Multiple Public-Key Algorithm X.509 Certificates
draft-truskovsky-lamps-pq-hybrid-x509-00

Document Type Active Internet-Draft (individual)
Last updated 2018-03-01
Stream (None)
Intended RFC status (None)
Formats plain text xml pdf html bibtex
Stream Stream state (No stream defined)
Consensus Boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date
Responsible AD (None)
Send notices to (None)
LAMPS                                                      A. Truskovsky
Internet-Draft                                               P. Lafrance
Intended status: Standards Track                            D. Van Geest
Expires: September 1, 2018                             ISARA Corporation
                                                              S. Fluhrer
                                                           P. Kampanakis
                                                           Cisco Systems
                                                            M. Ounsworth
                                                               S. Mister
                                                   Entrust Datacard, Ltd
                                                       February 28, 2018

            Multiple Public-Key Algorithm X.509 Certificates
                draft-truskovsky-lamps-pq-hybrid-x509-00

Abstract

   This document describes a method of embedding alternative sets of
   cryptographic materials into X.509v3 digital certificates, X.509v2
   Certificate Revocation Lists (CRLs), and PKCS #10 Certificate Signing
   Requests (CSRs).  The embedded alternative cryptographic materials
   allow a Public Key Infrastructure (PKI) to use multiple cryptographic
   algorithms in a single object, and allow it to transition to the new
   cryptographic algorithms while maintaining backwards compatibility
   with systems using the existing algorithms.  Three X.509 extensions
   and three PKCS #10 attributes are defined, and the signing and
   verification procedures for the alternative cryptographic material
   contained in the extensions and attributes are detailed.

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/.

   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 September 1, 2018.

Truskovsky, et al.      Expires September 1, 2018               [Page 1]
Internet-Draft                    MPKAC                    February 2018

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Alternative Public-Key Algorithm Objects  . . . . . . . . . .   5
     2.1.  OIDs  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  CSR Attributes  . . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Subject Alt Public Key Info Attribute . . . . . . . .   5
       2.2.2.  Alt Signature Algorithm Attribute . . . . . . . . . .   5
       2.2.3.  Alt Signature Value Attribute . . . . . . . . . . . .   6
     2.3.  X.509v3 Extensions  . . . . . . . . . . . . . . . . . . .   6
       2.3.1.  Subject Alt Public Key Info Extension . . . . . . . .   6
       2.3.2.  Alt Signature Algorithm Extension . . . . . . . . . .   7
       2.3.3.  Alt Signature Value Extension . . . . . . . . . . . .   7
   3.  Multiple Public-Key Algorithm Certificate Signing Requests  .   7
     3.1.  Creating Multiple Public-Key Algorithm CSRs . . . . . . .   8
     3.2.  Verifying Multiple Public-Key Algorithm CSRs  . . . . . .   9
   4.  Multiple Public-Key Algorithm Certificates  . . . . . . . . .  10
     4.1.  Creating Multiple Public-Key Algorithm Certificates . . .  11
     4.2.  Verifying Multiple Public-Key Algorithm Certificates  . .  13
   5.  Multiple Public-Key Algorithm Certificate Revocation Lists  .  15
     5.1.  Creating Multiple Public-Key Algorithm Certificate
           Revocation Lists  . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Verifying Multiple Public-Key Algorithm Certificate
           Revocation Lists  . . . . . . . . . . . . . . . . . . . .  18
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     8.1.  Post-Quantum Security Considerations  . . . . . . . . . .  20
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  21
   Appendix A.  ASN.1 Structures and OIDs  . . . . . . . . . . . . .  21
   Appendix B.  Upgrading PKI and Dependent Systems  . . . . . . . .  22

Truskovsky, et al.      Expires September 1, 2018               [Page 2]
Internet-Draft                    MPKAC                    February 2018

   Appendix C.  Options for Alternative Algorithms . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   Modern Public Key Infrastructure (PKI) extensively relies on
   classical signature algorithms such as RSA or ECDSA to achieve secure
   authentication.  The security of these algorithms is based on the
   time-tested difficulty of certain number-theoretic problems.
   However, it is well known that such schemes offer insufficient
   security against an adversary in possession of a universal quantum
   computer.  Such an adversary can efficiently recover the private key
   from the public key and impersonate any entity in the system -- even
   a root Certification Authority (CA).  Hence, it is necessary to
   upgrade these PKIs to utilize algorithms that are secure against such
   adversaries.

   An obvious solution is for relying parties to require multiple
   certificated to establish trust in an entity.  One could
   theoretically continue to use certificates as they currently are and
   introduce separate certificates that utilize the new algorithms.
   However, managing different cryptographic algorithms within a single
   PKI in this way requires multiple certificate chains.  This would
   greatly increase the complexity of the already complex system.
   Furthermore, some systems rely on physical solutions for credential
   storage.  These physical solutions may be limited in terms of
   capacity as well as in terms of how such systems are interacted with.
   Instead, it is far simpler to keep only a single identity and employ
   a single certificate chain for each user.

