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Internet INTAREA RFC Request for Comments I-D Internet-Draft DRIP PKIX The DRIP Entity Tag (DET) public Key Infrastructure (DKI) is a specific variant of classic Public Key Infrastructures (PKI) where the organization is around the DET, in place of X.520 Distinguished Names. Further, the DKI uses DRIP Endorsements in place of X.509 certificates for establishing trust within the DKI. There are two X.509 profiles for shadow PKI behind the DKI, with many of their X.509 fields mirroring content in the DRIP Endorsements. This PKI can at times be used where X.509 is expected and non-constrained communication links are available that can handle their larger size. C509 (CBOR) encoding of all X.509 certificates are also provided as an alternative for where there are gains in reduced object size.

Introduction A DRIP Entity Tag (DET, ) public Key Infrastructure (DKI) is designed as a strict hierarchy, governed by the administrator of the DET prefix and having the authority to authorize RAAs. RAAs in turn authorize HDAs within their domain. This authorization is managed via a set of DETs whose sole use is to define the DKI. The RAA Authorization DETs MUST reside in HID = RAA#|0 (Apex Authorization DET in HID = 0|0). There are three main classifications/types of DETs:

• Authorization DETs

  Used to assert the authorization of a DKI level.

  Issuing DETs

  Used to assert operations within DKI level.

  Operational DETs

  Used by operational entities within DKI level

All DETs exist in DET-Endorsements (). These DET-Endorsements provide the proof of registration and thus trust. These DETs, through chained Endorsements define the DKI as follows:

The DKI Endorsements

The Authorization DETs exist in a set of DET-Authorization-Endorsements. The lifetime of these endorsements SHOULD be no less than 1 year, recommended 5 years, and should not exceed 10 years. Endorsements SHOULD be reissued prior to expiry (may be for a new DET). DETs used to define this authorization are replaced per undetermined policy (note these DETs do very little signing, see ). This separation of DET type roles reduce the risk of private key loss for the critical Authentication DETs by making them infrequently used and only used in offline operations. It does make the chain of trust for a HDA customers' Operational DETs to be 4 Endorsements.

The DKI without an Apex Entity The hierarchical design of the DKI is the most efficient possible with the least data transmission overhead. But it requires the participation of an Entity, in the role of the Apex, trusted by all the RAAs. The logical Entity for this role is the International Civil Aviation Authority (ICAO), but the processes for ICAO to take on this role are complex. Work is ongoing with the ICAO, but timing is indeterminate and immediately implementable alternatives are needed. The DKI can work by the RAAs establishing mutual trust within a geographic region. It is envisioned that the initial RAA assignments will follow , Table 1. Without an Apex, each RAA self-endorses its Authentication DET, acting as its own apex. However, RAAs issued DETs (via their HDAs) will not exist in the air by themselves (except perhaps for some small island nations), thus a geographic regional consortium of RAAs will need to deploy some mechanism for mutual trust for their End Entities to fly together. There are three reasonable approaches for RAAs to manage their mutual trust and it is likely that all will occur:

• 1. RAA Trust lists
  2. RAA Cross-endorsements
  3. Bridge RAA with cross-endorsements to RAAs

It is recommended that the RAA Trust List be used during initial DKI testing. The cross-endorsing options will need their own testing to work out how best to deploy them.

RAA Trust lists A consortium of RAAs MAY choose to maintain a list of RAAs they trust. It is recommended that this list consist of the RAA's Authentication DET and HI. Each RAA in the consortium SHOULD maintain its own list, signed with its Authentication DET. This Trust List MAY contain each RAA's Authentication DET self-endorsement validity dates. If a trusted RAA has more than one self-endorsement (most likely to support key rollover), including these dates makes it easier to have an RAA duplicated in the list. How the RAAs communicate between themselves to maintain these lists is out of scope here. Each RAA SHOULD include validity dates in its Trust List. Frequency of Trust List updates is also out of scope here. Trust Lists is the simplest method to implement, but may not be the simplest to maintain over time. There is a natural Trust List of ALL RAAs, based on what is allocated in the DRIP DNS tree.

