Roxen Community: RFC 2510 Internet X.509 Public Key Infrastructure Certificate Management Protocols (Standards Track)

[Text version]

This RFC is obsoleted by: RFC 4210

Network Working Group
Request for Comments: 2510
Category: Standards Track C. Adams
Entrust Technologies
S. Farrell
SSE
March 1999
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 © 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.

3. Data Structures

This section contains descriptions of the data structures required for PKI management messages. Section 4 describes constraints on their values and the sequence of events for each of the various PKI management operations. Section 5 describes how these may be encapsulated in various transport mechanisms.

3.1 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 Certificate OPTIONAL }
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.

3.1.1 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:
PKIHeader ::= SEQUENCE { pvno INTEGER { ietf-version2 (1) }, sender GeneralName, -- identifies the sender recipient GeneralName, -- identifies the intended recipient messageTime [0] GeneralizedTime OPTIONAL, -- time of production of this message (used when sender -- believes that the transport will be "suitable"; i.e., -- that the time will still be meaningful upon receipt) protectionAlg [1] AlgorithmIdentifier OPTIONAL, -- algorithm used for calculation of protection bits senderKID [2] KeyIdentifier OPTIONAL, recipKID [3] KeyIdentifier OPTIONAL, -- to identify specific keys used for protection transactionID [4] OCTET STRING OPTIONAL, -- identifies the transaction; i.e., this will be the same in -- corresponding request, response and confirmation messages senderNonce [5] OCTET STRING OPTIONAL, recipNonce [6] OCTET STRING OPTIONAL, -- nonces used to provide replay protection, senderNonce
-- is inserted by the creator of this message; recipNonce -- is a nonce previously inserted in a related message by -- the intended recipient of this message freeText [7] PKIFreeText OPTIONAL, -- this may be used to indicate context-specific instructions -- (this field is intended for human consumption) generalInfo [8] SEQUENCE SIZE (1..MAX) OF InfoTypeAndValue OPTIONAL -- this may be used to convey context-specific information -- (this field not primarily intended for human consumption) }
PKIFreeText ::= SEQUENCE SIZE (1..MAX) OF UTF8String -- text encoded as UTF-8 String (note: each UTF8String SHOULD -- include an RFC 1766 language tag to indicate the language -- of the contained text)
The pvno field is fixed (at one) 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 usable 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). The transactionID field within the message header MAY be used to allow the recipient of a response message to correlate this with a previously issued request. For example, in the case of an RA there may be many requests "outstanding" at a given moment.
The senderNonce and recipNonce fields protect the PKIMessage against replay attacks.
The messageTime field contains the time at which the sender created the message. This may be useful to allow end entities to correct their local time to be consistent with the time on a central system.
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.

3.1.2 PKI Message Body
PKIBody ::= CHOICE { -- message-specific body elements ir [0] CertReqMessages, --Initialization Request ip [1] CertRepMessage, --Initialization Response cr [2] CertReqMessages, --Certification Request cp [3] CertRepMessage, --Certification Response p10cr [4] CertificationRequest, --PKCS #10 Cert. Req. -- the PKCS #10 certification request (see [PKCS10]) 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 Request krp [10] KeyRecRepContent, --Key Recovery Response rr [11] RevReqContent, --Revocation Request rp [12] RevRepContent, --Revocation Response ccr [13] CertReqMessages, --Cross-Cert. Request ccp [14] CertRepMessage, --Cross-Cert. Response ckuann [15] CAKeyUpdAnnContent, --CA Key Update Ann. cann [16] CertAnnContent, --Certificate Ann. rann [17] RevAnnContent, --Revocation Ann. crlann [18] CRLAnnContent, --CRL Announcement conf [19] PKIConfirmContent, --Confirmation nested [20] NestedMessageContent, --Nested Message genm [21] GenMsgContent, --General Message genp [22] GenRepContent, --General Response error [23] ErrorMsgContent --Error Message }
The specific types are described in Section 3.3 below.

