| ACME | Y. Sheffer |
| Internet-Draft | Intuit |
| Intended status: Standards Track | D. Lopez |
| Expires: August 21, 2020 | A. Pastor Perales |
| Telefonica I+D | |
| T. Fossati | |
| ARM | |
| February 18, 2020 |
An ACME Profile for Generating Delegated STAR Certificates
draft-ietf-acme-star-delegation-02
This memo proposes a profile of the ACME protocol that allows the owner of an identifier (e.g., a domain name) to delegate to a third party access to a certificate associated with said identifier. A primary use case is that of a CDN (the third party) terminating TLS sessions on behalf of a content provider (the owner of a domain name). The presented mechanism allows the owner of the identifier to retain control over the delegation and revoke it at any time by cancelling the associated STAR certificate renewal with the ACME CA. Another key property of this mechanism is it does not require any modification to the deployed TLS ecosystem.
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This document is a companion document to [I-D.ietf-acme-star]. To avoid duplication, we give here a bare-bones description of the motivation for this solution. For more details and further use cases, please refer to the introductory sections of [I-D.ietf-acme-star].
An Identifier Owner (IdO), that we can associate in the primary use case to a content provider (also referred to as Domain Name Owner, DNO), has agreements in place with one or more NDC (Name Delegation Consumer) to use and attest its identity. In the primary use case, we consider a CDN provider contracted to serve the IdO content over HTTPS. The CDN terminates the HTTPS connection at one of its edge cache servers and needs to present its clients (browsers, mobile apps, set-top-boxes) a certificate whose name matches the authority of the URL that is requested, i.e., that of the IdO. Understandably, most IdOs balk at sharing their long-term private keys with another organization and, equally, delegates would rather not have to handle other parties’ long-term secrets.
Other relevant use cases are discussed in Section 4.
This document describes a profile of the ACME protocol [I-D.ietf-acme-acme] that allows the NDC to request the IdO, acting as a profiled ACME server, a certificate for a delegated identity - i.e., one belonging to the IdO. The IdO then uses the ACME protocol (with the extensions described in [I-D.ietf-acme-star]) to request issuance of a STAR certificate for the same delegated identity. The generated short-term certificate is automatically renewed by the ACME Certification Authority (CA), periodically fetched by the NDC and used to terminate HTTPS connections in lieu of the IdO. The IdO can end the delegation at any time by simply instructing the CA to stop the automatic renewal and letting the certificate expire shortly thereafter.
In case the delegated identity is a domain name, this document also provides a way for the NDC to inform the IdO about the CNAME mappings that need to be installed in the IdO’s DNS zone to enable the aliasing of the delegated name, thus allowing the complete name delegation workflow to be handled using a single interface.
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This section presents the protocol flow. For completeness, we include the ACME profile proposed in this draft as well as the extended ACME protocol described in [I-D.ietf-acme-star].
The protocol assumes the following preconditions are met:
Note that even if the IdO implements the ACME server role, it is not acting as a CA: in fact, from the point of view of the certificate issuance process, the IdO only works as a “policing” forwarder of the NDC’s key-pair and is responsible for completing the identity verification process towards the ACME CA.
The interaction between the NDC and the IdO is governed by the profiled ACME workflow detailed in Section 2.3. The interaction between the IdO and the CA is ruled by ACME STAR [I-D.ietf-acme-star] as well as any other ACME extension that applies (e.g., [I-D.ietf-acme-authority-token-tnauthlist] for STIR).
The outline of the combined protocol is as follow (Figure 1):
The NDC can now download, install and use the certificate bearing the name delegated by the IdO.
Note that, because the identity validation is suppressed, the NDC sends the finalize request, including the CSR, to the IdO immediately after the Order has been acknowledged. The IdO must buffer a (valid) CSR until the Validation phase completes successfully.
