Nonce-based Freshness for Remote Attestation in Certificate Signing Requests (CSRs) for the Certification Management Protocol (CMP) and for Enrollment over Secure Transport (EST)

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Table of Contents

1. Introduction

Several protocols have been standardized to automate the management of certificates, such as certificate issuance, CA certificate provisioning, certificate renewal and certificate revocation.¶

The Certificate Management Protocol (CMP) [I-D.ietf-lamps-rfc4210bis] defines protocol messages for X.509v3 certificate creation and management. CMP provides interactions between end entities and PKI components, such as a Registration Authority (RA) and a Certification Authority (CA).¶

Enrollment over Secure Transport (EST) [RFC7030][RFC8295] is another certificate management protocol, which provides a sub-set of the features offered by CMP.¶

An end entity requesting a certificate from a Certification Authority (CA) may wish to offer believable claims about the protections afforded to the corresponding private key, such as whether the private key resides on a hardware security module or the protection capabilities provided by the hardware, and claims about the platform itself.¶

To convey these claims in Evidence, as part of remote attestation, the remote attestation extension [I-D.ietf-lamps-csr-attestation] has been defined. It describes how to encode Evidence produced by an Attester for inclusion in Certificate Signing Requests (CSRs), and any certificates necessary for validating it.¶

A Verifier or Relying Party might need to learn the point in time an Evidence has been produced. This is essential in deciding whether the included Claims can be considered fresh, meaning they still reflect the latest state of the Attester.¶

Attestation technology available today, such as [TPM20] and [I-D.tschofenig-rats-psa-token], often accomplish freshness with the help of nonces. More details about ensuring freshness of Evidence can be found in Section 10 of [RFC9334].¶

Since an end entity needs to obtain a nonce from the Verifier via the Relying Party, this leads to an additional roundtrip. The CSR is, however, a one-shot message. For this reason CMP and EST are used by the end entity to obtain the nonce from the RA/CA.¶

CMP and EST conveniently offer a mechanism to request information from the RA/CA prior to a certification request.¶

Once the nonce is obtained the end entity can issue an API call to the Attester to request Evidence and passes the nonce as an input parameter into the API call. The returned Evidence is then embedded into a CSR and returned to the RA/CA in a certification request message.¶

Figure 1 shows this interaction graphically. The nonce is obtained in step (1) using the extension to CMP/EST defined in this document. The CSR extension of [I-D.ietf-lamps-csr-attestation] is used to convey Evidence to the RA/CA in step (2). The Verifier processes the received information and returns an Attestation Result to the Relying Party in step (3).¶

The functionality of this document is described in two sections, namely:¶

• Section 3 describes how to convey the nonce CMP, and¶

• Section 4 offers the equivalent functionality for EST.¶

2. Terminology and Requirements Language

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 [RFC2119].¶

The terms Attester, Relying Party, Verifier and Evidence are defined in [RFC9334]. The terms end entity, certification authority (CA), and registration authority (RA) are defined in [RFC5280].¶

We use the terms Certificate Signing Request (CSR) and certification request interchangeably.¶

3. Conveying a Nonce in CMP

Section 5.3.19.16 of [I-D.ietf-lamps-rfc4210bis] defines the general request message (genm) and general response (genp). The NonceRequest payload of the genm message, which is send by the end entity to request a nonce, contains optional details on the length of nonce the Attester requires. The NonceResponse payload of the genp message, which is sent by the CA/RA in response to a request message by the end entity, contains the nonce.¶

 GenMsg:    {id-it TBD1}, NonceRequestValue
 GenRep:    {id-it TBD2}, NonceResponseValue | < absent >

 id-it-NonceRequest OBJECT IDENTIFIER ::= { id-it TBD1 }
 NonceRequestValue ::= SEQUENCE {
    len INTEGER OPTIONAL,
    -- indicates the required length of the requested nonce
    hint EvidenceHint OPTIONAL
    -- indicates which Verifier to request a nonce from
 }

 id-it-NonceResponse OBJECT IDENTIFIER ::= { id-it TBD2 }
 NonceResponseValue ::= SEQUENCE {
    nonce OCTET STRING
    -- contains the nonce of length len
    -- provided by the Verifier indicated with hint
    expiry Time OPTIONAL
    -- indicates how long the Verifier considers the
    -- nonce valid
 }

