: confirmed or not.
4.2.2 Mandatory Schemes
The criteria above allow for a large number of initial registration /
certification schemes. This specification mandates that conforming CA
equipment, RA equipment, and EE equipment MUST support the second
scheme listed below (Section 4.2.2.2) Any entity MAY additionally
support other schemes, if desired.
4.2.2.1 Centralized Scheme
In terms of the classification above, this scheme is, in some ways,
the simplest possible, where:
o initiation occurs at the certifying CA;
o no on-line message authentication is required;
o "key generation" occurs at the certifying CA (see Section
4.2.1.3);
o no confirmation message is required.
In terms of message flow, this scheme means that the only message
required is sent from the CA to the end entity. The message must
contain the entire PSE for the end entity. Some out-of-band means
must be provided to allow the end entity to authenticate the message
received and decrypt any encrypted values.
4.2.2.2 Basic Authenticated Scheme
In terms of the classification above, this scheme is where:
o initiation occurs at the end entity;
o message authentication is REQUIRED;
o "key generation" occurs at the end entity (see Section 4.2.1.3);
o a confirmation message is REQUIRED.
In terms of message flow, the basic authenticated scheme is as
follows:
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End entity RA/CA
========== =============
out-of-band distribution of Initial Authentication
Key (IAK) and reference value (RA/CA -> EE)
Key generation
Creation of certification request
Protect request with IAK
-->>-- certification request -->>--
verify request
process request
create response
--<<-- certification response --<<--
handle response
create confirmation
-->>-- cert conf message -->>--
verify confirmation
create response
--<<-- conf ack (optional) --<<--
handle response
(Where verification of the cert confirmation message fails, the RA/CA
MUST revoke the newly issued certificate if it has been published or
otherwise made available.)
4.3 Proof of Possession (POP) of Private Key
In order to prevent certain attacks and to allow a CA/RA to properly
check the validity of the binding between an end entity and a key
pair, the PKI management operations specified here make it possible
for an end entity to prove that it has possession of (i.e., is able
to use) the private key corresponding to the public key for which a
certificate is requested. A given CA/RA is free to choose how to
enforce POP (e.g., out-of-band procedural means versus PKIX-CMP in-
band messages) in its certification exchanges (i.e., this may be a
policy issue). However, it is REQUIRED that CAs/RAs MUST enforce POP
by some means because there are currently many non-PKIX operational
protocols in use (various electronic mail protocols are one example)
that do not explicitly check the binding between the end entity and
the private key. Until operational protocols that do verify the
binding (for signature, encryption, and key agreement key pairs)
exist, and are ubiquitous, this binding can only be assumed to have
been verified by the CA/RA. Therefore, if the binding is not verified
by the CA/RA, certificates in the Internet Public-Key Infrastructure
end up being somewhat less meaningful.
POP is accomplished in different ways depending upon the type of key
for which a certificate is requested. If a key can be used for
multiple purposes (e.g., an RSA key) then any appropriate method MAY
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be used (e.g., a key which may be used for signing, as well as other
purposes, SHOULD NOT be sent to the CA/RA in order to prove
possession).
This specification explicitly allows for cases where an end entity
supplies the relevant proof to an RA and the RA subsequently attests
to the CA that the required proof has been received (and validated!).
For example, an end entity wishing to have a signing key certified
could send the appropriate signature to the RA which then simply
notifies the relevant CA that the end entity has supplied the
required proof. Of course, such a situation may be disallowed by some
policies (e.g., CAs may be the only entities permitted to verify POP
during certification).
4.3.1 Signature Keys
For signature keys, the end entity can sign a value to prove
possession of the private key.
4.3.2 Encryption Keys
For encryption keys, the end entity can provide the private key to
the CA/RA, or can be required to decrypt a value in order to prove
possession of the private key (see Section 5.2.8). Decrypting a value
can be achieved either directly or indirectly.
The direct method is for the RA/CA to issue a random challenge to
which an immediate response by the EE is required.
The indirect method is to issue a certificate which is encrypted for
the end entity (and have the end entity demonstrate its ability to
decrypt this certificate in the confirmation message). This allows a
CA to issue a certificate in a form which can only be used by the
intended end entity.
This specification encourages use of the indirect method because this
requires no extra messages to be sent (i.e., the proof can be
demonstrated using the {request, response, confirmation} triple of
messages).
4.3.3 Key Agreement Keys
For key agreement keys, the end entity and the PKI management entity
(i.e., CA or RA) must establish a shared secret key in order to prove
that the end entity has possession of the private key.
Note that this need not impose any restrictions on the keys that can
be certified by a given CA -- in particular, for Diffie-Hellman keys
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the end entity may freely choose its algorithm parameters -- provided
that the CA can generate a short-term (or one-time) key pair with the
appropriate parameters when necessary.
4.4 Root CA Key Update
This discussion only applies to CAs that are directly trusted by some
end entities. Self-signed CAs SHALL be considered as directly trusted
CAs. Recognizing whether a non self-signed CA is supposed to be
directly trusted for some end entities is a matter of CA policy and
is thus beyond the scope of this document.
The basis of the procedure described here is that the CA protects its
new public key using its previous private key and vice versa. Thus
when a CA updates its key pair it must generate two extra
cACertificate attribute values if certificates are made available
using an X.500 directory (for a total of four: OldWithOld;
OldWithNew; NewWithOld; and NewWithNew).
