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Null Scheme for Signed Objects in the Resource Public Key Infrastructure (RPKI)
draft-doesburg-sidrops-nullscheme-00

Document Type Active Internet-Draft (individual)
Author Dirk Doesburg
Last updated 2025-10-05
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draft-doesburg-sidrops-nullscheme-00
SIDR Operations                                              D. Doesburg
Internet-Draft                                          6 September 2025
Intended status: Informational                                          
Expires: 10 March 2026

Null Scheme for Signed Objects in the Resource Public Key Infrastructure
                                 (RPKI)
                  draft-doesburg-sidrops-nullscheme-00

Abstract

   This document specifies the Null Scheme for use in Signed Objects in
   the Resource Public Key Infrastructure (RPKI).  The Null Scheme is a
   niche signature scheme that can replace the redundant and costly use
   of actual digital signatures from so-called "one-time-use" key pairs
   in Signed Objects.  The Null Scheme has as public key the digest of
   the message to be signed, and the signature is always empty.  When a
   Null Scheme public key is the subject of a Signed Object's one-time-
   use End-Entity (EE) certificate, it establishes a secure binding
   between the issuer of the EE certificate and the message to be
   signed.  This is cheaper in terms of size and verification time than
   using a real signature scheme, while providing the same security
   guarantees.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-doesburg-sidrops-nullscheme/.

   Discussion of this document takes place on the SIDR Operations
   Working Group mailing list (mailto:sidrops@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/sidrops/.
   Subscribe at https://www.ietf.org/mailman/listinfo/sidrops/.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 10 March 2026.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Related Work  . . . . . . . . . . . . . . . . . . . . . .   4
       1.2.1.  Attribute Certificates  . . . . . . . . . . . . . . .   4
       1.2.2.  No-Signature  . . . . . . . . . . . . . . . . . . . .   5
       1.2.3.  Relying on digest in Manifests  . . . . . . . . . . .   5
   2.  Definition  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Public Key and Signature Generation . . . . . . . . . . .   6
     2.2.  Signature Verification  . . . . . . . . . . . . . . . . .   6
   3.  ASN.1 Module  . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
     4.1.  Security Reduction to Second-Preimage Resistance  . . . .   8
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Test Vectors . . . . . . . . . . . . . . . . . . . .  10
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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1.  Introduction

   This document specifies the Null Scheme for use in Signed Objects in
   the Resource Public Key Infrastructure (RPKI) [RFC6480].  The Null
   Scheme is a niche signature scheme that can replace the redundant and
   costly use of actual digital signatures from so-called "one-time-use"
   key pairs in RPKI Signed Objects [RFC6488].

   Signed Objects contain an End-Entity (EE) certificate issued by a
   Certificate Authority (CA).  This EE certificate usually contains a
   public key corresponding to a one-time-use key pair, which is used to
   sign a single CMS signed-data object [RFC5652].  The practice of
   using each key pair for only one Signed Object enables the use of a
   CRL [RFC5280] to revoke individual objects.  However, it means that
   each Signed Object consists of two signatures and a public key,
   whereas, intuitively, only one signature should be needed to bind the
   object to its issuer.

   The Null Scheme is _not_ an actual digital signature algorithm, or
   even a One-Time Signature [OTS]: it requires the (single) message to
   be signed to be known before the public key can be generated.

   Essentially, the Null Scheme works as follows:

   *  the public key is the digest of the single message to be signed,

   *  there is no private key, and

   *  the signature is always empty.

   Signature generation has to happen together with generation of the
   public key, taking the message to be signed as input.  A public key
   cannot be generated without the message being known in advance.
   Verification is done by simply comparing the message digest with the
   public key.

   As the input to a signing algorithm when signing a CMS signed-data
   object is the output of the Message Digest Calculation Process
   defined in Section 5.4 of [RFC5652], the Null Scheme's public key is
   technically directly the input of the signing algorithm, rather than
   a digest of that input.  This avoids an unnecessary extra hashing
   step.

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1.1.  Requirements Language

   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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Related Work

   The Null Scheme is inspired by the idea that the one-time-use key
   pairs in RPKI Signed Objects could be replaced using One-Time
   Signature (OTS) algorithms, such as hash-based Lamport or Winternitz
   signatures as used in XMSS [RFC8391] and LMS [RFC8554].  While this
   could be a suitable post-quantum alternative to current signature
   schemes, these hash-based OTS algorithms have large signatures.  It
   was then observed that the requirements for the one-time-use key
   pairs in Signed Objects are even weaker than those offered by OTS
   algorithms: it is possible to know the message to be signed before
   generating the key pair.

