dnsop W. Hardaker
Internet-Draft USC/ISI
Updates: 7583 (if approved) W. Kumari
Intended status: Standards Track Google
Expires: March 17, 2018 September 13, 2017
Security Considerations for RFC5011 Publishers
draft-ietf-dnsop-rfc5011-security-considerations-04
Abstract
This document extends the RFC5011 rollover strategy with timing
advice that must be followed in order to maintain security.
Specifically, this document describes the math behind the minimum
time-length that a DNS zone publisher must wait before signing
exclusively with recently added DNSKEYs. It contains much math and
complicated equations, but the summary is that the key rollover /
revocation time is much longer than intuition would suggest. If you
are not both publishing a DNSSEC trust anchor, and using RFC5011 to
update that trust anchor, you probably don't need to read this
document.
This document also describes the minimum time-length that a DNS zone
publisher must wait after publishing a revoked DNSKEY before assuming
that all active RFC5011 resolvers should have seen the revocation-
marked key and removed it from their list of trust anchors.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 17, 2018.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Document History and Motivation . . . . . . . . . . . . . 3
1.2. Safely Rolling the Root Zone's KSK in 2017/2018 . . . . . 3
1.3. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Timing Associated with RFC5011 Processing . . . . . . . . . . 4
4.1. Timing Associated with Publication . . . . . . . . . . . 5
4.2. Timing Associated with Revocation . . . . . . . . . . . . 5
5. Denial of Service Attack Considerations . . . . . . . . . . . 5
5.1. Enumerated Attack Example . . . . . . . . . . . . . . . . 6
5.1.1. Attack Timing Breakdown . . . . . . . . . . . . . . . 6
6. Minimum RFC5011 Timing Requirements . . . . . . . . . . . . . 8
6.1. Timing Requirements For Adding a New KSK . . . . . . . . 8
6.1.1. addHoldDownTime . . . . . . . . . . . . . . . . . . . 8
6.1.2. sigExpirationTime . . . . . . . . . . . . . . . . . . 8
6.1.3. activeRefresh . . . . . . . . . . . . . . . . . . . . 8
6.1.4. activeRefreshOffset . . . . . . . . . . . . . . . . . 9
6.1.5. safetyMargin . . . . . . . . . . . . . . . . . . . . 9
6.1.6. Fully expanded equation . . . . . . . . . . . . . . . 9
6.1.7. Timing Constraint Summary . . . . . . . . . . . . . . 10
6.1.8. Additional Considerations . . . . . . . . . . . . . . 10
6.2. Timing Requirements For Revoking an Old KSK . . . . . . . 11
6.2.1. Example Results . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Operational Considerations . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
11. Normative References . . . . . . . . . . . . . . . . . . . . 13
Appendix A. Real World Example: The 2017 Root KSK Key Roll . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
[RFC5011] defines a mechanism by which DNSSEC validators can update
their list of trust anchors when they've seen a new key published in
a zone. However, RFC5011 [intentionally] provides no guidance to the
publishers of DNSKEYs about how long they must wait before switching
to exclusively using recently published keys for signing records, or
how long they must wait before ceasing publication of a revoked key.
Because of this lack of guidance, zone publishers may derive
incorrect assumptions about safe usage of the RFC5011 DNSKEY
advertising, rolling and revocation process. This document describes
the minimum security requirements from a publisher's point of view
and is intended to complement the guidance offered in RFC5011 (which
is written to provide timing guidance solely to a Validating
Resolver's point of view).
1.1. Document History and Motivation
To verify this lack of understanding is wide-spread, the authors
reached out to 5 DNSSEC experts to ask them how long they thought
they must wait before signing a zone exclusively with a new KSK
[RFC4033] that was being introduced according to the 5011 process.
All 5 experts answered with an insecure value, and we determined that
this lack of operational guidance is causing security concerns today
and wrote this companion document to RFC5011. We hope that this
document will rectify this understanding and provide better guidance
to zone publishers that wish to make use of the RFC5011 rollover
process.
1.2. Safely Rolling the Root Zone's KSK in 2017/2018
One important note about ICANN's [currently upcoming] 2017/2018 KSK
rollover plan for the root zone: the timing values chosen for rolling
the KSK in the root zone appear completely safe, and are not affected
by the timing concerns introduced by this draft
1.3. Requirements notation
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].
2. Background
The RFC5011 process describes a process by which a RFC5011 Validating
Resolver may accept a newly published KSK as a trust anchor for
validating future DNSSEC signed records. It also describes the
process for publicly revoking a published KSK. This document
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augments that information with additional constraints, from the
DNSKEY publication and revocation's points of view. Note that this
document does not define any other operational guidance or
recommendations about the RFC5011 process and restricts itself to
solely the security and operational ramifications of switching to
exclusively using recently added keys or removing a revoked keys too
soon.
Failure of a DNSKEY publisher to follow the minimum recommendations
associated with this draft will result in potential denial-of-service
attack opportunities against validating resolvers. Failure of a
DNSKEY publisher to publish a revoked key for a long enough period of
time may result in RFC5011 Validating Resolvers leaving that key in
their trust anchor storage beyond the key's expected lifetime.
3. Terminology
Trust Anchor Publisher The entity responsible for publishing a
DNSKEY that can be used as a trust anchor.
Zone Signer The owner of a zone intending to publish a new Key-
Signing-Key (KSK) that will become a trust anchor by validators
following the RFC5011 process.
RFC5011 Validating Resolver A DNSSEC Validating Resolver that is
using the RFC5011 processes to track and update trust anchors.
Sometimes referred to as a "RFC5011 Resolver"
Attacker An entity intent on foiling the RFC5011 Validator's ability
to successfully adopt the Zone Signer's new DNSKEY as a new trust
anchor or to prevent the RFC5011 Validator from removing an old
DNSKEY from its list of trust anchors.
sigExpirationTime The amount of time remaining before a RRSIG's
Signature Expiration time is reached. This will fundamentally be
the RRSIG's Signature Expiration time minus the RRSIG's Signature
Inception time when the signature is created.
Also see Section 2 of [RFC4033] and [RFC7719] for additional
terminology.
4. Timing Associated with RFC5011 Processing
These sections define a high-level overview of [RFC5011] processing.
These steps are not sufficient for proper RFC5011 implementation, but
provide enough background for the reader to follow the discussion in
this document. Readers need to fully understand [RFC5011] as well to
fully comprehend the importance of this document.
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4.1. Timing Associated with Publication
RFC5011's process of safely publishing a new key and then making use
of that key falls into a number of high-level steps to be performed
by the Trust Anchor Publisher. This document discusses the following
scenario, which is one of many possible combinations of operations
defined in Section 6 of RFC5011:
1. Publish a new DNSKEY in the zone, but continue to sign the zone
with the old one.
2. Wait a period of time.
3. Begin to exclusively use recently published DNSKEYs to sign the
appropriate resource records.
This document discusses step 2 of the above process. Some
interpretations of RFC5011 have erroneously determined that the wait
time is equal to RFC5011's "hold down time". Section 5 describes an
attack based on this (common) erroneous belief, which can result in a
denial of service attack against the zone.
4.2. Timing Associated with Revocation
RFC5011's process of advertising that an old key is to be revoked
from RFC5011 validating resolvers falls into a number of high-level
steps:
1. Set the revoke bit on the DNSKEY to be revoked.
2. Sign the revoked DNSKEY with itself.
3. Wait a period of time.
4. Remove the revoked key from the zone.
This document discusses step 3 of the above process. Some
interpretations of RFC5011 have erroneously determined that the wait
time is equal to RFC5011's "hold down time". This document describes
an attack based on this (common) erroneous belief, which results in a
revoked DNSKEY potentially remaining as a trust anchor in a RFC5011
validating resolver long past its expected usage.
5. Denial of Service Attack Considerations
If an attacker is able to provide a RFC5011 Validating Resolver with
past responses, such as when it is in-path or able to perform any
number of cache poisoning attacks, the attacker may be able to leave
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compliant RFC5011-Validating Resolvers without an appropriate DNSKEY
trust anchor. This scenario will remain until an administrator
manually fixes the situation.
The time-line below illustrates this situation.
5.1. Enumerated Attack Example
The following example settings are used in the example scenario
within this section:
TTL (all records) 1 day
sigExpirationTime 10 days
Zone resigned every 1 day
Given these settings, the sequence of events in Section 5.1.1 depicts
how a Trust Anchor Publisher that waits for only the RFC5011 hold
time timer length of 30 days subjects its users to a potential Denial
of Service attack. The timing schedule listed below is based on a
Trust Anchor Publisher publishing a new Key Signing Key (KSK), with
the intent that it will later become a trust anchor. We label this
publication time as "T+0". All numbers in this sequence refer to
days before and after this initial publication event. Thus, T-1 is
the day before the introduction of the new key, and T+15 is the 15th
day after the key was introduced into the fictitious zone being
discussed.
In this dialog, we consider two keys within the example zone:
K_old An older KSK and Trust Anchor being replaced.
K_new A new KSK being transitioned into active use and expected to
become a Trust Anchor via the RFC5011 process.
5.1.1. Attack Timing Breakdown
The steps shows an attack that foils the adoption of a new DNSKEY by
a 5011 Validating Resolver when the Trust Anchor Publisher that
starts signing and publishing with the new DNSKEY too quickly.
T-1 The K_old based RRSIGs are being published by the Zone Signer.
[It may also be signing ZSKs as well, but they are not relevant to
this event so we will not talk further about them; we are only
considering the RRSIGs that cover the DNSKEYs in this document.]
The Attacker queries for, retrieves and caches this DNSKEY set and
corresponding RRSIG signatures.
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T+0 The Zone Signer adds K_new to their zone and signs the zone's
key set with K_old. The RFC5011 Validator (later to be under
attack) retrieves this new key set and corresponding RRSIGs and
notices the publication of K_new. The RFC5011 Validator starts
the (30-day) hold-down timer for K_new. [Note that in a more
real-world scenario there will likely be a further delay between
the point where the Zone Signer publishes a new RRSIG and the
RFC5011 Validator notices its publication; though not shown in
this example, this delay is accounted for in the final solution
below]
T+5 The RFC5011 Validator queries for the zone's keyset per the
RFC5011 Active Refresh schedule, discussed in Section 2.3 of
RFC5011. Instead of receiving the intended published keyset, the
Attacker successfully replays the keyset and associated signatures
recorded at T-1. Because the signature lifetime is 10 days (in
this example), the replayed signature and keyset is accepted as
valid (being only 6 days old, which is less than
sigExpirationTime) and the RFC5011 Validator cancels the hold-down
timer for K_new, per the RFC5011 algorithm.
T+10 The RFC5011 Validator queries for the zone's keyset and
discovers a signed keyset that includes K_new (again), and is
signed by K_old. Note: the attacker is unable to replay the
records cached at T-1, because they have now expired. Thus at
T+10, the RFC5011 Validator starts (anew) the hold-timer for
K_new.
T+11 through T+29 The RFC5011 Validator continues checking the
zone's key set at the prescribed regular intervals. During this
period, the attacker can no longer replay traffic to their
benefit.
T+30 The Zone Signer knows that this is the first time at which some
validators might accept K_new as a new trust anchor, since the
hold-down timer of a RFC5011 Validator not under attack that had
queried and retrieved K_new at T+0 would now have reached 30 days.
However, the hold-down timer of our attacked RFC5011 Validator is
only at 20 days.
T+35 The Zone Signer (mistakenly) believes that all validators
following the Active Refresh schedule (Section 2.3 of RFC5011)
should have accepted K_new as a the new trust anchor (since the
hold down time (30 days) + the query interval [which is just 1/2
the signature validity period in this example] would have passed).
However, the hold-down timer of our attacked RFC5011 Validator is
only at 25 days (T+35 minus T+10); thus the RFC5011 won't consider
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it a valid trust anchor addition yet, as the required 30 days have
not yet elapsed.
T+36 The Zone Signer, believing K_new is safe to use, switches their
active signing KSK to K_new and publishes a new RRSIG, signed with
K_new, covering the DNSKEY set. Non-attacked RFC5011 validators,
with a hold-down timer of at least 30 days, would have accepted
K_new into their set of trusted keys. But, because our attacked
RFC5011 Validator now has a hold-down timer for K_new of only 26
days, it failed to accept K_new as a trust anchor. Since K_old is
no longer being used to sign the zone's DNSKEYs, all the DNSKEY
records from the zone will be treated as invalid. Subsequently,
all of the records in the DNS tree below the zone's apex will be
deemed invalid by DNSSEC.
6. Minimum RFC5011 Timing Requirements
6.1. Timing Requirements For Adding a New KSK
Given the attack description in Section 5, the correct minimum length
of time required for the Zone Signer to wait after publishing K_new
but before exclusively using it and newer keys is:
addWaitTime = addHoldDownTime
+ sigExpirationTime
+ activeRefresh
+ activeRefreshOffset
+ safetyMargin
6.1.1. addHoldDownTime
The addHoldDownTime is defined in Section 2.4.1 of [RFC5011] as:
The add hold-down time is 30 days or the expiration time of the
original TTL of the first trust point DNSKEY RRSet that contained
the new key, whichever is greater. This ensures that at least
two validated DNSKEY RRSets that contain the new key MUST be seen
by the resolver prior to the key's acceptance.
6.1.2. sigExpirationTime
sigExpirationTime is defined in Section 3.
6.1.3. activeRefresh
activeRefresh time is defined by RFC5011 by
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A resolver that has been configured for an automatic update
of keys from a particular trust point MUST query that trust
point (e.g., do a lookup for the DNSKEY RRSet and related
RRSIG records) no less often than the lesser of 15 days, half
the original TTL for the DNSKEY RRSet, or half the RRSIG
expiration interval and no more often than once per hour.
This translates to:
activeRefresh = MAX(1 hour,
MIN(sigExpirationTime / 2,
MAX(TTL of K_old DNSKEY RRSet) / 2,
15 days)
)
6.1.4. activeRefreshOffset
The activeRefreshOffset term must be added for situations where the
activeRefresh value is not a factor of "30 days". Specifically,
activeRefreshOffset will be "(30 days) % activeRefresh", where % is
the mathematical mod operator (which calculates the remainder in a
division problem). This will frequently be zero, but could be nearly
as large as activeRefresh itself. For simplicity, setting the
activeRefreshOffset to the activeRefresh value itself is safe.
6.1.5. safetyMargin
The safetyMargin is an extra period of time to account for caching,
network delays, etc. A suggested operational value for this is 2 *
MAX(TTL of all records)
RFC5011 also discusses a retryTime value for failed queries. Our
equation cannot take into account undeterministic failure situations,
so it might be wise to extend the safetyMargin by some factor of
retryTime, which is defined in RFC5011 as:
retryTime = MAX (1 hour,
MIN (1 day,
.1 * TTL of K_old DNSKEY RRset,
.1 * sigExpirationTime))
6.1.6. Fully expanded equation
The full expanded equation, with activeRefreshOffset set to
activeRefresh for simplicity, is:
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addWaitTime = addHoldDownTime
+ sigExpirationTime
+ 2 * MAX(1 hour,
MIN(sigExpirationTime / 2,
MAX(TTL of K_old DNSKEY RRSet) / 2,
15 days)
)
+ 2 * MAX(TTL of all records)
6.1.7. Timing Constraint Summary
The important timing constraint introduced by this memo relates to
the last point at which a validating resolver may have received a
replayed original DNSKEY set, containing K_old and not K_new. The
next query of the RFC5011 validator at which K_new will be seen
without the potential for a replay attack will occur after the
publication time plus sigExpirationTime. Thus, the latest time that
a RFC5011 Validator may begin their hold down timer is an "Active
Refresh" period after the last point that an attacker can replay the
K_old DNSKEY set. The worst case scenario of this attack is if the
attacker can replay K_old seconds before the (DNSKEY RRSIG Signature
Validity) field of the last K_old only RRSIG.
6.1.8. Additional Considerations
Note: our notion of addWaitTime is called "Itrp" in Section 3.3.4.1
of [RFC7583]. The equation for Itrp in RFC7583 is insecure as it
does not include the sigExpirationTime listed above. The Itrp
equation in RFC7583 also does not include the 2*TTL safety margin,
though that is an operational consideration and not necessarily as
critical.
6.1.8.1. Example Results
For the parameters listed in Section 5.1, the activeRefreshOffset is
0, since 30 days is evenly divisible by activeRefresh (1/2 day), and
our resulting addWaitTime is:
addWaitTime = 30
+ 10
+ 1 / 2
+ 2 * (1) (days)
addWaitTime = 42.5 (days)
This addWaitTime of 42.5 days is 12.5 days longer than just the hold
down timer.
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6.2. Timing Requirements For Revoking an Old KSK
It is important to note that this issue affects not just the
publication of new DNSKEYs intended to be used as trust anchors, but
also the length of time required to continuously publish a DNSKEY
with the revoke bit set. Both of these publication timing
requirements are affected by the attacks described in this document,
but with revocation the key is revoked immediately and the
addHoldDown timer does not apply. Thus the minimum amount of time
that a Trust Anchor Publisher must wait before removing a revoked key
from publication is:
remWaitTime = sigExpirationTime
+ MAX(1 hour,
MIN((sigExpirationTime) / 2,
MAX(TTL of K_old DNSKEY RRSet) / 2,
15 days),
1 hour)
+ 2 * MAX(TTL of all records)
Note that the activeRefreshOffset time does not apply to this
equation.
Note that our notion of remWaitTime is called "Irev" in
Section 3.3.4.2 of [RFC7583]. The equation for Irev in RFC7583 is
insecure as it does not include the sigExpirationTime listed above.
The Irev equation in RFC7583 also does not include the 2*TTL safety
margin, though that is an operational consideration and not
necessarily as critical.
Note also that adding retryTime intervals to the remWaitTime may be
wise, just as it was for addWaitTime in Section 6.
6.2.1. Example Results
For the parameters listed in Section 5.1, our example:
remwaitTime = 10
+ 1 / 2
+ 2 * (1) (days)
remwaitTime = 12.5 (days)
Note that for the values in this example produce a length shorter
than the recommended 30 days in RFC5011's section 6.6, step 3. Other
values of sigExpirationTime and the original TTL of the K_old DNSKEY
RRSet, however, can produce values longer than 30 days.
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Note that because revocation happens immediately, an attacker has a
much harder job tricking a RFC5011 Validator into leaving a trust
anchor in place, as the attacker must successfully replay the old
data for every query a RFC5011 Validator sends, not just one.
7. IANA Considerations
This document contains no IANA considerations.
8. Operational Considerations
A companion document to RFC5011 was expected to be published that
describes the best operational practice considerations from the
perspective of a zone publisher and Trust Anchor Publisher. However,
this companion document has yet to be published. The authors of this
document hope that it will at some point in the future, as RFC5011
timing can be tricky as we have shown, and a BCP is clearly
warranted. This document is intended only to fill a single
operational void which, when left misunderstood, can result in
serious security ramifications. This document does not attempt to
document any other missing operational guidance for zone publishers.
9. Security Considerations
This document, is solely about the security considerations with
respect to the Trust Anchor Publisher of RFC5011 trust anchors /
DNSKEYs. Thus the entire document is a discussion of Security
Considerations when adding or removing DNSKEYs from trust anchor
storage using the RFC5011 process.
For simplicity, this document assumes that the Trust Anchor Publisher
will use a consistent RRSIG validity period. Trust Anchor Publishers
that vary the length of RRSIG validity periods will need to adjust
the sigExpirationTime value accordingly so that the equations in
Section 6 and Section 6.2 use a value that coincides with the last
time a replay of older RRSIGs will no longer succeed.
10. Acknowledgements
The authors would like to especially thank to Michael StJohns for his
help and advice and the care and thought he put into RFC5011 itself.
We would also like to thank Bob Harold, Shane Kerr, Matthijs Mekking,
Duane Wessels, Petr Petr Spacek, Ed Lewis, and the dnsop working
group who have assisted with this document.
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11. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
September 2007, <http://www.rfc-editor.org/info/rfc5011>.
[RFC7583] Morris, S., Ihren, J., Dickinson, J., and W. Mekking,
"DNSSEC Key Rollover Timing Considerations", RFC 7583,
DOI 10.17487/RFC7583, October 2015, <https://www.rfc-
editor.org/info/rfc7583>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <http://www.rfc-editor.org/info/rfc7719>.
Appendix A. Real World Example: The 2017 Root KSK Key Roll
In 2017, ICANN expects to (or has, depending on when you're reading
this) roll the key signing key (KSK) for the root zone. The relevant
parameters associated with the root zone at the time of this writing
is as follows:
addHoldDownTime: 30 days
Old DNSKEY sigExpirationTime: 21 days
Old DNSKEY TTL: 2 days
Thus, sticking this information into the equation in
Section Section 6 yields (in days):
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addWaitTime = 30
+ (21)
+ MAX(MIN((21) / 2,
MAX(2 / 2,
15 days)),
1 hour)
+ 2 * MAX(2)
addWaitTime = 30 + 21 + MAX(MIN(11.5, 1, 15)), 1 hour) + 4
addWaitTime = 30 + 21 + 1 + 4
addWaitTime = 56 days
Note that we use a activeRefreshOffset of 0, since 30 days is evenly
divisible by activeRefresh (1 day).
Thus, ICANN should wait a minimum of 56 days before switching to the
newly published KSK (and 26 days before removing the old revoked key
once it is published as revoked). ICANN's current plans are to wait
70 days before using the new KEY and 69 days before removing the old,
revoked key. Thus, their current rollover plans are sufficiently
secure from the attack discussed in this memo.
Authors' Addresses
Wes Hardaker
USC/ISI
P.O. Box 382
Davis, CA 95617
US
Email: ietf@hardakers.net
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
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