When a CA changes its key pair those entities who have acquired the
old CA public key via "out-of-band" means are most affected. It is
these end entities who will need access to the new CA public key
protected with the old CA private key. However, they will only
require this for a limited period (until they have acquired the new
CA public key via the "out-of-band" mechanism). This will typically
be easily achieved when these end entities' certificates expire.
The data structure used to protect the new and old CA public keys is
a standard certificate (which may also contain extensions). There are
no new data structures required.
Note 1. This scheme does not make use of any of the X.509 v3
extensions as it must be able to work even for version 1
certificates. The presence of the KeyIdentifier extension would make
for efficiency improvements.
Note 2. While the scheme could be generalized to cover cases where
the CA updates its key pair more than once during the validity period
of one of its end entities' certificates, this generalization seems
of dubious value. Not having this generalization simply means that
the validity periods of certificates issued with the old CA key pair
cannot exceed the end of the OldWithNew validity period.
Note 3. This scheme ensures that end entities will acquire the new CA
public key, at the latest by the expiry of the last certificate they
owned that was signed with the old CA private key (via the
"out-of-band" means). Certificate and/or key update operations
occurring at other times do not necessarily require this (depending
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on the end entity's equipment).
4.4.1 CA Operator Actions
To change the key of the CA, the CA operator does the following:
1. Generate a new key pair;
2. Create a certificate containing the old CA public key signed with
the new private key (the "old with new" certificate);
3. Create a certificate containing the new CA public key signed with
the old private key (the "new with old" certificate);
4. Create a certificate containing the new CA public key signed with
the new private key (the "new with new" certificate);
5. Publish these new certificates via the repository and/or other
means (perhaps using a CAKeyUpdAnn message);
6. Export the new CA public key so that end entities may acquire it
using the "out-of-band" mechanism (if required).
The old CA private key is then no longer required. The old CA public
key will however remain in use for some time. The time when the old
CA public key is no longer required (other than for non-repudiation)
will be when all end entities of this CA have securely acquired the
new CA public key.
The "old with new" certificate must have a validity period starting
at the generation time of the old key pair and ending at the expiry
date of the old public key.
The "new with old" certificate must have a validity period starting
at the generation time of the new key pair and ending at the time by
which all end entities of this CA will securely possess the new CA
public key (at the latest, the expiry date of the old public key).
The "new with new" certificate must have a validity period starting
at the generation time of the new key pair and ending at or before
the time by which the CA will next update its key pair.
4.4.2 Verifying Certificates
Normally when verifying a signature, the verifier verifies (among
other things) the certificate containing the public key of the
signer. However, once a CA is allowed to update its key there are a
range of new possibilities. These are shown in the table below.
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Repository contains NEW Repository contains only OLD
and OLD public keys public key (due to, e.g.,
delay in publication)
PSE PSE Contains PSE Contains PSE Contains
Contains OLD public NEW public OLD public
NEW public key key key
key
Signer's Case 1: Case 3: Case 5: Case 7:
certifi- This is In this case Although the In this case
cate is the the verifier CA operator the CA
protected standard must access has not operator has
using NEW case where the updated the not updated
public the repository in repository the the repository
key verifier order to get verifier can and so the
can the value of verify the verification
directly the NEW certificate will FAIL
verify the public key directly -
certificate this is thus
without the same as
using the case 1.
repository
Signer's Case 2: Case 4: Case 6: Case 8:
certifi- In this In this case The verifier Although the
cate is case the the verifier thinks this CA operator
protected verifier can directly is the has not
using OLD must verify the situation of updated the
public access the certificate case 2 and repository the
key repository without will access verifier can
in order using the the verify the
to get the repository repository; certificate
value of however, the directly -
the OLD verification this is thus
public key will FAIL the same as
case 4.
4.4.2.1 Verification in Cases 1, 4, 5 and 8
In these cases the verifier has a local copy of the CA public key
which can be used to verify the certificate directly. This is the
same as the situation where no key change has occurred.
Note that case 8 may arise between the time when the CA operator has
generated the new key pair and the time when the CA operator stores
the updated attributes in the repository. Case 5 can only arise if
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the CA operator has issued both the signer's and verifier's
certificates during this "gap" (the CA operator SHOULD avoid this as
it leads to the failure cases described below)
4.4.2.2 Verification in Case 2
In case 2 the verifier must get access to the old public key of the
CA. The verifier does the following:
1. Look up the caCertificate attribute in the repository and pick
the OldWithNew certificate (determined based on validity periods;
note that the subject and issuer fields must match);
2. Verify that this is correct using the new CA key (which the
verifier has locally);
3. If correct, check the signer's certificate using the old CA key.
Case 2 will arise when the CA operator has issued the signer's
certificate, then changed key and then issued the verifier's
certificate, so it is quite a typical case.
4.4.2.3 Verification in Case 3
In case 3 the verifier must get access to the new public key of the
CA. The verifier does the following:
1. Look up the CACertificate attribute in the repository and pick
the NewWithOld certificate (determined based on validity periods;
note that the subject and issuer fields must match);
2. Verify that this is correct using the old CA key (which the
verifier has stored locally);
3. If correct, check the signer's certificate using the new CA key.
Case 3 will arise when the CA operator has issued the verifier's
certificate, then changed key and then issued the signer's
certificate, so it is also quite a typical case.
4.4.2.4 Failure of Verification in Case 6
In this case the CA has issued the verifier's PSE containing the new
key without updating the repository attributes. This means that the
verifier has no means to get a trustworthy version of the CA's old
key and so verification fails.
Note that the failure is the CA operator's fault.