3.1.3 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 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:
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); specification of external protection using PKCS #7 will be provided in a separate document. 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:
- 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:
PasswordBasedMac ::= OBJECT IDENTIFIER --{1 2 840 113533 7 66 13} PBMParameter ::= SEQUENCE { salt OCTET STRING, owf AlgorithmIdentifier, -- AlgId for a One-Way Function (SHA-1 recommended) iterationCount INTEGER, -- number of times the OWF is applied mac AlgorithmIdentifier -- the MAC AlgId (e.g., DES-MAC, Triple-DES-MAC [PKCS11], } -- or HMAC [RFC2104, RFC2202])
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 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.]
- 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:
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.]
- signature
Where the sender possesses a signature key pair it may simply sign the PKI message. PKIProtection will contain the signature value and the protectionAlg will be an AlgorithmIdentifier for a digital signature (e.g., md5WithRSAEncryption or dsaWithSha-1).
- 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 ::= PKIMessage
3.2 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.

3.2.1 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 Confirmation message may be withheld, or an Error Message may be sent (with a PKIStatus of "rejection").
See [CRMF] for CertTemplate syntax.

3.2.2 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.

3.2.3 Status codes and Failure Information for PKI messages

All response messages will include some status information. The following values are defined.
PKIStatus ::= INTEGER { granted (0), -- you got exactly what you asked for grantedWithMods (1), -- you got something like what you asked for; the -- requester is responsible for ascertaining the differences rejection (2), -- you don't get it, more information elsewhere in the message
waiting (3), -- the request body part has not yet been processed, -- expect to hear more later revocationWarning (4), -- this message contains a warning that a revocation is -- imminent revocationNotification (5), -- notification that a revocation has occurred keyUpdateWarning (6) -- update already done for the oldCertId specified in -- the key update request message }
Responders may use the following syntax to provide more information about failure cases.
PKIFailureInfo ::= BIT STRING { -- since we can fail in more than one way! -- More codes may be added in the future if/when required. badAlg (0), -- unrecognized or unsupported Algorithm Identifier badMessageCheck (1), -- integrity check failed (e.g., signature did not verify) badRequest (2), -- transaction not permitted or supported badTime (3), -- messageTime was not sufficiently close to the system time, -- as defined by local policy badCertId (4), -- no certificate could be found matching the provided criteria badDataFormat (5), -- the data submitted has the wrong format wrongAuthority (6), -- the authority indicated in the request is different from the -- one creating the response token incorrectData (7), -- the requester's data is incorrect (used for notary services) missingTimeStamp (8), -- when the timestamp is missing but should be there (by policy) badPOP (9) -- the proof-of-possession failed } PKIStatusInfo ::= SEQUENCE { status PKIStatus, statusString PKIFreeText OPTIONAL, failInfo PKIFailureInfo OPTIONAL }
3.2.4 Certificate Identification

In order to identify particular certificates the CertId data structure is used.

See [CRMF] for CertId syntax.

3.2.5 "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 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:
- The certificate MUST be self-signed (i.e., the signature must be verifiable using the SubjectPublicKeyInfo field); - The subject and issuer fields MUST be identical; - If the subject field is NULL then both subjectAltNames and issuerAltNames extensions MUST be present and have exactly the same value; - 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 -- hashVal is calculated over the self-signed -- certificate with the identifier certID. }
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.

3.2.6 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.

3.2.7 Publication Information

Requesters may indicate that they wish the PKI to publish a certificate using the PKIPublicationInfo structure.

See [CRMF] for PKIPublicationInfo syntax.