   The goal of this document is to profile new X.509v3 certificate
   extensions, X.509v2 CRL extensions and PKCS #10 CSR attributes that
   facilitate the use of a simple and efficient approach for executing
   this upgrade.  A key design requirement for this approach is to not
   affect the behavior of non-upgraded systems and ensure they can
   process any new attributes or extensions without breaking.

   By placing an alternative public key and alternative signature into
   custom extensions, one effectively embeds multiple certificate chains
   within a single chain.  By utilizing these multiple public-key
   algorithm certificates, legacy applications can continue using their
   current choices of cryptographic algorithms and upgraded applications
   can use new algorithms while remaining interoperable with the legacy
   systems.

   It is useful to observe that even though the motivation for this
   document is to upgrade PKIs to use quantum-safe cryptography, the
   same methodology can be used to upgrade such systems to any new

Truskovsky, et al.      Expires September 1, 2018               [Page 3]
Internet-Draft                    MPKAC                    February 2018

   algorithm.  For this reason, this document does not specify that
   quantum-safe algorithms are the new technology the PKI is being
   upgraded to use.

   The remainder of this document is organized as follows.

   Section 2 profiles the three new PKCS #10 attributes and three new
   X.509 extensions.  Sections 3, 4 and 5 profile methods for signing
   and verifying CSRs, certificates and CRLs respectively using the new
   extensions.

1.1.  Requirements Language

   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].

1.2.  Terminology

   The following terms are defined:

   o  alternative algorithm: The algorithm, whose usage is profiled in
      this document, which can be used to sign and verify a certificate
      instead of, or in addition to, the conventional algorithm.

   o  alternative [public, private] key: The keys, whose usage is
      profiled in this document, which can be used to create or verify a
      signature instead of, or in addition to, the conventional keys.

   o  alternative signature: The signature, whose usage is profiled in
      this document, which can be used to validate a certificate instead
      of, or in addition to, the conventional signature.

   o  conventional algorithm: The algorithm specified in the
      signatureAlgorithm field of an X.509v3 certificate.

   o  conventional [public, private] key: The key used to create or
      verify a conventional signature in an X.509v3 certificate.

   o  conventional signature: The value specified in the signature field
      of an X.509v3 certificate.

   o  multiple public-key algorithm certificate: A certificate which is
      equipped with the extensions introduced in this document.  Thus,
      the certificate is signed and can be verified using two different
      public-key algorithms.  One public-key algorithm (the
      "conventional" one) uses the keys, signatures and algorithms
      specified in the standard X.509v3 fields.  The other

Truskovsky, et al.      Expires September 1, 2018               [Page 4]
Internet-Draft                    MPKAC                    February 2018

      ("alternative") public-key algorithm uses the keys, signatures and
      algorithms in the extensions defined in this document.

   o  upgraded [application, system]: An application or system which is
      capable of understanding and using the extensions introduced in
      this document.

2.  Alternative Public-Key Algorithm Objects

2.1.  OIDs

   The following OIDs are used to identify the CSR attributes and
   X.509v3 extensions defined in the following sections.

   id-subjectAltPublicKeyInfo  OBJECT IDENTIFIER  ::=  { TBD }

   id-altSignatureAlgorithm    OBJECT IDENTIFIER  ::=  { TBD }

   id-altSignatureValue        OBJECT IDENTIFIER  ::=  { TBD }

2.2.  CSR Attributes

   Three new CSR attributes are used to submit an alternative public key
   for certification.  Each of these attributes mirror existing fields
   within a CSR and serve the same purpose as those fields, but with the
   alternative algorithms.  An entity creating a CSR MUST include either
   all three of these attributes or none.

2.2.1.  Subject Alt Public Key Info Attribute

   The Subject Alt Public Key Info Attribute corresponds to the
   SubjectPublicKeyInfo type defined in Section 4.1 of [RFC2986].  This
   attribute carries information about the alternative public key being
   certified.  The information also identifies the entity's alternative
   public-key algorithm (and any associated parameters).

   This attribute is identified using the id-subjectAltPublicKeyInfo
   OID.

   SubjectAltPublicKeyInfoAttr ATTRIBUTE  ::=  {
        WITH SYNTAX SubjectPublicKeyInfo
        ID id-subjectAltPublicKeyInfo }

2.2.2.  Alt Signature Algorithm Attribute

   The Alt Signature Algorithm attribute corresponds to the
   signatureAlgorithm field of the CertificationRequest type described
   in Section 4.2 of [RFC2986].  This attribute contains the identifier

Truskovsky, et al.      Expires September 1, 2018               [Page 5]
Internet-Draft                    MPKAC                    February 2018

   for the alternative cryptographic algorithm used by the requesting
   entity to sign the CertificationRequestInfo.