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4.4.2.5 Failure of Verification in Case 7
In this case the CA has issued the signer's certificate protected
with the new key without updating the repository attributes. This
means that the verifier has no means to get a trustworthy version of
the CA's new key and so verification fails.
Note that the failure is again the CA operator's fault.
4.4.3 Revocation - Change of CA Key
As we saw above the verification of a certificate becomes more
complex once the CA is allowed to change its key. This is also true
for revocation checks as the CA may have signed the CRL using a newer
private key than the one that is within the user's PSE.
The analysis of the alternatives is as for certificate verification.
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5. Data Structures
This section contains descriptions of the data structures required
for PKI management messages. Section 6 describes constraints on their
values and the sequence of events for each of the various PKI
management operations.
5.1 Overall PKI Message
All of the messages used in this specification for the purposes of
PKI management use the following structure:
PKIMessage ::= SEQUENCE {
header PKIHeader,
body PKIBody,
protection [0] PKIProtection OPTIONAL,
extraCerts [1] SEQUENCE SIZE (1..MAX) OF CMPCertificate
OPTIONAL
}
PKIMessages ::= SEQUENCE SIZE (1..MAX) OF PKIMessage
The PKIHeader contains information which is common to many PKI
messages.
The PKIBody contains message-specific information.
The PKIProtection, when used, contains bits that protect the PKI
message.
The extraCerts field can contain certificates that may be useful to
the recipient. For example, this can be used by a CA or RA to present
an end entity with certificates that it needs to verify its own new
certificate (if, for example, the CA that issued the end entity's
certificate is not a root CA for the end entity). Note that this
field does not necessarily contain a certification path - the
recipient may have to sort, select from, or otherwise process the
extra certificates in order to use them.
5.1.1 PKI Message Header
All PKI messages require some header information for addressing and
transaction identification. Some of this information will also be
present in a transport-specific envelope; however, if the PKI message
is protected then this information is also protected (i.e., we make
no assumption about secure transport).
The following data structure is used to contain this information:
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PKIHeader ::= SEQUENCE {
pvno INTEGER { cmp1999(1), cmp2000(2) },
sender GeneralName,
recipient GeneralName,
messageTime [0] GeneralizedTime OPTIONAL,
protectionAlg [1] AlgorithmIdentifier OPTIONAL,
senderKID [2] KeyIdentifier OPTIONAL,
recipKID [3] KeyIdentifier OPTIONAL,
transactionID [4] OCTET STRING OPTIONAL,
senderNonce [5] OCTET STRING OPTIONAL,
recipNonce [6] OCTET STRING OPTIONAL,
freeText [7] PKIFreeText OPTIONAL,
generalInfo [8] SEQUENCE SIZE (1..MAX) OF
InfoTypeAndValue OPTIONAL
}
PKIFreeText ::= SEQUENCE SIZE (1..MAX) OF UTF8String
The pvno field is fixed (at 2) for this version of this
specification.
The sender field contains the name of the sender of the PKIMessage.
This name (in conjunction with senderKID, if supplied) should be
sufficient to indicate the key to use to verify the protection on the
message. If nothing about the sender is known to the sending entity
(e.g., in the init. req. message, where the end entity may not know
its own Distinguished Name (DN), e-mail name, IP address, etc.), then
the "sender" field MUST contain a "NULL" value; that is, the SEQUENCE
OF relative distinguished names is of zero length. In such a case the
senderKID field MUST hold an identifier (i.e., a reference number)
which indicates to the receiver the appropriate shared secret
information to use to verify the message.
The recipient field contains the name of the recipient of the
PKIMessage. This name (in conjunction with recipKID, if supplied)
should be usable to verify the protection on the message.
The protectionAlg field specifies the algorithm used to protect the
message. If no protection bits are supplied (note that PKIProtection
is OPTIONAL) then this field MUST be omitted; if protection bits are
supplied then this field MUST be supplied.
senderKID and recipKID are usable to indicate which keys have been
used to protect the message (recipKID will normally only be required
where protection of the message uses Diffie-Hellman (DH) keys).
These fields MUST be used if required to uniquely identify a key
(e.g., if more than one key is associated with a given sender name)
and SHOULD be omitted otherwise.
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The transactionID field within the message header is to be used to
allow the recipient of a message to correlate this with an ongoing
transaction. This is needed for all transactions that consist of more
than just a single request/response pair. For transactions that
consist of a single request/response pair the rules are as follows.
A client MAY populate the transactionID field of the request. If a
server receives such a request which has the transactionID field set,
then it MUST set the transactionID field of the response to the same
value; if a server receives such request with a missing transactionID
field then it MAY set transactionID field of the response.
For transactions that consist of more than just a single request/
response pair the rules are as follows. Clients SHOULD generate a
transactionID for the first request. If a server receives such a
request which has the transactionID field set, then it MUST set the
transactionID field of the response to the same value; if a server
receives such request with a missing transactionID field then it MUST
populate transactionID field of the response with a server-generated
ID. Subsequent requests and responses MUST all set the transactionID
field to the thus established value. In all cases where a
transactionID is being used, a given client MUST NOT have more than
one transaction with the same transactionID in progress at any time
(to a given server). Servers are free to require uniqueness of the
transactionID or not, as long as they are able to correctly associate
messages with the corresponding transaction. Typically this means
that a server will require the {client, transactionID} tuple to be
unique, or even the transactionID alone to be unique if it cannot
distinguish clients based on transport level information. A server
receiving the first message of a transaction (which requires more
than a single request/response pair) that contains a transactionID
that does not allow it to meet the above constraints (typically
because the transactionID is already in use) MUST send back an
ErrorMsgContent with a PKIFailureInfo of transactionIdInUse. It is
RECOMMENDED that the clients fill the transactionID field with 128
bits of (pseudo-) random data for the start of a transaction to
reduce the probability of having the transactionID in use at the
server.