3.2.8 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 [CRMF] for POPOSigningKey syntax, but note that POPOSigningKeyInput has the following semantic stipulations in this specification.
POPOSigningKeyInput ::= SEQUENCE { authInfo CHOICE { sender [0] GeneralName, -- from PKIHeader (used only if an authenticated identity -- has been established for the sender (e.g., a DN from a -- previously-issued and currently-valid certificate)) publicKeyMAC [1] PKMACValue -- used if no authenticated GeneralName currently exists for -- the sender; publicKeyMAC contains a password-based MAC -- (using the protectionAlg AlgId from PKIHeader) on the -- DER-encoded value of publicKey }, publicKey SubjectPublicKeyInfo -- from CertTemplate }
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.
1) By the inclusion of the private key (encrypted) in the CertRequest (in the PKIArchiveOptions control structure).
2) 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 2.3.2. The end entity proves knowledge of the private decryption key to the CA by MACing the PKIConfirm message using a key derived from this symmetric key. [Note that if more than one CertReqMsg is included in the PKIMessage, then the CA uses a different symmetric key for each CertReqMsg and the MAC uses a key derived from the concatenation of all these keys.] The MACing procedure uses the PasswordBasedMac AlgId defined in Section 3.1.
3) 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 2.3.2. [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 ---> <--- rep ----- ---- conf --->
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.
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, 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 Appendix B6) 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 nonces used in the PKIHeader and by the protection (MACing or signing) applied to the PKIMessage.
POPODecKeyChallContent ::= SEQUENCE OF Challenge -- One Challenge per encryption key certification request (in the -- same order as these requests appear in CertReqMessages).
Challenge ::= SEQUENCE { owf AlgorithmIdentifier OPTIONAL, -- MUST be present in the first Challenge; MAY be omitted in any -- subsequent Challenge in POPODecKeyChallContent (if omitted, -- then the owf used in the immediately preceding Challenge is -- to be used). witness OCTET STRING, -- the result of applying the one-way function (owf) to a -- randomly-generated INTEGER, A. [Note that a different -- INTEGER MUST be used for each Challenge.] challenge OCTET STRING -- the encryption (under the public key for which the cert. -- request is being made) of Rand, where Rand is specified as -- Rand ::= SEQUENCE { -- int INTEGER, -- - the randomly-generated INTEGER A (above) -- sender GeneralName -- - the sender's name (as included in PKIHeader) -- } }
POPODecKeyRespContent ::= SEQUENCE OF INTEGER -- One INTEGER per encryption key certification request (in the -- same order as these requests appear in CertReqMessages). The -- retrieved INTEGER A (above) is returned to the sender of the -- corresponding Challenge.
3.3 Operation-Specific Data Structures

3.3.1 Initialization Request

An Initialization request message contains as the PKIBody an 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 B profiles for further information). This message is intended to be used for entities first initializing into the PKI.

See [CRMF] for CertReqMessages syntax.

3.3.2 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 protocolEncKey).

See Section 3.3.4 for CertRepMessage syntax. Note that if the PKI Message Protection is "shared secret information" (see Section 3.1.3), then any certificate transported in the caPubs field may be directly trusted as a root CA certificate by the initiator.

3.3.3 Registration/Certification Request

A Registration/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 [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.

3.3.4 Registration/Certification Response

A registration 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 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, -- to match this response with corresponding request (a value -- of -1 is to be used if certReqId is not specified in the -- corresponding request) status PKIStatusInfo, certifiedKeyPair CertifiedKeyPair OPTIONAL, rspInfo OCTET STRING OPTIONAL -- analogous to the id-regInfo-asciiPairs OCTET STRING defined -- for regInfo in CertReqMsg [CRMF] }
CertifiedKeyPair ::= SEQUENCE { certOrEncCert CertOrEncCert, privateKey [0] EncryptedValue OPTIONAL, 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 PKIConfirm 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.

3.3.5 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.

See [CRMF] for CertReqMessages syntax.

3.3.6 Key Update response content

For key update responses the CertRepMessage syntax is used. The response is identical to the initialization response.

See Section 3.3.4 for CertRepMessage syntax.

3.3.7 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 B profiles for further information).

See [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.

3.3.8 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 }
3.3.9 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, -- allows requester to specify as much as they can about -- the cert. for which revocation is requested -- (e.g., for cases in which serialNumber is not available) revocationReason ReasonFlags OPTIONAL, -- the reason that revocation is requested badSinceDate GeneralizedTime OPTIONAL, -- indicates best knowledge of sender crlEntryDetails Extensions OPTIONAL -- requested crlEntryExtensions }
3.3.10 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, -- in same order as was sent in RevReqContent revCerts [0] SEQUENCE SIZE (1..MAX) OF CertId OPTIONAL, -- IDs for which revocation was requested (same order as status) crls [1] SEQUENCE SIZE (1..MAX) OF CertificateList OPTIONAL -- the resulting CRLs (there may be more than one) }
3.3.11 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.

See [CRMF] for CertReqMessages syntax.

3.3.12 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 3.3.4 for CertRepMessage syntax.

3.3.13 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, -- old pub signed with new priv newWithOld Certificate, -- new pub signed with old priv newWithNew Certificate -- new pub signed with new priv }
3.3.14 Certificate Announcement

This structure MAY be used to announce the existence of certificates.

