Network Working Group J. Schaad

Network Working Group J. Schaad
Request for Comments: 5274 Soaring Hawk Consulting
Category: Standards Track M. Myers
TraceRoute Security, Inc.
June 2008

Certificate Management Messages over CMS (CMC): Compliance Requirements

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document provides a set of compliance statements about the CMC
(Certificate Management over CMS) enrollment protocol. The ASN.1
structures and the transport mechanisms for the CMC enrollment
protocol are covered in other documents. This document provides the
information needed to make a compliant version of CMC.

Table of Contents

1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Requirements Terminology . . . . . . . . . . . . . . . . . . . 3
4. Requirements for All Entities . . . . . . . . . . . . . . . . 3
4.1. Cryptographic Algorithm Requirements . . . . . . . . . . . 4
4.2. Controls . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. CRMF Feature Requirements . . . . . . . . . . . . . . . . 8
4.4. Requirements for Clients . . . . . . . . . . . . . . . . . 8
5. Requirements for Servers . . . . . . . . . . . . . . . . . . . 8
6. Requirements for EEs . . . . . . . . . . . . . . . . . . . . . 8
7. Requirements for RAs . . . . . . . . . . . . . . . . . . . . . 8
8. Requirements for CAs . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 11

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1. Overview

The CMC (Certificate Management over CMS) protocol is designed in
terms of a client/server relationship. In the simplest case, the
client is the requestor of the certificate (i.e., the End Entity
(EE)) and the server is the issuer of the certificate (i.e., the
Certification Authority (CA)). The introduction of a Registration
Authority (RA) into the set of agents complicates the picture only
slightly. The RA becomes the server with respect to the certificate
requestor, and it becomes the client with respect to the certificate
issuer. Any number of RAs can be inserted into the picture in this
manner.

The RAs may serve specialized purposes that are not currently covered
by this document. One such purpose would be a Key Escrow agent. As
such, all certificate requests for encryption keys would be directed
through this RA and it would take appropriate action to do the key
archival. Key recovery requests could be defined in the CMC
methodology allowing for the Key Escrow agent to perform that
operation acting as the final server in the chain of agents.

If there are multiple RAs in the system, it is considered normal that
not all RAs will see all certificate requests. The routing between
the RAs may be dependent on the content of the certificate requests
involved.

This document is divided into six sections, each section specifying
the requirements that are specific to a class of agents in the CMC
model. These are 1) All agents, 2) all servers, 3) all clients, 4)
all End-Entities, 5) all Registration Entities, 6) all Certificate
Authorities.

2. Terminology

There are several different terms, abbreviations, and acronyms used
in this document that we define here for convenience and consistency
of usage:

End-Entity (EE) refers to the entity that owns a key pair and for
whom a certificate is issued.

Registration Authority (RA) or Local RA (LRA) refers to an entity
that acts as an intermediary between the EE and the CA. Multiple
RAs can exist between the End-Entity and the Certification
Authority. RAs may perform additional services such as key
generation or key archival. This document uses the term RA for
both RA and LRA.

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Certification Authority (CA) refers to the entity that issues
certificates.

Client refers to an entity that creates a PKI Request. In this
document, both RAs and EEs can be clients.

Server refers to the entities that process PKI Requests and create
PKI Responses. In this document both CAs and RAs can be servers.

PKCS #10 refers to the Public Key Cryptography Standard #10
[PKCS10], which defines a certification request syntax.

CRMF refers to the Certificate Request Message Format RFC [CRMF].
CMC uses this certification request syntax defined in this
document as part of the protocol.

CMS refers to the Cryptographic Message Syntax RFC [CMS]. This
document provides for basic cryptographic services including
encryption and signing with and without key management.

PKI Request/Response refers to the requests/responses described in
this document. PKI Requests include certification requests,
revocation requests, etc. PKI Responses include certs-only
messages, failure messages, etc.

Proof-of-Identity refers to the client proving they are who they say
that they are to the server.

Proof-of-Possession (POP) refers to a value that can be used to
prove that the private key corresponding to a public key is in the
possession and can be used by an end-entity.

Transport wrapper refers to the outermost CMS wrapping layer.

3. Requirements 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 [MUST].

4. Requirements for All Entities

All [CMC-STRUCT] and [CMC-TRANS] compliance statements MUST be
adhered to unless specifically stated otherwise in this document.

All entities MUST support Full PKI Requests, Simple PKI Responses,
and Full PKI Responses. Servers SHOULD support Simple PKI Requests.

