Other Certificates Extension

RFC 3280 defines a profile for the use of public key certificates for Internet applications. Some applications may require a way to link together a set of certificates belonging to the same end entity so this memo defines a new public key certificate extension that supports such linkage. Other than asserting that the set of certificates belong to the same end entity, the semantics of the actual linkage of certifcates is not defined here, that is a matter for application developers and the operators of certification authorities (CAs). In particular we do not define how a CA can validate that the same end entity is the holder of the various private keys, nor how any application should make use of this information. This memo simply defines the relevant syntax. 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.

Public key certificates expire, typically about a year after they are created. Some applications might need to know that the same entity is the subject of this certificate and a previously used certificate. For example, if a web server certificate expires, it could be useful for a web browser to know that the server currently presenting a certificate in a TLS handshake represents the same web server that previously presented a certificate. This could be used for example to allow the browser to automatically fill in form fields for the server in question, even if the server certificate has been replaced. While the same effect can be achieved based on the use of the same issuer and subject fields in a certificate there could be security issues involved in such comparisons, e.g. if the subject name includes a DNS name and the ownership of that DNS domain has changed. The use of the new extension provides a way for the CA to signal to the application that the same end entity is involved, regardless of name changes. The new extension could also allow the web site operator to more easily change CA when renewing its certificate.

This section defines the syntax for the other certificates extension. The new extension is simply a list of other issuer/serial number pairs from the linked certificates. The IssuerAndSerialNumber construct is taken from CMS. When this extension is present the CA is asserting that the same end entity is the subject of the relevant certificates. Mechanisms for how this assertion is validated by the CA or used by consumers of the certificate are out of scope of this memo. This extension MUST NOT be marked critical. id-ce-otherCerts OBJECT IDENTIFIER ::== { id-ce XXX } OtherCertificates ::= SEQUENCE OF IssuerAndSerialNumber

The use case motivating this was contributed to the W3C web security context (WSC) working group by Tyler Close. See http://www.w3.org/2006/WSC/wiki/SafeWebFormEditor for details.

This memo includes no request to IANA.

TBD. Some warnings for CAs and applications needed.