   This attribute is identified using the id-altSignatureAlgorithm OID.

   AltSignatureAlgorithmAttr ATTRIBUTE  ::=  {
        WITH SYNTAX AlgorithmIdentifier
        ID id-altSignatureAlgorithm }

2.2.3.  Alt Signature Value Attribute

   The Alt Signature Value attribute corresponds to the signature field
   of the CertificationRequest type described in Section 4.2 of
   [RFC2986].  This attribute contains a digital signature computed upon
   the ASN.1 DER encoded PreCertificationRequestInfo as described in
   Section 3 of this document.

   By generating this alternative signature, a certification request
   subject proves possession of the alternative private key.

   This attribute is identified using the id-altSignatureValue OID.

   AltSignatureValueAttr ATTRIBUTE  ::=  {
        WITH SYNTAX BIT STRING
        EQUALITY MATCHING RULE bitStringMatch
        ID id-altSignatureValue }

2.3.  X.509v3 Extensions

   Three new X.509v3 extensions are used to authenticate a certificate
   using alternative algorithms.  Each of these extensions mirror
   existing fields within an X.509v3 certificate and serve the same
   purpose as those fields, but with the alternative algorithms.

2.3.1.  Subject Alt Public Key Info Extension

   The Subject Alt Public Key Info extension corresponds to the Subject
   Public Key Info field described in Section 4.1.2.7 of [RFC5280].
   This extension carries the alternative public key, and identifies the
   algorithm with which the key is used.

   This extension is identified using the id-subjectAltPublicKeyInfo
   OID.

   SubjectAltPublicKeyInfoExt  ::=  SEQUENCE  {
        algorithm                  AlgorithmIdentifier,
        subjectAltPublicKey        BIT STRING }

Truskovsky, et al.      Expires September 1, 2018               [Page 6]
Internet-Draft                    MPKAC                    February 2018

2.3.2.  Alt Signature Algorithm Extension

   The Alt Signature Algorithm extension corresponds to the signature
   field described in Section 4.1.2.3 of [RFC5280].  It also corresponds
   to the signatureAlgorithm field described in Section 4.1.1.2 of
   [RFC5280] since both those fields have the same values.  This
   extension contains the identifier for the alternative digital
   signature algorithm used by the CA to sign the preTBSCertificate.

   This extension is identified using the id-altSignatureAlgorithm OID.

   AltSignatureAlgorithmExt  ::=  AlgorithmIdentifier

2.3.3.  Alt Signature Value Extension

   The Alt Signature Value extension corresponds to the signatureValue
   field described in Section 4.1.1.3 of [RFC5280].  This extension
   contains a digital signature computed upon the ASN.1 DER encoded
   preTBSCertificate as described in Section 4.

   By generating this alternative signature, a CA certifies the validity
   of the preTBSCertificate data.  In particular, the CA certifies the
   binding between the alternative public key material and the subject
   of the certificate.

   This extension is identified using the id-altSignatureValue OID.

   AltSignatureValueExt  ::=  BIT STRING

3.  Multiple Public-Key Algorithm Certificate Signing Requests

   A Certificate Signing Request (CSR) is a sequence of three required
   fields as defined in Section 4.2 of [RFC2986].

   CertificationRequest  ::=  SEQUENCE  {
        certificationRequestInfo    CertificationRequestInfo,
        signatureAlgorithm          AlgorithmIdentifier,
        signature                   BIT STRING }

   A CSR's signature is calculated on the ASN.1 DER encoding of the
   CertificationRequestInfo object as defined in Section 4.2 of
   [RFC2986].

   CertificationRequestInfo  ::=  SEQUENCE  {
        version       INTEGER  { v1(0) } (v1,...),
        subject       Name,
        subjectPKInfo SubjectPublicKeyInfo{{ PKInfoAlgorithms }},
        attributes    [0] Attributes{{ CRIAttributes }} }

Truskovsky, et al.      Expires September 1, 2018               [Page 7]
Internet-Draft                    MPKAC                    February 2018

   The alternative signature is calculated on the ASN.1 DER encoding of
   the identical PreCertificationRequestInfo object.

