]> Certificate Management over CMS (CMC): Compliance Requirements AKAYLA, Inc.
joe@akayla.com
sn3rd
sean@sn3rd.com
SEC lamps Public Key Infrastructure Cryptographic Message Syntax Certificate Management Compliance This document provides a set of compliance statements about the Certificate Management over CMS (CMC) enrollment protocol. The ASN.1 structures and the transport mechanisms for the CMC enrollment protocol are covered in other documents (RFCs 10002 and 10003). This document provides the information needed to make a compliant version of CMC. This document obsoletes RFCs 5274 and 6402.
Introduction The Certificate Management over CMS (CMC) 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 entities, 2) all servers, 3) all clients, 4) all EEs, 5) all RAs, and 6) all CAs. This document obsoletes RFCs 5274 and 6402 .
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 EE and the CA. RAs may perform additional services such as key generation or key archival. This document uses the term RA for both RA and LRA.
Certification Authority (CA):
Refers to the entity that issues certificates.
Client:
Refers to an entity that creates a Public Key Infrastructure (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 , which defines a certification request syntax.
CRMF:
Refers to the Certificate Request Message Format . CMC uses this certification request syntax defined in this document as part of the protocol.
CMS:
Refers to the Cryptographic Message Syntax . 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 EE. See .
Transport wrapper:
Refers to the outermost CMS wrapping layer.
Entity:
Refers to EE, RA (or LRA), or CA.
HMAC:
Refers to the Hashed Message Authentication Code. CMC uses the ASN.1 module defined in .
Requirements Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.
Changes Since RFCs 5274 and 6402 Merged text. Updated the Introduction, changed "all agents" to "all entities" in the overview to maintain consistency throughout the document, and renumbered the section headers. Added RA Identity Proof Witness and Response Body controls to . Updated the Cryptographic Algorithm Requirements:
  • Replaced SHA-1 with SHA-256
  • Replaced HMAC-SHA-1 with HMAC-SHA-256
  • Added algorithms for AuthEnvelopedData
Added a paragraph to maintain backward algorithm compatibility.
Requirements for All Entities All and 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. 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 . Other transport mechanisms MAY be implemented.
Cryptographic Algorithm Requirements All entities MUST verify RSA-SHA256 signatures in SignedData; (see ). Entities MAY verify other signature algorithms. All entities MUST generate RSA-SHA256 signatures for SignedData; (see ). Other signature algorithms MAY be used for generation. All entities MUST support Advanced Encryption Standard (AES) as the content encryption algorithm for EnvelopedData; (see ). Other content encryption algorithms MAY be implemented. All entities MUST support AES-GCM (Galois/Counter Mode) as the authenticated content encryption algorithm for AuthEnvelopedData; (see ). They MUST also support a 12-octet nonce size and a 12-octet Integrity Check Value (ICV) length. Other content encryption algorithms MAY be implemented. All entities MUST support RSA as a key transport algorithm for EnvelopedData and AuthEnvelopedData; see . Other key transport algorithms MAY be implemented. If an entity supports key agreement for EnvelopedData or AuthEnvelopedData, it MUST support Diffie-Hellman (DH); (see ). If an entity supports PasswordRecipientInfo for EnvelopedData, AuthenticatedData, or AuthEnvelopedData, it MUST support Password-Based Key Derivation Function 2 (PBKDF2) for key derivation algorithms. It MUST support AES key wrap (see ) as the key encryption algorithm. If AuthenticatedData is supported, PasswordRecipientInfo MUST be supported. Algorithm requirements for the Identity Proof Version 2 control () are as follows:
  • SHA-256 MUST be implemented for hashAlgId.
  • HMAC-SHA256 MUST be implemented for macAlgId.
Algorithm requirements for the Pop Link Witness Version 2 control () are as follows:
  • SHA-256 MUST be implemented for keyGenAlgorithm.
  • PBKDF2 MAY be implemented for keyGenAlgorithm.
  • HMAC-SHA256 MUST be implemented for macAlgorithm.
Algorithm requirements for the Encrypted POP and Decrypted POP controls () are as follows:
  • SHA-256 MUST be implemented for witnessAlgID.
  • HMAC-SHA256 MUST be implemented for thePOPAlgID.
Algorithm requirements for Publish Trust Anchors control () are as follows:
  • SHA-256 MUST be implemented for hashAlgorithm.
If an EE generates DH keys for certification, it MUST support . EEs MAY support . CAs and RAs that do POP verification MUST support and SHOULD support . EEs that need to use a signature algorithm for keys that cannot produce a signature MUST support 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. For backward compatibility with the previous version of CMC, servers MAY offer the algorithms specified therein, but SHOULD use the CMC requests to identify which certificates should be transitioned to more secure algorithms, if possible.
Controls The following table lists the name and level of support required for each control. CMC Control Attributes
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
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)
Authenticated 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)
RA Identity Proof Witness N/A MUST (2)
Response Body (6) (6) N/A
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.
  6. EEs SHOULD implement this if designed to work with RAs and MUST implement if intended to be used in environments where RAs are used for identity validation or key generation. RAs SHOULD implement and validate responses for consistency.
Strong consideration should be given to implementing the Authenticated Data and Publish Trust Anchors controls as this gives a simple method for distributing trust anchors to clients without user intervention.
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 control's identification and identityProof will perform the same service and are more specifically defined.
  • The control token id-regCtrl-pkiArchiveOptions SHOULD NOT be supported. See and for alternative methods.
  • 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.
Requirements for Clients There are no additional requirements.
Requirements for Servers There are no additional requirements.
Requirements for EEs If an EE implements Diffie-Hellman, it MUST implement either the DH POP Proof-of-Possession as defined in or the challenge-response POP controls id-cmc-encryptedPOP and id-cmc-decryptedPOP.
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 .
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 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 and the challenge-response POP controls id-cmc-encryptedPOP and id-cmc-decryptedPOP.
Security Considerations This document uses and as building blocks. 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 and .
IANA Considerations This document has no IANA actions.
References Normative References Certificate Management over CMS (CMC) AKAYLA, Inc. sn3rd Certificate Management over CMS (CMC): Transport Protocols AKAYLA, Inc. sn3rd Informative References
Acknowledgements Obviously, the authors would like to thank and for their work on . Thank you to , , and for reviewing the document and providing comments. The Acknowledgments section from RFC 5274 follows:
The authors and the PKIX Working Group are grateful for the participation of and in helping to author the original versions of this document. The authors would like to thank for his work in developing and writing up many of the concepts presented in this document. The authors would also like to thank and for their contributions.
Contributors August Cellars
TraceRoute Security, Inc.