Note that this message is intended to be used for those cases (if any) where there is no pre-existing method for publication of certificates; it is not intended to be used where, for example, X.500 is the method for publication of certificates.
CertAnnContent ::= Certificate
3.3.15 Revocation Announcement

When a CA has revoked, or is about to revoke, a particular certificate it MAY issue an announcement of this (possibly upcoming) event.
RevAnnContent ::= SEQUENCE { status PKIStatus, certId CertId, willBeRevokedAt GeneralizedTime, badSinceDate GeneralizedTime, crlDetails Extensions OPTIONAL -- extra CRL details(e.g., crl number, reason, location, etc.) }
A CA MAY use such an announcement to warn (or notify) a subject that its certificate is about to be (or has been) revoked. This would typically be used where the request for revocation did not come from the subject concerned.
The willBeRevokedAt field contains the time at which a new entry will be added to the relevant CRLs.

3.3.16 CRL Announcement

When a CA issues a new CRL (or set of CRLs) the following data structure MAY be used to announce this event.
CRLAnnContent ::= SEQUENCE OF CertificateList
3.3.17 PKI Confirmation content

This data structure is used in three-way protocols as the final PKIMessage. Its content is the same in all cases - actually there is no content since the PKIHeader carries all the required information.
PKIConfirmContent ::= NULL
3.3.18 PKI General Message content
InfoTypeAndValue ::= SEQUENCE { infoType OBJECT IDENTIFIER, infoValue ANY DEFINED BY infoType OPTIONAL } -- Example InfoTypeAndValue contents include, but are not limited to: -- { CAProtEncCert = {id-it 1}, Certificate } -- { SignKeyPairTypes = {id-it 2}, SEQUENCE OF AlgorithmIdentifier } -- { EncKeyPairTypes = {id-it 3}, SEQUENCE OF AlgorithmIdentifier } -- { PreferredSymmAlg = {id-it 4}, AlgorithmIdentifier } -- { CAKeyUpdateInfo = {id-it 5}, CAKeyUpdAnnContent } -- { CurrentCRL = {id-it 6}, CertificateList } -- where {id-it} = {id-pkix 4} = {1 3 6 1 5 5 7 4} -- This construct MAY also be used to define new PKIX Certificate -- Management Protocol request and response messages, or general- -- purpose (e.g., announcement) messages for future needs or for -- specific environments.
GenMsgContent ::= SEQUENCE OF InfoTypeAndValue -- May be sent by EE, RA, or CA (depending on message content). -- The OPTIONAL infoValue parameter of InfoTypeAndValue will typically -- be omitted for some of the examples given above. The receiver is -- free to ignore any contained OBJ. IDs that it does not recognize. -- If sent from EE to CA, the empty set indicates that the CA may send -- any/all information that it wishes.

3.3.19 PKI General Response content

GenRepContent ::= SEQUENCE OF InfoTypeAndValue -- The receiver is free to ignore any contained OBJ. IDs that it does -- not recognize.

3.3.20 Error Message content
ErrorMsgContent ::= SEQUENCE { pKIStatusInfo PKIStatusInfo, errorCode INTEGER OPTIONAL, -- implementation-specific error codes errorDetails PKIFreeText OPTIONAL -- implementation-specific error details }
4. Mandatory PKI Management functions

The PKI management functions outlined in Section 1 above are described in this section.

This section deals with functions that are "mandatory" in the sense that all end entity and CA/RA implementations MUST be able to provide the functionality described (perhaps via one of the transport mechanisms defined in Section 5). This part is effectively the profile of the PKI management functionality that MUST be supported.
Note that not all PKI management functions result in the creation of a PKI message.

4.1 Root CA initialization

[See Section 1.2.2 for this document's definition of "root CA".]

A newly created root CA must produce a "self-certificate" which is a Certificate structure with the profile defined for the "newWithNew" certificate issued following a root CA key update.
In order to make the CA's self certificate useful to end entities that do not acquire the self certificate via "out-of-band" means, the CA must also produce a fingerprint for its public key. End entities that acquire this fingerprint securely via some "out-of-band" means can then verify the CA's self-certificate and hence the other attributes contained therein. The data structure used to carry the fingerprint is the OOBCertHash.

