LAMPS Working Group H. Brockhaus
Internet-Draft Siemens
Updates: 4210 (if approved) February 17, 2020
Intended status: Standards Track
Expires: August 20, 2020

CMP Updates


This document contains a set of updates to the base syntax of Certificate Management Protocol (CMP) version 2. This document updates RFC 4210.

Specifically, the CMP services updated in this document comprise the enabling of using EnvelopedData instead of EncryptedValue and the definition of extended key usages to identify certificates of CMP endpoints on certification and registration authorities.

Status of This Memo

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This Internet-Draft will expire on August 20, 2020.

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Table of Contents

1. History of changes

From draft-brockhaus-lamps-cmp-updates-03 -> draft-brockhaus-lamps-cmp-updates-03:

From version 02 -> 03:

From version 01 -> 02:

From version 00 -> 01:

2. Introduction

While using CMP in industrial and IoT environments and developing the Lightweight CMP Profile some limitations were identified in the original CMP specification. This document updates RFC 4210 to overcome these limitations.

In general, this document aims to improve the crypto agility of CMP to be flexible to react on future advances in cryptography.

This document also introduces new extended key usages to identify CMP endpoints on registration and certification authorities.

2.1. Convention and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

In this document, these words will appear with that interpretation only when in ALL CAPS. Lower case uses of these words are not to be interpreted as carrying significance described in RFC 2119.

Technical terminology is used in conformance with RFC 4210, RFC 4211, and RFC 5280. The following key words are used:

Certification authority, which issues certificates.
Registration authority, an optional system component to which a CA delegates certificate management functions such as authorization checks.
Key generation authority, which generates key pairs on behalf of an EE. The KGA could be co-located with an RA or a CA.
End entity, a user, device, or service that holds a PKI certificate. An identifier for the EE is given as its subject of the certificate.

3. Updates to RFC 4210 - Certificate Management Protocol (CMP)

3.1. New Section 1.1. - Changes since RFC 4210

The following subsections describe feature updates to RFC 4210. They are always related to the base specification. Hence references to the original sections in RFC 4210 are used whenever possible.

Insert this section at the end of the current Section 1.

The following updates are made in draft-brockhaus-lamps-cmp-updates:

3.2. New Section 4.5 - Extended Key Usage

Insert this section.

The Extended Key Usage (EKU) extension indicates the purposes for which the certified public key may be used. It therefore restricts the use of a certificate to specific applications.

A CA may want to delegate parts of their duties to other PKI management entities. The mechanism to prove this delegation explained in this section offers zero-touch means to check the authorization of such delegation. Such delegation could also be expressed by other means, e.g., explicit configuration.

To offer automatic validation means for the delegation of a role by a CA, the certificates used by PKI management entities 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 the authorization of this entity by the delegating CA to act as the PKI management entity as described below.

The ASN.1 to define these EKUs is:

id-kp-cmpCA OBJECT IDENTIFIER ::= { id-kp 27 }
id-kp-cmpRA OBJECT IDENTIFIER ::= { id-kp 28 }
id-kp-cmpKGA OBJECT IDENTIFIER ::= { id-kp ... }

< TBD: id-kp-cmpKGA to be defined. >

Note: RFC 6402 section 2.10 specifies OIDs for a CMC CA and a CMC RA. As the functionality of a CA and RA is not specific to whether use CMC or CMP as certificate management protocol, the same OIDs SHALL be used for a cmpCA and a cmpRA.

< TBD: It needs to be clarified, if the Name and Description of the OIDs can be adapted or extended to avoid confusion as they currently only refer to CMC endpoints. >

The description of the PKI management entity for each of the EKUs is as follows:

CMP Certification Authorities are CMP endpoints on CA equipment as described in section The key used in the context of CMP management operations, especially CMP message protection, need not be the same key that signs the certificates. It is necessary, however, to ensure that the entity acting as cmpCA is authorized to do so. Therefore, the cmpCA MUST do one of the following,

For automatic validation of such delegation it MUST be indicated by the id-kp-cmpCA extended key usage. This extended key usage MUST be placed into the certificate used on the CA equipment and the CA that delegates this role MUST issue the cmpCA certificate.