NDC IdO CA
Client Server Client Server
Order
Signature ------->
[ No identity validation ]
CSR
Signature ------->
Order'
Signature ------->
<------- Required
Authorizations
Responses
Signature ------->
<~~~~~~~~Validation~~~~~~~~>
CSR
Signature ------->
<~~~~~~Await issuance~~~~~~> <~~~~~~Await issuance~~~~~~>
<------------------------------------ Certificate
Figure 1: End to end flow
The Order object created by the NDC:
POST /acme/new-order HTTP/1.1
Host: acme.dno.example
Content-Type: application/jose+json
{
"protected": base64url({
"alg": "ES256",
"kid": "https://acme.dno.example/acme/acct/evOfKhNU60wg",
"nonce": "5XJ1L3lEkMG7tR6pA00clA",
"url": "https://acme.dno.example/acme/new-order"
}),
"payload": base64url({
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.dno.example.",
"delegated": true,
"cname": "abc.ndc.example."
}
],
}),
"signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
}
The Order object that is created on the IdO:
{
"status": "ready",
"expires": "2016-01-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.dno.example.",
"delegated": true,
"cname": "abc.ndc.example."
}
],
"authorizations": [],
"finalize": "https://acme.dno.example/acme/order/TO8rfgo/finalize"
}
The IdO SHOULD copy any “recurrent-*” field from the NDC request into the related STAR request to the ACME CA.
When the validation of the identifiers has been successfully completed and the certificate has been issued by the CA, the IdO:
The latter indirectly includes (via the NotBefore and NotAfter HTTP headers) the renewal timers needed by the NDC to inform its certificate reload logic.
{
"status": "valid",
"expires": "2016-01-01T00:00:00Z",
"identifiers": [
{
"type": "dns",
"value": "abc.ndc.dno.example.",
"delegated": true,
"cname": "abc.ndc.example."
}
],
"authorizations": [],
"finalize": "https://acme.dno.example/acme/order/TO8rfgo/finalize",
"star-certificate": "https://acme.ca.example/acme/order/yTr23sSDg9"
}
If an “identifier” object of type “dns” was included, the IdO MUST validate the specified CNAME at this point in the flow. The NDC and IdO may have a pre-established list of valid CNAME values. At the minimum, the IdO MUST verify that both DNS names are syntactically valid.
Following this validation, the IdO can add the CNAME records to its zone:
abc.ndc.dno.example. CNAME abc.ndc.example.
When sending the Order to the ACME CA, the IdO SHOULD strip the “delegated” and “cname” attributes sent by the NDC (Section 2.3.1). The IdO MUST add the necessary STAR extensions to the Order. In addition, to allow the NDC to download the certificate using unauthenticated GET, the IdO MUST add the recurrent-certificate-get attribute and set it to true.
In order to help a client to discover support for this profile, the directory object of an ACME server MUST contain the following attribute inside the “meta” field:
It is worth noting that cancelation of the ACME STAR certificate is a prerogative of the IdO. The NDC does not own the relevant account key on the ACME CA, therefore it can’t issue a cancelation request for the STAR cert. Potentially, since it holds the STAR cert private key, it could request the revocation of a single STAR certificate. However, STAR explicitly disables the revokeCert interface.
The CSR template is used to express and constrain the shape of the CSR that the NDC uses to request the certificate. The CSR is used for every certificate created under the same delegation. Its validation by the IdO is a critical element in the security of the whole delegation mechanism.
Instead of defining every possible CSR attribute, this document takes a minimalist approach by declaring only the minimum attribute set and deferring the registration of further, more specific, attributes to future documents.
The template is a JSON document. Each field denotes one of:
The NDC MUST NOT include in the CSR any fields that are not specified in the template, and in particular MUST NOT add any extensions unless those were previously negotiated out of band with the IdO.
The mapping between X.509 CSR fields and the template will be defined in a future revision of this document.
When the CSR is received by the IdO, it MUST verify that the CSR is consistent with the template that the IdO sent earlier. The IdO MAY enforce additional constraints, e.g. by restricting field lengths.
The CSR template in Figure 2 represents one possible CSR template governing the delegation exchanges provided in the rest of this document.