The senderNonce and recipNonce fields protect the PKIMessage against
replay attacks. The senderNonce will typically be 128 bits of
(pseudo-) random data generated by the sender, whereas the recipNonce
is copied from the senderNonce of the previous message in the
transaction.
The messageTime field contains the time at which the sender created
the message. This may be useful to allow end entities to correct/
check their local time for consistency with the time on a central
system.
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The freeText field may be used to send a human-readable message to
the recipient (in any number of languages). The first language used
in this sequence indicates the desired language for replies.
The generalInfo field may be used to send machine-processable
additional data to the recipient. The following generalInfo
extensions are defined and MAY be supported.
5.1.1.1 ImplicitConfirm
This is used by the EE to inform the CA that it does not wish to send
a certificate confirmation for issued certificates.
implicitConfirm OBJECT IDENTIFIER ::= {id-it 13}
ImplicitConfirmValue ::= NULL
If the CA grants the request to the EE, it MUST put the same
extension in the PKIHeader of the response. If the EE does not find
the extension in the response, it MUST send the certificate
confirmation.
5.1.1.2 ConfirmWaitTime
This is used by the CA to inform the EE how long it intends to wait
for the certificate confirmation before revoking the certificate and
deleting the transaction.
confirmWaitTime OBJECT IDENTIFIER ::= {id-it 14}
ConfirmWaitTimeValue ::= GeneralizedTime
5.1.2 PKI Message Body
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PKIBody ::= CHOICE {
ir [0] CertReqMessages, --Initialization Req
ip [1] CertRepMessage, --Initialization Resp
cr [2] CertReqMessages, --Certification Req
cp [3] CertRepMessage, --Certification Resp
p10cr [4] CertificationRequest, --PKCS #10 Cert. Req.
popdecc [5] POPODecKeyChallContent --pop Challenge
popdecr [6] POPODecKeyRespContent, --pop Response
kur [7] CertReqMessages, --Key Update Request
kup [8] CertRepMessage, --Key Update Response
krr [9] CertReqMessages, --Key Recovery Req
krp [10] KeyRecRepContent, --Key Recovery Resp
rr [11] RevReqContent, --Revocation Request
rp [12] RevRepContent, --Revocation Response
ccr [13] CertReqMessages, --Cross-Cert. Request
ccp [14] CertRepMessage, --Cross-Cert. Resp
ckuann [15] CAKeyUpdAnnContent, --CA Key Update Ann.
cann [16] CertAnnContent, --Certificate Ann.
rann [17] RevAnnContent, --Revocation Ann.
crlann [18] CRLAnnContent, --CRL Announcement
pkiconf [19] PKIConfirmContent, --Confirmation
nested [20] NestedMessageContent, --Nested Message
genm [21] GenMsgContent, --General Message
genp [22] GenRepContent, --General Response
error [23] ErrorMsgContent, --Error Message
certConf [24] CertConfirmContent, --Certificate confirm
pollReq [25] PollReqContent, --Polling request
pollRep [26] PollRepContent --Polling response
}
The specific types are described in Section Section 5.3 below.
5.1.3 PKI Message Protection
Some PKI messages will be protected for integrity. (Note that if an
asymmetric algorithm is used to protect a message and the relevant
public component has been certified already, then the origin of the
message can also be authenticated. On the other hand, if the public
component is uncertified then the message origin cannot be
automatically authenticated, but may be authenticated via out-of-band
means.)
When protection is applied the following structure is used:
PKIProtection ::= BIT STRING
The input to the calculation of PKIProtection is the DER encoding of
the following data structure:
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ProtectedPart ::= SEQUENCE {
header PKIHeader,
body PKIBody
}
There MAY be cases in which the PKIProtection BIT STRING is
deliberately not used to protect a message (i.e., this OPTIONAL field
is omitted) because other protection, external to PKIX, will instead
be applied. Such a choice is explicitly allowed in this
specification. Examples of such external protection include PKCS #7
[PKCS7] and Security Multiparts [RFC1847] encapsulation of the
PKIMessage (or simply the PKIBody (omitting the CHOICE tag), if the
relevant PKIHeader information is securely carried in the external
mechanism). It is noted, however, that many such external mechanisms
require that the end entity already possesses a public-key
certificate, and/or a unique Distinguished Name, and/or other such
infrastructure-related information. Thus, they may not be appropriate
for initial registration, key-recovery, or any other process with
"boot-strapping" characteristics. For those cases it may be necessary
that the PKIProtection parameter be used. In the future, if/when
external mechanisms are modified to accommodate boot-strapping
scenarios, the use of PKIProtection may become rare or non-existent.
Depending on the circumstances the PKIProtection bits may contain a
Message Authentication Code (MAC) or signature. Only the following
cases can occur:
5.1.3.1 Shared Secret Information
In this case the sender and recipient share secret information
(established via out-of-band means or from a previous PKI management
operation). PKIProtection will contain a MAC value and the
protectionAlg will be the following (see also Appendix D.2):
id-PasswordBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 13}
PBMParameter ::= SEQUENCE {
salt OCTET STRING,
owf AlgorithmIdentifier,
iterationCount INTEGER,
mac AlgorithmIdentifier
}
In the above protectionAlg the salt value is appended to the shared
secret input. The OWF is then applied iterationCount times, where the
salted secret is the input to the first iteration and, for each
successive iteration, the input is set to be the output of the
previous iteration. The output of the final iteration (called
"BASEKEY" for ease of reference, with a size of "H") is what is used
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to form the symmetric key. If the MAC algorithm requires a K-bit key
and K <= H, then the most significant K bits of BASEKEY are used. If
K > H, then all of BASEKEY is used for the most significant H bits of
the key, OWF("1" || BASEKEY) is used for the next most significant H
bits of the key, OWF("2" || BASEKEY) is used for the next most
significant H bits of the key, and so on, until all K bits have been
derived. [Here "N" is the ASCII byte encoding the number N and "||"
represents concatenation.]
