X.509 Certificate Extension for Secure/Multipurpose Internet Mail Extensions (S/MIME) Capabilities
Extracted elements (11)
The extension addresses the scenario where a sender of an encrypted message has no prior knowledge of the recipient's cryptographic capabilities through S/MIME exchanges. Since S/MIME encryption already requires possession of the recipient's certificate, embedding capabilities there avoids reliance on out-of-band means or potentially incompatible default configurations.
The specification is intentionally limited to a 'static' approach where asserted cryptographic capabilities remain unchanged until the certificate expires or is revoked. Dynamic approaches allowing retrieval of certified dynamically updateable capabilities are explicitly out of scope.
All content requirements defined for the SMIMECapabilities attribute in RFC 3851 apply equally to this certificate extension, since the data structure used is identical to the one defined in RFC 3851.
Applications using the S/MIME Capabilities extension SHOULD NOT use information in the extension if more reliable and relevant authenticated capabilities information is available to the application.
Certification Authorities SHOULD limit the type of included S/MIME Capabilities in this extension to types that are considered relevant to the intended use of the certificate.
Client applications processing this extension MAY at their own discretion ignore any present S/MIME Capabilities, and SHOULD always gracefully ignore any present S/MIME Capabilities that are not considered relevant to the particular use of the certificate.
The S/MIME Capabilities extension MUST NOT be marked critical.
The S/MIME Capabilities extension embeds an SMIMECapabilities data structure (identical to the SMIMECapabilities attribute from RFC 3851) inside an X.509 certificate. The structure is a SEQUENCE OF SMIMECapability, where each SMIMECapability contains a capabilityID OID and optional parameters.
The S/MIME Capabilities extension reflects the subject's capabilities at certificate issuance time. Changes in capabilities during the certificate's lifetime may require revocation, but revocation is only warranted if a listed algorithm is considered broken or too weak for the governing security policy.
Use of this extension does not change the responsibility of the sender to choose sufficiently strong encryption; it is always the sender's responsibility to select appropriate protection for its information disclosure regardless of capability hints in the certificate.
The SMIMECapabilities ASN.1 structure is defined as a SEQUENCE OF SMIMECapability, where SMIMECapability ::= SEQUENCE { capabilityID OBJECT IDENTIFIER, parameters ANY DEFINED BY capabilityID OPTIONAL }. The OID for smimeCapabilities is {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}.