   The Null Scheme takes advantage of this to achieve optimal size and
   verification time while preserving the structure and validation
   process of Signed Objects.  This makes it possible to introduce the
   Null Scheme in the RPKI without requiring any changes to its
   specifications such as [RFC6480] and [RFC6488]: only the algorithms
   specification [RFC7935] needs to be updated.

1.2.1.  Attribute Certificates

   Several other niche signature schemes exist that have some
   similarities to the Null Scheme, but serve a different purpose.
   [RFC5755] describes the use of 'Attribute Certificates' that can
   allow something other than a public key to be the subject of an X509
   certificate.  Through using the 'ObjectDigestInfo' holder type (see
   Section 7.3 of [RFC5755]) something equivalent to the Null Scheme can
   be achieved: the 'holder' of an Attribute Certificate is the digest
   of an RPKI Signed Object's payload.  However, compared to the Null
   Scheme, using Attribute Certificates in place of plain X.509 EE
   certificates would require much more extensive changes to the RPKI
   specifications, making it operationally less practical to introduce.

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1.2.2.  No-Signature

   Appendix C.1 of [RFC5272] and [I-D.ietf-lamps-x509-alg-none-00]
   define a No-Signature algorithm for signatures in X.509 certificates
   and CMS signed-data objects.  These specifications are intended for
   use-cases where a signature is not needed at all.  However, in RPKI
   Signed Objects, _something_ is needed to bind the Signed Object to an
   EE certificate.  This can be achieved by removing the signature from
   the CMS signed-data (as in the No-Signature scheme) only if the
   signed-data is linked to the EE certificate in some other way.  That
   binding is provided by the Null Scheme through the EE certificate's
   public key.  Many alternative ways to provide such a binding exist
   (such as using Attribute Certificates as mentioned above), but
   require more extensive changes to the structure of Signed Objects.

1.2.3.  Relying on digest in Manifests

   Another more radical way of reducing the cryptographic overhead in
   the RPKI is to remove the CMS signed-data wrapper and EE certificate
   from Signed Objects (other than Manifests [RFC6486]) entirely, and
   instead rely on the digest of the Signed Object being listed in a
   valid Manifest.  This is a major change to the workings of the RPKI.
   It would reduce the cryptographic overhead much more (removing a
   public key and not one but two signatures per object), but would be
   very hard to introduce in practice.

2.  Definition

   The Null Scheme MUST only be used to sign and verify the CMS signed-
   data object contained in an RPKI Signed Object [RFC6488].
   Consequently, when it is used, a Null Scheme public key appears as
   the subject of a one-time-use EE certificate attached in the
   certificates field of the Signed Object's CMS signed-data object.
   The Null Scheme signature appears in the single SignerInfo object
   included in the signed-data object's signerInfos field.

   As the Null Scheme requires the message to be signed to be known
   before the public key can be generated, it consists of two
   algorithms: SignOnce and Verify, rather than the usual KeyGen, Sign,
   and Verify algorithms.

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2.1.  Public Key and Signature Generation

   The SignOnce algorithm takes as input the message to be signed m.  It
   produces as output a public key pk and a signature sig.  As the Null
   Scheme must only be used to sign CMS signed-data objects, the input
   message m is the output of the Message Digest Calculation Process
   defined in Section 5.4 of [RFC5652].  Therefore, although the Null
   Scheme's public key is always a digest of the message, the SignOnce
   algorithm actually returns its input m directly.  The digest
   algorithm used is indicated by the SignerInfo object's
   digestAlgorithm field.

     1. pk = m    # The output of the message digest calculation process
     2. sig = ""  # Empty octet string
     3. return (pk, sig)

                      Figure 1: Algorithm SignOnce(m)

2.2.  Signature Verification

   The Verify algorithm takes as input a message m, a public key pk, and
   a signature sig.  Like SignOnce, the input m is the output of the
   Message Digest Calculation Process defined in Section 5.4 of
   [RFC5652].  It produces as output either "valid" or "invalid".

