| SAAG | Y. Nir |
| Internet-Draft | Dell EMC |
| Intended status: Informational | T. Fossati |
| Expires: April 18, 2018 | Nokia |
| Y. Sheffer | |
| Intuit | |
| October 15, 2017 |
Considerations For Using Short Term Certificates
draft-nir-saag-star-00
Recently there has been renewed interest in an old idea: Issue certificates with short validity periods and forego revocation processing, reasoning that expiration is a sufficient replacement for revocation as long as that expiration is not too far off.
This document covers considerations, both security and operational, for using such Short Term Auto Renewed (STAR) certificates for various scenarios where Using a revocation protocol is considered inappropriate.
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This Internet-Draft will expire on April 18, 2018.
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Certificates ([RFC5280]) are used in multiple protocols such as the Internet Key Exchange (IKEv2-[RFC7296]) and the Transport Layer Security protocol (TLS-[RFC5246]). Certificates are used to authenticate communicating parties to each other. Certificates are issued by Certificate Authorities (CAs) to End Entities (EE) to be used to authenticate them to Relying Parties (RPs) in security protocols. Systems that use secure communications typically include certificate authorities, end entities and relying parties, with some nodes in the network having more than one of these roles.
When deploying a system involving secure communications, one of the challenges is how to deal with an End Entity losing control of its private key or having its secrecy potentially compromised. The standardized ways of dealing with this is adding a protocol layer for revocation such as CRLs ([RFC5280]) or OCSP ([RFC6960]).
Such revocation protocols have drawbacks. Although caching of CRLs and OCSP responses is allowed, each setup of a secure channel may require accessing the CRL distribution point (DP) or the OCSP responder. This is both time consuming and provides the system with a few more modes of failure. Assuring reliability of the revocation service increases the cost, and overcoming the latency issue requires changes to the security protocols.
For these reasons it is attractive to forego revocation checking. Some deployed systems do this by either eliminating the CRL DP and OCSP extensions from the certificates, or ignoring network and timeout errors in fetching revocation information. Both practices reduce the efficacy of revocation.
An alternative solution to the revocation problem is to issue certificates with a short validity period. Normally certificates are issued with a validity period of between a few months and a few years. With a shorter validity period if the private key is compromised the potential for abuse is lower because the certificate and its private key expire within a short period of time - a few hours to a few days.
The rest of this document describes operational and security considerations with using short term certificates.
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 [RFC2119].
Throughout this document we will use the term DP to denote a server for revocation information, either a CRL distribution point or an OCSP Responder. For our purposes they are the same.
We use the term longevity for the period of time between certificate issuance and the time of its expiration as indicated in the notAfter field of the certificate. Note that issuance time may be different from the notBefore field in the certificate.
The text describes end entities as renewing their certificates because the usual operational model for certificates is one of "pull": end entities create certificate requests and send them to CAs for signature. Some systems are designed around a "push" operation where either the CA or a management function generates a new certificate and installs it on the end entity. The text in the document uses pull terminology, but is equally relevant for push design.
Short term certificates are like any other [RFC5280] certificates except that the period of time between their issuance and their notAfter date is relatively short. Whereas normally certificates are issued for a period of time between a few months and a few years, short term certificates usually expire after a few hours, a few days, or at a limit a couple of weeks.
While it is not a part of the definition, short term certificates typically have neither a CRL DP extension nor an OCSP authorityInformationAccess extension. In other words such certificates cannot be revoked. Instead, they are valid until they expire.
Automatic certificate renewal is getting ever more popular with enrollment protocols such as EST ([RFC7030]) or ACME ([I-D.ietf-acme-acme]). For short term certificates automatic renewal is essential as a human cannot be expected to flawlessly perform a manual renewal every few days or hours. This document does not recommend any particular automatic renewal method, but does recommend (Section 4.4) that some such method be used. Automatic renewal processing can roll over the keys from one certificate to its successor, or it can generate new keys with each Certificate generation. As revocation may not exist, multiple certificates for the same EE may be valid at any given time.
The solution for revocation in this scheme is to stop the automatic renewal. The existing compromised certificate will remain valid until it expires. See the considerations in Section 5.1 about revocation.
[Topalovic] describes the design of a system involving STAR certificates for the web, and analyzes its security and efficacy. It concludes that STAR certificates can be as secure as certificates with OCSP revocation.