   PreCertificationRequestInfo  ::=  SEQUENCE  {
        version       INTEGER  { v1(0) } (v1,...),
        subject       Name,
        subjectPKInfo SubjectPublicKeyInfo{{ PKInfoAlgorithms }},
        attributes    [0] Attributes{{ CRIAttributes }} }

   The PreCertificationRequestInfo type is the same as the
   CertificationRequestInfo type, however the
   PreCertificationRequestInfo object will have different attributes
   than the CertificationRequestInfo.  Specifically, the
   CertificationRequestInfo will include the AltSignatureValueAttr
   attribute, while the PreCertificationRequestInfo will not.

3.1.  Creating Multiple Public-Key Algorithm CSRs

   A multiple public-key algorithm CSR requires the applicant to
   generate two key pairs: one for the old algorithm (the conventional
   key pair), and another for the new algorithm (the alternative key
   pair).  All actions taken by the applicant with regards to the
   conventional algorithm and key pair are unchanged during this
   process.  Additional attributes are populated to prove that the
   applicant is in possession of the alternative private key.

   The PreCertificationRequestInfo object MUST contain the
   SubjectAltPublicKeyInfoAttr attribute carrying the alternative public
   key and algorithm for the CSR being created.

   The PreCertificationRequestInfo object MUST contain the
   AltSignatureAlgorithmAttr attribute, which specifies the algorithm
   identifier for the algorithm used to sign the
   PreCertificationRequestInfo object.

   The alternative signature of the PreCertificationRequestInfo MUST be
   calculated using the alternative private key of the certificate
   request subject, which is the private key associated with the public
   key found in the subject's SubjectAltPublicKeyInfoAttr attribute.

   After the alternative signature is calculated, the alternative
   signature MUST be added as an AltSignatureValueAttr attribute to
   create the CertificationRequestInfo object.

   The process of signing a multiple public-key algorithm CSR as
   described above can be summarized as follows:

Truskovsky, et al.      Expires September 1, 2018               [Page 8]
Internet-Draft                    MPKAC                    February 2018

   a.  Create a PreCertificationRequestInfo object, which is populated
       with all the data to be signed by the alternative private key,
       including the SubjectAltPublicKeyInfoAttr and
       AltSignatureAlgorithmAttr attributes.

   b.  Calculate the alternative signature on the DER encoding of the
       PreCertificationRequestInfo, using the certificate request
       subject's alternative private key with the algorithm specified in
       the AltSignatureAlgorithmAttr attribute.

   c.  Convert the PreCertificationRequestInfo to a
       CertificationRequestInfo by adding the calculated alternative
       signature to the PreCertificationRequestInfo object using the
       AltSignatureValueAttr attribute.

   d.  As per [RFC2986], calculate the conventional signature using the
       certificate request subject's conventional private key and create
       the CertificationRequest from the certificationRequestInfo,
       signatureAlgorithm and signature.

   An upgraded system MAY issue both multiple public-key algorithm and
   single public-key algorithm CSRs depending on their policies.  If the
   system issues a single public-key algorithm CSR, then that CSR MUST
   NOT contain any of the three attributes profiled in this section.

3.2.  Verifying Multiple Public-Key Algorithm CSRs

   The certificate issuer verifies the alternative signature of the
   multiple public-key algorithm CSR by reconstructing the
   PreCertificationRequestInfo object and using its ASN.1 DER encoding,
   alternative public key and alternative signature algorithm to verify
   the signature.

   To verify the alternative signature of a multiple public-key
   algorithm CSR, the following steps are taken:

   a.  ASN.1 DER decode the certificationRequestInfo field of the
       CertificationRequest to get a CertificationRequestInfo object.

   b.  Remove the AltSignatureValueAttr attribute from the
       CertificationRequestInfo object and set aside the alternative
       signature.  The object is now the same as the
       PreCertificationRequestInfo which the signature was generated on.

   c.  ASN.1 DER encode the PreCertificationRequestInfo object.

   d.  Using the algorithm specified in the AltSignatureAlgorithmAttr
       attribute of the PreCertificationRequestInfo, the alternative

Truskovsky, et al.      Expires September 1, 2018               [Page 9]
Internet-Draft                    MPKAC                    February 2018

       public key from the CSR's SubjectAltPublicKeyInfoAttr attribute
       and the ASN.1 DER encoded PreCertificationRequestInfo, verify the
       alternative signature from (b).

   During the process of ASN.1 DER decoding the
   CertificationRequestInfo, removing the AltSignatureValueAttr
   attribute from the PreCertificationRequestInfo, and ASN.1 DER
   encoding the PreCertificationRequestInfo, the relative ordering of
   the remaining attributes is not modified.  This is due to the DER
   encoding rules applied during signature generation as specified in
   RFC2986.  Thus, the resulting ASN.1 DER encoded
   PreCertificationRequestInfo is identical to the one the issuer used
   to generate the alternative signature.

4.  Multiple Public-Key Algorithm Certificates

   An X.509 digital certificate is a sequence of three fields as defined
   in [RFC5280].

