Certificate Limitation Policy

Binary trust model standardized as a set of trusted anchors and CRLs/OCSP services does not cover all corner cases in the modern crypto world. There is a need in more differentiated limitations. Some of them are suggested by Google when it limits the usage of Symantec’s certificates. The CRL profile does not fit the purpose of such limitations. The CRLs are issued by the same CAs that are subject to be limited. Currently the set of CAs trusted by OS or browsers can be used for the validation purposes. In case when a large enough CA becomes untrusted, it cannot be deleted from the storage of trusted CAs because it may cause error of validation of many certificates. The measures usually taken in such cases usually include application-level limitation of certificates lifetimes, refusing to accept EV-certificates in other way than DV, requirements to use Certificate Transparency, etc. This document suggests a cryptographically signed format dubbed Certificate Limitation Profile (CLP) designed for description of such limitations. This format can be used by applications that use system-wide set of trust anchors for validating purposes or by applications with own wide enough set of trusted anchors in case when the trust anchor for the entity found misbehaving cannot be revoked. Currently the only way to provide such limitations is hard coding them in application itself. Using of CLPs does not allow to completely avoid hard coding but allows to hard code only the minimal set of rarely changing data: the fact that application uses CLP the certificate to verify the signature under the CLP file minimal date of the CLP to be used for the current version of application. It will be possible to move the checks for the limitations to the external cryptographical libraries, such as OpenSSL, instead of checking them at the application level.

A proposed syntax and overall structure of CLP is very similar to the one defined for CRLs.

The ASN.1 format of particular limitations is provided in the corresponding sections. Anywhere below the Time in ASN.1 notation is treated according to RFC 5280:

CLP has general structure similar to CRLs. The upper-level fields are: TBSCertList - the sequence of individual certificates to be limited; signatureAlgorithm - the OID of the signature algorithm used for signature; signatureValue - the bit string representing signature of the TBSCertList.

The key used for signing the CLP files should have a special Key Usage value and/or an Extended Key Usage value.

Each entry in list contains the following fields: The issuer of the certificate with limited trust. The serial of the certificate with limited trust. The fingerprint of the certificate with limited trust (optional). and a subset of the following limitations: issuedNotAfter - do not trust the certs issued after the specified date trustNotAfter - do not trust the certs after the specified date validityPeriod, days - take minimal value from "native" validity period and specified in the limitation file ignoredX509Extensions - list of X.509 extensions of limited certificate that MUST be ignored for the specified certificate (e.g. EV-indicating extensions) requiredX509extensions - list of X.509 extensions that MUST be present in the certificate to be trusted. requiredNativeChecking - list of the CA-provided checks that MUST be applied applicationNameConstraints - list of domains allowed to be issued by this certificate excludedIssueIntermediatory - disallow issuing of the Intermediatory certificates The limitations are identified by OIDs

When this limitation is present, any certificate matching the entry and issued after the specified date MUST NOT be trusted The issuedNotAfter limitation is identified by OID TBA.

When this limitation is present, any certificate matching the entry MUST NOT be trusted after the specified date. The trustNotAfter limitation is identified by OID TBA.

When this limitation is present, no certificate matching the entry should be treated as valid after specified period from its validFrom. The validityPeriod is measured in days. The validityPeriod limitation is identified by OID TBA.

When this limitation is present, the extensions listed in this element should be ignored for the matching certificate. The ignoredX509Extensions limitation is identified by OID TBA.

When this limitation is present, the extensions listed in this element should be present for the matching certificate. The requiredX509extensions limitation is identified by OID TBA.

When this limitation is present, it specifies that the certificates issued by this CA SHOULD be checked against CRL and/or OCSP, depending on contents of the extension. The requiredNativeChecking limitation is identified by OID TBA.

This limitation are applied like Name Constraints limitation specified in RFC 5280. This section implies 2 variants of checks: The list of names that are allowed for the CA to issue certificates for The list of names that are forbidden for the CA to issue certificates for The applicationNameConstraints limitation is specified according to RFC 5280, 4.2.1.10 and reuses OID specified in RFC 5280.

When this limitation is present, the intermediate certificates issued by this CA MUST NOT be trusted. The excludedIssueIntermediatory limitation is identified by OID TBA.

The verification of CLP SHOULD be performed by the application. The application should check whether the provided CLP matches the internal requirements and is correclty signed by the specified key.

In case of using CLP the checks enforced by CLP should be applied after the other checks. The limitation provided by CLP MUST NOT extend the trustworthy of the checked certificate. The limitations are applied after cryptographic validation of the certificate and during building its chain of trust. If the certificate or any of its ascendants in the chain of trust matches any record in the CLP, the limitations are applied from the ascendant to descendants. The issuedNotAfter and trustNotAfter limitations are applied to find out the actual validity periods for the any certificate in the chain of trust. If the CLP prescribes to have a particular extension(s) and the certificate does not have it, the certificate MUST NOT be trusted. Application MAY use more than one CLPs (e.g. app-wide, set of system-wide, user-defined). When multiple CLPs are in use, the limitations are applied simultaneously. In case when more than one chain of trust are valid for a certificate, if any of this chains is valid after applying the limitations, the certificate MUST be treated as valid.

TBD

Public CLPs can be created and distributed by such parties as OS vendors, browser vendors and other parties treated as worth trusting. Usage of CLPs is reasonable for applications establishing TLS connections with unpredictable sets of peers. The main examples of such applications are web-browsers and MTAs.

To avoid uncontrolled growth of CLPs, the limitations are applied to root and intermediate CA certificates.

In case when an application uses CLP, it is recommended to specify the minimal date of issuing of the CLP document somewhere in code. It allows to avoid an attack of CLP rollback when the stale version of CLP is used. It is recommended to distribute CLPs using the channels that are used for distribution of the applications themselves to avoid possible DoS consequences. If application checks for fresh CLPs, it SHOULD check that nextUpdate field in a fresh one is newer than in the current one. The application MAY accept a CLP with nextUpdate in past. If an application is failing to get updates, then it can continue to run with what it has.

In case of trusted environment signing CLP can be reluctant. If CLP is delivered via application bundle, it can be verified together with other application data. But it makes sense to separate trust to the source of the content from trust to the content itself. On the other hand it is not a problem to create a local CLP signed by a locally created key.

TBD

Special thanks to Rich Salz, Igor Ustinov, Vasily Dolmatov, Stanislav Smyishlyaev, Patrik Fältström, Alexander Venedioukhin, Artem Chuprina, Viktor Dukhovni.

The current version of the document is available on GitHub https://github.com/beldmit/clp