Network Working Group
Internet-Draft July 1, 2017
Intended status: Experimental
Expires: January 2, 2018
Certificate Limitation Policy
draft-belyavskiy-certificate-limitation-policy-02
Abstract
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.
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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 [1] 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:
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.
2. Certificate Limitations Profile
A proposed syntax and overall structure of CLP is very similar to the
one defined for CRLs [2].
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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
TBD
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:
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).
limitationPropagation. This field indicates whether limitations
are applied to the certificate itself, to all of its descendants
in the chain of trust, or both.
and a subset of the following limitations:
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maxIssued - do not trust the certs issued after the specified date
maxValidity - 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.
The limitations are identified by OIDs
2.3.1. Limitations
2.3.1.1. maxIssued
When this limitation is present, any certificate matching the entry
and issued after the specified date MUST NOT be trusted
2.3.1.2. maxValidity
When this limitation is present, any certificate matching the entry
MUST NOT be trusted after the specified date.
2.3.1.3. validityPeriod
When this limitation is present, no certificate matching the entry
should be treated as valid after specified period from its validFrom.
2.3.1.4. ignoredX509Extensions
When this limitation is present, the extensions listed in this
element should be ignored for the matching certificate.
2.3.1.5. 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.
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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
TBD
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 Faeltstroem.
9. References
The current version of the document is available on GitHub
https://github.com/beldmit/clp
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10. References
10.1. URIs
[1] https://groups.google.com/a/chromium.org/forum/#!msg/blink-
dev/eUAKwjihhBs/rpxMXjZHCQAJ
[2] https://tools.ietf.org/html/rfc5280#section-5
Author's Address
Dmitry Belyavskiy
Email: beldmit@gmail.com
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