   Certificate  ::=  SEQUENCE  {
        tbsCertificate       TBSCertificate,
        signatureAlgorithm   AlgorithmIdentifier,
        signatureValue       BIT STRING }

   An X.509v3 certificate's signature is calculated on the ASN.1 DER
   encoding of the TBSCertificate object as defined in Section 4.1 of
   [RFC5280].  In this way, a CA certifies the validity of the
   information in the tbsCertificate field, in particular the binding
   between the conventional public key material and the subject of the
   certificate.

   The alternative signature is calculated on the ASN.1 DER encoding of
   the similar, but not identical, PreTBSCertificate defined below.
   This signature also certifies the validity of the information in the
   tbsCertificate field.  In particular, the binding between the
   alternative public key material and the subject of the certificate is
   validated.

Truskovsky, et al.      Expires September 1, 2018              [Page 10]
Internet-Draft                    MPKAC                    February 2018

   PreTBSCertificate  ::=  SEQUENCE  {
        version         [0]  EXPLICIT Version DEFAULT v1,
        serialNumber         CertificateSerialNumber,
        issuer               Name,
        validity             Validity,
        subject              Name,
        subjectPublicKeyInfo SubjectPublicKeyInfo,
        issuerUniqueID  [1]  IMPLICIT UniqueIdentifier OPTIONAL,
                             -- If present, version MUST be v2 or v3
        subjectUniqueID [2]  IMPLICIT UniqueIdentifier OPTIONAL,
                             -- If present, version MUST be v2 or v3
        extensions      [3]  EXPLICIT Extensions OPTIONAL
                             -- If present, version MUST be v3
        }

   The PreTBSCertificate type is similar to the TBSCertificate type,
   except that the PreTBSCertificate does not include the signature
   field (the third element in the TBSCertificate sequence).  In a
   TBSCertificate the signature field contains the AlgorithmIdentifier
   of the algorithm which will be used to sign the final certificate,
   and this value might not be known at the time that the alternative
   signature is calculated.  Additionally, since the AlgorithmIdentifier
   of the signature field is associated with the final signatureValue
   field in the certificate, it is outside the scope of the alternative
   public-key algorithm and does not need to be protected by the
   alternative signature.

   The PreTBSCertificate object also does not contain the
   AltSignatureValueExt extension in its extension list, while the
   TBSCertificate will.  Since the alternative signature is calculated
   on the encoding of the PreTBSCertificate it cannot be included in the
   PreTBSCertificate.

4.1.  Creating Multiple Public-Key Algorithm Certificates

   If a CA is issuing a subject certificate and the issuer certificate
   or root of trust contains an alternative public key, then the CA
   SHOULD add an alternative signature to the subject certificate.
   Failure to do so could result in a verifier rejecting the certificate
   as being malformed, especially if the verifier is concerned about
   quantum-enabled adversaries.  This is discussed further in
   Section 8.1.

   A multiple public-key algorithm certificate MAY contain the
   SubjectAltPublicKeyInfoExt extension.  If the certificate's subject
   has an alternative public key which they wish to bind to their
   identity, then the public key and algorithm MUST be placed in the
   SubjectAltPublicKeyInfoExt extension.  However, if the certificate's

Truskovsky, et al.      Expires September 1, 2018              [Page 11]
Internet-Draft                    MPKAC                    February 2018

   subject has no such alternative public key (e.g. the subject's
   application has not been upgraded to support multiple public-key
   algorithms) then the SubjectAltPublicKeyInfoExt extension will not be
   added to the certificate.

   If a CA is issuing a certificate with an alternative signature, the
   extensions field of the PreTBSCertificate MUST contain the
   AltSignatureAlgorithmExt extension, which specifies the algorithm
   identifier for the algorithm used to sign the PreTBSCertificate.

   The alternative signature of the PreTBSCertificate MUST be calculated
   using the alternative private key of the Issuer, which is the private
   key associated with the public key found in the Issuer's
   SubjectAltPublicKeyInfoExt extension.

   After the alternative signature is calculated, the alternative
   signature MUST be added as an AltSignatureValueExt extension to the
   extensions list of the PreTBSCertificate, resulting in the
   TBSCertificate.

   The process of signing an X.509v3 multiple public-key algorithm
   certificate as described above can be summarized as follows:

   a.  Create a PreTBSCertificate object, which is populated with all
       the data to be signed by the alternative private key, including
       the SubjectAltPublicKeyInfoExt and AltSignatureAlgorithmExt
       extensions.

   b.  Calculate the alternative signature on the DER encoding of the
       PreTBSCertificate, using the Issuer's alternative private key
       with the algorithm specified in the AltSignatureAlgorithmExt
       extension.

   c.  Add the calculated alternative signature to the PreTBSCertificate
       object using the AltSignatureValueExt extension.

   d.  Convert the PreTBSCertificate to a TBSCertificate by adding the
       signature field and populating it with the algorithm identifier
       of the conventional algorithm to be used to sign the certificate.

   e.  As per [RFC5280], calculate the conventional signature using the
       conventional private key associated with the Issuer's certificate
       and create the certificate from the tbsCertificate,
       signatureAlgorithm and signature.