{
"keyTypes": [
{
"PublicKeyType": "RSA",
"PublicKeyLength": 4096,
"SignatureType": "sha256WithRSAEncryption"
}
],
"subject": {
"country": "CA",
"stateOrProvince": "**",
"locality": "**",
"commonName": "**"
},
"extensions": {
"subjectAltName": {
"DNS": ["client1.ndc.dno.example"],
"IP": ["1.2.3.4", "13::17"]
},
"keyUsage": ["digitalSignature"],
"extendedKeyUsage": [
"serverAuth", "timeStamping"
]
}
}
Figure 2: Example CSR template
The template syntax is defined in Appendix B.
[I-D.ietf-cdni-interfaces-https-delegation] discusses several solutions addressing different delegation requirements for the CDNI (CDN Interconnection) environment. This section discusses two of the stated requirements in the context of the STAR delegation workflow.
In some cases the content owner (IdO) would like to delegate authority over a web site to multiple NDCs (CDNs). This could happen if the IdO has agreements in place with different regional CDNs for different geographical regions, or if a “backup” CDN is used to handle overflow traffic by temporarily altering some of the CNAME mappings in place. The STAR delegation flow enables this use case naturally, since each CDN can authenticate separately to the IdO (via its own separate account) specifying its CSR, and the IdO is free to allow or deny each certificate request according to its own policy.
In other cases, a content owner (IdO) delegates some domains to a large CDN (uCDN), which in turn delegates to a smaller regional CDN, dCDN. The DNO has a contractual relationship with uCDN, and uCDN has a similar relationship with dCDN. However IdO may not even know about dCDN.
The STAR protocol can be chained to support this use case: uCDN could forward requests from dCDN to DNO, and forward responses back to dCDN. Whether such proxying is allowed is governed by policy and contracts between the parties.
A mechanism is necessary at the interface between uCDN and dCDN by which the uCDN can advertise:
Note that such mechanism is provided by the CSR template.
As a second use case, we consider the delegation of credentials in the STIR ecosystem [I-D.ietf-stir-cert-delegation].
In the STIR “delegated” model, a service provider, the NDC, needs to sign PASSPorT’s [RFC8225] for telephone numbers (e.g., TN=+123) belonging to another service provider, the IdO. In order to do that, it needs a STIR certificate, and private key, that includes TN=+123 in the TNAuthList [RFC8226] cert extension.
The STAR delegation profile described in this document applies straightforwardly, the only extra requirement being the ability to instruct the NDC about the allowed TNAuthList values. This can be achieved by a simple extension of the CSR template.
[[RFC Editor: please replace XXXX below by the RFC number.]]
This document adds the following entries to the ACME Directory Metadata Fields:
| Field Name | Field Type | Reference |
|---|---|---|
| star-delegation-enabled | boolean | RFC XXXX |
TODO
When a web site is delegated to a CDN, the CDN can in principle modify the web site at will, create and remove pages. This means that a malicious or breached CDN can pass the ACME (as well as common non-ACME) HTTPS-based validation challenges and generate a certificate for the site. This is true regardless of whether the CNAME mechanisms defined in the current document is used or not.
In some cases, this is the desired behavior: the domain owner trusts the CDN to have full control of the cryptographic credentials for the site. The current document however assumes that the domain owner only wants to delegate restricted control, and wishes to retain the capability to cancel the CDN’s credentials at a short notice.
Following is the proposed solution where the IdO wishes to ensure that a rogue CDN cannot issue unauthorized certificates:
We note that the above solution may need to be tweaked depending on the exact capabilities and authorisation flows supported by the selected CAs.