4.2 Root CA key update

CA keys (as all other keys) have a finite lifetime and will have to be updated on a periodic basis. The certificates NewWithNew, NewWithOld, and OldWithNew (see Section 2.4.1) are issued by the CA to aid existing end entities who hold the current self-signed CA certificate (OldWithOld) to transition securely to the new self- signed CA certificate (NewWithNew), and to aid new end entities who will hold NewWithNew to acquire OldWithOld securely for verification of existing data.

4.3 Subordinate CA initialization

[See Section 1.2.2 for this document's definition of "subordinate CA".]

From the perspective of PKI management protocols the initialization of a subordinate CA is the same as the initialization of an end entity. The only difference is that the subordinate CA must also produce an initial revocation list.

4.4 CRL production

Before issuing any certificates a newly established CA (which issues CRLs) must produce "empty" versions of each CRL which is to be periodically produced.

4.5 PKI information request

When a PKI entity (CA, RA, or EE) wishes to acquire information about the current status of a CA it MAY send that CA a request for such information.

The CA must respond to the request by providing (at least) all of the information requested by the requester. If some of the information cannot be provided then an error must be conveyed to the requester.
If PKIMessages are used to request and supply this PKI information, then the request must be the GenMsg message, the response must be the GenRep message, and the error must be the Error message. These messages are protected using a MAC based on shared secret information (i.e., PasswordBasedMAC) or any other authenticated means (if the end entity has an existing certificate).

4.6 Cross certification

The requester CA is the CA that will become the subject of the cross-certificate; the responder CA will become the issuer of the cross-certificate.

The requester CA must be "up and running" before initiating the cross-certification operation.

4.6.1 One-way request-response scheme:

The cross-certification scheme is essentially a one way operation; that is, when successful, this operation results in the creation of one new cross-certificate. If the requirement is that cross- certificates be created in "both directions" then each CA in turn must initiate a cross-certification operation (or use another scheme).

This scheme is suitable where the two CAs in question can already verify each other's signatures (they have some common points of trust) or where there is an out-of-band verification of the origin of the certification request.
Detailed Description:
Cross certification is initiated at one CA known as the responder. The CA administrator for the responder identifies the CA it wants to cross certify and the responder CA equipment generates an authorization code. The responder CA administrator passes this authorization code by out-of-band means to the requester CA administrator. The requester CA administrator enters the authorization code at the requester CA in order to initiate the on- line exchange.
The authorization code is used for authentication and integrity purposes. This is done by generating a symmetric key based on the authorization code and using the symmetric key for generating Message Authentication Codes (MACs) on all messages exchanged.
The requester CA initiates the exchange by generating a random number (requester random number). The requester CA then sends to the responder CA the cross certification request (ccr) message. The fields in this message are protected from modification with a MAC based on the authorization code.
Upon receipt of the ccr message, the responder CA checks the protocol version, saves the requester random number, generates its own random number (responder random number) and validates the MAC. It then generates (and archives, if desired) a new requester certificate that contains the requester CA public key and is signed with the responder CA signature private key. The responder CA responds with the cross certification response (ccp) message. The fields in this message are protected from modification with a MAC based on the authorization code.
Upon receipt of the ccp message, the requester CA checks that its own system time is close to the responder CA system time, checks the received random numbers and validates the MAC. The requester CA responds with the PKIConfirm message. The fields in this message are protected from modification with a MAC based on the authorization code. The requester CA writes the requester certificate to the Repository.
Upon receipt of the PKIConfirm message, the responder CA checks the random numbers and validates the MAC.
Notes:
1. The ccr message must contain a "complete" certification request, that is, all fields (including, e.g., a BasicConstraints extension) must be specified by the requester CA. 2. The ccp message SHOULD contain the verification certificate of the responder CA - if present, the requester CA must then verify this certificate (for example, via the "out-of-band" mechanism).
4.7 End entity initialization

As with CAs, end entities must be initialized. Initialization of end entities requires at least two steps:
- acquisition of PKI information - out-of-band verification of one root-CA public key
(other possible steps include the retrieval of trust condition information and/or out-of-band verification of other CA public keys).

