Internet-Draft | RFC XML Examples | August 2023 |
Fieau, et al. | Expires 18 February 2024 | [Page] |
The delivery of content over HTTPS involving multiple CDNs raises credential management issues. This document defines metadata in the CDNI Control and Metadata interface to setup HTTPS delegation using Delegated Credentials from an Upstream CDN (uCDN) to a Downstream CDN (dCDN).¶
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Content delivery over HTTPS using one or more CDNs along the path requires credential management. This specifically applies when an entity delegates to another trusted entity delivery of content via HTTPS.¶
This document defines the CDNI Metadata interface to setup HTTPS delegation using Delegated Credentials between an upstream CDN (uCDN) and downstream CDN (dCDN).¶
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 [RFC2119].¶
This document uses terminology from CDNI framework documents: CDNI framework document [RFC7336], CDNI requirements [RFC7337] and CDNI interface specifications documents: CDNI Metadata interface [RFC8006] and CDNI Control interface / Triggers [RFC8007].¶
A dCDN should advertise its supported delegation methods using the Footprint and Capabilities interface (FCI) as defined in RFC8008. The FCI.Metadata object allows a dCDN to advertise its capabilities and the MI objects supported by the dCDN. Accordingly, to announce the support for delegated credentials, the dCDN should announce the support of MI.DelegatedCredentials as shown in below example.¶
{ "capabilities": [ { "capability-type": "FCI.Metadata", "capability-value": { "metadata": [ "MI.DelegatedCredentials", "... other supported MI objects ..." ] }, "footprints": [ "Footprint objects" ] } ] }¶
There is also a need to announce additional parameters related to the number of credentials supported by the dCDN. For that purpose we introduce the FCI object FCI.DelegationCredentials.¶
The FCI.DelegationCredentials object enables advertising the maximum number of delegated credentials supported by the dCDN. This number is typically (but not necessarily) linked with the number of servers in the dCDN.¶
number-delegated-certs-supported¶
The following is an example of the FCI.DelegatedCredentials.¶
{ "capabilities": [ { "capability-type": "FCI.DelegatedCredentials", "capability-value": { "number-delegated-certs-supported": 10 } "footprints": [ <Footprint objects> ] } ] }¶
The dCDN uses the FCI.DelegatedCredentials object to announce the number of endpoints as the number of supported delegated credentials.¶
When the uCDN receives the FCI.DelegatedCredentials object it can provide the supported number of delegated credentials to the dCDN. When configuring the dCDN, the uCDN MAY decide to provide less than the maximum supported delegated credentials of the dCDN. Note that, within a dCDN, different deployment possibilities of the delegated credentials on the endpoints exist. The dCDN may use one single delegated credential and deploy it on multiple endpoints. Alternatively, the dCDN may deploy a different delegated credential for each endpoint (provided that the uCDN delivers enough different delegated credentials). This choice is at the discretion of the dCDN and depends on the number of delegated credentials provided by the uCDN.¶
The FCI.DelegationCredentials object does not address expiry and renewal of delegated credentials. Once the uCDN has provided delegated credentials via the MI, uCDN SHOULD monitor the provided credentials and their expiry times. The uCDN should timely refresh dCDN credentials via the MI.¶
As expressed in [RFC9345], when an origin has delgated to a dCDN, the dCDN presents the "delegated_credential" during the TLS handshake [RFC8446] to the User Agent, instead of its own certificate. This implies that the dCDN is also in the possession of the private key corresponding to the public key in DelegatedCredential.cred [RFC9345]. This allows the User Agent to verify the signature in CertificateVerify message sent and signed by the dCDN.¶
This section defines the MI.DelegatedCredentials object containing an array of delegated credentials and optionally the corresponding private keys. The CDNI Metadata Interface [RFC8006] describes the CDNI metadata distribution mechanisms according to which a dCDN can retrieve the MI.DelegatedCredentials object from the uCDN.¶
