RFC 2510
Internet X.509 Public Key Infrastructure Certificate Management Protocols
Network Working Group C. Adams Request for Comments: 2510 Entrust Technologies Category: Standards Track S. Farrell SSE March 1999 Internet X.509 Public Key Infrastructure Certificate Management Protocols 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.Abstract
This document describes the Internet X.509 Public Key Infrastructure (PKI) Certificate Management Protocols. Protocol messages are defined for all relevant aspects of certificate creation and management. Note that "certificate" in this document refers to an X.509v3 Certificate as defined in [COR95, X509-AM]. 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]. Introduction The layout of this document is as follows: - Section 1 contains an overview of PKI management; - Section 2 contains discussion of assumptions and restrictions; - Section 3 contains data structures used for PKI management messages; - Section 4 defines the functions that are to be carried out in PKI management by conforming implementations; - Section 5 describes a simple protocol for transporting PKI messages; - the Appendices specify profiles for conforming implementations and provide an ASN.1 module containing the syntax for all messages defined in this specification.
1 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.1.1 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.1.2 Definitions of PKI Entities
The entities involved in PKI management include the end entity (i.e., the entity to be named in the subject field of a certificate) and the certification authority (i.e., the entity named in the issuer field of a certificate). A registration authority MAY also be involved in PKI management.1.2.1 Subjects and End Entities
The term "subject" is used here to refer to the entity named in the subject 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
field of a certificate or cross-certificate. Where 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. 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.1.2.2 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.
1.2.3 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.1.3 PKI Management Requirements
The protocols given here meet the following requirements on PKI management. 1. PKI management must conform to the ISO 9594-8 standard and the associated amendments (certificate extensions) 2. PKI management must conform to the other parts of this series. 3. It must be possible to regularly update any key pair without affecting any other key pair. 4. The use of confidentiality in PKI management protocols must be kept to a minimum in order to ease regulatory problems.
5. PKI management protocols must allow the use of different
industry-standard cryptographic algorithms, (specifically
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).
6. 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.
7. 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.
8. 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.
9. PKI management protocols must be usable over a variety of
"transport" mechanisms, specifically including mail, http,
TCP/IP and ftp.
10. 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 PKIConfirm message).
11. 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 the old CA public key is "hardwired" into
the end entity's cryptographic equipment).
12. 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 (but, of course, not the same key!) regardless of
whether the communication is with an RA or CA.
13. 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 2.3, "Proof of Possession
of Private Key", for details of the in-band methods defined
for the PKIX-CMP (i.e., Certificate Management Protocol)
messages.
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.
+---+ cert. publish +------------+ j
| | <--------------------- | End Entity | <-------
| C | g +------------+ "out-of-band"
| | | ^ loading
| e | | | initial
| r | a | | b registration/
| t | | | certification
| | | | key pair recovery
| / | | | key pair update
| | | | certificate update
| C | PKI "USERS" V | revocation request
| R | -------------------+-+-----+-+------+-+-------------------
| L | PKI MANAGEMENT | ^ | ^
| | ENTITIES a | | b a | | b
| | V | | |
| R | g +------+ d | |
| e | <------------ | RA | <-----+ | |
| p | cert. | | ----+ | | |
| o | publish +------+ c | | | |
| s | | | | |
| i | V | V |
| t | g +------------+ i
| o | <------------------------| CA |------->
| r | h +------------+ "out-of-band"
| y | 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.
3 Certification: various operations result in the creation of new
certificates:
3.1 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).
3.2 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.
3.3 certificate update: As certificates expire they may be
"refreshed" if nothing relevant in the environment has
changed.
3.4 CA key pair update: As with end entities, CA key pairs need
to be updated regularly; however, different mechanisms are
required.
3.5 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.
Notes:
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.
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.
Note 3. Issuance of cross-certificates may be, but is not
necessarily, mutual; that is, two CAs may issue cross-certificates
for each other.
3.6 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:
4.1 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 Sections 3.3.13 - 3.3.16, or MAY
involve other methods (LDAP, for example) as described in
the "Operational Protocols" documents of the PKIX series of
specifications.
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:
5.1 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:
6.1 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
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. The protocols for conveying these exchanges in different
environments (file based, on-line, E-mail, and WWW) is also
specified.
2. Assumptions and restrictions
2.1 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).
2.2 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.2.2.1 Criteria used
2.2.1.1 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.2.2.1.2 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 transaction) 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).2.2.1.3 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.2.2.1.4 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). This gives two further possibilities: confirmed or not.2.2.2 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. Any entity MAY additionally support other schemes, if desired.2.2.2.1 Centralized scheme
In terms of the classification above, this scheme is, in some ways, the simplest possible, where: - initiation occurs at the certifying CA; - no on-line message authentication is required; - "key generation" occurs at the certifying CA (see Section 2.2.1.3); - 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.
2.2.2.2 Basic authenticated scheme
In terms of the classification above, this scheme is where: - initiation occurs at the end entity; - message authentication is REQUIRED; - "key generation" occurs at the end entity (see Section 2.2.1.3); - a confirmation message is REQUIRED. In terms of message flow, the basic authenticated scheme is as follows: 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 -->>--confirmation message-->>-- verify confirmation (Where verification of the confirmation message fails, the RA/CA MUST revoke the newly issued certificate if it has been published or otherwise made available.)2.3 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 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).2.3.1 Signature Keys
For signature keys, the end entity can sign a value to prove possession of the private key.2.3.2 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 3.2.8). 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).
2.3.3 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 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.2.4 Root CA key update
This discussion only applies to CAs that are a root CA for some end entity. 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 period of a CA key pair must be greater than the validity period of any certificate issued by that CA using that key pair.
Note 3.This scheme forces end entities to acquire the new CA public key on 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 on the end entity's equipment).2.4.1 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 directory 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 the time by which the CA will next update its key pair.
2.4.2 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. 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 directory in directory the the directory 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. directory 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 directory the key directory without will access verifier can in order using the the verify the to get the directory directory; certificate value of however, the directly - the OLD verification this is thus public key will FAIL the same as case 4.
2.4.2.1 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 directory. Case 5 can only arise if 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).2.4.2.2 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 directory and pick the OldWithNew certificate (determined based on validity periods); 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.2.4.2.3 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 directory and pick the NewWithOld certificate (determined based on validity periods); 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.
2.4.2.4 Failure of verification in case 6.
In this case the CA has issued the verifier's PSE containing the new key without updating the directory 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.2.4.2.5 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 directory 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.2.4.3 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.
(page 19 continued on part 2)