Note: Using a separate key pair for protecting CMP management operations at the CA decreases the number of operations of the private key used to sign certificates.

CMP Registration Authorities are CMP endpoints on RA equipment as described in section A cmpRA is identified by the id-kp-cmpRA extended key usage. This extended key usage is placed into RA certificates. The CA that delegated this role is identified by the CA that issued the cmpRA certificate.

CMP Key Generation Authorities are identified by the id-kp-cmpKGA extended key usage. Though the cmpKGA knows the private key it generated on behalf of the end entity, this is a very sensible service and needs specific authorization. This authorization is either with the CA certificate itself, or indicated by placing the id-kp-cmpKGA extended key usage into the cmpRA or cmpCA certificate used to authenticate the origin of the private key to express the authorization to offer this service.

Note: In device PKIs, especially those issuing IDevID certificates, CA may have very long validity (including the GeneralizedTime value 99991231235959Z to indicate a not well-defined expiration date as specified in IEEE 802.1AR section 8.5 [IEEE802.1AR] and RFC 5280 Section Such validity periods SHOULD NOT be used for protection of CMP messages. Certificates for delegated CMP message protection (cmpCA, cmpRA, cmpKGA) MUST NOT use indefinite expiration date.

3.3. Replace Section - Multiple Protection

Section of RFC 4210 describes the nested message. This document opens the usage of nested messages also for batch transport of PKI messages between different PKI management entities.

Replace the text of the section with the following text.

In cases where an end entity sends a protected PKI message to an RA, the RA MAY forward that message to a CA, adding its own protection (which MAY be a MAC or a signature, depending on the information and certificates shared between the RA and the CA). There are different use cases for such multi protected messages.

These use cases are accomplished by nesting the messages sent by the end entity within a new PKI message. The structure used is as follows.

NestedMessageContent ::= PKIMessages

(The use of PKIMessages, a SEQUENCE OF PKIMessage, lets the RA batch the requests of several EEs in a single new message.)

3.4. Replace Section 5.2.2. - Encrypted Values

Section 5.2.2 of RFC 4210 describes the usage of EncryptedValue to transport encrypted data. This document extends the encryption of data to also use EnvelopedData.

Replace the text of the section with the following text.

Where encrypted data (restricted, in this specification, to be either private keys, certificates, or passwords) are sent in PKI messages, the EncryptedKey data structure is used.

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

See CRMF for EncryptedKey and EncryptedValue syntax and for EnvelopedData syntax see CMS. Using the EncryptedKey data structure, the choice to either use EncryptedValue (for backward compatibility only) or EnvelopedData is offered. The use of the EncryptedValue structure has been deprecated in favor of the EnvelopedData structure. Therefore, it is recommended to use EnvelopedData.

The EncryptedKey data structure is used in CMP to either transport a private key, certificate or revocation passphrase in encrypted form.

EnvelopedData is used as follows:

Note: When transferring a centrally generated private key in a certificate response message to the EE, the algorithm identifier and the associated public key will anyhow be transported in this response message. Therefore, the private key will not be delivered in a key package structure as specified in [RFC5958] and [RFC6032]. But the wrapping of the private key in a SignedData structure that is wrapped in the EnvelopedData structure as specified in [RFC6032] is applied.

The content of the EnvelopedData structure, as specified in CMS section 3, MUST be encrypted using a newly generated symmetric content-encryption key. This content-encryption key MUST be securely provided to the recipient using one of three key management techniques.

The choice of the key management technique to be used by the sender depends on the credential available for the recipient:

3.5. Update Section 5.3.4. - Certification Response

Section 5.3.4 of RFC 4210 describes the Certification Response. This document updates the syntax by using the parent structure EncryptedKey instead of EncryptedValue as described in Section 3.1 above.