RAA Cross-endorsements A consortium of RAAs MAY choose to cross-endorse each's Authentication DET. This is done by one RAA endorsing for its community, an other's Authentication DET. This establishes one-way trust; thus, in practice, each RAA needs to cross-endorse each RAA's Authentication DET within the consortium. RAA Cross-endorsements definitely has a scaling (n^2) problem. It works for a starting point or for a very small group of RAAs. How these RAA Cross-endorsements are discovered has not been defined at this point. One potential is via a to-be-defined DNS HHIT RR within the endorsing RAA's zone. This information would need to be cached by any potential offline entity.

Bridge RAA with cross-endorsements to RAAs A consortium of RAAs MAY select one RAA to function as a "Bridge" between all members of the consortium. In this approach, the "Bridge RAA" does not authorize any sub-HDAs. Its sole purpose is the cross-endorse to member RAAs. The Bridge and each RAA cross endorse as in . Bridge RAA Cross-endorsementing reduces the scaling challenge to only the number of RAAs in the consortium. Plus there is little need to communicate any changes in the cross-endorsementing to the various parties within the consortium. Thus this option scales the best out of the three alternatives to DKI Apex hierarchy. How these RAA Cross-endorsements are discovered has not been defined at this point. The Bridge RAA will have to be known to all parties within the consortium. One potential, as above, is via a to-be-defined DNS HHIT RR () within the endorsing RAA's zone. This information would need to be cached by any potential offline entity.

The C509 encoding of X.509 Certificates A price in object size is paid in the ASN.1 encoding of X.509 certificates. This is often a barrier for use over constrained links and even storage demands on constrained processing platforms. The provides an alternative encoding in two different manners:

• 1. An invertible CBOR re-encoding of DER encoded X.509 certificates , which can be reversed to obtain the original DER encoded X.509 certificate.
  2. Natively signed C509 certificates, where the signature is calculated over the CBOR encoding instead of over the DER encoding as in 1. This removes the need for ASN.1 and DER parsing and the associated complexity but they are not backwards compatible with implementations requiring DER encoded X.509.

The invertible CBOR encoding is recommended for use here. This can be readily implemented through libraries that do the translation, as needed, between X.509 and c509.

Terms and Definitions

Requirements Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Definitions This document uses the terms defined in Drip Requirements and Terminology and in Drip Architecture. The following new terms are used in the document:

Authorization DETs

DETs whose use is to define a hierarchy level and endorse lower hierarchy level Authorization DETs and finally Issuing DETs at this hierarchy level. They the DETs in the Authentication Endorsements and X.509 certificates.

DKI

A DRIP Entity Tag (DET) public Key Infrastructure. Similar to an X.509 PKI, but built on the DRIP Endorsements.

International Aviation Trust Framework (IATF)

The ICAO IATF is comprised of a set of policies, requirements, and best practices that will enable resilient and secured ground-ground, air-ground, and air-air exchange of digital information, and among both traditional and newly-emerging system stakeholders.

Issuing DETs

DETs whose use is to sign Endorsements and X.509 certificates for Operational DETs that are at the same hierarchy level as the Issuing DET.

Operational DETs

DETs used by various entities in DRIP protocols and as non-routable IPv6 addresses. A partial list of such entities includes: GCS, Infrastructure (e.g. wireless tower systems), UAS Operators, Pilots-in-command, Servers, UA.

​System Wide Information Management (SWIM)

The ICAO SWIM consists of Standards, Infrastructure and Governance enabling the management of Air Navigation Systems (ANS) related information and its exchange between qualified parties via interoperable services.

The DET public Key Infrastructure (DKI)

The DKI Levels

The Apex The Apex Authorization DET is used to endorse RAA Authorization DETs and its own Apex Issuing DETs; it has no other use. This is the case for all Authorization DETs. Apex Issuing DETs are used to endorse DETs, with HID= 0|0, used by Apex services. The DET Apex may be only theoretical if no Entity steps forward to provide this role.

The RAAs Each RAA use its Authorization DET (HID = RAA#|0) to endorse its RAA Issuing DET(s) (also HID = RAA#|0) and for signing its HDA Authorization DETs (HID = RAA#|HDA#). An RAA may have multiple Issuing DETs (HID = RAA#|0), each for a different use (e.g. CRL signing, RAA server signing). It is expected that, over time, an RAA will rollover its Issuing DETs, thus at times there will be more than ONE Issuing DET per role in use. These Issuing DETs, like those at the Apex level, constitute an implicit HDA. There is no Authorization DET for this implicit HDA, but other than only signing for entities like servers needed by the RAA, it should be considered as an HDA in terms of policies. The initial RAA range assignments are defined in , Table 1. It is anticipated that DRIP usage will expand to use into General/Civil Aviation. Thus at some point a block of RAAs will be set aside much like for the CTA-2063A range.

