RFC 9810: Internet X.509 Public Key Infrastructure -- Certificate Management Protocol (CMP)

1.1. Changes Made by RFC 4210

[RFC4210] differs from [RFC2510] in the following areas:¶

1.2. Updates Made by RFC 9480

CMP Updates [RFC9480] and CMP Algorithms [RFC9481] updated [RFC4210], supporting the PKI management operations specified in the Lightweight CMP Profile [RFC9483], in the following areas:¶

1.3. Changes Made by This Document

This document obsoletes [RFC4210] and [RFC9480].¶

Backward compatibility with CMP version 2 is maintained wherever possible. Updates to CMP version 2 improve crypto agility, extend the polling mechanism, add new general message types, and add EKUs to identify special CMP server authorizations. CMP version 3 is introduced for changes to the ASN.1 syntax, which support EnvelopedData, certConf with hashAlg, POPOPrivKey with agreeMAC, and RootCaKeyUpdateContent in ckuann messages.¶

The updates made in this document include the changes specified by Section 2 and Appendix A.2 of [RFC9480] as described in Section 1.2. Additionally, this document updates the content of [RFC4210] in the following areas:¶

3.1. PKI Management Model

Before specifying particular message formats and procedures, we first define the entities involved in PKI management and their interactions (in terms of the PKI management functions required). We then group these functions in order to accommodate different identifiable types of end entities.¶

4.1. End Entity Initialization

The first step for an end entity in dealing with PKI management entities is to request information about the PKI functions supported and to securely acquire a copy of the relevant root CA public key(s).¶

4.2. Initial Registration/Certification

There are many schemes that can be used to achieve initial registration and certification of end entities. No one method is suitable for all situations due to the range of policies that a CA may implement and the variation in the types of end entity that can occur.¶

However, we can 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, except having received the root CA certificate of that PKI by some zero-touch method like BRSKI [RFC8995] [RFC9733] or SZTP [RFC8572]. In case 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 that will arise in real use, whilst the optional schemes are available for special cases that arise less frequently. In this way, we achieve a balance between flexibility and ease of implementation.¶

Further classification of mandatory and optional schemes addressing different environments is available, e.g., in Appendices C and D of this specification on managing human user certificates as well as in the Lightweight CMP Profile [RFC9483] on fully automating certificate management in a machine-to-machine and Internet of Things (IoT) environment. Industry standards such as [ETSI-3GPP.33.310] for mobile networks and [UNISIG.Subset-137] for railroad automation have adopted CMP and defined a series of mandatory schemes for their use cases.¶

We will now describe the classification of initial registration/certification schemes.¶

4.3. POP of Private Key

POP is where a PKI management entity (CA/RA) verifies if an end entity has access to the private key corresponding to a given public key. The question of whether, and in what circumstances, POPs add value to a PKI is a debate as old as PKI itself! See Section 8.1 for a further discussion on the necessity of POP in PKI.¶

The PKI management operations specified here make it possible for an end entity to prove to a CA/RA 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 (see Section 5.2.8 for different POP methods). 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, key agreement, and KEM 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 PKI 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 that may be used for signing, as well as other purposes, MUST NOT be sent to the CA/RA in order to prove possession unless archival of the private key is explicitly desired).¶

This specification explicitly allows for cases where an end entity supplies the relevant proof to an RA and the RA subsequently attests to the CA that the required proof has been received (and validated!). For example, an end entity wishing to have a signing key certified could send the appropriate signature to the RA, which then simply notifies the relevant CA that the end entity has supplied the required proof. Of course, such a situation may be disallowed by some policies (e.g., CAs may be the only entities permitted to verify POP during certification).¶

4.4. Root CA Key Update

This discussion only applies to CAs that are directly trusted by some end entities. Recognizing whether a self-signed or non-self-signed CA is supposed to be directly trusted for some end entities is a matter of CA policy and end entity configuration. Thus, this is beyond the scope of this document.¶

The basis of the procedure described here is that the CA protects its new public key using its previous private key and vice versa. Thus, when a CA updates its key pair, it may generate two link certificates: "old with new" and "new with old".¶

Note: The usage of link certificates has been shown to be very specific for each use case, and no assumptions are done on this aspect. RootCaKeyUpdateContent is updated to specify these link certificates as optional.¶

Note: When an LDAP directory is used to publish root CA updates, the old and new root CA certificates together with the two link certificates are stored as cACertificate attribute values.¶

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. These end entities need to acquire the new CA public key in a trusted way. This may be achieved "out-of-band" by using a repository or by using online messages also containing the link certificates "new with old". Once the end entity acquired and properly verified the new CA public key, it must load the new trust anchor information into its trusted store.¶

The data structure used to protect the new and old CA public keys is typically a standard X.509v3 certificate (which may also contain extensions). There are no new data structures required.¶

Note: Sometimes self-signed root CA certificates do not make use of X.509v3 extensions and may be X.509v1 certificates. Therefore, a root CA key update must be able to work for version 1 certificates. The use of the X.509v3 KeyIdentifier extension is recommended for easier path building.¶

Note: While the scheme could be generalized to cover cases where the CA updates its key pair more than once during the validity period of one of its end entities' certificates, this generalization seems of dubious value. Not having this generalization simply means that the validity periods of certificates issued with the old CA key pair cannot exceed the end of the "old with new" certificate validity period.¶

Note: This scheme offers a mechanism to ensures that end entities will acquire the new CA public key, at the latest by the expiry of the last certificate they owned that was signed with the old CA private key. Certificate and/or key update operations occurring at other times do not necessarily require this (depending on the end entity's equipment).¶

Note: In practice, a new root CA may have a slightly different subject Distinguished Name (DN), e.g., indicating a generation identifier like the year of issuance or a version number, for instance, in an Organizational Unit (OU) element. How to bridge trust to the new root CA certificate in a CA DN change or a cross-certificate scenario is out of scope for this document.¶

4.5. EKU for PKI Entities

The EKU extension indicates the purposes for which the certified key pair may be used. Therefore, it restricts the use of a certificate to specific applications.¶

A CA may want to delegate parts of its duties to other PKI management entities. This section provides a mechanism to both prove this delegation and enable automated means for checking the authorization of this delegation. Such delegation may also be expressed by other means, e.g., explicit configuration.¶

To offer automatic validation for the delegation of a role by a CA to another entity, the certificates used for CMP message protection or signed data for central key generation MUST be issued by the delegating CA and MUST contain the respective EKUs. This proves that the delegating CA authorized this entity to act in the given role, as described below.¶

The OIDs to be used for these EKUs are:¶

  id-kp-cmcCA OBJECT IDENTIFIER ::= {
     iso(1) identified-organization(3) dod(6) internet(1)
     security(5) mechanisms(5) pkix(7) kp(3) 27 }

  id-kp-cmcRA OBJECT IDENTIFIER ::= {
     iso(1) identified-organization(3) dod(6) internet(1)
     security(5) mechanisms(5) pkix(7) kp(3) 28 }

  id-kp-cmKGA OBJECT IDENTIFIER ::= {
     iso(1) identified-organization(3) dod(6) internet(1)
     security(5) mechanisms(5) pkix(7) kp(3) 32 }

Note: Section 2.10 of [RFC6402] specifies OIDs for a Certificate Management over CMS (CMC) CA and a CMC RA. As the functionality of a CA and RA is not specific to any protocol used for managing certificates (such as CMC or CMP), these EKUs are reused by CMP.¶

The meaning of the id-kp-cmKGA EKU is as follows:¶

CMP KGA:

CMP KGAs are CAs or are identified by the id-kp-cmKGA EKU. The CMP KGA knows the private key it generated on behalf of the end entity. This is a very sensitive service and needs specific authorization, which by default is with the CA certificate itself. The CA may delegate its authorization by placing the id-kp-cmKGA EKU in the certificate used to authenticate the origin of the generated private key. The authorization may also be determined through local configuration of the end entity.¶

5.1. Overall PKI Message

All of the messages used in this specification for the purposes of PKI management use the following structure:¶