   If the upgraded CA's policy allows it to process single public-key
   algorithm CSRs and issue single public-key algorithm certificates,
   and the issuer's certificate has an alternative public key, and the

Truskovsky, et al.      Expires September 1, 2018              [Page 12]
Internet-Draft                    MPKAC                    February 2018

   CA receives a single-algorithm CSR, the CA SHOULD still include
   properly calculated AltSignatureValueExt and AltSignatureAlgorithmExt
   extensions in the certificate.  This ensures that when an upgraded
   system verifies the subject's certificate and sees that the issuer
   certificate contains the SubjectAltPublicKeyInfoExt extension that it
   will verify the subject's alternative signature.  Otherwise it might
   treat the subject's certificate as invalid.  This is discussed
   further in the Security Considerations section.

   Note - A certificate issuer would typically mark the
   SubjectAltPublicKeyInfoExt, AltSignatureAlgorithmExt and
   AltSignatureValueExt extensions as non-critical, allowing the
   multiple public-key algorithm certificate to be treated like a
   regular certificate by non-upgraded entities.  However, the issuer
   MAY mark the extensions as critical, for example if it is part of a
   PKI which requires entities to understand both the conventional and
   alternative signatures.

4.2.  Verifying Multiple Public-Key Algorithm Certificates

   Users wishing to verify a multiple public-key algorithm certificate
   using the alternative public-key algorithm will need to convert the
   tbsCertificate field in the certificate to a PreTBSCertificate object
   identical to the PreTBSCertificate object which the issuer used to
   create the alternative signature.  Then the user can use the issuer's
   alternative public key with the alternative signature algorithm to
   verify the alternative signature of the PreTBSCertificate.

   To verify the alternative signature of the multiple public-key
   algorithm certificate, the following steps are taken:

   a.  ASN.1 DER decode the tbsCertificate field of the certificate to
       get a TBSCertificate object.

   b.  Remove the AltSignatureValueExt extension from the TBSCertificate
       object and set aside the alternative signature.

   c.  Remove the signature field from the TBSCertificate object,
       converting it to a PreTBSCertificate object.

   d.  ASN.1 DER encode the PreTBSCertificate object.

   e.  Using the algorithm specified in the AltSignatureAlgorithmExt
       extension of the PreTBSCertificate, the alternative public key
       from the Issuer's SubjectAltPublicKeyInfoExt extension and the
       ASN.1 DER encoded PreTBSCertificate, verify the alternative
       signature from (b).

Truskovsky, et al.      Expires September 1, 2018              [Page 13]
Internet-Draft                    MPKAC                    February 2018

   The issuer's alternative public key comes from the issuing
   certificate's SubjectAltPublicKeyInfoExt extension, unless the issuer
   is a trust anchor.  In that case, the trust anchor's alternative
   public key may come from a self-signed certificate's
   SubjectAltPublicKeyInfoExt extension, or it may come from elsewhere.
   [RFC5280] section 6.1.1 (d) lists the trust anchor information as
   including:

   a.  the trusted issuer name,

   b.  the trusted public key algorithm,

   c.  the trusted public key, and

   d.  optionally, the trusted public key parameters associated with the
       public key.

   When validating a multiple public-key algorithm certificate, the
   trust anchor information also includes:

   a.  the trusted alternative public key algorithm,

   b.  the trusted alternative public key, and

   c.  optionally, the trusted alternative public key parameters
       associated with the alternative public key.

   During the process of ASN.1 DER decoding the TBSCertificate, removing
   the AltSignatureValueExt extension from the PreTBSCertificate and
   ASN.1 DER encoding the PreTBSCertificate, the relative ordering of
   the remaining extensions is not modified.  Thus, the resulting ASN.1
   DER encoded PreTBSCertificate is identical to the one the issuer used
   to generate the alternative signature.

   A certificate that contains an AltSignatureValueExt extension but
   does not contain an AltSignatureAlgorithmExt extension cannot be
   verified under the alternative public-key algorithm and so SHOULD be
   rejected as being malformed.  Similarly, a certificate that contains
   an AltSignatureAlgorithmExt extension but does not contain an
   AltSignatureValueExt extension SHOULD be rejected.