Relying parties can also avoid the need for contacting the DP at connection setup by having the End Entities implement OCSP stapling. This feature has the EEs rather than the RPs retrieve the OCSP response and send it as part of the protocol. OCSP stapling is described for TLS in [RFC6961] and [RFC6066], and for IKE in [RFC4806].
STAR has two advantages over OCSP stapling:
This section lists some use cases where STAR certificates seem to be more appropriate than long-lived certificates with revocation checking. The purpose of this section is only motivational. None of the following sections are intended to be a definition of the use case or the standard by which future documents or implementations will be measured for sufficiency.
TBA
This is a system installed in multiple hosts in one or more data centers that fulfills some task and requires mutual authentication of its components. An example of such a system is a Storage Area Network (SAN).
This example of a distributed system is multiple network security functions (NSF) [RFC8192] where the SDN controller needs to authenticate the NSFs with which it communicates, and some NSFs need to communicate with each other.
TBA
The motivations for using short-term certificates are operational. We don't want the latency introduced by fetching the CRL from the DP; we don't want the cost of making the DP 99.999% reliable, and we don't want the cost of making the network paths from all RPs to the DP always available.
Deploying short term certificates comes with its own set of operational considerations, and some of these are enumerated in the following sub-sections.
Since we do not assume the CA to be close to 100% available it makes sense for End Entities to renew their certificates well in advance. While the security considerations in Section 5.2 set an upper limit on the longevity of a STAR certificate, operational necessity sets the frequency of renewal. It is necessary to strike a balance between renewing too often which leads to increased load on the CA and renewing too seldom which increases the risk of having the certificate expire while either the CA or the End Entity are down.
Individual system properties play a significant role here. Systems where both the CA and the EEs are expected to be up all of the time absent a fault may choose to renew a day or even an hour before expiration, while systems with nodes that are only up infrequently and for short periods of time may choose to renew the certificates whenever the EEs happen to be up.
As a general rule of thumb for systems where the CA is mostly available it makes sense for the EE to make the first attempt to renew its certificate about half-way through its lifetime. If that attempt fails because the CA is not available an EE SHOULD retry at regular intervals until it succeeds. Shortly before expiration, the EE SHOULD increase the frequency of retires.
For example, suppose a STAR certificate is issued for 8 days. The EE will first attempt to renew the certificate 4 days before expiration. If that fails it will retry every three hours until only six hours are left before expiration. At that point it will increase the frequency and retry every five minutes. If this is part of the system design, at this point it should also alert the user that something is wrong.
While the STAR design does not require 99.999% availability, the CA does need to be available for renewing certificates. Downtimes of more than a quarter of the certificate longevity SHOULD NOT happen. For most modern hardware this is entirely possible even without exotic clustering solutions, but when configuring the system administrators should consider that the longevity of the certificates constrains the required availability of the CA.
When setting the longevity for certificates administrators SHOULD consider how long it takes to recover from a failure of the CA. That length of time can be seconds with a good clustering solution, but can span hours or days without one, especially if the fault happens at a bad time. A failure of a CA should be considered a conceivable occurrence, and longevity should be set so that such a failure does not lead to expiration and outage.
Conversely, if short longevity is required by security targets, the CA should be made more reliable with clustering solutions.
Despite NTP ([RFC5905]) being over thirty years old and implemented in every major operating system clock skew is a fact of life and many deployed systems don't have the right time. It is also not possible to just mandate the use of NTP because the systems that use STAR certificates are often installed on hosts and networks where NTP is either not configured or blocked. We cannot assume that these systems can enable NTP at will.
Skewed clocks have always been a problem for certificates. Because STAR certificates are always just a few days or hours from expiration they are more sensitive to clock skew. A sufficiently skewed clock can cause three different disfunctions and for STAR certificate such disfunction happens with considerably less skew than with long term certificates:
There are several common modes of clock skew:
Because of the prevalence of systems with a relatively small skew it is RECOMMENDED to set the notBefore field to a time 72 hours before the actual issuance date.
End Entities MUST NOT use expired certificates and Relying Parties SHOULD alert whenever an expired certificate is presented. This will help the users keep their host clocks set or encourage them to enable NTP.
Automatic enrollment and renewal is recommended for any system using certificates. While it is possible to renew certificates manually on time, even organizations with the best of IT departments occasionally miss this: [cert-expires]
With short term certificates, this becomes even more important. Renewing a certificate manually every few days or hours is extremely labor intensive, especially when the system contains hundreds, thousands or more end entities, and the risk of outages becomes a certainty.