  PKIMessage ::= SEQUENCE {
     header           PKIHeader,
     body             PKIBody,
     protection   [0] PKIProtection OPTIONAL,
     extraCerts   [1] SEQUENCE SIZE (1..MAX) OF CMPCertificate
                       OPTIONAL
  }

  PKIMessages ::= SEQUENCE SIZE (1..MAX) OF PKIMessage

The PKIHeader contains information that 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 (for example, if 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.¶

  PKIHeader ::= SEQUENCE {
     pvno                INTEGER     { cmp1999(1), cmp2000(2),
                                       cmp2021(3) },
     sender              GeneralName,
     recipient           GeneralName,
     messageTime     [0] GeneralizedTime         OPTIONAL,
     protectionAlg   [1] AlgorithmIdentifier{ALGORITHM, {...}}
                         OPTIONAL,
     senderKID       [2] KeyIdentifier           OPTIONAL,
     recipKID        [3] KeyIdentifier           OPTIONAL,
     transactionID   [4] OCTET STRING            OPTIONAL,
     senderNonce     [5] OCTET STRING            OPTIONAL,
     recipNonce      [6] OCTET STRING            OPTIONAL,
     freeText        [7] PKIFreeText             OPTIONAL,
     generalInfo     [8] SEQUENCE SIZE (1..MAX) OF
                         InfoTypeAndValue     OPTIONAL
  }

  PKIFreeText ::= SEQUENCE SIZE (1..MAX) OF UTF8String

  id-it-implicitConfirm OBJECT IDENTIFIER ::= {id-it 13}
  ImplicitConfirmValue ::= NULL

  id-it-confirmWaitTime OBJECT IDENTIFIER ::= {id-it 14}
  ConfirmWaitTimeValue ::= GeneralizedTime

  id-it-origPKIMessage OBJECT IDENTIFIER ::= {id-it 15}
  OrigPKIMessageValue ::= PKIMessages

  id-it-certProfile OBJECT IDENTIFIER ::= {id-it 21}
  CertProfileValue ::= SEQUENCE SIZE (1..MAX) OF UTF8String

  id-it-KemCiphertextInfo OBJECT IDENTIFIER ::= { id-it 24 }
  KemCiphertextInfoValue ::= KemCiphertextInfo

  PKIBody ::= CHOICE {
     ir       [0]  CertReqMessages,        --Initialization Req
     ip       [1]  CertRepMessage,         --Initialization Resp
     cr       [2]  CertReqMessages,        --Certification Req
     cp       [3]  CertRepMessage,         --Certification Resp
     p10cr    [4]  CertificationRequest,   --PKCS #10 Cert.  Req.
     popdecc  [5]  POPODecKeyChallContent, --pop Challenge
     popdecr  [6]  POPODecKeyRespContent,  --pop Response
     kur      [7]  CertReqMessages,        --Key Update Request
     kup      [8]  CertRepMessage,         --Key Update Response
     krr      [9]  CertReqMessages,        --Key Recovery Req
     krp      [10] KeyRecRepContent,       --Key Recovery Resp
     rr       [11] RevReqContent,          --Revocation Request
     rp       [12] RevRepContent,          --Revocation Response
     ccr      [13] CertReqMessages,        --Cross-Cert.  Request
     ccp      [14] CertRepMessage,         --Cross-Cert.  Resp
     ckuann   [15] CAKeyUpdContent,        --CA Key Update Ann.
     cann     [16] CertAnnContent,         --Certificate Ann.
     rann     [17] RevAnnContent,          --Revocation Ann.
     crlann   [18] CRLAnnContent,          --CRL Announcement
     pkiconf  [19] PKIConfirmContent,      --Confirmation
     nested   [20] NestedMessageContent,   --Nested Message
     genm     [21] GenMsgContent,          --General Message
     genp     [22] GenRepContent,          --General Response
     error    [23] ErrorMsgContent,        --Error Message
     certConf [24] CertConfirmContent,     --Certificate Confirm
     pollReq  [25] PollReqContent,         --Polling Request
     pollRep  [26] PollRepContent          --Polling Response
  }

  PKIProtection ::= BIT STRING

  ProtectedPart ::= SEQUENCE {
     header    PKIHeader,
     body      PKIBody
  }

  id-PasswordBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 13}
  PBMParameter ::= SEQUENCE {
     salt                OCTET STRING,
     owf                 AlgorithmIdentifier,
     iterationCount      INTEGER,
     mac                 AlgorithmIdentifier
  }

  id-DHBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 30}

  DHBMParameter ::= SEQUENCE {
     owf                 AlgorithmIdentifier,
     -- AlgId for an OWF
     mac                 AlgorithmIdentifier
     -- the MAC AlgId
  }

  id-KemBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 16}

  KemBMParameter ::= SEQUENCE {
    kdf              AlgorithmIdentifier{KEY-DERIVATION, {...}},
    kemContext   [0] OCTET STRING OPTIONAL,
    len              INTEGER (1..MAX),
    mac              AlgorithmIdentifier{MAC-ALGORITHM, {...}}
  }

  NestedMessageContent ::= PKIMessages

5.2. Common Data Structures

Before specifying the specific types that may be placed in a PKIBody, we define some data structures that are used in more than one case.¶

  id-regCtrl-altCertTemplate OBJECT IDENTIFIER ::= { iso(1)
     identified-organization(3) dod(6) internet(1) security(5)
     mechanisms(5) pkix(7) pkip(5) regCtrl(1) 7}

  AltCertTemplate ::= AttributeTypeAndValue

  EncryptedKey ::= CHOICE {
     encryptedValue       EncryptedValue, -- deprecated
     envelopedData    [0] EnvelopedData }

  PKIStatus ::= INTEGER {
     accepted               (0),
     grantedWithMods        (1),
     rejection              (2),
     waiting                (3),
     revocationWarning      (4),
     revocationNotification (5),
     keyUpdateWarning       (6)
  }

  PKIFailureInfo ::= BIT STRING {
     badAlg                 (0),
     badMessageCheck        (1),
     badRequest             (2),
     badTime                (3),
     badCertId              (4),
     badDataFormat          (5),
     wrongAuthority         (6),
     incorrectData          (7),
     missingTimeStamp       (8),
     badPOP                 (9),
     certRevoked            (10),
     certConfirmed          (11),
     wrongIntegrity         (12),
     badRecipientNonce      (13),
     timeNotAvailable       (14),
     unacceptedPolicy       (15),
     unacceptedExtension    (16),
     addInfoNotAvailable    (17),
     badSenderNonce         (18),
     badCertTemplate        (19),
     signerNotTrusted       (20),
     transactionIdInUse     (21),
     unsupportedVersion     (22),
     notAuthorized          (23),
     systemUnavail          (24),
     systemFailure          (25),
     duplicateCertReq       (26)
  }

  PKIStatusInfo ::= SEQUENCE {
     status        PKIStatus,
     statusString  PKIFreeText     OPTIONAL,
     failInfo      PKIFailureInfo  OPTIONAL
  }

  OOBCert ::= Certificate

  OOBCertHash ::= SEQUENCE {
     hashAlg     [0] AlgorithmIdentifier     OPTIONAL,
     certId      [1] CertId                  OPTIONAL,
     hashVal         BIT STRING
  }

   ProofOfPossession ::= CHOICE {
      raVerified      [0] NULL,
      signature       [1] POPOSigningKey,
      keyEncipherment [2] POPOPrivKey,
      keyAgreement    [3] POPOPrivKey
   }

   POPOSigningKey ::= SEQUENCE {
      poposkInput [0] POPOSigningKeyInput OPTIONAL,
      algorithmIdentifier AlgorithmIdentifier,
      signature BIT STRING
   }

   POPOSigningKeyInput ::= SEQUENCE {
      authInfo CHOICE {
         sender [0] GeneralName,
         publicKeyMAC PKMACValue
      },
      publicKey SubjectPublicKeyInfo
   }

   PKMACValue ::= SEQUENCE {
      algId AlgorithmIdentifier,
      value BIT STRING
   }