Note: The EvidenceHint structure is defined in [I-D.ietf-lamps-csr-attestation]. The hint is intended for an Attester to indicate to the EST server which Verifier should be invoked to request a nonce.¶

The use of the general request/response message exchange leads to an extra roundtrip to convey the nonce from the CA/RA to the end entity (and ultimately to the Attester inside the end entity).¶

The end entity MUST construct a NonceRequest request message to trigger the RA/CA to transmit a nonce in the response.¶

[Open Issue: Should request message indicate the remote attestation capability of the end entity rather than relying on "policy information"? This may also allow to inform the CA/RA about the type of attestation technology/technologies available to the end entity.]¶

If the end entity supports remote attestation and the policy requires Evidence in a CSR to be provided, the RA/CA issues an NonceResponse response message containing a nonce.¶

Figure 2 showns the interaction graphically.¶

End Entity                                          RA/CA
==========                                      =============

              -->>-- NonceRequest -->>--
                                                Verify request
                                                Generate nonce*
                                                Create response
              --<<-- NonceResponse --<<--
                      (nonce, expiry)

Generate key pair
Generate Evidence*
Generate certification request message
              -->>-- certification request -->>--
                    (+Evidence including nonce)
                                               Verify request
                                               Verify Evidence*
                                               Check for replay*
                                               Issue certificate
                                               Create response
              --<<-- certification response --<<--
Handle response
Store certificate

*: These steps require interactions with the Attester
(on the EE side) and with the Verifier (on the RA/CA side).

4. Conveying a Nonce in EST

The EST client can request a nonce for its Attester. This function is generally performed after requesting CA certificates and before other EST functions.¶

The EST server MUST support the use of the path-prefix of "/.well- known/" as defined in [RFC5785] and the registered name of "est". Thus, a valid EST server URI path begins with "https://www.example.com/.well-known/est". Each EST operation is indicated by a path-suffix that indicates the intended operation.¶

The following operation is defined by this specificaion:¶

 +------------------------+-----------------+-------------------+
 | Operation              |Operation path   | Details           |
 +========================+=================+===================+
 | Retrieval of a nonce   | /nonce          | This section      |
 +------------------------+-----------------+-------------------+

The operation path is appended to the path-prefix to form the URI used with HTTP GET or POST to perform the desired EST operation. An example valid URI absolute path for the "/nonce" operation is "/.well-known/est/nonce".¶

An EST client uses a GET or a POST depending on whether parameters are included:¶

• A GET request MUST be used when the EST client does not want to convey extra parameters.¶

• A POST request MUST be used when parameters, like nonce length or a hint about the verification service, are included in the request.¶

 +-------------------+---------------------------------+---------------+
 | Message type      | Media type(s)                   | Reference     |
 | (per operation)   |                                 |               |
 +===================+=================================+===============+
 | Nonce Request     | N/A (for GET) or                | This section  |
 |                   | application/json (for POST)     |               |
 +===================+=================================+===============+
 | Nonce Response    | application/json                | This section  |
 |                   |                                 |               |
 +===================+=================================+===============+

To retrieve a nonce using a GET, the EST client would use the following HTTP request-line:¶

GET /.well-known/est/nonce HTTP/1.1

To retrieve a nonce by specifying the size of the requested nonce (and/or by including a hint about the Verification service) a POST message is used, as shown below:¶

POST /.well-known/est/nonce HTTP/1.1
Content-Type: application/json
{
  "len": 8,
  "hint": "https://example.com"
}

The payload in a POST request MUST be of content-type of "application/json" and MUST contain a JSON object [RFC7159] with the member "len" and/or "hint". The value of the "len" member indicates the length of the requested nonce value in bytes. The "hint" member contains either be a rfc822Name, a dNSName, a uri, or a test value (based on the definition in the EvidenceHint structure from [I-D.ietf-lamps-csr-attestation]).¶