4.7.1 Acquisition of PKI information

The information REQUIRED is:
- the current root-CA public key - (if the certifying CA is not a root-CA) the certification path from the root CA to the certifying CA together with appropriate revocation lists - the algorithms and algorithm parameters which the certifying CA supports for each relevant usage
Additional information could be required (e.g., supported extensions or CA policy information) in order to produce a certification request which will be successful. However, for simplicity we do not mandate that the end entity acquires this information via the PKI messages. The end result is simply that some certification requests may fail (e.g., if the end entity wants to generate its own encryption key but the CA doesn't allow that).
The required information MAY be acquired as described in Section 4.5.

4.7.2 Out-of-Band Verification of Root-CA Key

An end entity must securely possess the public key of its root CA. One method to achieve this is to provide the end entity with the CA's self-certificate fingerprint via some secure "out-of-band" means. The end entity can then securely use the CA's self-certificate.

See Section 4.1 for further details.

4.8 Certificate Request

An initialized end entity MAY request a certificate at any time (as part of an update procedure, or for any other purpose). This request will be made using the certification request (cr) message. If the end entity already possesses a signing key pair (with a corresponding verification certificate), then this cr message will typically be protected by the entity's digital signature. The CA returns the new certificate (if the request is successful) in a CertRepMessage.

4.9 Key Update

When a key pair is due to expire the relevant end entity MAY request a key update - that is, it MAY request that the CA issue a new certificate for a new key pair. The request is made using a key update request (kur) message. If the end entity already possesses a signing key pair (with a corresponding verification certificate), then this message will typically be protected by the entity's digital signature. The CA returns the new certificate (if the request is successful) in a key update response (kup) message, which is syntactically identical to a CertRepMessage.

5. Transports

The transport protocols specified below allow end entities, RAs and CAs to pass PKI messages between them. There is no requirement for specific security mechanisms to be applied at this level if the PKI messages are suitably protected (that is, if the OPTIONAL PKIProtection parameter is used as specified for each message).

5.1 File based protocol

A file containing a PKI message MUST contain only the DER encoding of one PKI message, i.e., there MUST be no extraneous header or trailer information in the file.

Such files can be used to transport PKI messages using, e.g., FTP.

5.2 Direct TCP-Based Management Protocol

The following simple TCP-based protocol is to be used for transport of PKI messages. This protocol is suitable for cases where an end entity (or an RA) initiates a transaction and can poll to pick up the results.

If a transaction is initiated by a PKI entity (RA or CA) then an end entity must either supply a listener process or be supplied with a polling reference (see below) in order to allow it to pick up the PKI message from the PKI management component.
The protocol basically assumes a listener process on an RA or CA which can accept PKI messages on a well-defined port (port number 829). Typically an initiator binds to this port and submits the initial PKI message for a given transaction ID. The responder replies with a PKI message and/or with a reference number to be used later when polling for the actual PKI message response.
If a number of PKI response messages are to be produced for a given request (say if some part of the request is handled more quickly than another) then a new polling reference is also returned.
When the final PKI response message has been picked up by the initiator then no new polling reference is supplied.
The initiator of a transaction sends a "direct TCP-based PKI message" to the recipient. The recipient responds with a similar message.
A "direct TCP-based PKI message" consists of:
length (32-bits), flag (8-bits), value (defined below)
The length field contains the number of octets of the remainder of the message (i.e., number of octets of "value" plus one). All 32-bit values in this protocol are specified to be in network byte order.
Message name flag value
pkiMsg '00'H DER-encoded PKI message
-- PKI message pollRep '01'H polling reference (32 bits), time-to-check-back (32 bits) -- poll response where no PKI message response ready; use polling -- reference value (and estimated time value) for later polling pollReq '02'H polling reference (32 bits) -- request for a PKI message response to initial message negPollRep '03'H '00'H -- no further polling responses (i.e., transaction complete) partialMsgRep '04'H next polling reference (32 bits), time-to-check-back (32 bits), DER-encoded PKI message -- partial response to initial message plus new polling reference -- (and estimated time value) to use to get next part of response finalMsgRep '05'H DER-encoded PKI message -- final (and possibly sole) response to initial message errorMsgRep '06'H human readable error message -- produced when an error is detected (e.g., a polling reference is -- received which doesn't exist or is finished with)
Where a PKIConfirm message is to be transported (always from the initiator to the responder) then a pkiMsg message is sent and a negPollRep is returned.
The sequence of messages which can occur is then:
a) end entity sends pkiMsg and receives one of pollRep, negPollRep, partialMsgRep or finalMsgRep in response. b) end entity sends pollReq message and receives one of negPollRep, partialMsgRep, finalMsgRep or errorMsgRep in response.
The "time-to-check-back" parameter is a 32-bit integer, defined to be the number of seconds which have elapsed since midnight, January 1, 1970, coordinated universal time. It provides an estimate of the time that the end entity should send its next pollReq.