Replace the ASN.1 syntax of CertifiedKeyPair and CertOrEncCert with the following text.

CertifiedKeyPair ::= SEQUENCE {
    certOrEncCert       CertOrEncCert,
    privateKey      [0] EncryptedKey        OPTIONAL,
    -- see [CRMF] for comment on encoding
    publicationInfo [1] PKIPublicationInfo  OPTIONAL

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

Add the following paragraphs to the end of the section.

The use of EncryptedKey is described in section 5.2.2.

3.6. Replace Section - Revocation Passphrase

Section of RFC 4210 describes the provisioning of a revocation passphrase for authenticating a later revocation request. This document updates the handling by using the parent structure EncryptedKey instead of EncryptedValue to transport this information as described in Section 3.1 above.

Replace the text of the section with the following text.

This MAY be used by the EE to send a passphrase to a CA/RA for the purpose of authenticating a later revocation request (in the case that the appropriate signing private key is no longer available to authenticate the request). See Appendix B for further details on the use of this mechanism.

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

The use of EncryptedKey is described in section 5.2.2.

3.7. Update Section 5.3.22 - Polling Request and Response

Section 5.3.22 of RFC 4210 describes when and how polling messages are used. This document adds the polling mechanism also to outstanding p10cr transactions.

Replace all paragraphs in front of the state machine diagram with the following text.

This pair of messages is intended to handle scenarios in which the client needs to poll the server in order to determine the status of an outstanding ir, cr, p10cr, or kur transaction (i.e., when the "waiting" PKIStatus has been received).

    certReqId    INTEGER }

    certReqId    INTEGER,
    checkAfter   INTEGER,  -- time in seconds
    reason       PKIFreeText OPTIONAL }

The following clauses describe when polling messages are used, and how they are used. It is assumed that multiple certConf messages can be sent during transactions. There will be one sent in response to each ip, cp, or kup that contains a CertStatus for an issued certificate.

In response to an ip, cp, or kup message, an EE will send a certConf for all issued certificates and, following the ack, a pollReq for all pending certificates.
In response to a pollReq, a CA/RA will return an ip, cp, or kup if one or more of the pending certificates is ready; otherwise, it will return a pollRep.
If the EE receives a pollRep, it will wait for at least as long as the checkAfter value before sending another pollReq.
If an ip, cp, or kup is received in response to a pollReq, then it will be treated in the same way as the initial response.

Note: A p10cr message contains exactly one CertificationRequestInfo data structure as specified in PKCS#10 but not certificate request number. Therefore, the certReqId MUST be set to 0 in all following messages of this transaction.

3.8. Update Appendix B - The Use of Revocation Passphrase

Appendix B of RFC 4210 describes the usage of the revocation passphrase. As this document updates RFC 4210 to utilize the parent structure EncryptedKey instead of EncryptedValue as described in Section 3.1 above, the description is updated accordingly.

Replace the first bullet point of this section with the following text.

Replace the third bullet point of this section with the following text.

< TBD: The attribute structure containing the key identifier in the unprotectedAttr field could either be pkcs-9-at-friendlyName or pkcs-9-at-localKeyId as specified in RFC 2985 section 5.5. Are there preferences for either one? >

3.9. Update Appendix C - Request Message Behavioral Clarifications

Appendix C of RFC 4210 provides clarifications to the request message behavior. As this document updates RFC 4210 to utilize the parent structure EncryptedKey instead of EncryptedValue as described in Section 3.1 above, the description is updated accordingly.

Replace the note coming after the ASN.1 syntax of POPOPrivKey of this section with the following text.