Note: it is RECOMMENDED that the fields of PBMParameter remain
constant throughout the messages of a single transaction (e.g., ir/
ip/certConf/pkiConf) in order to reduce the overhead associated with
PasswordBasedMac computation).
5.1.3.2 DH Key Pairs
Where the sender and receiver possess Diffie-Hellman certificates
with compatible DH parameters, then in order to protect the message
the end entity must generate a symmetric key based on its private DH
key value and the DH public key of the recipient of the PKI message.
PKIProtection will contain a MAC value keyed with this derived
symmetric key and the protectionAlg will be the following:
id-DHBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 30}
DHBMParameter ::= SEQUENCE {
owf AlgorithmIdentifier,
-- AlgId for a One-Way Function (SHA-1 recommended)
mac AlgorithmIdentifier
-- the MAC AlgId (e.g., DES-MAC, Triple-DES-MAC [PKCS11],
} -- or HMAC [RFC2104, RFC2202])
In the above protectionAlg OWF is applied to the result of the
Diffie-Hellman computation. The OWF output (called "BASEKEY" for ease
of reference, with a size of "H") is what is used to form the
symmetric key. If the MAC algorithm requires a K-bit key and K <= H,
then the most significant K bits of BASEKEY are used. If K > H, then
all of BASEKEY is used for the most significant H bits of the key,
OWF("1" || BASEKEY) is used for the next most significant H bits of
the key, OWF("2" || BASEKEY) is used for the next most significant H
bits of the key, and so on, until all K bits have been derived. [Here
"N" is the ASCII byte encoding the number N and "||" represents
concatenation.]
5.1.3.3 Signature
In this case the sender possesses a signature key pair and simply
signs the PKI message. PKIProtection will contain the signature value
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and the protectionAlg will be an AlgorithmIdentifier for a digital
signature (e.g., md5WithRSAEncryption or dsaWithSha-1).
5.1.3.4 Multiple Protection
In cases where an end entity sends a protected PKI message to an RA,
the RA MAY forward that message to a CA, attaching its own protection
(which MAY be a MAC or a signature, depending on the information and
certificates shared between the RA and the CA). This is accomplished
by nesting the entire message sent by the end entity within a new PKI
message. The structure used is as follows.
NestedMessageContent ::= PKIMessages
(The use of PKIMessages, a SEQUENCE OF PKIMessage, lets the RA batch
the requests of several EEs in a single new message. For simplicity,
all messages in the batch MUST be of the same type (e.g., ir)). If
the RA wishes to modify the message(s) in some way (e.g., add
particular field values or new extensions), then it MAY create its
own desired PKIBody. The original PKIMessage from the EE MAY be
included in the generalInfo field of PKIHeader (to accommodate, for
example, cases in which the CA wishes to check POP or other
information on the original EE message). The infoType to be used in
this situation is {id-it 15} (see Section 5.3.19 for the value of
id-it) and the infoValue is PKIMessages (contents MUST be in the same
order as the requests in PKIBody).
5.2 Common Data Structures
Before specifying the specific types that may be placed in a PKIBody
we define some data structures that are used in more than one case.
5.2.1 Requested Certificate Contents
Various PKI management messages require that the originator of the
message indicate some of the fields that are required to be present
in a certificate. The CertTemplate structure allows an end entity or
RA to specify as much as it wishes about the certificate it requires.
CertTemplate is identical to a Certificate but with all fields
optional.
Note that even if the originator completely specifies the contents of
a certificate it requires, a CA is free to modify fields within the
certificate actually issued. If the modified certificate is
unacceptable to the requester, the requester MUST send back a
certConf message which either does not include this certificate (via
a CertHash), or does include this certificate (via a CertHash) along
with a status of "rejected". See Section 5.3.18 for the definition
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and use of CertHash and the certConf message.
See Appendix C and [CRMF] for CertTemplate syntax.
5.2.2 Encrypted Values
Where encrypted values (restricted, in this specification, to be
either private keys or certificates) are sent in PKI messages the
EncryptedValue data structure is used.
See [CRMF] for EncryptedValue syntax.
Use of this data structure requires that the creator and intended
recipient respectively be able to encrypt and decrypt. Typically,
this will mean that the sender and recipient have, or are able to
generate, a shared secret key.
If the recipient of the PKIMessage already possesses a private key
usable for decryption, then the encSymmKey field MAY contain a
session key encrypted using the recipient's public key.
5.2.3 Status codes and Failure Information for PKI Messages
All response messages will include some status information. The
following values are defined.
PKIStatus ::= INTEGER {
accepted (0),
grantedWithMods (1),
rejection (2),
waiting (3),
revocationWarning (4),
revocationNotification (5),
keyUpdateWarning (6)
}
Responders may use the following syntax to provide more information
about failure cases.
