Network Working Group
Internet-Draft July 2, 2017
Intended status: Experimental
Expires: January 3, 2018

Certificate Limitation Policy


The document provides a specification of the application-level trust model. Being provided at the application level, the limitations of trust can be distributed separately using cryptographically protected format instead of hardcoding the checks into the application itself.

Status of This Memo

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This Internet-Draft will expire on January 3, 2018.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

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1. Introduction

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 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:

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.

2. Certificate Limitations Profile

A proposed syntax and overall structure of CLP is very similar to the one defined for CRLs. CertificateList ::= SEQUENCE { tbsCertList TBSCertList, signatureAlgorithm AlgorithmIdentifier, signatureValue BIT STRING } TBSCertList ::= SEQUENCE { version Version OPTIONAL, -- if present, MUST be v1 signature AlgorithmIdentifier, issuer Name, thisUpdate Time, limitedCertificates SEQUENCE OF SEQUENCE { userCertificate CertificateSerialNumber, sertificateIssuer Name, limitationDate Time, limitationPropagation Enum, fingerprint SEQUENCE { fingerprintAlgorithm AlgorithmIdentifier, fingerprintValue OCTET STRING } OPTIONAL, limitations SEQUENCE, } OPTIONAL, };

2.1. CLP fields


2.2. CLP signature

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

2.3. CLP entry fields

Each entry in list contains the following fields:

and a subset of the following limitations:

The limitations are identified by OIDs

2.3.1. Limitations maxIssued

When this limitation is present, any certificate matching the entry and issued after the specified date MUST NOT be trusted maxValidity

When this limitation is present, any certificate matching the entry MUST NOT be trusted after the specified date. validityPeriod

When this limitation is present, no certificate matching the entry should be treated as valid after specified period from its validFrom. ignoredX509Extensions

When this limitation is present, the extensions listed in this element should be ignored for the matching certificate. requiredX509extensions

When this limitation is present, the extensions listed in this element should be present for the matching certificate.

3. Verification of CLP

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.

4. Verification with CLP

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 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 maxIssued and maxValidity 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.

5. ASN.1 notation


6. Security considerations

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.

7. IANA considerations

8. Acknoledgements

Special thaks to Rich Salz, Igor Ustinov, Vasily Dolmatov, Stanislav Smyishlyaev, Patrik Fältström.

9. References

The current version of the document is available on GitHub

Author's Address

Dmitry Belyavskiy EMail:

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