This document does not mandate any particular enrollment or renewal mechanism. Any of a myriad of standard and proprietary methods can be used and systems with proprietary methods have been shipping for years. The IETF is in the process of standardizing the ACME protocol for enrollment and renewal ([I-D.ietf-acme-acme]) and an extension is proposed to make it more suitable for STAR certificates ([I-D.ietf-acme-star]).
Certificate Transparency (CT), [RFC6962] is about keeping a log of all issued certificates.
A system that issues a certificate every few days to thousands or end entities will create more records for a CT log than a web host that gets one certificate every year.
TBA: Discussion about this.
STAR certificates eliminate an important security feature of PKI which is the ability to revoke certificates. Revocation allows the administrator to limit the damage done by a rogue node or an adversary who has control of the private key. With STAR certificates expiration replaces revocation so there is a timeliness issue.
It should be noted that revocation also has timeliness issues, because both CRLs and OCSP responses have nextUpdate fields that tell RPs how long they should trust this revocation data. These fields are typically set to hours, days, or even weeks in the future. Any revocation that happens before the time in nextUpdate goes unnoticed by the RP.
Section 5.1 discusses the reasons why a certificate would be revoked if revocation was available and how STAR certificates do the same. Section 5.2 discusses considerations for setting the longevity of a certificate, and Section 5.3 discusses how longevity should be adjusted to deal with clock skew.
More discussion of the security of STAR certificates is available in [Topalovic].
There are two types of compromise that require administrators to revoke a certificate:
When a node "goes rogue" or an adversary gets control of the private key it is important to block renewal or these certificates or else the attack can persist forever. No matter how short-term these short term certificates are, there is a certain window of time when the attacker can use the certificate. This can often be mitigated with application-level measures.
With most systems relying parties are configured with the names of nodes with which they are allowed to communicate. When revocation is not available changing the configuration so that the rogue node cannot connect is RECOMMENDED. This is useful even when revocation is available because timeliness issues are common to both revocation and expiration.
There is always a period of time between when a compromise is discovered and when RPs stop trusting the certificate. With revocation this has to do with the time it takes to process the revocation and the span of time between the thisUpdate and nextUpdate fields. With STAR certificates this is controlled by the time it takes to inhibit renewals and the longevity of the certificates.
For this reason it makes sense to set the longevity to a period of time similar to the span of time that we would set for the CRL or OCSP updates. Typically a few days is an appropriate time. For some cases this can be as low as a few hours. Setting the renewal time too short may cause operational problems as discussed in Section 4.3 and Section 4.2. In general longevity should not be set shorter than the availability of the CA allows.
Fortunately modern hardware is powerful enough and reliable enough that even a system with tens of thousands of end entities with longevity of 1-2 days should not suffer an outage because of expired certificates.
As discussed in Section 4.3 clock skew can lead to expired certificates being treated as valid. While even the use of NTP may leave clocks with a few seconds of inaccuracy, all installations MUST take steps to limit the clock skew on their hosts.
An upper bound for the amount of skew allowed for hosts in a particular system is one of the parameters for such a system. For systems using NTP this can be 2 seconds. For systems where the clocks are set manually, this tends to be far greater, but without an upper bound no guarantees can be made about the security of certificate use.
This upper bound is also a limit on the target certificate longevity. For example, if hosts and CAs can each have a clock skew of 24 hours then it is impossible to achieve a longevity of under 48 hours. With a reasonable skew and a reasonable target longevity we can achieve our security targets by reducing the certificate longevity by twice the upper bound for skew. So if skew is bounded by 24 hours (the bad timezone case) and target longevity is 7 days, it makes sense to set the longevity on the CA to 5 days.
A successful Denial of Service (DoS) attack against a CA prevents it from issuing certificates. With short-term certificates this could quickly lead to outages as certificates expire.
The important period of time here is the time between when the EE first attempts to renew the certificate and the time that the certificate expires. For example, if the EE attempts to renew the certificates a mere five minutes before expiration, then a five-minute CA outage can lead to an invalid certificate and failed connections.
This issue is no different from DoS attacks against the DP for certificates with revocation. The methods of protection are also similar:
There are no requests to IANA in this document.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC5280] | Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R. and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008. |