The EST server MAY request HTTP-based client authentication, as explained in Section 3.2.3 of [RFC7030].¶

If the request is successful, the EST server response MUST contain a HTTP 200 response code with a content-type of "application/json" and a JSON object [RFC7159] with the member nonce. The expiry member is optional and indicates the time the nonce is considered valid. After the expiry time is expired, the session is likely garbage collected.¶

Below is a non-normative example of the response given by the EST server:¶

HTTP/1.1 200 OK
Content-Type: application/json

{
    "nonce": "MTIzNDU2Nzg5MDEyMzQ1Njc4OTAxMjM0NTY3ODkwMTI=",
    "expiry": "2031-10-12T07:20:50.52Z"
}

[Open Issue: Should we also register a content type for use with EST over CoAP where the nonce and the expiry field is encoded in a CBOR structure?]¶

5. Nonce Processing Guidelines

When the RA/CA is requested or configured to transmit a nonce to an end entity then it interacts with the Verifier. The Verifier is, according to the IETF RATS architecture [RFC9334], "a role performed by an entity that appraises the validity of Evidence about an Attester and produces Attestation Results to be used by a Relying Party." Since the Verifier validates Evidence it is also the source of the nonce to check for replay.¶

The nonce value MUST contain a random byte sequence whereby the length depends on the used remote attestation technology. Since the nonce is relayed with the RA/CA, it MUST be copied to the respective structure, as described in Section 4 and Section 3, for transmission to the Attester.¶

For example, the PSA attestation token [I-D.tschofenig-rats-psa-token] supports nonces of length 32, 48 and 64 bytes. Other attestation technologies use nonces of similar length. The assumption in this specification is that the RA/CA have out-of-band knowledge about the required nonce length required for the attestation technology used by the end entity. The nonces of incorrect length will cause the remote attestation protocol to fail.¶

When the end entity receives the nonce it MUST use it, if remote attestation is available and supports nonces. It is better for an RA/CA to be aggressive in sending a nonce, at least where there is a reasonable chance the client supports remote attestation and the client should be allowed to ignore the nonce if it either does not support it or cannot use it for policy reasons.¶

While the semantics of the attestation API and the software/hardware architecture is out-of-scope of this specification, the API will return Evidence from the Attester in a format specific to the attestation technology utilized. The encoding of the returned evidence varies but will be placed inside the CSR, as specified in [I-D.ietf-lamps-csr-attestation]. The software creating the CSR will not have to interpret the Evidence format - it is treated as an opaque blob. It is important to note that the nonce is carried in the Evidence, either implicitly or explicitly, and it MUST NOT be conveyed in CSR structures outside the Evidence payload.¶

The processing of the CSR containing Evidence is described in [I-D.ietf-lamps-csr-attestation]. Note that the issued certificates does not contain the nonce, as explained in [I-D.ietf-lamps-csr-attestation].¶

6. IANA Considerations

TBD:¶

7. Security Considerations

This specification defines how to obtain a nonce via CMP and EST and assumes that the nonce does not require confidentiality protection without impacting the security property of the remote attestation protocol. [RFC9334] defines the IETF remote attestation architecture discusses nonce-based freshness in great detail.¶

Section 8.4 of [I-D.ietf-rats-eat] places randomness and privacy requirement on the generation of the nonce for use with an Entity Attestation Token (EAT). These requirements have been adopted by attestation technologies, such as the PSA attestation token [I-D.tschofenig-rats-psa-token] and are of general utility:¶

• The nonce MUST have at least 64 bits of entropy.¶

• To avoid the conveyance of privacy-related information in the nonce, it should be derived using a salt that originates from a true and reliable random number generator or any other source of randomness.¶

Since each specification of an attestation technology offers guidance for replay protection with nonces (and other techniques) this document needs to defer specific guidance to the respective specifications.¶

For the use of Evidence in a CSR the security considerations of [I-D.ietf-lamps-csr-attestation] are relevant to this document.¶

Appendix A. Acknowledgments

We would like to thank Russ Housley, Thomas Fossati, Watson Ladd, Ionut Mihalcea and Carl Wallace for their review comments.¶

Authors' Addresses