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PKIFailureInfo ::= BIT STRING {
badAlg (0),
badMessageCheck (1),
badRequest (2),
badTime (3),
badCertId (4),
badDataFormat (5),
wrongAuthority (6),
incorrectData (7),
missingTimeStamp (8),
badPOP (9),
certRevoked (10),
certConfirmed (11),
wrongIntegrity (12),
badRecipientNonce (13),
timeNotAvailable (14),
unacceptedPolicy (15),
unacceptedExtension (16),
addInfoNotAvailable (17),
badSenderNonce (18),
badCertTemplate (19),
signerNotTrusted (20),
transactionIdInUse (21),
unsupportedVersion (22),
notAuthorized (23),
systemUnavail (24),
systemFailure (25),
duplicateCertReq (26)
}
PKIStatusInfo ::= SEQUENCE {
status PKIStatus,
statusString PKIFreeText OPTIONAL,
failInfo PKIFailureInfo OPTIONAL
}
5.2.4 Certificate Identification
In order to identify particular certificates the CertId data
structure is used.
See [CRMF] for CertId syntax.
5.2.5 Out-of-band root CA Public Key
Each root CA must be able to publish its current public key via some
"out-of-band" means. While such mechanisms are beyond the scope of
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this document, we define data structures which can support such
mechanisms.
There are generally two methods available: either the CA directly
publishes its self-signed certificate; or this information is
available via the Directory (or equivalent) and the CA publishes a
hash of this value to allow verification of its integrity before use.
OOBCert ::= Certificate
The fields within this certificate are restricted as follows:
o The certificate MUST be self-signed (i.e., the signature must be
verifiable using the SubjectPublicKeyInfo field);
o The subject and issuer fields MUST be identical;
o If the subject field is NULL then both subjectAltNames and
issuerAltNames extensions MUST be present and have exactly the
same value;
o The values of all other extensions must be suitable for a self-
signed certificate (e.g., key identifiers for subject and issuer
must be the same).
OOBCertHash ::= SEQUENCE {
hashAlg [0] AlgorithmIdentifier OPTIONAL,
certId [1] CertId OPTIONAL,
hashVal BIT STRING
}
The intention of the hash value is that anyone who has securely
received the hash value (via the out-of-band means) can verify a
self-signed certificate for that CA.
5.2.6 Archive Options
Requesters may indicate that they wish the PKI to archive a private
key value using the PKIArchiveOptions structure.
See [CRMF] for PKIArchiveOptions syntax.
5.2.7 Publication Information
Requesters may indicate that they wish the PKI to publish a
certificate using the PKIPublicationInfo structure.
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See [CRMF] for PKIPublicationInfo syntax.
5.2.8 Proof-of-Possession Structures
If the certification request is for a signing key pair (i.e., a
request for a verification certificate), then the proof of possession
of the private signing key is demonstrated through use of the
POPOSigningKey structure.
See Appendix C and [CRMF] for POPOSigningKey syntax, but note that
POPOSigningKeyInput has the following semantic stipulations in this
specification.
POPOSigningKeyInput ::= SEQUENCE {
authInfo CHOICE {
sender [0] GeneralName,
publicKeyMAC PKMACValue
},
publicKey SubjectPublicKeyInfo
}
On the other hand, if the certification request is for an encryption
key pair (i.e., a request for an encryption certificate), then the
proof of possession of the private decryption key may be demonstrated
in one of three ways.
5.2.8.1 Inclusion of the Private Key
By the inclusion of the private key (encrypted) in the CertRequest
(in the thisMessage field of POPOPrivKey (see Appendix C) or in the
PKIArchiveOptions control structure, depending upon whether or not
archival of the private key is also desired).
5.2.8.2 Indirect Method
By having the CA return not the certificate, but an encrypted
certificate (i.e., the certificate encrypted under a randomly-
generated symmetric key, and the symmetric key encrypted under the
public key for which the certification request is being made) -- this
is the "indirect" method mentioned previously in Section 4.3.2. The
end entity proves knowledge of the private decryption key to the CA
by providing the correct CertHash for this certificate in the
certConf message. This demonstrates POP because the EE can only
compute the correct CertHash if it is able to recover the
certificate, and it can only recover the certificate if it is able to
decrypt the symmetric key using the required private key. Clearly,
for this to work, the CA MUST NOT publish the certificate until the
certConf message arrives (when certHash is to be used to demonstrate
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POP). See Section 5.3.18 for further details.
5.2.8.3 Challenge-Response Protocol
By having the end entity engage in a challenge-response protocol
(using the messages POPODecKeyChall and POPODecKeyResp; see below)
between CertReqMessages and CertRepMessage -- this is the "direct"
method mentioned previously in Section 4.3.2 [This method would
typically be used in an environment in which an RA verifies POP and
then makes a certification request to the CA on behalf of the end
entity. In such a scenario, the CA trusts the RA to have done POP
correctly before the RA requests a certificate for the end entity.]