This work is partially supported by the European Commission under Horizon 2020 grant agreement no. 688421 Measurement and Architecture for a Middleboxed Internet (MAMI). This support does not imply endorsement.
| [I-D.ietf-acme-authority-token-tnauthlist] | Wendt, C., Hancock, D., Barnes, M. and J. Peterson, "TNAuthList profile of ACME Authority Token", Internet-Draft draft-ietf-acme-authority-token-tnauthlist-05, November 2019. |
| [I-D.ietf-cdni-interfaces-https-delegation] | Fieau, F., Emile, S. and S. Mishra, "CDNI extensions for HTTPS delegation", Internet-Draft draft-ietf-cdni-interfaces-https-delegation-02, November 2019. |
| [I-D.ietf-stir-cert-delegation] | Peterson, J., "STIR Certificate Delegation", Internet-Draft draft-ietf-stir-cert-delegation-01, November 2019. |
| [RFC8225] | Wendt, C. and J. Peterson, "PASSporT: Personal Assertion Token", RFC 8225, DOI 10.17487/RFC8225, February 2018. |
| [RFC8226] | Peterson, J. and S. Turner, "Secure Telephone Identity Credentials: Certificates", RFC 8226, DOI 10.17487/RFC8226, February 2018. |
[[Note to RFC Editor: please remove before publication.]]
Following is a JSON Schema definition of the CSR template. The syntax used is that of draft 7 of [[json-schema]], which may not be the latest version of the corresponding Internet Draft [I-D.handrews-json-schema] at the time of publication.
While the CSR template must follow the syntax defined here, neither the IdO nor the NDC are expected to validate it at run-time.
{
"title": "JSON Schema for the STAR Delegation CSR template",
"$schema": "http://json-schema.org/draft-07/schema#",
"$id": "http://ietf.org/acme/drafts/star-delegation/csr-template",
"$def": {
"distinguished-name": {
"$id": "#distinguished-name",
"type": "object",
"properties": {
"country": {"type": "string"},
"stateOrProvince": {"type": "string"},
"locality": {"type": "string"},
"organization": {"type": "string"},
"organizationalUnit": {"type": "string"},
"emailAddress": {"type": "string"},
"commonName": {"type": "string"}
},
"additionalProperties": false
},
"rsaKeyType": {
"$id": "#rsaKeyType",
"type": "object",
"properties": {
"PublicKeyType": {
"type": "string",
"const": "RSA"
},
"PublicKeyLength": {
"type": "integer"
},
"SignatureType": {
"type": "string",
"enum": ["sha256WithRSAEncryption"]
}
},
"additionalProperties": false
},
"ecKeyTYpe": {
"$id": "#ecKeyType",
"type": "object",
"properties": {
"PublicKeyType": {
"type": "string",
"const": "ecPublicKey"
},
"Curve": {
"type": "string",
"enum": ["secp521r1"]
},
"SignatureType": {
"type": "string",
"enum": ["ecdsa-with-SHA256"]
}
},
"additionalProperties": false
}
},
"type": "object",
"properties": {
"keyTypes": {
"type": "array",
"items": {
"oneOf": [
{"$ref": "#rsaKeyType"},
{"$ref": "#ecKeyType"}
]
}
},
"subject": {
"$ref": "#distinguished-name"
},
"extensions": {
"type": "object",
"properties": {
"keyUsage": {
"type": "array",
"items": {
"type": "string",
"enum": ["digitalSignature", "nonRepudiation", "keyEncipherment", "dataEncipherment", "keyAgreement", "keyCertSign", "cRLSign", "encipherOnly", "decipherOnly"]
}
},
"extendedKeyUsage": {
"type": "array",
"items": {
"type": "string",
"enum": ["serverAuth", "clientAuth", "codeSigning", "emailProtection", "timeStamping", "OCSPSigning"]
}
},
"subjectAltName": {
"type": "object",
"properties": {
"DNS": {
"type": "array",
"items": {
"type": "string",
"format": "hostname"
}
},
"IP": {
"type": "array",
"items": {
"oneOf": [
{
"type": "string",
"format": "ipv4"
},
{
"type": "string",
"format": "ipv6"
}
]
}
},
"Email": {
"type": "array",
"items": {
"type": "string",
"format": "email"
}
}
},
"additionalProperties": false
}
},
"additionalProperties": false
}
},
"additionalProperties": false
}