5.3 Management Protocol via E-mail

This subsection specifies a means for conveying ASN.1-encoded messages for the protocol exchanges described in Section 4 via Internet mail.

A simple MIME object is specified as follows.
Content-Type: application/pkixcmp Content-Transfer-Encoding: base64
<<the ASN.1 DER-encoded PKIX-CMP message, base64-encoded>>
This MIME object can be sent and received using common MIME processing engines and provides a simple Internet mail transport for PKIX-CMP messages. Implementations MAY wish to also recognize and use the "application/x-pkixcmp" MIME type (specified in earlier versions of this document) in order to support backward compatibility wherever applicable.

5.4 Management Protocol via HTTP

This subsection specifies a means for conveying ASN.1-encoded messages for the protocol exchanges described in Section 4 via the HyperText Transfer Protocol.

A simple MIME object is specified as follows.
Content-Type: application/pkixcmp
<<the ASN.1 DER-encoded PKIX-CMP message>>
This MIME object can be sent and received using common HTTP processing engines over WWW links and provides a simple browser- server transport for PKIX-CMP messages. Implementations MAY wish to also recognize and use the "application/x-pkixcmp" MIME type (specified in earlier versions of this document) in order to support backward compatibility wherever applicable.

SECURITY CONSIDERATIONS

This entire memo is about security mechanisms.

One cryptographic consideration is worth explicitly spelling out. In the protocols specified above, when an end entity is required to prove possession of a decryption key, it is effectively challenged to decrypt something (its own certificate). This scheme (and many others!) could be vulnerable to an attack if the possessor of the decryption key in question could be fooled into decrypting an arbitrary challenge and returning the cleartext to an attacker. Although in this specification a number of other failures in security are required in order for this attack to succeed, it is conceivable that some future services (e.g., notary, trusted time) could potentially be vulnerable to such attacks. For this reason we re- iterate the general rule that implementations should be very careful about decrypting arbitrary "ciphertext" and revealing recovered "plaintext" since such a practice can lead to serious security vulnerabilities. Note also that exposing a private key to the CA/RA as a proof-of- possession technique can carry some security risks (depending upon whether or not the CA/RA can be trusted to handle such material appropriately). Implementers are advised to exercise caution in selecting and using this particular POP mechanism.

References
[COR95] ISO/IEC JTC 1/SC 21, Technical Corrigendum 2 to ISO/IEC 9594-8: 1990 & 1993 (1995:E), July 1995.
[CRMF] Myers, M., Adams, C., Solo, D. and D. Kemp, "Certificate Request Message Format", RFC 2511, March 1999.
[MvOV97] A. Menezes, P. van Oorschot, S. Vanstone, "Handbook of Applied Cryptography", CRC Press, 1997.
[PKCS7] RSA Laboratories, "The Public-Key Cryptography Standards (PKCS)", RSA Data Security Inc., Redwood City, California, November 1993 Release.
[PKCS10] RSA Laboratories, "The Public-Key Cryptography Standards (PKCS)", RSA Data Security Inc., Redwood City, California, November 1993 Release.
[PKCS11] RSA Laboratories, "The Public-Key Cryptography Standards - PKCS #11: Cryptographic token interface standard", RSA Data Security Inc., Redwood City, California, April 28, 1995.
[RFC1847] Galvin, J., Murphy, S. Crocker, S. and N. Freed, "Security Multiparts for MIME: Multipart/Signed and Multipart/ Encrypted", RFC 1847, October 1995.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed Hashing for Message Authentication", RFC 2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2202] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and HMAC- SHA-1", RFC 2202, September 1997.
[X509-AM] ISO/IEC JTC1/SC 21, Draft Amendments DAM 4 to ISO/IEC 9594-2, DAM 2 to ISO/IEC 9594-6, DAM 1 to ISO/IEC 9594-7, and DAM 1 to ISO/IEC 9594-8 on Certificate Extensions, 1 December, 1996.
Acknowledgements

The authors gratefully acknowledge the contributions of various members of the PKIX Working Group. Many of these contributions significantly clarified and improved the utility of this specification.