   A certificate MAY have AltSignatureValueExt and
   AltSignatureAlgorithmExt extensions without having a
   SubjectAltPublicKeyInfoExt extension.  This case could arise if a
   non-upgraded subject requests a certificate from an upgraded CA who
   has a multiple public-key algorithm CA certificate.

Truskovsky, et al.      Expires September 1, 2018              [Page 14]
Internet-Draft                    MPKAC                    February 2018

   If an issuer certificate or root of trust has an alternative public
   key, but a subject certificate issued by the issuer certificate or
   root of trust doesn't contain an alternative signature then the
   verifier SHOULD reject the subject certificate.  This is especially
   important if the verifier is concerned about quantum-enabled
   adversaries.  This is discussed further in the Section 8.1.
   Accepting such a subject certificate SHOULD be limited to cases where
   the verifier has been explicitly configured to ignore missing
   alternative signatures for a given issuing CA, for subject
   certificates matching a given wildcard, or similar whitelisting
   mechanisms.

5.  Multiple Public-Key Algorithm Certificate Revocation Lists

   In certain situations, certificates must be revoked and no longer
   used.  This can happen for a variety of reasons including, but not
   limited to: key compromise, CA compromise, or due to a change in
   affiliation.  Roughly speaking, Certificate Revocation Lists (CRLs)
   are authenticated lists of revoked certificates.

   An X.509v2 Certificate Revocation List (CRL) is a sequence of three
   fields as defined in [RFC5280].

   CertificateList  ::=  SEQUENCE  {
       tbsCertList          TBSCertList,
       signatureAlgorithm   AlgorithmIdentifier,
       signatureValue       BIT STRING  }

   An X.509v2 CRL's signature is calculated on the ASN.1 DER encoding of
   the TBSCertList object as defined in Section 5.1 of [RFC5280].

   The alternative signature is calculated on the ASN.1 DER encoding of
   the similar, but not identical, PreTBSCertList object defined here.

Truskovsky, et al.      Expires September 1, 2018              [Page 15]
Internet-Draft                    MPKAC                    February 2018

   PreTBSCertList  ::=  SEQUENCE  {
        version                 Version OPTIONAL,
                                     -- if present, MUST be v2
        issuer                  Name,
        thisUpdate              Time,
        nextUpdate              Time OPTIONAL,
        revokedCertificates     SEQUENCE OF SEQUENCE  {
             userCertificate         CertificateSerialNumber,
             revocationDate          Time,
             crlEntryExtensions      Extensions OPTIONAL
                                      -- if present, version MUST be v2
                                  }  OPTIONAL,
        crlExtensions           [0]  EXPLICIT Extensions OPTIONAL
                                      -- if present, version MUST be v2
                                  }

   The PreTBSCertList object is similar to the TBSCertList object,
   except that the PreTBSCertList does not include the signature field
   (the second element in the TBSCertList sequence).  In a TBSCertList
   the signature field contains the AlgorithmIdentifier of the algorithm
   which will sign the final certificate revocation list, and this value
   might not be known at the time that the alternative signature is
   calculated.  Additionally, since the AlgorithmIdentifier of the
   signature field is associated with the final signatureValue field in
   the CRL, it is outside the scope of the alternative public-key
   algorithm and does not need to be protected by the alternative
   signature.

   The PreTBSCertList object also does not contain the
   AltSignatureValueExt extension in its extension list, while the
   TBSCertList will.  Since the alternative signature is calculated on
   the encoding of the PreTBSCertList, it cannot be included in the
   TBSCertList.

   If a multiple public-key algorithm certificate is revoked, whether
   because the classical key is compromised, the alternative key is
   compromised or or other reason, both the classical and alternative
   keys SHOULD be considered revoked.  This avoids any unneeded
   complexity in dealing with a certificate where one key is compromised
   but the other isn't.

5.1.  Creating Multiple Public-Key Algorithm Certificate Revocation
      Lists

   To create a multiple public-key algorithm CRL, one creates a CRL as
   specified in Section 5 of [RFC5280] and includes the additional
   extensions as specified in this section.

Truskovsky, et al.      Expires September 1, 2018              [Page 16]
Internet-Draft                    MPKAC                    February 2018

   If the CRL issuer's certificate has a SubjectAltPublicKeyInfoExt
   extension, the CRL SHOULD be created with an alternative signature.
   Otherwise, some upgraded systems may fail to validate the CRL because
   it is not trusted under the alternative public-key algorithm.

   The extensions field of the PreTBSCertList MUST contain the
   AltSignatureAlgorithmExt extension, which specifies the algorithm
   identifier for the algorithm used to sign the PreTBSCertList.

   The alternative signature of the PreTBSCertList MUST be calculated
   using the alternative private key of the CRL issuer, which is the
   private key associated with the public key found in the CRL issuer
   X.509v3 certificate's SubjectAltPublicKeyInfoExt extension.