-- **********
-- *  the type of "thisMessage" is given as BIT STRING in RFC 4211
-- *  [RFC4211]; it should be "EncryptedKey" (in accordance with
-- *  Section 5.2.2, "Encrypted Values", of this specification).
-- *  Therefore, this document makes the behavioral clarification of
-- *  specifying that the contents of "thisMessage" MUST be encoded
-- *  either as EnvelopedData or EncryptedValue (only for backward
-- *  compatibility) and then wrapped in a BIT STRING.  This allows
-- *  the necessary conveyance and protection of the private key
-- *  while maintaining bits-on-the-wire compatibility with RFC 4211
-- *  [RFC4211].
-- **********

3.10. Update Appendix D.4. - Initial Registration/Certification (Basic Authenticated Scheme)

Appendix D.4 of RFC 4210 provides the initial registration/certification scheme. This scheme shall continue to use EncryptedValue for backward compatibility reasons.

Replace the comment after the privateKey field of crc[1].certifiedKeyPair in the syntax of the Initialization Response message with the following text.

-- see Appendix C, Request Message Behavioral Clarifications
-- for backward compatibility reasons, use EncryptedValue 

4. IANA Considerations

< TBD: Add any IANA considerations >

5. Security Considerations

No changes are made to the existing security considerations of RFC 4210.

6. Acknowledgements

Special thank goes to Jim Schaad for his guidance and the inspiration on structuring and writing this document I got from [RFC6402] that updates CMC.

I also like to thank all reviewers of this document for their valuable feedback.

7. References

7.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[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.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification Request Syntax Specification Version 1.7", RFC 2986, DOI 10.17487/RFC2986, November 2000.
[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.
[RFC4211] Schaad, J., "Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", RFC 4211, DOI 10.17487/RFC4211, September 2005.
[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.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009.
[RFC6402] Schaad, J., "Certificate Management over CMS (CMC) Updates", RFC 6402, DOI 10.17487/RFC6402, November 2011.

7.2. Informative References

[I-D.brockhaus-lamps-lightweight-cmp-profile] Brockhaus, H., Fries, S. and D. Oheimb, "Lightweight CMP Profile", Internet-Draft draft-brockhaus-lamps-lightweight-cmp-profile-03, January 2020.
[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, DOI 10.17487/RFC5958, August 2010.
[RFC6032] Turner, S. and R. Housley, "Cryptographic Message Syntax (CMS) Encrypted Key Package Content Type", RFC 6032, DOI 10.17487/RFC6032, December 2010.

Appendix A. ASN.1 Modules

Changes to the following parts are needed

friendlyName, localKeyId
    FROM PKCS-9 {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
                 pkcs-9(9) modules(0) pkcs-9(1)}

< TBD: Either friendlyName or localKeyId need to be imported here. >

CertTemplate, PKIPublicationInfo, EncryptedKey, CertId,
    FROM PKIXCRMF-2005 {iso(1) identified-organization(3)
    dod(6) internet(1) security(5) mechanisms(5) pkix(7)
    id-mod(0) id-mod-crmf2005(36)}

CertifiedKeyPair ::= SEQUENCE {
    certOrEncCert       CertOrEncCert,
    privateKey      [0] EncryptedKey        OPTIONAL,
    -- see [CRMF] for comment on encoding
    publicationInfo [1] PKIPublicationInfo  OPTIONAL

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

--   id-it-revPassphrase    OBJECT IDENTIFIER ::= {id-it 12}
--      RevPassphraseValue      ::= EncryptedKey

-- Extended Key Usage extension for PKI entities used in
-- CMP operations

id-kp-cmpCA OBJECT IDENTIFIER ::= { id-kp 27 }
id-kp-cmpRA OBJECT IDENTIFIER ::= { id-kp 28 }
id-kp-cmpKGA OBJECT IDENTIFIER ::= { id-kp ... }

< TBD: id-kp-cmpKGA to be defined. >

< TBD: If needed the complete ASN.1 Module from RFC 4210 section needs to be copied here. >

Author's Address

Hendrik Brockhaus Siemens AG Otto-Hahn-Ring 6 Munich, 81739 Germany EMail: URI: