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General Internet Engineering Task Force Internet-Draft DNS DNSSEC Keytag Collision Free DNSSEC employs a Key Tag field in the RRSIG and DS resource records in order to efficiently identify the key that produced a DNSSEC signature and the key that should be used as a secure entry point into a delegated zone. The Key Tag was not intended to be a unique identifier. Key tag collisions can occur in practice for keys in the same zone, though they are relatively rare in normal operation. Colliding key tags impose additional work on a validating resolver because it then has to check signatures for each of the candidate set of keys identified by the Key Tag. Furthermore, they open up resolvers to computational denial of service attacks by adversaries deploying specially crafted zones with many intentionally colliding key tags. This document specifies updates to the DNSSEC protocol and the process of key generation to avoid colliding key tags. Discussion Venues Source for this draft and an issue tracker can be found at .

Introduction DNSSEC employs a Key Tag field in the RRSIG and DS resource records in order to efficiently identify the key that produced a DNSSEC signature the key that should be used as secure entry point into a delegated zone. The Key Tag was not intended to be a unique identifier. Key tag collisions can occur in practice for keys in the same zone, though they are relatively rare in normal operation. Colliding key tags impose additional work on a validating resolver because it then has to check signatures for each of the candidate set of keys identified by the Key Tag. Furthermore, they open up resolvers to computational denial of service attacks by adversaries deploying specially crafted zones with many intentionally colliding key tags . This document specifies updates to the DNSSEC protocol and the process of key generation to avoid colliding key tags.

Protocol Behavior

• New DNSKEY algorithms MUST have DNSKEY RRsets that do not have colliding key tags
• What about existing algorithms? Should we have new aliases for existing algorithms that allow us to incorporate the non collision requirement?
• Can we propose a future flag date after which existing algorithms will be required to enforce this requirement?
• Outline the general process by which key generation software should ensure uniqueness of keytags.
• Special considerations for multi-signer configurations, where multiple distinct parties generate their own keys for the same zone (i) partition the keytag space between each signer/provider, and have each provider re-generate keys if necessary until they obtain one whose keytag is contained in their partition (ii) Use a central key broker to enforce keytag uniqueness, (iii) each signer when generating new keys, queries all DNSKEYs in the multi-signer group to avoid colliding keys. To avoid race conditions, ideally the providers should not generate keys at the same time, and plausibly the zone owner could enforce non-conflicting key generation schedules across the multi-signer group.
• Describe what to do when a validator encounters a zone with both old and new DNSKEY algorithm numbers.
• For possible discussion: recommend the use DNS cookies to avoid offpath computational DoS attacks.

Updates to RFCs TBD

Security Considerations Lorem ipsum.

IANA Considerations Lorem ipsum.

References Normative References Informative References