   POPOPrivKey ::= CHOICE {
      thisMessage [0] BIT STRING, -- deprecated
      subsequentMessage [1] SubsequentMessage,
      dhMAC [2] BIT STRING, -- deprecated
      agreeMAC [3] PKMACValue,
      encryptedKey [4] EnvelopedData
   }

   SubsequentMessage ::= INTEGER {
      encrCert (0),
      challengeResp (1)
   }

   POPODecKeyChallContent ::= SEQUENCE OF Challenge

   Challenge ::= SEQUENCE {
      owf AlgorithmIdentifier OPTIONAL,
      witness OCTET STRING,
      challenge OCTET STRING, -- deprecated
      encryptedRand [0] EnvelopedData OPTIONAL
   }

   Rand ::= SEQUENCE {
      int INTEGER,
      sender GeneralName
   }

   POPODecKeyRespContent ::= SEQUENCE OF INTEGER

5.3. Operation-Specific Data Structures

  CertRepMessage ::= SEQUENCE {
     caPubs          [1] SEQUENCE SIZE (1..MAX) OF CMPCertificate
                         OPTIONAL,
     response            SEQUENCE OF CertResponse
  }

  CertResponse ::= SEQUENCE {
     certReqId           INTEGER,
     status              PKIStatusInfo,
     certifiedKeyPair    CertifiedKeyPair     OPTIONAL,
     rspInfo             OCTET STRING         OPTIONAL
     -- analogous to the id-regInfo-utf8Pairs string defined
     -- for regInfo in CertReqMsg [RFC4211]
  }

  CertifiedKeyPair ::= SEQUENCE {
     certOrEncCert       CertOrEncCert,
     privateKey      [0] EncryptedKey         OPTIONAL,
     -- See [RFC4211] for comments on encoding.
     publicationInfo [1] PKIPublicationInfo   OPTIONAL
  }

  CertOrEncCert ::= CHOICE {
     certificate     [0] CMPCertificate,
     encryptedCert   [1] EncryptedKey
  }

  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
  }

  RevReqContent ::= SEQUENCE OF RevDetails

  RevDetails ::= SEQUENCE {
     certDetails         CertTemplate,
     crlEntryDetails     Extensions       OPTIONAL
  }

  RevRepContent ::= SEQUENCE {
     status        SEQUENCE SIZE (1..MAX) OF PKIStatusInfo,
     revCerts  [0] SEQUENCE SIZE (1..MAX) OF CertId OPTIONAL,
     crls      [1] SEQUENCE SIZE (1..MAX) OF CertificateList
                   OPTIONAL
  }

  RootCaKeyUpdateContent ::= SEQUENCE {
     newWithNew              CMPCertificate,
     newWithOld          [0] CMPCertificate OPTIONAL,
     oldWithNew          [1] CMPCertificate OPTIONAL
  }

CAKeyUpdContent ::= CHOICE {
    cAKeyUpdAnnV2      CAKeyUpdAnnContent, -- deprecated
    cAKeyUpdAnnV3  [0] RootCaKeyUpdateContent
}

  CertAnnContent ::= Certificate

  RevAnnContent ::= SEQUENCE {
     status              PKIStatus,
     certId              CertId,
     willBeRevokedAt     GeneralizedTime,
     badSinceDate        GeneralizedTime,
     crlDetails          Extensions  OPTIONAL
  }

  CRLAnnContent ::= SEQUENCE OF CertificateList

  PKIConfirmContent ::= NULL

  CertConfirmContent ::= SEQUENCE OF CertStatus

  CertStatus ::= SEQUENCE {
     certHash    OCTET STRING,
     certReqId   INTEGER,
     statusInfo  PKIStatusInfo OPTIONAL,
     hashAlg [0] AlgorithmIdentifier{DIGEST-ALGORITHM, {...}}
                 OPTIONAL
  }

  InfoTypeAndValue ::= SEQUENCE {
     infoType               OBJECT IDENTIFIER,
     infoValue              ANY DEFINED BY infoType  OPTIONAL
  }

  -- where {id-it} = {id-pkix 4} = {1 3 6 1 5 5 7 4}
  GenMsgContent ::= SEQUENCE OF InfoTypeAndValue

  GenMsg:    {id-it 1}, < absent >
  GenRep:    {id-it 1}, Certificate | < absent >

  GenMsg:    {id-it 2}, < absent >
  GenRep:    {id-it 2}, SEQUENCE SIZE (1..MAX) OF
                          AlgorithmIdentifier

  GenMsg:    {id-it 3}, < absent >
  GenRep:    {id-it 3}, SEQUENCE SIZE (1..MAX) OF
                          AlgorithmIdentifier

  GenMsg:    {id-it 4}, < absent >
  GenRep:    {id-it 4}, AlgorithmIdentifier

  GenMsg:    {id-it 18}, RootCaKeyUpdateValue

  GenMsg:    {id-it 6}, < absent >
  GenRep:    {id-it 6}, CertificateList

  GenRep:    {id-it 7}, SEQUENCE SIZE (1..MAX) OF OBJECT IDENTIFIER

  GenMsg:    {id-it 10}, OBJECT IDENTIFIER -- (Algorithm object-id)
  GenRep:    {id-it 11}, AlgorithmIdentifier | < absent >

  GenMsg:    {id-it 12}, EncryptedKey
  GenRep:    {id-it 12}, < absent >

  GenMsg:    {id-it 16}, SEQUENCE SIZE (1..MAX) OF UTF8String
  GenRep:    {id-it 16}, SEQUENCE SIZE (1) OF UTF8String

  GenMsg:    {id-it 17}, < absent >
  GenRep:    {id-it 17}, SEQUENCE SIZE (1..MAX) OF
                           CMPCertificate | < absent >

  GenMsg:    {id-it 20}, RootCaCertValue | < absent >
  GenRep:    {id-it 18}, RootCaKeyUpdateValue | < absent >

  RootCaCertValue ::= CMPCertificate

  RootCaKeyUpdateValue ::= RootCaKeyUpdateContent

  RootCaKeyUpdateContent ::= SEQUENCE {
     newWithNew              CMPCertificate,
     newWithOld          [0] CMPCertificate OPTIONAL,
     oldWithNew          [1] CMPCertificate OPTIONAL
  }

  GenMsg:    {id-it 19}, < absent >
  GenRep:    {id-it 19}, CertReqTemplateContent | < absent >

  CertReqTemplateValue  ::= CertReqTemplateContent

  CertReqTemplateContent ::= SEQUENCE {
     certTemplate           CertTemplate,
     keySpec                Controls OPTIONAL }

  Controls  ::= SEQUENCE SIZE (1..MAX) OF AttributeTypeAndValue

  id-regCtrl-algId OBJECT IDENTIFIER ::= { iso(1)
     identified-organization(3) dod(6) internet(1) security(5)
     mechanisms(5) pkix(7) pkip(5) regCtrl(1) 11 }

  AlgIdCtrl ::= AlgorithmIdentifier{ALGORITHM, {...}}

  id-regCtrl-rsaKeyLen OBJECT IDENTIFIER ::= { iso(1)
     identified-organization(3) dod(6) internet(1) security(5)
     mechanisms(5) pkix(7) pkip(5) regCtrl(1) 12 }

  RsaKeyLenCtrl ::= INTEGER (1..MAX)

  GenMsg:    {id-it 22}, SEQUENCE SIZE (1..MAX) OF CRLStatus
  GenRep:    {id-it 23}, SEQUENCE SIZE (1..MAX) OF
                           CertificateList  |  < absent >

  CRLSource ::= CHOICE {
     dpn          [0] DistributionPointName,
     issuer       [1] GeneralNames }

  CRLStatus ::= SEQUENCE {
     source       CRLSource,
     thisUpdate   Time OPTIONAL }

  GenMsg:    {id-it 24}, < absent >
  GenRep:    {id-it 24}, KemCiphertextInfo

  KemCiphertextInfo ::= SEQUENCE {
    kem              AlgorithmIdentifier{KEM-ALGORITHM, {...}},
    ct               OCTET STRING
  }