     1. if sig == "" and pk == m then return "valid"
     2. return "invalid"

                   Figure 2: Algorithm Verify(m, pk, sig)

3.  ASN.1 Module

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   RPKINullScheme2025
     { iso(1) member-body(2) us(840) rsadsi(113549)
       pkcs(1) pkcs-9(9) smime(16) modules(0) null-scheme-2025(TBD) }
   --
   -- TODO: module ID to be replaced by IANA
   --

   DEFINITIONS IMPLICIT TAGS ::= BEGIN

   EXPORTS ALL;

   IMPORTS
     PUBLIC-KEY, SIGNATURE-ALGORITHM
       FROM AlgorithmInformation-2009  -- RFC 5912
         { iso(1) i id-mod(0)
           id-mod-algorithmInformation-02(58) }
   ;

   --
   -- TODO: OIDs to be replaced by IANA
   --
   id-RPKI-NULL-SCHEME OBJECT IDENTIFIER ::= {
       identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) alg(6) TBD
     }

   pk-RPKI-NULL-SCHEME PUBLIC-KEY ::= {
       IDENTIFIER id-RPKI-NULL-SCHEME
       -- The digest algorithm used to determine the input to the signing / verification
       -- algorithms is determined by the SignerInfo's digestAlgorithm field. The signing
       -- and verification algorithms themselves are not dependent on the digest algorithm,
       -- so we don't need PARAMS or distinct OIDs for pairing different digest algorithms.
       PARAMS ARE absent
       -- A Null Scheme public key should only be certified by EE certificates.
       -- So, in accordance with RFC6487, only the digitalSignature bit is valid.
       CERT-KEY-USAGE {digitalSignature}
     }

   sa-RPKI-NULL-SCHEME SIGNATURE-ALGORITHM ::= {
       IDENTIFIER id-RPKI-NULL-SCHEME
       PARAMS ARE absent
       PUBLIC-KEYS {pk-RPKI-NULL-SCHEME}
     }

   END

4.  Security Considerations

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4.1.  Security Reduction to Second-Preimage Resistance

   Although the Null Scheme cannot be used as a general-purpose digital
   signature algorithm, it does provably provide the same security
   properties that are expected from normal digital signatures.

   Given a public key pk and corresponding message-signature pair (m,
   sig), finding another valid message-signature pair (m', sig') is
   clearly impossible: a pair (m', sig') is only valid under public key
   pk if m' == pk and sig' == "", and therefore m' == m.

   As the Null Scheme is used to sign CMS signed-data objects, it is, as
   with any other signature scheme, possible for two distinct messages
   to lead to the same message digest.  Finding such a second message m'
   given a message m that is valid under public key pk is breaking the
   second-preimage resistance of H.  This would not only allow forging
   (or reusing) a Null Scheme signature on m', but also reusing the
   signature of any other signature scheme.  This makes the Null Scheme
   strictly no less secure than any other signature scheme paired with
   the same digest algorithm H.

5.  IANA Considerations

   IANA is requested to allocate a value from the "SMI Security for S/
   MIME Module Identifier" registry [RFC7299] for the ASN.1 module
   RPKINullScheme2025 defined in this document, and a value for id-RPKI-
   NULL-SCHEME-SHA256 from the "SMI Security for PKIX Algorithms"
   registry [RFC7299].

   Editorial note: the assigned OID values will need to be added in the
   ASN.1 module, and test vectors regenerated using the definitive value
   for id-RPKI-NULL-SCHEME-SHA256.

6.  References

6.1.  Normative References

   [FIPS.180-4]
              "Secure hash standard", National Institute of Standards
              and Technology (U.S.), DOI 10.6028/nist.fips.180-4, 2015,
              <https://doi.org/10.6028/nist.fips.180-4>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

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   [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,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/rfc/rfc5652>.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <https://www.rfc-editor.org/rfc/rfc6480>.

   [RFC6487]  Huston, G., Michaelson, G., and R. Loomans, "A Profile for
              X.509 PKIX Resource Certificates", RFC 6487,
              DOI 10.17487/RFC6487, February 2012,
              <https://www.rfc-editor.org/rfc/rfc6487>.

   [RFC6488]  Lepinski, M., Chi, A., and S. Kent, "Signed Object
              Template for the Resource Public Key Infrastructure
              (RPKI)", RFC 6488, DOI 10.17487/RFC6488, February 2012,
              <https://www.rfc-editor.org/rfc/rfc6488>.