The properties of the MI.DelegatedCredentials object are as follows.¶
delegated-credentials¶
The DelegatedCredentialObject object is composed of the following two properties:¶
delegated-credential¶
private-key¶
The private-key property is not mandatory. If not specified, it is assumed that the dCDN generated the public-private key pair for the delegated credential itself and provided the public key information with an out-of-band mechanism to the uCDN. As discussed in Section 7, it is NOT RECOMMENDED to communicate private keys to the dCDN using MI.¶
Below, please find an example MI.DelegatedCredential object.¶
{ "generic-metadata-type": "MI.DelegatedCredentials", "generic-metadata-value": { "delegated-credentials": [ {"delegated-credential": "cBBfm8KK6pPz/tdgKyedwA... iXCCIAmzMM0R8FLI3Ba0UQ=="}, {"delegated-credential": "4pyIGtjFdys1+9y/4sS/Fg... J+h9lnRY/xgmi65RLGKoRw=="}, {"delegated-credential": "6PWFO0g2AXvUaULXLObcVA... HXoldT/qaYCCNEyCc8JM2A=="} ] } }¶
An example call-flow using delegated credentials in CDNI is depicted in Figure 1.¶
1. It is assumed that the uCDN has been provisioned and configured with a certificate. Note that it is out of scope of CDNI and the present document how and from where (e.g., CSP) the uCDN acquired its certificate.¶
2. The uCDN generates a set of delegated credentials (here it is assumed that public keys of the dCDN are known). Note, that the uCDN may generate this material at different points in time, e.g., in advance to have a pool of delegated credentials or on-demand when the dCDN announces its maximum number of supported delegated crednetials.¶
3. Using the CDNI Footprint and Capabilities interface [RFC8008], the dCDN advertises MI.DelegatedCredentials capabilities to the uCDN. The dCDN further uses FCI.DelegatedCredentials to inform on the maximum number of supported delegated credentials.¶
4. Using the CDNI Metadata interface [RFC8006], the dCDN acquires the MI.DelegatedCredentials, therefore retrieving an array of delegated credentials.¶
5. The client establishes a TLS connection with an endpoint of the dCDN according to [RFC9345] using the delegated credentials retrieved in step 4.¶
6. When some delegated credentials are about to expire, the uCDN uses the CDNI MI [RFC8006] to provide new, valid delegated credentials.¶
This document requests the registration of the following entries under the "CDNI Payload Types" registry hosted by IANA regarding "CDNI delegation":¶
Payload Type | Specification |
MI.DelegatedCredentials | RFCthis |
FCI.DelegatedCredentials | RFCthis |
[RFC Editor: Please replace RFCthis with the published RFC number for this document.]¶
The extensions defined in the present document enable providing delegated credentials to dCDNs. A delegated credentials can only be used by a dCDN if it is in possession of the associated private key. Similarly, an attacker requires access to the private key in order to exploit delegated credential and impersonate dCDN nodes. Thus, leakage of only the delegated credential without the private key represents a limited security risk.¶
The delegated credentials and associated private keys are short-lived and as such a single leaked delegated credential with its private key represents a limited security risk. Still, it is NOT RECOMMENDED to send private keys through the MI as omitting the private key further limits the possibility exploits by an attacker to exploit the delegated credential.¶
It is also important to ensure that an attacker is not able to systematically retrieve a consecutive or consistent set of delegated credentials and associated private keys. Such an attack would allow the attacker to systematically impersonate dCDN nodes. The FCI and MI objects defined in the present document are transferred via the interfaces defined in CDNI [RFC8006]. [RFC8006] describes how to secure these interfaces, protecting the integrity, confidentiality and ensuring the authenticity of the dCDN and uCDN, which should prevent an attacker to systematically retrieve delegated credential and associated private keys.¶
The information, FCI, and MI objects defined in the present document do not contain any personally identifiable information (PII). As such this document does not change or alter the Confidentiality and Privacy Consideration outlined in the CDNI Metadata and Footprint and Capabilities RFCs [RFC8006].¶