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All entities MUST support the use of the CRMF syntax for
certification requests. Support for the PKCS #10 syntax for
certification requests SHOULD be implemented by servers.

The extendedFailInfo field SHOULD NOT be populated in the
CMCStatusInfoV2 object; the failInfo field SHOULD be used to relay
this information. If the extendedFailInfo field is used, it is
suggested that an additional CMCStatusInfoV2 item exist for the same
body part with a failInfo field.

All entities MUST implement the HTTP transport mechanism as defined
in [CMC-TRANS]. Other transport mechanisms MAY be implemented.

4.1. Cryptographic Algorithm Requirements

All entities MUST verify DSA-SHA1 and RSA-SHA1 signatures in
SignedData (see [CMS-ALG]). Entities MAY verify other signature
algorithms. It is strongly suggested that RSA-PSS with SHA-1 be
verified (see [CMS-RSA-PSS]). It is strongly suggested that SHA-256
using RSA and RSA-PSS be verified (see [RSA-256]).

All entities MUST generate either DSA-SHA1 or RSA-SHA1 signatures for
SignedData (see [CMS-ALG]). Other signatures algorithms MAY be used
for generation.

All entities MUST support Advanced Encryption Standard (AES) as the
content encryption algorithm for EnvelopedData (see [CMS-AES]).
Other content encryption algorithms MAY be implemented.

All entities MUST support RSA as a key transport algorithm for
EnvelopedData (see [CMS-ALG]). All entities SHOULD support RSA-OAEP
(see [CMS-RSA-OAEP]) as a key transport algorithm. Other key
transport algorithms MAY be implemented.

If an entity supports key agreement for EnvelopedData, it MUST
support Diffie-Hellman (see [CMS-DH]).

If an entity supports PasswordRecipientInfo for EnvelopedData or
AuthenticatedData, it MUST support PBKDF2 [PBKDF2] for key derivation
algorithms. It MUST support AES key wrap (see [AES-WRAP] as the key
encryption algorithm.

If AuthenticatedData is supported, PasswordRecipientInfo MUST be
supported.

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Algorithm requirements for the Identity Proof Version 2 control
(Section 6.2.1 of [CMC-STRUCT]) are: SHA-1 MUST be implemented for
hashAlgId. SHA-256 SHOULD be implemented for hashAlgId. HMAC-SHA1
MUST be implemented for macAlgId. HMAC-SHA256 SHOULD be implemented
for macAlgId.

Algorithm requirements for the Pop Link Witness Version 2 control
(Section 6.3.1 of [CMC-STRUCT]) are: SHA-1 MUST be implemented for
keyGenAlgorithm. SHA-256 SHOULD be implemented for keyGenAlgorithm.
PBKDF2 [PBKDF2] MAY be implemented for keyGenAlgorithm. HMAC-SHA1
MUST be implemented for macAlgorithm. HMAC-SHA256 SHOULD be
implemented for macAlgorithm.

Algorithm requirements for the Encrypted POP and Decrypted POP
controls (Section 6.7 of [CMC-STRUCT]) are: SHA-1 MUST be implemented
for witnessAlgID. SHA-256 SHOULD be implemented for witnessAlgID.
HMAC-SHA1 MUST be implemented for thePOPAlgID. HMAC-SHA256 SHOULD be
implemented for thePOPAlgID.

Algorithm requirements for Publish Trust Anchors control (Section
6.15 of [CMC-STRUCT]) are: SHA-1 MUST be implemented for
hashAlgorithm. SHA-256 SHOULD be implemented for hashAlgorithm.

If an EE generates DH keys for certification, it MUST support section
4 of [DH-POP]. EEs MAY support Section 3 of [DH-POP]. CAs and RAs
that do POP verification MUST support Section 4 of [DH-POP] and
SHOULD support Section 3 of [DH-POP].

EEs that need to use a signature algorithm for keys that cannot
produce a signature MUST support Appendix C of [CMC-STRUCT] and MUST
support the Encrypted/Decrypted POP controls. CAs and RAs that do
POP verification MUST support this signature algorithm and MUST
support the Encrypted/Decrypted POP controls.

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4.2. Controls

The following table lists the name and level of support required for
each control.