   [RFC7299]  Housley, R., "Object Identifier Registry for the PKIX
              Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
              <https://www.rfc-editor.org/rfc/rfc7299>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

6.2.  Informative References

   [I-D.ietf-lamps-x509-alg-none-00]
              Benjamin, D., "Unsigned X.509 Certificates", Work in
              Progress, Internet-Draft, draft-ietf-lamps-x509-alg-none-
              00, 20 March 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-lamps-x509-alg-none-00>.

   [OTS]      Lamport, L., "Constructing Digital Signatures from a One
              Way Function", SRI International, CSL-98, 18 October 1979.

   [PQC-for-the-RPKI]
              Doesburg, D., "Post-Quantum Cryptography for the RPKI",
              MSc Thesis, Radboud University, 27 June 2025.

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   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
              <https://www.rfc-editor.org/rfc/rfc5272>.

   [RFC5755]  Farrell, S., Housley, R., and S. Turner, "An Internet
              Attribute Certificate Profile for Authorization",
              RFC 5755, DOI 10.17487/RFC5755, January 2010,
              <https://www.rfc-editor.org/rfc/rfc5755>.

   [RFC6486]  Austein, R., Huston, G., Kent, S., and M. Lepinski,
              "Manifests for the Resource Public Key Infrastructure
              (RPKI)", RFC 6486, DOI 10.17487/RFC6486, February 2012,
              <https://www.rfc-editor.org/rfc/rfc6486>.

   [RFC7935]  Huston, G. and G. Michaelson, Ed., "The Profile for
              Algorithms and Key Sizes for Use in the Resource Public
              Key Infrastructure", RFC 7935, DOI 10.17487/RFC7935,
              August 2016, <https://www.rfc-editor.org/rfc/rfc7935>.

   [RFC8391]  Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
              Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
              RFC 8391, DOI 10.17487/RFC8391, May 2018,
              <https://www.rfc-editor.org/rfc/rfc8391>.

   [RFC8554]  McGrew, D., Curcio, M., and S. Fluhrer, "Leighton-Micali
              Hash-Based Signatures", RFC 8554, DOI 10.17487/RFC8554,
              April 2019, <https://www.rfc-editor.org/rfc/rfc8554>.

Appendix A.  Test Vectors

   The following test vector is a base-64 encoded RPKI Signed Object
   containing an EE certificate with as subject a Null Scheme public key
   matching the signed content.  The EE certificate is issued by an RSA
   key pair, whose public key is also added below.

   Editorial note: the test vectors below are generated using
   placeholder OID value 1.3.6.1.4.1.64241.1.1 for id-RPKI-NULL-SCHEME-
   SHA256.  Once IANA has assigned a value, the test vectors will need
   to be regenerated using that definitive OID value.