  GenRepContent ::= SEQUENCE OF InfoTypeAndValue

  ErrorMsgContent ::= SEQUENCE {
     pKIStatusInfo          PKIStatusInfo,
     errorCode              INTEGER           OPTIONAL,
     errorDetails           PKIFreeText       OPTIONAL
  }

  PollReqContent ::= SEQUENCE OF SEQUENCE {
     certReqId    INTEGER }

  PollRepContent ::= SEQUENCE OF SEQUENCE {
     certReqId    INTEGER,
     checkAfter   INTEGER,  -- time in seconds
     reason       PKIFreeText OPTIONAL }

6.1. Root CA Initialization

[See Section 3.1.1.2 for this document's definition of "root CA".]¶

If a newly created root CA is at the top of a PKI hierarchy, it usually produces 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 certificate. 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 may be the OOBCertHash (see Section 5.2.5).¶

6.2. 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 4.4.1) MAY be issued by the CA to aid existing end entities who hold the current root CA certificate (OldWithOld) to transition securely to the new root CA certificate (NewWithNew) and to aid new end entities who will hold NewWithNew to acquire OldWithOld securely for verification of existing data.¶

6.3. Subordinate CA Initialization

[See Section 3.1.1.2 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.¶

6.4. CRL Production

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

6.5. PKI Information Request

When a PKI entity (CA, RA, or end entity) 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 (e.g., password-based MAC; see Section 6.1 of "CMP Algorithms" [RFC9481]) or using any asymmetric authentication means such as a signature (if the end entity has an existing certificate).¶

6.6. 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.¶

6.7. End Entity Initialization

As with CAs, end entities must be initialized. Initialization of end entities requires at least two steps:¶

(Other possible steps include the retrieval of trust condition information and/or out-of-band verification of other CA public keys.)¶

6.8. Certificate Request

An initialized end entity MAY request an additional certificate at any time (for any 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.¶

6.9. 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 (or, in certain circumstances, a new certificate for the same key pair). The request is made using a key update request (kur) message (referred to, in some environments, as a "Certificate Update" operation). 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.¶

7.1. Supporting RFC 2510 Implementations

[RFC2510] did not specify the behavior of implementations receiving versions they did not understand since there was only one version in existence. With the introduction of the revision in [RFC4210], the following versioning behavior is recommended.¶

8.1. On the Necessity of POP

It is well established that the role of a CA is to verify that the name and public key belong to the end entity prior to issuing a certificate. If an entity holding a private key obtains a certificate containing the corresponding public key issued for a different entity, it can authenticate as the entity named in the certificate. This facilitates masquerading. It is not entirely clear what security guarantees are lost if an end entity is able to obtain a certificate containing a public key that they do not possess the corresponding private key for. There are some scenarios, described as "forwarding attacks" in Appendix A of [Gueneysu], in which this can lead to protocol attacks against a naively implemented sign-then-encrypt protocol, but in general, it merely results in the end entity obtaining a certificate that they cannot use.¶

8.2. POP with a Decryption Key

Some cryptographic considerations are 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 reiterate 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.¶

The client MUST return the decrypted values only if they match the expected content type. In an indirect method, the decrypted value MUST be a valid certificate, and in a direct method, the decrypted value MUST be a Rand as defined in Section 5.2.8.3.3.¶

8.3. POP by Exposing the Private Key

Note also that exposing a private key to the CA/RA as a POP 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:¶

8.4. Attack Against DH Key Exchange

A small subgroup attack during a DH key exchange may be carried out as follows. A malicious end entity may deliberately choose DH parameters that enable it to derive (a significant number of bits of) the DH private key of the CA during a key archival or key recovery operation. Armed with this knowledge, the end entity would then be able to retrieve the decryption private key of another unsuspecting end entity, EE2, during EE2's legitimate key archival or key recovery operation with that CA. In order to avoid the possibility of such an attack, two courses of action are available. (1) The CA may generate a fresh DH key pair to be used as a protocol encryption key pair for each end entity with which it interacts. (2) The CA may enter into a key validation protocol (not specified in this document) with each requesting end entity to ensure that the end entity's protocol encryption key pair will not facilitate this attack. Option (1) is clearly simpler (requiring no extra protocol exchanges from either party) and is therefore RECOMMENDED.¶

8.5. Perfect Forward Secrecy

Long-term security typically requires perfect forward secrecy (pfs). When transferring encrypted long-term confidential values such as centrally generated private keys or revocation passphrases, pfs is likely important. Yet, it is not needed for CMP message protection providing integrity and authenticity because transfer of PKI messages is usually completed in very limited time. For the same reason, it is not typically required for the indirect method to provide a POP (Section 5.2.8.3.2) delivering the newly issued certificate in encrypted form.¶

Encrypted values (Section 5.2.2) are transferred using CMS EnvelopedData [RFC5652], which does not offer pfs. In cases where long-term security is needed, CMP messages SHOULD be transferred over a mechanism that provides pfs, such as TLS with appropriate cipher suites selected.¶

8.6. Private Keys for Certificate Signing and CMP Message Protection

A CA should not reuse its certificate signing key for other purposes, such as protecting CMP responses and TLS connections. This way, exposure to other parts of the system and the number of uses of this particularly critical key are reduced to a minimum.¶

8.7. Entropy of Random Numbers, Key Pairs, and Shared Secret Information

Implementations must generate nonces and private keys from random input. The use of inadequate pseudorandom number generators (PRNGs) to generate cryptographic keys can result in little or no security. An attacker may find it much easier to reproduce the PRNG environment that produced the keys and to search the resulting small set of possibilities than brute-force searching the whole key space. As an example of predictable random numbers, see [CVE-2008-0166]; consequences of low-entropy random numbers are discussed in Mining Your Ps and Qs [MiningPsQs]. The generation of quality random numbers is difficult. ISO/IEC 20543:2019 [ISO.20543-2019], NIST SP 800-90A Rev.1 [NIST.SP.800_90Ar1], BSI AIS 31 V2.0 [AIS31], and other specifications offer valuable guidance in this area.¶

If shared secret information is generated by a cryptographically secure random number generator (CSRNG), it is safe to assume that the entropy of the shared secret information equals its bit length. If no CSRNG is used, the entropy of shared secret information depends on the details of the generation process and cannot be measured securely after it has been generated. If user-generated passwords are used as shared secret information, their entropy cannot be measured. Passwords generated from user-supplied entropy are typically insufficient for protected delivery of centrally generated keys or trust anchors.¶

If the entropy of shared secret information protecting the delivery of a centrally generated key pair is known, it should not be less than the security strength of that key pair; if the shared secret information is reused for different key pairs, the security of the shared secret information should exceed the security strength of each individual key pair.¶

For the case of a PKI management operation that delivers a new trust anchor (e.g., a root CA certificate), using caPubs or genp that is (a) not concluded in a timely manner or (b) where the shared secret information is reused for several key management operations, the entropy of the shared secret information, if known, should not be less than the security strength of the trust anchor being managed by the operation. The shared secret information should have an entropy that at least matches the security strength of the key material being managed by the operation. Certain use cases may require shared secret information that may be of a low security strength, e.g., a human-generated password. It is RECOMMENDED that such secret information be limited to a single PKI management operation.¶

Importantly for this section, further information about algorithm use profiles and their security strength is available in Section 7 of CMP Algorithms [RFC9481].¶

8.8. Recurring Usage of KEM Keys for Message Protection

For each PKI management operation using MAC-based message protection involving KEM (see Section 5.1.3.4), the KEM Encapsulate() function, providing a fresh KEM ciphertext (ct) and shared secret (ss), MUST be invoked.¶

It is assumed that the overall data size of the CMP messages in a PKI management operation protected by a single shared secret key is small enough not to introduce extra security risks.¶