The HDAs Each HDA use its Authorization DET to endorse its HDA Issuing DETs (e.g. RAA=267, HDA=567). An HDA Issuing DET is used to endorse Operational DETs; those used by the HDA for its services (e.g. USS) and for Devices (e.g. UA, GCS, ground infrastructure) partaking in the HDA's services. If the Operational DET is a Manufacturer DET, the "valid not after" date (vna) MUST be 99991231235959Z.

The Offline Requirement for Authentication DETs The Authentication DETs private keys MUST NEVER be on a system with any network connectivity. Also efforts MUST be taken to limit any external digital media connections to these offline systems. Compromise of an Authentication DET compromises its and all lower hierarchy levels. Such a compromise could result in a major re-signing effort with a new Authentication DET. Also, during the time of compromise, fraudulent additions to the DKI could have occurred. This means that the process whereby the Authentication DET is used to sign the Endorsement/X.509 certificate of its level's Issuing DET(s) and lower level Authentication DETs MUST be conducted in an offline manner. This offline process need not be onerous. For example, QR codes could be used to pass CSR objects to the offline Authentication DET system, and this system could produce QR codes containing the Endorsements and X.509 certificates it signed. A video conference between the parties could have one side show its QR code and the other copy and print it to move between the video conferencing system and the offline system. This is a simplification of a larger signing operation, but shows how such a signing need not require travel and expensive hand-off methodologies. It should be noted that the endorsement of Issuing DETs follow the same restriction, as it is done with the Authentication DET. It MUST be conducted in an offline manner.

DNS view of DKI The primary view of the DKI is within DNS. This is in the 3.0.0.1.0.0.2.ip6.arpa zone (Apex level of the DRIP IPv6 DET format). In the DET DNS structure, only the Apex and RAA levels MUST be DNSSEC signed. The HDA level may be too dynamic for DNSSEC signing (e.g. hundreds of new EE Operational DETs per hour); trust in the EE Operational DETs within the HDA level comes through inclusion of the HDA Endorsement of EE object. A slow-churn HDA MAY use DNSSEC. The RAA and HDA levels MUST contain their Endorsement by higher object; this provides the needed trust in the Endorsement of EE objects. The Apex level Endorsement is self-signed, thus trust in it is only possible via DNSSEC. Endorsements are expected to be stored in DNS HHIT RR () will soon provide an alternative and specifically designed RR for this purpose. Other RR within these levels will vary. There also may be HIP, TLSA, and/or URI RR. Each level continues on down the 3.0.0.1.0.0.2.ip6.arpa zone for its Authorization DET and Issuing DET(s). RR with FQDNs for services offered may also be present in various forms (e.g. a URI for the commercial FQDN for the DKI Entity). TLSA RR of DET SPKI may be directly included here. Same with HIP RR. The Authorization Endorsement SHOULD be present, as SHOULD be Issuing Endorsements.

Managing DET Revocation For Operational DETs, there is no direct concept of DET revocation. Operational DETs are either discoverable via DNS or not valid despite being in a non-expired Endorsement signed an Issuing DET. Thus if an Issuing Entity needs to "revoke" an Operational DET it removes all entries for it from DNS, so a short TTL on those records is recommended. Authorization and Issuing DETs are not so easily "revoked"; something akin to an X.509 CRL mechanism is needed. This could best be dealt with by Endorsements managed in the new DET RR that includes revocation status. Thus defines the specific RR for Endorsements that will be used here. Minimally, at least the revocation status and revocation date(s) need to be in this RR. Until this RR is available, there is no mechanism, other than removal for Authorization and Issuing DET revocations.

The Offline cache of HDA Issuing Endorsements The Offline cache of HDA Issuing Endorsements, used to verify various EE signed objects without needing DNS access, SHOULD consist of the HDA Authentication DET Endorsements of the HDA Issuing DETs. Thus the receiver has a trusted source of the HDA Issuing DET Public Key (HI) in a DRIP standard object (136 bytes). If the DKI DNS tree includes GEO location data and coverage, a receiver could query some service for a trusted cache within some radius of its location. Such as, please tell me of all HDAs within 100KM of... This cache MAY contain the full chain up to the Apex. This could be helpful in limited connectivity environments when encountering an HDA Issuing DET under a unknown Authenticated HDA or RAA. The needed trust chain could be shorter.