The complete protocol then looks as follows (note that req' does not
necessarily encapsulate req as a nested message):
EE RA CA
---- req ---->
<--- chall ---
---- resp --->
---- req' --->
<--- rep -----
---- conf --->
<--- ack -----
<--- rep -----
---- conf --->
<--- ack -----
This protocol is obviously much longer than the 3-way exchange given
in choice (2) above, but allows a local Registration Authority to be
involved and has the property that the certificate itself is not
actually created until the proof of possession is complete. In some
environments a different order of the above messages may be required,
such as the following (this may be determined by policy):
EE RA CA
---- req ---->
<--- chall ---
---- resp --->
---- req' --->
<--- rep -----
<--- rep -----
---- conf --->
---- conf --->
<--- ack -----
<--- ack -----
If the cert. request is for a key agreement key (KAK) pair, then the
POP can use any of the 3 ways described above for enc. key pairs,
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with the following changes: (1) the parenthetical text of bullet 2)
is replaced with "(i.e., the certificate encrypted under the
symmetric key derived from the CA's private KAK and the public key
for which the certification request is being made)"; (2) the first
parenthetical text of the challenge field of "Challenge" below is
replaced with "(using PreferredSymmAlg (see Section 5.3.19.4 and
Appendix E.5) and a symmetric key derived from the CA's private KAK
and the public key for which the certification request is being
made)". Alternatively, the POP can use the POPOSigningKey structure
given in [CRMF] (where the alg field is DHBasedMAC and the signature
field is the MAC) as a fourth alternative for demonstrating POP if
the CA already has a D-H certificate that is known to the EE.
The challenge-response messages for proof of possession of a private
decryption key are specified as follows (see [MvOV97], p.404 for
details). Note that this challenge-response exchange is associated
with the preceding cert. request message (and subsequent cert.
response and confirmation messages) by the transactionID used in the
PKIHeader and by the protection (MACing or signing) applied to the
PKIMessage.
POPODecKeyChallContent ::= SEQUENCE OF Challenge
Challenge ::= SEQUENCE {
owf AlgorithmIdentifier OPTIONAL,
witness OCTET STRING,
challenge OCTET STRING
}
Note that the size of Rand needs to be appropriate for encryption
under the public key of the requester. Given that "int" will
typically not be longer than 64 bits, this leaves well over 100 bytes
of room for the "sender" field when the modulus is 1024 bits. If, in
some environment, names are so long that they cannot fit (e.g., very
long DNs), then whatever portion will fit should be used (as long as
it includes at least the common name, and as long as the receiver is
able to deal meaningfully with the abbreviation).
POPODecKeyRespContent ::= SEQUENCE OF INTEGER
5.2.8.4 Summary of PoP Options
The text in this section provides several options with respect to POP
techniques. Using "SK" for "signing key", "EK" for "encryption key",
and "KAK" for "key agreement key", the techniques may be summarized
as follows:
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RAVerified;
SKPOP;
EKPOPThisMessage;
KAKPOPThisMessage;
KAKPOPThisMessageDHMAC;
EKPOPEncryptedCert;
KAKPOPEncryptedCert;
EKPOPChallengeResp; and
KAKPOPChallengeResp.
Given this array of options, it is natural to ask how an end entity
can know what is supported by the CA/RA (i.e., which options it may
use when requesting certificates). The following guidelines should
clarify this situation for EE implementers.
RAVerified. This is not an EE decision; the RA uses this if and only
if it has verified POP before forwarding the request on to the CA, so
it is not possible for the EE to choose this technique.
SKPOP. If the EE has a signing key pair, this is the only POP method
specified for use in the request for a corresponding certificate.
EKPOPThisMessage and KAKPOPThisMessage. It is an EE decision whether
or not to give up its private key to the CA/RA. If the EE decides to
reveal its key, then these are the only POP methods available in this
specification to achieve this (and the key pair type will determine
which of these two methods to use).
KAKPOPThisMessageDHMAC. The EE can only use this method if (1) the
CA has a DH certificate available for this purpose, and (2) the EE
already has a copy of this certificate. If both these conditions
hold, then this technique is clearly supported and may be used by the
EE, if desired.
EKPOPEncryptedCert, KAKPOPEncryptedCert, EKPOPChallengeResp,
KAKPOPChallengeResp. The EE picks one of these (in the
subsequentMessage field) in the request message, depending upon
preference and key pair type. The EE is not doing POP at this point;
it is simply indicating which method it wants to use. Therefore, if
the CA/RA replies with a "badPOP" error, the EE can re-request using
the other POP method chosen in subsequentMessage. Note, however,
that this specification encourages the use of the EncryptedCert
choice and, furthermore, says that the challenge-response would
typically be used when an RA is involved and doing POP verification.
Thus, the EE should be able to make an intelligent decision regarding
which of these POP methods to choose in the request message.
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5.3 Operation-Specific Data Structures
5.3.1 Initialization Request
An Initialization request message contains as the PKIBody a
CertReqMessages data structure which specifies the requested
certificate(s). Typically, SubjectPublicKeyInfo, KeyId, and Validity
are the template fields which may be supplied for each certificate
requested (see Appendix D profiles for further information). This
message is intended to be used for entities first initializing into
the PKI.
See Appendix C and [CRMF] for CertReqMessages syntax.
5.3.2 Initialization Response
An Initialization response message contains as the PKIBody an
CertRepMessage data structure which has for each certificate
requested a PKIStatusInfo field, a subject certificate, and possibly
a private key (normally encrypted with a session key, which is itself
encrypted with the protocolEncrKey).
See Section 5.3.4 for CertRepMessage syntax. Note that if the PKI
Message Protection is "shared secret information" (see Section
5.1.3), then any certificate transported in the caPubs field may be
directly trusted as a root CA certificate by the initiator.
5.3.3 Certification Request
A Certification request message contains as the PKIBody a
CertReqMessages data structure which specifies the requested
certificates. This message is intended to be used for existing PKI
entities who wish to obtain additional certificates.
See Appendix C and [CRMF] for CertReqMessages syntax.
Alternatively, the PKIBody MAY be a CertificationRequest (this
structure is fully specified by the ASN.1 structure
CertificationRequest given in [PKCS10]). This structure may be
required for certificate requests for signing key pairs when
interoperation with legacy systems is desired, but its use is
strongly discouraged whenever not absolutely necessary.