Authors' Addresses
Carlisle Adams Entrust Technologies 750 Heron Road, Suite E08, Ottawa, Ontario Canada K1V 1A7
EMail: cadams@entrust.com
Stephen Farrell Software and Systems Engineering Ltd. Fitzwilliam Court Leeson Close Dublin 2 IRELAND
EMail: stephen.farrell@sse.ie
APPENDIX A: Reasons for the presence of RAs

The reasons which justify the presence of an RA can be split into those which are due to technical factors and those which are organizational in nature. Technical reasons include the following.
-If hardware tokens are in use, then not all end entities will have the equipment needed to initialize these; the RA equipment can include the necessary functionality (this may also be a matter of policy).
-Some end entities may not have the capability to publish certificates; again, the RA may be suitably placed for this.
-The RA will be able to issue signed revocation requests on behalf of end entities associated with it, whereas the end entity may not be able to do this (if the key pair is completely lost).
Some of the organizational reasons which argue for the presence of an RA are the following.
-It may be more cost effective to concentrate functionality in the RA equipment than to supply functionality to all end entities (especially if special token initialization equipment is to be used).
-Establishing RAs within an organization can reduce the number of CAs required, which is sometimes desirable.
-RAs may be better placed to identify people with their "electronic" names, especially if the CA is physically remote from the end entity.
-For many applications there will already be in place some administrative structure so that candidates for the role of RA are easy to find (which may not be true of the CA).
Appendix B. PKI Management Message Profiles.

This appendix contains detailed profiles for those PKIMessages which MUST be supported by conforming implementations (see Section 4).

Profiles for the PKIMessages used in the following PKI management operations are provided:
- root CA key update - information request/response - cross-certification request/response (1-way) - initial registration/certification - basic authenticated scheme - certificate request - key update
<<Later versions of this document may extend the above to include profiles for the operations listed below (along with other operations, if desired).>>
- revocation request - certificate publication - CRL publication
B1. General Rules for interpretation of these profiles.
1. Where OPTIONAL or DEFAULT fields are not mentioned in individual profiles, they SHOULD be absent from the relevant message (i.e., a receiver can validly reject a message containing such fields as being syntactically incorrect). Mandatory fields are not mentioned if they have an obvious value (e.g., pvno). 2. Where structures occur in more than one message, they are separately profiled as appropriate. 3. The algorithmIdentifiers from PKIMessage structures are profiled separately. 4. A "special" X.500 DN is called the "NULL-DN"; this means a DN containing a zero-length SEQUENCE OF RelativeDistinguishedNames (its DER encoding is then '3000'H). 5. Where a GeneralName is required for a field but no suitable value is available (e.g., an end entity produces a request before knowing its name) then the GeneralName is to be an X.500 NULL-DN (i.e., the Name field of the CHOICE is to contain a NULL-DN). This special value can be called a "NULL-GeneralName". 6. Where a profile omits to specify the value for a GeneralName then the NULL-GeneralName value is to be present in the relevant PKIMessage field. This occurs with the sender field of the PKIHeader for some messages.
7. Where any ambiguity arises due to naming of fields, the profile names these using a "dot" notation (e.g., "certTemplate.subject" means the subject field within a field called certTemplate). 8. Where a "SEQUENCE OF types" is part of a message, a zero-based array notation is used to describe fields within the SEQUENCE OF (e.g., crm[0].certReq.certTemplate.subject refers to a subfield of the first CertReqMsg contained in a request message). 9. All PKI message exchanges in Sections B7-B10 require a PKIConfirm message to be sent by the initiating entity. This message is not included in some of the profiles given since its body is NULL and its header contents are clear from the context. Any authenticated means can be used for the protectionAlg (e.g., password-based MAC, if shared secret information is known, or signature).
B2. Algorithm Use Profile

The following table contains definitions of algorithm uses within PKI management protocols.

The columns in the table are:
Name: an identifier