HDA Offline Trust cache There are situations where a list of specific HDAs for an entity to trust for some application is needed. This can best be met by maintaining a cache as above but only of the trusted HDA Issuing Endorsements. How a list of this limited trust is maintain and distributed is out of scope of this document and is left to those needing this specific feature.

The DKI's Shadow PKI The following defines the components of a DKI's shadow PKI built from X.509 certificates with content that mirrors that in the DKI Endorsements. There are two profiles provided; both may be used, or the community may select one for deployment. In both cases, the PKI tree mirrors that of the DKI levels (). At this point in defining the shadow PKIs, alternatives to a strict hierarchy is still an open work item. This work will follow the pattern set in .

Shadow Lite-PKI with minimal content Certificates The Lite-PKI is designed to fully mirror the DKI in the smallest reasonable X.509 certificates (e.g. 240 bytes for DER), but still adhere to MUST field usage.

DRIP Lite X.509 certificate profile The following is the profile for the DRIP X.509 Lite certificates

The X.509 Lite Profile

DRIP Lite Manditory Certificate Content The following detail the Manditory to include content in all DRIP Lite certificates.

Serial Number The Serial Number is a MUST field, but it has no usage in this Lite-PKI. It is 1-byte in size and thus duplicates are guaranteed. To drop this field could make many X.509 parsing libraries fail. However, CA certificate's Serial Number MAY be the common 20 bytes. This is to avoid possible issues with general softward expecting this size Serial Numbers for CAs.

Subject The Subject field is only used in Authentication and Issuing Certificates. For Entity Certificates, the Subject is Empty and the DET will be in Subject Alternative Name (SAN). In the SAN, the DET can be properly encoded as an IPv6 address. The Subject field in Authentication and Issuing Certificates uses the following format:

Lite CA Certificate Subject Name Format

The CA Subject Name serves a duo purpose: foremostly, to place the CA within the DKI tree, but secondly for outside of DRIP usage to tag that this CA's function is to serve DRIP Entities.

Subject Alternative Name Subject Alternative Name is only used in Operational (End Entity) certificates. It is used to provide the DET as an IP address with an Empty Subject (SAN MUST be flagged as Critical). The Subject Alternative Name is also used in Manufacturer DET certificates. These may contain the hardwareModuleName as described in that references . Per and , Manufacturer DET certificates with hardwareModuleName MUST have the notAfter date as 99991231235959Z.

Issuer The Issuer MUST be the higher level's DET. The Issuer for the Apex Authentication certificate MUST be the Subject (indicating self-signed). The Issuer content of its DET assists in finding this specific Issuer in the DRIP ip6.arpa. DNS tree and any additional information.

The test DKI and Lite PKI The test DKI and Lite PKI, (/), were created using the python scripts at . First csr-gen.py is used to create an X.509 CSR (and optionally the EdDSA keypair). This CSR is minimal in content. For example, a UA might only have knowledge of its Manufacturer Serial Number and can generate its keypair. Per , this CSR may be sent to the CA with additional information provided by the Operator, for example desired validityDates. The raa-endorse.py and hda-endorse.py scripts are provided to produce the DRIP Endorsements and X.509 certificates. At this time, with no Apex level, each RAA Authorization CA is self-signed. These are created using the RAA's CSR and its own keypair as input to the raa-endorse.py script. Normally, the raa-endorse.py script is used to create the HDA's Authorization and Issuing CAs and the hda-endorse.py script for the End Entity certificates.

DRIP Lite Optional CA Certificate Content The following detail the Optional content for DRIP Lite CA certificates. Inclusion of these objects are based on the policies of the CA using them and CAs higher in the trust chain.

CA Subject Alternative Name URI This is the one exception to . A CA certificate MAY have a SAN URI, containing a pointer where additional information on the CA and certificates under its control. For example, an authorized authority may access EE PII like an Oberator phone number through this URI link.

CA Policy OIDs The CA MAY have policy OIDs defining rules for EEs within its domain. The OIDs used here will tend to be within ICAO's arc of 1.3.9.16.

Abandoned: Shadow PKI with PKIX-like Certificates Author's Note: The PKIX-like Certificate model has been abandoned for now due to lack of interest in these larger ~400 bytes for DER) size. For information on these certificates, please review the first draft of this document.