5.3.4 Certification Response
A Certification response message contains as the PKIBody a
CertRepMessage data structure which has a status value for each
certificate requested, and optionally has a CA public key, failure
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information, a subject certificate, and an encrypted private key.
CertRepMessage ::= SEQUENCE {
caPubs [1] SEQUENCE SIZE (1..MAX) OF Certificate
OPTIONAL,
response SEQUENCE OF CertResponse
}
CertResponse ::= SEQUENCE {
certReqId INTEGER,
status PKIStatusInfo,
certifiedKeyPair CertifiedKeyPair OPTIONAL,
rspInfo OCTET STRING OPTIONAL
-- analogous to the id-regInfo-utf8Pairs string defined
-- for regInfo in CertReqMsg [CRMF]
}
CertifiedKeyPair ::= SEQUENCE {
certOrEncCert CertOrEncCert,
privateKey [0] EncryptedValue OPTIONAL,
-- see [CRMF] for comment on encoding
publicationInfo [1] PKIPublicationInfo OPTIONAL
}
CertOrEncCert ::= CHOICE {
certificate [0] Certificate,
encryptedCert [1] EncryptedValue
}
Only one of the failInfo (in PKIStatusInfo) and certificate (in
CertifiedKeyPair) fields can be present in each CertResponse
(depending on the status). For some status values (e.g., waiting)
neither of the optional fields will be present.
Given an EncryptedCert and the relevant decryption key the
certificate may be obtained. The purpose of this is to allow a CA to
return the value of a certificate, but with the constraint that only
the intended recipient can obtain the actual certificate. The benefit
of this approach is that a CA may reply with a certificate even in
the absence of a proof that the requester is the end entity which can
use the relevant private key (note that the proof is not obtained
until the certConf message is received by the CA). Thus the CA will
not have to revoke that certificate in the event that something goes
wrong with the proof of possession (but MAY do so anyway, depending
upon policy).
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5.3.5 Key Update Request Content
For key update requests the CertReqMessages syntax is used.
Typically, SubjectPublicKeyInfo, KeyId, and Validity are the template
fields which may be supplied for each key to be updated. This
message is intended to be used to request updates to existing (non-
revoked and non-expired) certificates (therefore, it is sometimes
referred to as a "Certificate Update" operation). An update is a
replacement certificate containing either a new subject public key or
the current subject public key (although the latter practice may not
be appropriate for some environments).
See Appendix C and [CRMF] for CertReqMessages syntax.
5.3.6 Key Update Response Content
For key update responses the CertRepMessage syntax is used. The
response is identical to the initialization response.
See Section 5.3.4 for CertRepMessage syntax.
5.3.7 Key Recovery Request Content
For key recovery requests the syntax used is identical to the
initialization request CertReqMessages. Typically,
SubjectPublicKeyInfo and KeyId are the template fields which may be
used to supply a signature public key for which a certificate is
required (see Appendix D profiles for further information).
See Appendix C and [CRMF] for CertReqMessages syntax. Note that if a
key history is required, the requester must supply a Protocol
Encryption Key control in the request message.
5.3.8 Key Recovery Response Content
For key recovery responses the following syntax is used. For some
status values (e.g., waiting) none of the optional fields will be
present.
KeyRecRepContent ::= SEQUENCE {
status PKIStatusInfo,
newSigCert [0] Certificate OPTIONAL,
caCerts [1] SEQUENCE SIZE (1..MAX) OF
Certificate OPTIONAL,
keyPairHist [2] SEQUENCE SIZE (1..MAX) OF
CertifiedKeyPair OPTIONAL
}
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5.3.9 Revocation Request Content
When requesting revocation of a certificate (or several certificates)
the following data structure is used. The name of the requester is
present in the PKIHeader structure.
RevReqContent ::= SEQUENCE OF RevDetails
RevDetails ::= SEQUENCE {
certDetails CertTemplate,
crlEntryDetails Extensions OPTIONAL
}
5.3.10 Revocation Response Content
The response to the above message. If produced, this is sent to the
requester of the revocation. (A separate revocation announcement
message MAY be sent to the subject of the certificate for which
revocation was requested.)
RevRepContent ::= SEQUENCE {
status SEQUENCE SIZE (1..MAX) OF PKIStatusInfo,
revCerts [0] SEQUENCE SIZE (1..MAX) OF CertId OPTIONAL,
crls [1] SEQUENCE SIZE (1..MAX) OF CertificateList
OPTIONAL
}
5.3.11 Cross Certification Request Content
Cross certification requests use the same syntax (CertReqMessages) as
for normal certification requests with the restriction that the key
pair MUST have been generated by the requesting CA and the private
key MUST NOT be sent to the responding CA. This request MAY also be
used by subordinate CAs to get their certificates signed by the
parent CA.
See Appendix C and [CRMF] for CertReqMessages syntax.
5.3.12 Cross Certification Response Content
Cross certification responses use the same syntax (CertRepMessage) as
for normal certification responses with the restriction that no
encrypted private key can be sent.
See Section 5.3.4 for CertRepMessage syntax.
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5.3.13 CA Key Update Announcement Content
When a CA updates its own key pair the following data structure MAY
be used to announce this event.
CAKeyUpdAnnContent ::= SEQUENCE {
oldWithNew Certificate,
newWithOld Certificate,
newWithNew Certificate
}
5.3.14 Certificate Announcement
This structure MAY be us