To be appropriate for use with this specification, the KEM algorithm MUST explicitly be designed to be secure when the public key is used many times. For example, a KEM algorithm with a single-use public key is not appropriate because the public key is expected to be carried in a long-lived certificate [RFC5280] and used over and over. Thus, KEM algorithms that offer indistinguishability under adaptive chosen ciphertext attack (IND-CCA2) security are appropriate. A common design pattern for obtaining IND-CCA2 security with public key reuse is to apply the Fujisaki-Okamoto (FO) transform [Fujisaki] or a variant of the FO transform [Hofheinz].¶

Therefore, given a long-term public key using an IND-CCA2-secure KEM algorithm, there is no limit to the number of CMP messages that can be authenticated using KEM keys for MAC-based message protection.¶

8.9. Trust Anchor Provisioning Using CMP Messages

A provider of trust anchors, which may be an RA involved in configuration management of its clients, MUST NOT include to-be-trusted CA certificates in a CMP message unless the specific deployment scenario can ensure that it is adequate that the receiving end entity trusts these certificates, e.g., by loading them into its trust store.¶

Whenever an end entity receives in a CMP message a CA certificate to be used as a trust anchor (for example, in the caPubs field of a certificate response or in a general response), it MUST properly authenticate the message sender with existing trust anchors without requiring new trust anchor information included in the message.¶

Additionally, the end entity MUST verify that the sender is an authorized source of trust anchors. This authorization is governed by local policy and typically indicated using shared secret information or with a signature-based message protection using a certificate issued by a PKI that is explicitly authorized for this purpose.¶

8.10. Authorizing Requests for Certificates with Specific EKUs

When a CA issues a certificate containing EKU extensions as defined in Section 4.5, this expresses delegation of an authorization that originally is only with the CA certificate itself. Such delegation is a very sensitive action in a PKI, and therefore, special care must be taken when approving such certificate requests to ensure that only legitimate entities receive a certificate containing such an EKU.¶

8.11. Usage of CT Logs

CAs that support indirect POP MUST NOT also publish final certificates to CT logs [RFC9162] before having received the certConf message containing the certHash of that certificate to complete the POP. The risk is that a malicious actor could fetch the final certificate from the CT log and use that to spoof a response to the implicit POP challenge via a certConf response. This risk does not apply to CT precertificates, so those are OK to publish.¶

If a certificate or its precertificate was published in a CT log, it must be revoked if a required certConf message could not be verified, especially when the implicit POP was used.¶

10.1. Normative References

[MvOV97]

Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook of Applied Cryptography", CRC Press ISBN 0-8493-8523-7, 1996, <https://cacr.uwaterloo.ca/hac/>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>.

[RFC2985]

Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object Classes and Attribute Types Version 2.0", RFC 2985, DOI 10.17487/RFC2985, November 2000, <https://www.rfc-editor.org/info/rfc2985>.

[RFC2986]

Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, DOI 10.17487/RFC2986, November 2000, <https://www.rfc-editor.org/info/rfc2986>.

[RFC3629]

Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003, <https://www.rfc-editor.org/info/rfc3629>.

[RFC4211]

Schaad, J., "Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", RFC 4211, DOI 10.17487/RFC4211, September 2005, <https://www.rfc-editor.org/info/rfc4211>.

[RFC5280]

Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <https://www.rfc-editor.org/info/rfc5280>.

[RFC5480]

Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk, "Elliptic Curve Cryptography Subject Public Key Information", RFC 5480, DOI 10.17487/RFC5480, March 2009, <https://www.rfc-editor.org/info/rfc5480>.

[RFC5646]

Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646, September 2009, <https://www.rfc-editor.org/info/rfc5646>.

[RFC5652]

Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009, <https://www.rfc-editor.org/info/rfc5652>.

[RFC5958]

Turner, S., "Asymmetric Key Packages", RFC 5958, DOI 10.17487/RFC5958, August 2010, <https://www.rfc-editor.org/info/rfc5958>.

[RFC6402]

Schaad, J., "Certificate Management over CMS (CMC) Updates", RFC 6402, DOI 10.17487/RFC6402, November 2011, <https://www.rfc-editor.org/info/rfc6402>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8933]

Housley, R., "Update to the Cryptographic Message Syntax (CMS) for Algorithm Identifier Protection", RFC 8933, DOI 10.17487/RFC8933, October 2020, <https://www.rfc-editor.org/info/rfc8933>.

[RFC9045]

Housley, R., "Algorithm Requirements Update to the Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", RFC 9045, DOI 10.17487/RFC9045, June 2021, <https://www.rfc-editor.org/info/rfc9045>.

[RFC9481]

Brockhaus, H., Aschauer, H., Ounsworth, M., and J. Gray, "Certificate Management Protocol (CMP) Algorithms", RFC 9481, DOI 10.17487/RFC9481, November 2023, <https://www.rfc-editor.org/info/rfc9481>.

[RFC9629]

Housley, R., Gray, J., and T. Okubo, "Using Key Encapsulation Mechanism (KEM) Algorithms in the Cryptographic Message Syntax (CMS)", RFC 9629, DOI 10.17487/RFC9629, August 2024, <https://www.rfc-editor.org/info/rfc9629>.

10.2. Informative References

[AIS31]

Killmann, W. and W. Schindler, "A proposal for: Functionality classes for random number generators - Version 2.0", Federal Office for Information Security (BSI), September 2011, <https://www.bsi.bund.de/SharedDocs/Downloads/DE/BSI/Zertifizierung/Interpretationen/AIS_31_Functionality_classes_for_random_number_generators_e.pdf>.

[CVE-2008-0166]

National Institute of Science and Technology (NIST), "National Vulnerability Database - CVE-2008-0166", May 2008, <https://nvd.nist.gov/vuln/detail/CVE-2008-0166>.

[ETSI-3GPP.33.310]

3GPP, "Network Domain Security (NDS); Authentication Framework (AF)", 3GPP TS 33.310 16.6.0, December 2020, <http://www.3gpp.org/ftp/Specs/html-info/33310.htm>.

[Fujisaki]

Fujisaki, E. and T. Okamoto, "Secure Integration of Asymmetric and Symmetric Encryption Schemes", Journal of Cryptology, vol. 26, no. 1, pp. 80-101, DOI 10.1007/s00145-011-9114-1, December 2011, <https://doi.org/10.1007/s00145-011-9114-1>.

[Gueneysu]

Gueneysu, T., Hodges, P., Land, G., Ounsworth, M., Stebila, D., and G. Zaverucha, "Proof-of-possession for KEM certificates using verifiable generation", Cryptology ePrint Archive, Paper 2022/703, 2022, <https://eprint.iacr.org/2022/703>.

[Hofheinz]

Hofheinz, D., Hövelmanns, K., and E. Kiltz, "A Modular Analysis of the Fujisaki-Okamoto Transformation", Theory of Cryptography (TCC 2017), Lecture Notes in Computer Science, vol. 10677, pp. 341-371, DOI 10.1007/978-3-319-70500-2_12, November 2017, <https://doi.org/10.1007/978-3-319-70500-2_12>.

[IEEE.802.1AR-2018]

IEEE, "IEEE Standard for Local and Metropolitan Area Networks - Secure Device Identity", IEEE Std 802.1AR-2018, DOI 10.1109/ieeestd.2018.8423794, August 2018, <https://doi.org/10.1109/ieeestd.2018.8423794>.

[ISO.20543-2019]

ISO/IEC, "Information technology -- Security techniques -- Test and analysis methods for random bit generators within ISO/IEC 19790 and ISO/IEC 15408", ISO/IEC 20543:2019, October 2019, <https://www.iso.org/standard/68296.html>.

[MiningPsQs]

Heninger, N., Durumeric, Z., Wustrow, E., and J. A. Halderman, "Mining Your Ps and Qs: Detection of Widespread Weak Keys in Network Devices", 21st USENIX Security Symposium (USENIX Security 12), August 2012, <https://www.usenix.org/conference/usenixsecurity12/technical-sessions/presentation/heninger>.

[ML-KEM]

Turner, S., Kampanakis, P., Massimo, J., and B. Westerbaan, "Internet X.509 Public Key Infrastructure - Algorithm Identifiers for the Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM)", Work in Progress, Internet-Draft, draft-ietf-lamps-kyber-certificates-11, 22 July 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-kyber-certificates-11>.