The DKI and the ICAO IAC PKI The ICAO has defined an International Aviation Common PKI (IAC) PKI in their ICAO Doc 10169 Aviation Common Certificate Policy (ACCP). A test version of this PKI is rolling out for testing the Aviation System Wide Information Management (SWIM) environment. Currently, this PKI is using ECDSA P-256 in its certificates. This is equivalent to DET SuiteID of "3". The subjectNames in use can readily by mapped to RAAs (, Table 1) and HDAs. Thus it is a potential straight-forward technical work item to add DET support into the PKI. The DETs can readily be stored in subjectAltName or more interestingly in subjectKeyIdentifier (and thus authorityKeyIdentifier). There are a number of advantages in the IATF and SWIM to have DETs and the matching DNS available. For example, the "cost" of adding DETs to these certificates could result in moving much of their content into DNS SRV RR and potentially reduce their size by 1/3rd. DETs as the authorityKeyIdentifier would enable DNS for Trust Chain discovery. Another approach is direct inclusion in this PKI of the DET "Lite" EE certificates for constrained A2A communications. Discussions are ongoing with those involved with the IATF PKI and this could open up DET usage into General/Civil Aviation.

IANA Considerations TBD

Security Considerations Risks in the DKI are similar to those in any X.509 PKI. The methodologies to mitigate risk in PKI management should be considered and implemented as appropriate. The DKI presents a tree-breath problem that is rarely seen in PKIs and needs practical solutions to minimize cost of operations and not introduce risks needlessly. Consider that there can be 16,384 RAAs. Assume only 10,000 RAAs, each of which Authentication DET Endorsement has a 10 year validity period. This means that, on average, 1,000 RAAs per year need to rekey their Authentication DET Endorsement, or on average, 3 per day. Current witnessed key signing processes will not scale to this volume. Some virtual method (like in ) is needed.

Protecting against DKI/PKI compromise There is always a risk of key compromise that could be a major setback to the operation of a PKI and likewise the DRIP DKI. To mitigate this risk, the Authentication DETs MUST only be used in offline signing operations. They MUST NEVER be used on connected systems. The information needed to create the Endorsements and X.509 certificates are brought to them on media that cannot transfer code, for example in a QR code. The objects that are created are then transferred away from the offline system to be used where needed. It should be noted that this offline process MUST be followed down the DKI/PKI tree. That is, the Apex has offline operations that include signing the RAA Authentication DET that will be used in the RAA's set up.

References IANA ANSI/CTA

Test DETs and Endorsements The following are test DETs and Endorsements for the test DKI. This testing environment is open to all. There are 4 RAAs available for others to build out. HDAs under the 4 preset RAAs, or under any of the 4, built out by others, are available. Finally the test HDA is available for setting up a handful of entities. Any tester wanting more than a few DETs for entities should plan on doing that under their own HDA. The following are the test values and objects. They were generated using the csr-gen.py, raa-endorse.py, and hda-endorse.py scripts available at . Note, that as there is no APEX level at this time, the RAA Endorsement is self-signed.

Test DKI DETs and Endorsements

Test DNS The DNS tree(s) for the above test data is still in limbo and will be added in a later version of this draft with the proposed DET RR from .

Test X.509 certificates

Test Lite X.509 certificates The following the test DRIP X.509 certificates that mirror the test Endorsements.

Test Lite X.509 certificates

Test Lite C509 certificates The CBOR Encoded X.509 Certificates (C509 Certificates) provides a standards-based approach to reduce the size of X.509 certificates both on-the-wire and in storage. These C509 certificates MAY be stored in the DET RR, but are more likely to by used in over-the-air protocols and exist only for transmission, being converted from/to their source X.509 certificates. Author's Note: This section is still a Work in Progress. The CBOR diagnostic tool is currently not working, and that content should be added back in on a later revision. Further note that we think there is a bug in the c509 code, making the certificate version = 1, not 3. The following are examples of a C509 cert.

Test Lite C.509 certificates

Acknowledgments Many people assisted in creating the python scripts for making DETs and DRIP Endorsements. Any roughness in the scripts is all my doing. The openssl-user mailing list provided needed help in getting openssl command line to do what was needed to build the test PKI. The COSE C509 authors are providing active help in creating the C509 equivalent objects.