+----------------------------+----------+----------+----------+
| Control | EE | RA | CA |
+----------------------------+----------+----------+----------+
| Extended CMC Status Info | MUST | MUST | MUST |
| | | | |
| CMC Status Info | SHOULD | SHOULD | SHOULD |
| | | | |
| Identity Proof Version 2 | MUST | MUST | MUST |
| | | | |
| Identity Proof | SHOULD | SHOULD | SHOULD |
| | | | |
| Identification | MUST | MUST | MUST |
| | | | |
| POP Link Random | MUST | MUST | MUST |
| | | | |
| POP Link Witness Version 2 | MUST | MUST | MUST |
| | | | |
| POP Link Witness | SHOULD | MUST | MUST |
| | | | |
| Data Return | MUST | MUST | MUST |
| | | | |
| Modify Cert Request | N/A | MUST | (2) |
| | | | |
| Add Extensions | N/A | MAY | (1) |
| | | | |
| Transaction ID | MUST | MUST | MUST |
| | | | |
| Sender Nonce | MUST | MUST | MUST |
| | | | |
| Recipient Nonce | MUST | MUST | MUST |
| | | | |
| Encrypted POP | (4) | (5) | SHOULD |
| | | | |
| Decrypted POP | (4) | (5) | SHOULD |
| | | | |
| RA POP Witness | N/A | SHOULD | (1) |
| | | | |
| Get Certificate | optional | optional | optional |
| | | | |
| Get CRL | optional | optional | optional |
| | | | |
| Revocation Request | SHOULD | SHOULD | MUST |
| | | | |

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| Registration Info | SHOULD | SHOULD | SHOULD |
| | | | |
| Response Information | SHOULD | SHOULD | SHOULD |
| | | | |
| Query Pending | MUST | MUST | MUST |
| | | | |
| Confirm Cert. Acceptance | MUST | MUST | MUST |
| | | | |
| Publish Trust Anchors | (3) | (3) | (3) |
| | | | |
| Authenticate Data | (3) | (3) | (3) |
| | | | |
| Batch Request | N/A | MUST | (2) |
| | | | |
| Batch Responses | N/A | MUST | (2) |
| | | | |
| Publication Information | optional | optional | optional |
| | | | |
| Control Processed | N/A | MUST | (2) |
+----------------------------+----------+----------+----------+

Table 1: CMC Control Attributes

Notes:

1. CAs SHOULD implement this control if designed to work with RAs.

2. CAs MUST implement this control if designed to work with RAs.

3. Implementation is optional for these controls. We strongly
suggest that they be implemented in order to populate client
trust anchors.

4. EEs only need to implement this if (a) they support key agreement
algorithms or (b) they need to operate in environments where the
hardware keys cannot provide POP.

5. RAs SHOULD implement this if they implement RA POP Witness.

Strong consideration should be given to implementing the Authenticate
Data and Publish Trust Anchors controls as this gives a simple method
for distributing trust anchors into clients without user
intervention.

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4.3. CRMF Feature Requirements

The following additional restrictions are placed on CRMF features:

The registration control tokens id-regCtrl-regToken and id-regCtrl-
authToken MUST NOT be used. No specific CMC feature is used to
replace these items, but generally the CMC controls identification
and identityProof will perform the same service and are more
specifically defined.

The control token id-regCtrl-pkiArchiveOptions SHOULD NOT be
supported. An alternative method is under development to provide
this functionality.

The behavior of id-regCtrl-oldCertID is not presently used. It is
replaced by issuing the new certificate and using the id-cmc-
publishCert to remove the old certificate from publication. This
operation would not normally be accompanied by an immediate
revocation of the old certificate; however, that can be accomplished
by the id-cmc-revokeRequest control.

The id-regCtrl-protocolEncrKey is not used.

4.4. Requirements for Clients

There are no additional requirements.

5. Requirements for Servers

There are no additional requirements.

6. Requirements for EEs

If an entity implements Diffie-Hellman, it MUST implement either the
DH-POP Proof-of-Possession as defined in [DH-POP], Section 4, or the
challenge-response POP controls id-cmc-encryptedPOP and id-cmc-
decryptedPOP.

7. Requirements for RAs

RAs SHOULD be able to do delegated POP. RAs implementing this
feature MUST implement the id-cmc-lraPOPWitness control.

All RAs MUST implement the promotion of the id-aa-cmc-unsignedData as
covered in Section 3.2.3 of [CMC-STRUCT].

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8. Requirements for CAs

Providing for CAs to work in an environment with RAs is strongly
suggested. Implementation of such support is strongly suggested as
this permits the delegation of substantial administrative interaction
onto an RA rather than at the CA.

CAs MUST perform at least minimal checks on all public keys before
issuing a certificate. At a minimum, a check for syntax would occur
with the POP operation. Additionally, CAs SHOULD perform simple
checks for known bad keys such as small subgroups for DSA-SHA1 and DH
keys [SMALL-SUB-GROUP] or known bad exponents for RSA keys.