[NIST.SP.800_90Ar1]

Barker, E. B. and J. M. Kelsey, "Recommendation for Random Number Generation Using Deterministic Random Bit Generators", NIST SP 800-90Ar1, DOI 10.6028/NIST.SP.800-90Ar1, June 2015, <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.800-90Ar1.pdf>.

[RFC1847]

Galvin, J., Murphy, S., Crocker, S., and N. Freed, "Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847, October 1995, <https://www.rfc-editor.org/info/rfc1847>.

[RFC2510]

Adams, C. and S. Farrell, "Internet X.509 Public Key Infrastructure Certificate Management Protocols", RFC 2510, DOI 10.17487/RFC2510, March 1999, <https://www.rfc-editor.org/info/rfc2510>.

[RFC2585]

Housley, R. and P. Hoffman, "Internet X.509 Public Key Infrastructure Operational Protocols: FTP and HTTP", RFC 2585, DOI 10.17487/RFC2585, May 1999, <https://www.rfc-editor.org/info/rfc2585>.

[RFC4210]

Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)", RFC 4210, DOI 10.17487/RFC4210, September 2005, <https://www.rfc-editor.org/info/rfc4210>.

[RFC4212]

Blinov, M. and C. Adams, "Alternative Certificate Formats for the Public-Key Infrastructure Using X.509 (PKIX) Certificate Management Protocols", RFC 4212, DOI 10.17487/RFC4212, October 2005, <https://www.rfc-editor.org/info/rfc4212>.

[RFC4303]

Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005, <https://www.rfc-editor.org/info/rfc4303>.

[RFC4511]

Sermersheim, J., Ed., "Lightweight Directory Access Protocol (LDAP): The Protocol", RFC 4511, DOI 10.17487/RFC4511, June 2006, <https://www.rfc-editor.org/info/rfc4511>.

[RFC5912]

Hoffman, P. and J. Schaad, "New ASN.1 Modules for the Public Key Infrastructure Using X.509 (PKIX)", RFC 5912, DOI 10.17487/RFC5912, June 2010, <https://www.rfc-editor.org/info/rfc5912>.

[RFC6268]

Schaad, J. and S. Turner, "Additional New ASN.1 Modules for the Cryptographic Message Syntax (CMS) and the Public Key Infrastructure Using X.509 (PKIX)", RFC 6268, DOI 10.17487/RFC6268, July 2011, <https://www.rfc-editor.org/info/rfc6268>.

[RFC6712]

Kause, T. and M. Peylo, "Internet X.509 Public Key Infrastructure -- HTTP Transfer for the Certificate Management Protocol (CMP)", RFC 6712, DOI 10.17487/RFC6712, September 2012, <https://www.rfc-editor.org/info/rfc6712>.

[RFC7296]

Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T. Kivinen, "Internet Key Exchange Protocol Version 2 (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October 2014, <https://www.rfc-editor.org/info/rfc7296>.

[RFC7299]

Housley, R., "Object Identifier Registry for the PKIX Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014, <https://www.rfc-editor.org/info/rfc7299>.

[RFC8446]

Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.

[RFC8572]

Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero Touch Provisioning (SZTP)", RFC 8572, DOI 10.17487/RFC8572, April 2019, <https://www.rfc-editor.org/info/rfc8572>.

[RFC8649]

Housley, R., "Hash Of Root Key Certificate Extension", RFC 8649, DOI 10.17487/RFC8649, August 2019, <https://www.rfc-editor.org/info/rfc8649>.

[RFC8995]

Pritikin, M., Richardson, M., Eckert, T., Behringer, M., and K. Watsen, "Bootstrapping Remote Secure Key Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995, May 2021, <https://www.rfc-editor.org/info/rfc8995>.

[RFC9147]

Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, <https://www.rfc-editor.org/info/rfc9147>.

[RFC9162]

Laurie, B., Messeri, E., and R. Stradling, "Certificate Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162, December 2021, <https://www.rfc-editor.org/info/rfc9162>.

[RFC9480]

Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate Management Protocol (CMP) Updates", RFC 9480, DOI 10.17487/RFC9480, November 2023, <https://www.rfc-editor.org/info/rfc9480>.

[RFC9482]

Sahni, M., Ed. and S. Tripathi, Ed., "Constrained Application Protocol (CoAP) Transfer for the Certificate Management Protocol", RFC 9482, DOI 10.17487/RFC9482, November 2023, <https://www.rfc-editor.org/info/rfc9482>.

[RFC9483]

Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight Certificate Management Protocol (CMP) Profile", RFC 9483, DOI 10.17487/RFC9483, November 2023, <https://www.rfc-editor.org/info/rfc9483>.

[RFC9733]

von Oheimb, D., Ed., Fries, S., and H. Brockhaus, "BRSKI with Alternative Enrollment (BRSKI-AE)", RFC 9733, DOI 10.17487/RFC9733, March 2025, <https://www.rfc-editor.org/info/rfc9733>.

[RFC9811]

Brockhaus, H., von Oheimb, D., Ounsworth, M., and J. Gray, "Internet X.509 Public Key Infrastructure -- HTTP Transfer for the Certificate Management Protocol (CMP)", RFC 9811, July 2025, <https://www.rfc-editor.org/info/rfc9811>.

[UNISIG.Subset-137]

UNISIG, "ERTMS/ETCS On-line Key Management FFFIS", Subset-137, V1.0.0, December 2015, <https://www.era.europa.eu/system/files/2023-01/sos3_index083_-_subset-137_v100.pdf>.

[X509.2019]

ITU-T, "Information technology - Open Systems Interconnection - The Directory: Public-key and attribute certificate frameworks", ITU-T Recommendation X.509 (10/2019), October 2019, <https://handle.itu.int/11.1002/1000/14033>.

C.1. General Rules for Interpretation of These Profiles

C.2. Algorithm Use Profile

For specifications of algorithm identifiers and respective conventions for conforming implementations, please refer to Section 7.1 of CMP Algorithms [RFC9481].¶

C.3. POP Profile

The table below describes the POP fields for use (in signature field of pop field of ProofOfPossession structure) when proving possession of a private signing key that corresponds to a public verification key for which a certificate has been requested.¶

Note: For examples of MSG_SIG_ALG OIDs, see Section 3 of CMP Algorithms [RFC9481].¶

POP of a private decryption key that corresponds to a public encryption key for which a certificate has been requested does not use this profile; the CertHash field of the certConf message is used instead.¶

Not every CA/RA will do POP (of signing key, decryption key, or key agreement key) in the PKIX-CMP in-band certification request protocol (how POP is done MAY ultimately be a policy issue that is made explicit for any given CA in its publicized Policy OID and Certification Practice Statement). However, this specification mandates that CA/RA entities MUST do POP (by some means) as part of the certification process. All end entities MUST be prepared to provide POP (i.e., these components of the PKIX-CMP protocol MUST be supported).¶

C.4. Initial Registration/Certification (Basic Authenticated Scheme)

An (uninitialized) end entity requests a (first) certificate from a CA. When the CA responds with a message containing a certificate, the end entity replies with a certificate confirmation. The CA sends a pkiconf message back, closing the transaction. All messages are authenticated.¶

This scheme allows the end entity to request certification of a locally generated public key (typically a signature key). The end entity MAY also choose to request the centralized generation and certification of another key pair (typically an encryption key pair).¶

Certification may only be requested for one locally generated public key (for more, use separate PKIMessages).¶

The end entity MUST support POP of the private key associated with the locally generated public key.¶

Preconditions:¶

Message Flow:¶

For this profile, we mandate that the end entity MUST include all (i.e., one or two) CertReqMsg in a single PKIMessage and that the PKI (CA) MUST produce a single response PKIMessage that contains the complete response (i.e., including the OPTIONAL second key pair, if it was requested and if centralized key generation is supported). For simplicity, we also mandate that this message MUST be the final one (i.e., no use of "waiting" status value).¶

The end entity has an out-of-band interaction with the CA/RA. This transaction established the shared secret, the referenceNumber, and optionally the DN used for both the sender and subject name in the certificate template. See Section 8.7 for security considerations on quality of shared secret information.¶

Initialization Request -- ir¶

Field                Value

recipient            CA name
  -- the name of the CA who is being asked to produce a certificate
protectionAlg        MSG_MAC_ALG
  -- only MAC protection is allowed for this request, based
  -- on initial authentication key
senderKID            referenceNum
  -- the reference number that the CA has previously issued
  -- to the end entity (together with the MACing key)
transactionID        present
  -- implementation-specific value, meaningful to end
  -- entity.
  -- [If already in use at the CA, then a rejection message MUST
  -- be produced by the CA]

senderNonce          present
  -- 128 (pseudo-)random bits
freeText             any valid value
body                 ir (CertReqMessages)
                     only one or two CertReqMsg
                     are allowed
  -- if more certificates are required, requests MUST be
  -- packaged in separate PKIMessages

CertReqMsg           one or two present
  -- see below for details, note: crm[0] means the first
  -- (which MUST be present), crm[1] means the second (which
  -- is OPTIONAL, and used to ask for a centrally generated key)

crm[0].certReq.      fixed value of zero
   certReqId
  -- this is the index of the template within the message
crm[0].certReq       present
   certTemplate
  -- MUST include subject public key value, otherwise unconstrained
crm[0].pop...        optionally present if public key
   POPOSigningKey    from crm[0].certReq.certTemplate is
                     a signing key
  -- POP MAY be required in this exchange
  -- (see Appendix D.3 for details)
crm[0].certReq.      optionally present
   controls.archiveOptions
  -- the end entity MAY request that the locally generated
  -- private key be archived

crm[0].certReq.      optionally present
   controls.publicationInfo
  -- the end entity MAY ask for publication of resulting cert.

crm[1].certReq       fixed value of one
      certReqId
     -- the index of the template within the message
   crm[1].certReq       present
      certTemplate
      -- MUST NOT include actual public key bits, otherwise
      -- unconstrained (e.g., the names need not be the same as in
      -- crm[0]).  Note that subjectPublicKeyInfo MAY be present
      -- and contain an AlgorithmIdentifier followed by a
      -- zero-length BIT STRING for the subjectPublicKey if it is
      -- desired to inform the CA/RA of algorithm and parameter
      -- preferences regarding the to-be-generated key pair.

   crm[1].certReq.      present [object identifier MUST be
                                 PROT_ENC_ALG]

      controls.protocolEncrKey
     -- if centralized key generation is supported by this CA,
     -- this short-term asymmetric encryption key (generated by
     -- the end entity) will be used by the CA to encrypt (a
     -- symmetric key used to encrypt) a private key generated by
     -- the CA on behalf of the end entity

crm[1].certReq.      optionally present
   controls.archiveOptions
crm[1].certReq.      optionally present
   controls.publicationInfo
protection           present
  -- bits calculated using MSG_MAC_ALG

Initialization Response -- ip¶

Field                Value

sender               CA name
  -- the name of the CA who produced the message
messageTime          present
  -- time at which CA produced message
protectionAlg        MSG_MAC_ALG
  -- only MAC protection is allowed for this response
senderKID             referenceNum
  -- the reference number that the CA has previously issued to the
  -- end entity (together with the MACing key)
transactionID        present
  -- value from corresponding ir message
senderNonce          present
  -- 128 (pseudo-)random bits
recipNonce           present
  -- value from senderNonce in corresponding ir message
freeText             any valid value
body                 ip (CertRepMessage)
                     contains exactly one response
                     for each request
  -- The PKI (CA) responds to either one or two requests as
  -- appropriate.  crc[0] denotes the first (always present);
  -- crc[1] denotes the second (only present if the ir message
  -- contained two requests and if the CA supports centralized
  -- key generation).
crc[0].              fixed value of zero
   certReqId
  -- MUST contain the response to the first request in the
  -- corresponding ir message
crc[0].status.       present, positive values allowed:
   status               "accepted", "grantedWithMods"
                     negative values allowed:
                        "rejection"
crc[0].status.       present if and only if
   failInfo          crc[0].status.status is "rejection"
crc[0].              present if and only if
   certifiedKeyPair  crc[0].status.status is
                        "accepted" or "grantedWithMods"
certificate          present unless end entity's public
                     key is an encryption key and POP
                     is done in this in-band exchange
encryptedCert        present if and only if end entity's
                     public key is an encryption key and
                     POP done in this in-band exchange
publicationInfo      optionally present

  -- indicates where certificate has been published (present
  -- at discretion of CA)

crc[1].              fixed value of one
   certReqId
  -- MUST contain the response to the second request in the
  -- corresponding ir message
crc[1].status.       present, positive values allowed:
   status               "accepted", "grantedWithMods"
                     negative values allowed:
                        "rejection"
crc[1].status.       present if and only if
   failInfo          crc[0].status.status is "rejection"
crc[1].              present if and only if
   certifiedKeyPair  crc[0].status.status is "accepted"
                     or "grantedWithMods"
certificate          present
privateKey           present
  -- Use EnvelopedData; if backward compatibility is required,
  -- use EncryptedValue, see Section 5.2.2
publicationInfo      optionally present
  -- indicates where certificate has been published (present
  -- at discretion of CA)

protection           present
  -- bits calculated using MSG_MAC_ALG
extraCerts           optionally present
  -- the CA MAY provide additional certificates to the end
  -- entity

Certificate confirm -- certConf¶

Field                Value

sender               present
  -- same as in ir
recipient            CA name
  -- the name of the CA who was asked to produce a certificate
transactionID        present
  -- value from corresponding ir and ip messages
senderNonce          present
  -- 128 (pseudo-)random bits
recipNonce           present
  -- value from senderNonce in corresponding ip message
protectionAlg        MSG_MAC_ALG
  -- only MAC protection is allowed for this message.  The
  -- MAC is based on the initial authentication key shared
  -- between the end entity and the CA.

senderKID            referenceNum
  -- the reference number that the CA has previously issued
  -- to the end entity (together with the MACing key)

body                 certConf
  -- see Section 5.3.18, "PKI Confirmation Content", for the
  -- contents of the certConf fields.
  -- Note: two CertStatus structures are required if both an
  -- encryption and a signing certificate were sent.

protection           present
  -- bits calculated using MSG_MAC_ALG

Confirmation -- pkiconf¶

Field                Value

sender               present
  -- same as in ip
recipient            present
  -- sender name from certConf
transactionID        present
  -- value from certConf message
senderNonce          present
  -- 128 (pseudo-)random bits
recipNonce           present
  -- value from senderNonce from certConf message
protectionAlg        MSG_MAC_ALG
  -- only MAC protection is allowed for this message.
senderKID            referenceNum
body                 pkiconf
protection           present
  -- bits calculated using MSG_MAC_ALG

C.5. Certificate Request

An (initialized) end entity requests a certificate from a CA (for any reason). When the CA responds with a message containing a certificate, the end entity replies with a certificate confirmation. The CA replies with a pkiconf message to close the transaction. All messages are authenticated.¶

The profile for this exchange is identical to that given in Appendix C.4, with the following exceptions:¶

C.6. Key Update Request

An (initialized) end entity requests a certificate from a CA (to update the key pair and/or corresponding certificate that it already possesses). When the CA responds with a message containing a certificate, the end entity replies with a certificate confirmation. The CA replies with a PKIConfirm to close the transaction. All messages are authenticated.¶

The profile for this exchange is identical to that given in Appendix C.4, with the following exceptions:¶

D.1. General Rules for Interpretation of These Profiles

Identical to Appendix C.1.¶

D.2. Algorithm Use Profile

Identical to Appendix C.2.¶

D.3. Self-Signed Certificates

The table below is a profile of how a certificate structure may be "self-signed". These structures are used for distribution of new root CA public keys. This can occur in one of three ways (see Section 4.4 above for a description of the use of these structures):¶

A newWithNew certificate (including relevant extensions) must contain "sensible" values for all fields. For example, when present, subjectAltName MUST be identical to issuerAltName, and, when present, keyIdentifiers must contain appropriate values, et cetera.¶

D.4. Root CA Key Update

A root CA updates its key pair. It then produces a CA key update announcement message that can be made available (via some transport mechanism) to the relevant entities. A confirmation message is not required from the end entities.¶

ckuann message:¶

D.5. PKI Information Request/Response

The end entity sends a general message to the PKI requesting details that will be required for later PKI management operations. The RA/CA responds with a general response. If an RA generates the response, then it will simply forward the equivalent message that it previously received from the CA, with the possible addition of certificates to the extraCerts fields of the PKIMessage. A confirmation message is not required from the end entity.¶

Message Flows:¶

genM:¶

Field               Value

recipient           CA name
  -- the name of the CA as contained in issuerAltName
  -- extensions or issuer fields within certificates
protectionAlg       MSG_MAC_ALG or MSG_SIG_ALG
  -- any authenticated protection alg.
SenderKID           present if required
  -- must be present if required for verification of message
  -- protection
freeText            any valid value
body                genr (GenReqContent)
GenMsgContent       empty SEQUENCE
  -- all relevant information requested
protection          present
  -- bits calculated using MSG_MAC_ALG or MSG_SIG_ALG

genP:¶

Field                Value

sender               CA name
  -- name of the CA that produced the message
protectionAlg        MSG_MAC_ALG or MSG_SIG_ALG
  -- any authenticated protection alg.
senderKID            present if required
  -- must be present if required for verification of message
  -- protection
body                 genp (GenRepContent)
CAProtEncCert        present (object identifier one
                     of PROT_ENC_ALG), with relevant
                     value
  -- to be used if end entity needs to encrypt information for
  -- the CA (e.g., private key for recovery purposes)

SignKeyPairTypes     present, with relevant value
  -- the set of signature algorithm identifiers that this CA will
  -- certify for subject public keys
EncKeyPairTypes      present, with relevant value
  -- the set of encryption / key agreement algorithm identifiers that
  -- this CA will certify for subject public keys
PreferredSymmAlg     present (object identifier one
                     of PROT_SYM_ALG) , with relevant
                     value
  -- the symmetric algorithm that this CA expects to be used
  -- in later PKI messages (for encryption)
RootCaKeyUpdate      optionally present, with
                     relevant value
  -- Use RootCaKeyUpdate; if backward compatibility with cmp2000 is
  -- required, use CAKeyUpdateInfo.
  -- The CA MAY provide information about a relevant root CA
  -- key pair using this field (note that this does not imply
  -- that the responding CA is the root CA in question)
CurrentCRL           optionally present, with relevant value
  -- the CA MAY provide a copy of a complete CRL (i.e.,
  -- fullest possible one)
protection           present
  -- bits calculated using MSG_MAC_ALG or MSG_SIG_ALG
extraCerts           optionally present
  -- can be used to send some certificates to the end
  -- entity. An RA MAY add its certificate here.

D.6. Cross-Certification Request/Response (1-way)

This section describes the creation of a single cross-certificate (i.e., not two at once). The requesting CA MAY choose who is responsible for publication of the cross-certificate created by the responding CA through use of the PKIPublicationInfo control.¶

Preconditions:¶

The use of certificate confirmation and the corresponding server confirmation is determined by the generalInfo field in the PKIHeader (see Section 5.1.1). The following profile does not mandate support for either confirmation.¶

Message Flows:¶

ccr:¶

Field                 Value

sender                Requesting CA name
  -- the name of the CA who produced the message
recipient             Responding CA name
  -- the name of the CA who is being asked to produce a certificate
messageTime           time of production of message
  -- current time at requesting CA
protectionAlg         MSG_SIG_ALG
  -- only signature protection is allowed for this request
senderKID             present if required
  -- must be present if required for verification of message
  -- protection
recipKID             present if required
  -- must be present if required for verification of message
  -- protection
transactionID         present
  -- implementation-specific value, meaningful to requesting CA.
  -- [If already in use at responding CA, then a rejection message
  -- MUST be produced by responding CA]
senderNonce           present
  -- 128 (pseudo-)random bits
freeText              any valid value
body                  ccr (CertReqMessages)
                      only one CertReqMsg
                      allowed
  -- if multiple cross-certificates are required, they MUST be
  -- packaged in separate PKIMessages
certTemplate          present
  -- details follow
version               v1 or v3
  -- v3 STRONGLY RECOMMENDED
signingAlg            present
  -- the requesting CA must know in advance with which algorithm it
  -- wishes the certificate to be signed

subject               present
  -- may be NULL-DN only if subjectAltNames extension value proposed
validity              present
  -- MUST be completely specified (i.e., both fields present)
issuer                present
  -- may be NULL-DN only if issuerAltNames extension value proposed
publicKey             present
  -- the key to be certified (which must be for a signing algorithm)
extensions            optionally present
  -- a requesting CA must propose values for all extensions
  -- that it requires to be in the cross-certificate
POPOSigningKey        present
  -- see Appendix C.3: POP Profile
protection            present
  -- bits calculated using MSG_SIG_ALG
extraCerts            optionally present
  -- MAY contain any additional certificates that requester wishes
  -- to include

ccp:¶

Field                 Value

sender                Responding CA name
  -- the name of the CA who produced the message
recipient             Requesting CA name
  -- the name of the CA who asked for production of a certificate
messageTime           time of production of message
  -- current time at responding CA
protectionAlg         MSG_SIG_ALG
  -- only signature protection is allowed for this message
senderKID             present if required
  -- must be present if required for verification of message
  -- protection
recipKID              present if required
transactionID         present
  -- value from corresponding ccr message
senderNonce           present
  -- 128 (pseudo-)random bits
recipNonce            present
-- senderNonce from corresponding ccr message
freeText              any valid value
body                  ccp (CertRepMessage)
                      only one CertResponse allowed
  -- if multiple cross-certificates are required, they MUST be
  -- packaged in separate PKIMessages
response              present
status                present

PKIStatusInfo.status  present
  -- if PKIStatusInfo.status is one of:
  --   accepted, or
  --   grantedWithMods,
  -- then certifiedKeyPair MUST be present and failInfo MUST
  -- be absent

failInfo              present depending on
                      PKIStatusInfo.status
  -- if PKIStatusInfo.status is:
  --   rejection,
  -- then certifiedKeyPair MUST be absent and failInfo MUST be
  -- present and contain appropriate bit settings

certifiedKeyPair      present depending on
                      PKIStatusInfo.status
certificate           present depending on
                      certifiedKeyPair
  -- content of actual certificate must be examined by requesting CA
  -- before publication
protection            present
  -- bits calculated using MSG_SIG_ALG
extraCerts            optionally present
  -- MAY contain any additional certificates that responder wishes
  -- to include

D.7. In-Band Initialization Using External Identity Certificate

An (uninitialized) end entity wishes to initialize into the PKI with a CA, CA-1. It uses, for authentication purposes, a pre-existing identity certificate issued by another (external) CA, CA-X. A trust relationship must already have been established between CA-1 and CA-X so that CA-1 can validate the end entity identity certificate signed by CA-X. Furthermore, some mechanism must already have been established within the TEE, also known as PSE, of the end entity that would allow it to authenticate and verify PKIMessages signed by CA-1 (as one example, the TEE may contain a certificate issued for the public key of CA-1, signed by another CA that the end entity trusts on the basis of out-of-band authentication techniques).¶

The end entity sends an initialization request to start the transaction. When CA-1 responds with a message containing the new certificate, the end entity replies with a certificate confirmation. CA-1 replies with a pkiconf message to close the transaction. All messages are signed (the end entity messages are signed using the private key that corresponds to the public key in its external identity certificate; the CA-1 messages are signed using the private key that corresponds to the public key in a certificate that can be chained to a trust anchor in the end entity's TEE).¶

The profile for this exchange is identical to that given in Appendix C.4, with the following exceptions:¶