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   MIIExwYJKoZIhvcNAQcCoIIEuDCCBLQCAQMxDTALBglghkgBZQMEAgEwKAYLKoZI
   hvcNAQkQARigGQQXMBUCAQUwEDAOBAIAATAIMAYDBAB7DCKgggPLMIIDxzCCAq+g
   AwIBAgIUfsjuEOuYMPFgVb8W26Oa9upWd3owDQYJKoZIhvcNAQELBQAwMzExMC8G
   A1UEAxMoRTU4QzAzRjlGQUNDM0I5QUIxMEY3OEJEQTZDODZDNkQ1N0IyNTBGMzAe
   Fw0yNTA5MTkxODQ5MzNaFw0yNjA5MTgxODU0MzNaMDMxMTAvBgNVBAMTKDhGQjJE
   NkY0MjkzNTNCMENBRTQ0MDM5NjRDOUI3MDk3N0I5M0RDMDAwMTAMBgorBgEEAYP1
   cQEBAyEAs4nZPXR8JArehzIBiIrCTWPw2Hw2w+m5L+uO+7aW7FmjggHEMIIBwDAd
   BgNVHQ4EFgQUj7LW9Ck1OwyuRAOWTJtwl3uT3AAwHwYDVR0jBBgwFoAU5YwD+frM
   O5qxD3i9pshsbVeyUPMwDgYDVR0PAQH/BAQDAgeAMFwGA1UdHwRVMFMwUaBPoE2G
   S3JzeW5jOi8vbG9jYWxob3N0L3JlcG8vY2hpbGQvMC9FNThDMDNGOUZBQ0MzQjlB
   QjEwRjc4QkRBNkM4NkM2RDU3QjI1MEYzLmNybDBoBggrBgEFBQcBAQRcMFowWAYI
   KwYBBQUHMAKGTHJzeW5jOi8vbG9jYWxob3N0L3JlcG8vb25saW5lLzAvRTU4QzAz
   RjlGQUNDM0I5QUIxMEY3OEJEQTZDODZDNkQ1N0IyNTBGMy5jZXIwawYIKwYBBQUH
   AQsEXzBdMFsGCCsGAQUFBzALhk9yc3luYzovL2xvY2FsaG9zdC9yZXBvL2NoaWxk
   LzAvMzEzMjMzMmUzMTMyMmUzMzM0MmUzMDJmMzIzNDJkMzIzNDIwM2QzZTIwMzUu
   cm9hMBgGA1UdIAEB/wQOMAwwCgYIKwYBBQUHDgIwHwYIKwYBBQUHAQcBAf8EEDAO
   MAwEAgABMAYDBAB7DCIwDQYJKoZIhvcNAQELBQADggEBAI/6f6QVtlCDdeUtSaDY
   +hcI3MQy8I68o3u7dBK492bzhEHX94EbXF8vJCqqckBCHUUJwOb2MfUiUq0ZjUyG
   baBksk+4k55CaFTi0QAgbnCYZ+08bTCd61yc7bYiyPpHOW3MWX9evTir+h2uFJyT
   ETWtkb7APGJgTA9KtIxCj2rNveKv+gmci/mi3FMELLn/y0QAexBMVPIyU142KUrM
   X5SeHvHYz9QzXwpyhKVeyqD3wMTe+I70mEMnCjMZTHGSYsfykXbXKmAmnk76SB7n
   C1MCy8BhCUBLBAn+QcK4aTpqVi3i/2V2a3EZm+nfcNB/kb/wMCCYawrS6XjFpWL8
   LoYxgaYwgaMCAQOAFI+y1vQpNTsMrkQDlkybcJd7k9wAMAsGCWCGSAFlAwQCAaBr
   MBoGCSqGSIb3DQEJAzENBgsqhkiG9w0BCRABGDAcBgkqhkiG9w0BCQUxDxcNMjUw
   OTE5MTg1NDMzWjAvBgkqhkiG9w0BCQQxIgQgaoUbRc3K1OCIGMdQhz9HjaFxJhrc
   dEXSKTTashDvSv0wDAYKKwYBBAGD9XEBAQQA

        Figure 3: RPKI Signed Object with Null Scheme EE Certificate

   The EE certificate in the Signed Object is issued with the following
   RSA public key:

   -----BEGIN PUBLIC KEY-----
   MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA2nyEbcU+KLPQYfV+NYVD
   2GVy06sXSoWSMbHl4BNgpkyaVT+wmXfiBq5RT33Nb5bm6mNsX6rZzRBm9BT6p5wb
   y+eMpVHqbrLb4JfBXSQnq+7132iTkfgBbIt5v5Z2Ly6UfiBb/Ftpz/5NeYpDpd65
   A0qngooYjY6Loyjvfoa53Rc40/bfWGkValseQ0qrww9t3ztejpWUIY8p5FzNdjeV
   XfkB1DgMmllLAUcRb+yC844oEGNJyZL1Stspo9+HVLUOUUXpoylXjd7rOStuq8Y1
   JGa6VWHq6q0hwd67oEcZRwGmExJKhvuRPy+Udk1a2S3X60s9pqMOd4Soo5cKtul4
   ZQIDAQAB
   -----END PUBLIC KEY-----

             Figure 4: RSA Public Key of EE Certificate Issuer

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Internet-Draft          Null Scheme for the RPKI          September 2025

Acknowledgments

   The author would like to thank Moritz Müller, Ralph Koning and Lisa
   Bruder who supervised the thesis that led to the idea of the Null
   Scheme, and especially to Job Snijders for his valuable comments and
   suggestions on earlier drafts of this document.

Author's Address

   Dirk Doesburg
   Email: dirk@ddoesburg.nl

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