CAs MUST enforce POP checking before issuing any certificate. CAs
MAY delegate the POP operation to an RA for those cases where 1) a
challenge/response message pair must be used, 2) an RA performs
escrow of a key and checks for POP in that manner, or 3) an unusual
algorithm is used and that validation is done at the RA.

CAs SHOULD implement both the DH-POP Proof-of-Possession as defined
in [DH-POP], Section 4, and the challenge-response POP controls id-
cmc-encryptedPOP and id-cmc-decryptedPOP.

9. Security Considerations

This document uses [CMC-STRUCT] and [CMC-TRANS] as building blocks to
this document. The security sections of those two documents are
included by reference.

Knowledge of how an entity is expected to operate is vital in
determining which sections of requirements are applicable to that
entity. Care needs to be taken in determining which sections apply
and fully implementing the necessary code.

Cryptographic algorithms have and will be broken or weakened.
Implementers and users need to check that the cryptographic
algorithms listed in this document make sense from a security level.
The IETF from time to time may issue documents dealing with the
current state of the art. Two examples of such documents are
[SMALL-SUB-GROUP] and [HASH-ATTACKS].

10. Acknowledgements

The authors and the PKIX Working Group are grateful for the
participation of Xiaoyi Liu and Jeff Weinstein in helping to author
the original versions of this document.

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The authors would like to thank Brian LaMacchia for his work in
developing and writing up many of the concepts presented in this
document. The authors would also like to thank Alex Deacon and Barb
Fox for their contributions.

11. References

11.1. Normative References

[CMC-STRUCT] Schaad, J. and M. Myers, "Certificate Management
over CMS (CMC)", RFC 5272, June 2008.

[CMC-TRANS] Schaad, J. and M. Myers, "Certificate Management
over CMS (CMC): Transport Protocols", RFC 5273,
June 2008.

[CMS] Housley, R., "Cryptographic Message Syntax (CMS)",
RFC 3852, July 2004.

[CMS-AES] Schaad, J., "Use of the Advanced Encryption
Standard (AES) Encryption Algorithm in
Cryptographic Message Syntax (CMS)", RFC 3565,
July 2003.

[CMS-ALG] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.

[CMS-DH] Rescorla, E., "Diffie-Hellman Key Agreement
Method", RFC 2631, June 1999.

[CRMF] Schaad, J., "Internet X.509 Public Key
Infrastructure Certificate Request Message Format
(CRMF)", RFC 4211, September 2005.

[CMS-RSA-OAEP] Housley, R., "Use of the RSAES-OAEP Key Transport
Algorithm in the Cryptographic Message Syntax
(CMS)", RFC 3560, July 2003.

[CMS-RSA-PSS] Schaad, J., "Use of the RSASSA-PSS Signature
Algorithm in Cryptographic Message Syntax (CMS)",
RFC 4056, June 2005.

[DH-POP] Prafullchandra, H. and J. Schaad, "Diffie-Hellman
Proof-of-Possession Algorithms", RFC 2875,
June 2000.

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[MUST] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", RFC 2119, BCP 14,
March 1997.

[RSA-256] Schaad, J., Kaliski, B., and R. Housley,
"Additional Algorithms and Identifiers for RSA
Cryptography for use in the Internet X.509 Public
Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", RFC 4055,
June 2005.

[PBKDF2] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification Version 2.0", RFC 2898,
September 2000.

[AES-WRAP] Schaad, J. and R. Housley, "Advanced Encryption
Standard (AES) Key Wrap Algorithm", RFC 3394,
September 2002.

11.2. Informative References

[PKCS10] Nystrom, M. and B. Kaliski, "PKCS #10:
Certification Request Syntax Specification v1.7",
RFC 2986, November 2000.

[SMALL-SUB-GROUP] Zuccherato, R., "Methods for Avoiding the "Small-
Subgroup" Attacks on the Diffie-Hellman Key
Agreement Method for S/MIME", RFC 2785,
March 2000.

[HASH-ATTACKS] Hoffman, P. and B. Schneier, "Attacks on
Cryptographic Hashes in Internet Protocols",
RFC 4270, November 2005.

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Authors' Addresses

Jim Schaad
Soaring Hawk Consulting
PO Box 675
Gold Bar, WA 98251

Phone: (425) 785-1031
EMail: [email protected]

Michael Myers
TraceRoute Security, Inc.

EMail: [email protected]

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RFC 5274 CMC: Compliance June 2008

Full Copyright Statement

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