   After the alternative signature is calculated, the alternative
   signature MUST be added as an AltSignatureValueExt extension to the
   extensions list of the PreTBSCertList, resulting in the TBSCertList.

   The process of signing an X.509v2 multiple public-key algorithm CRL
   as described above can be summarized as follows:

   a.  Create a TBSCertList object, which is populated with all the data
       to be signed by the alternative private key, including the
       AltSignatureAlgorithmExt extension.

   b.  Calculate the alternative signature on the DER encoding of the
       PreTBSCertList, using the CRL issuer's alternative private key
       with the algorithm specified in the AltSignatureAlgorithmExt
       extension.

   c.  Add the calculated alternative signature to the PreTBSCertList
       object using the AltSignatureValueExt extension.

   d.  Convert the PreTBSCertList to a TBSCertList by adding the
       signature field and populating it with the algorithm identifier
       of the conventional algorithm to be used to sign the certificate.

   e.  As per [RFC5280], calculate the conventional signature using the
       conventional private key associated with the CRL issuer's
       certificate and create the CRL from the tbsCertList,
       signatureAlgorithm and signature.

   Note - A CRL issuer would typically mark the AltSignatureAlgorithmExt
   and AltSignatureValueExt extensions as non-critical, allowing the
   multiple public-key algorithm CRL to be treated like a regular CRL by
   non-upgraded entities.  However, the issuer may be part of a PKI
   which requires entities to understand both the conventional and

Truskovsky, et al.      Expires September 1, 2018              [Page 17]
Internet-Draft                    MPKAC                    February 2018

   alternative signatures, in which case it would mark the extensions as
   critical.

5.2.  Verifying Multiple Public-Key Algorithm Certificate Revocation
      Lists

   Users wishing to verify the alternative signature of a multiple
   public-key algorithm CRL will need to convert the tbsCertList field
   in the CRL to a PreTBSCertList identical to the PreTBSCertList which
   the issuer used to create the alternative signature.  Then the user
   can use the CRL issuer certificate's alternative public key with the
   alternative signature algorithm to verify the alternative signature
   of the PreTBSCertList.

   To verify the alternative signature of the multiple public-key
   algorithm CRL, the following steps are taken:

   a.  ASN.1 DER decode the tbsCertList field of the certificate to get
       a TBSCertList object.

   b.  Remove the AltSignatureValueExt extension from the TBSCertList
       object and set aside the alternative signature.

   c.  Remove the signature field from the TBSCertList object,
       converting it to a PreTBSCertList object.

   d.  ASN.1 DER encode the PreTBSCertList object.

   e.  Using the algorithm specified in the AltSignatureAlgorithmExt
       extension of the PreTBSCertList, the alternative public key from
       the CRL issuer certificate's SubjectAltPublicKeyInfoExt extension
       and the ASN.1 DER encoded PreTBSCertList, verify the alternative
       signature from (b).

   During the process of ASN.1 DER decoding the TBSCertList, removing
   the AltSignatureValueExt extension from the PreTBSCertList and ASN.1
   DER encoding the PreTBSCertList, the relative ordering of the
   remaining extensions will not be modified.  Thus, the resulting ASN.1
   DER encoded PreTBSCertList is identical to the one the issuer used to
   generate the alternative signature.

   In addition to verifying the alternative signature of a CRL, an
   implementation also needs to validate the CRL issuer's certificate
   and the certificate chain it is a part of.  Implementations SHOULD
   use the same method as profiled in Section 6 of [RFC5280] with the
   following modifications to the CRL processing algorithm of that
   document's Section 6.3.3.  Step (f) of the CRL processing algorithm
   requires certificate path validation for the issuer of the complete

Truskovsky, et al.      Expires September 1, 2018              [Page 18]
Internet-Draft                    MPKAC                    February 2018

   CRL.  To validate multiple public-key algorithm CRLs, upgraded
   entities SHOULD additionally verify the alternative signatures along
   the path as described in Section 4.2 of this document.  Step (g) of
   the CRL Processing algorithm requires the verification of a single
   signature on the complete CRL.  To verify multiple public-key
   algorithm CRLs, this step MUST be modified to instead verify dual
   signatures on the complete CRL.  Similarly, in step (h) of the same
   algorithm, if use-deltas is set and if the delta CRL is a multiple
   public-key algorithm CRL, then the verifying peer should validate the
   signature on the delta CRL via the method described above, and use
   standard practice otherwise - using the public key(s) validated in
   step (f).

6.  Acknowledgements

7.  IANA Considerations

   Extensions in certificates and CRLs are identified using object
   Identifiers (OIDs).  The creation and delegation of these arcs is to
   be determined.

8.  Security Considerations

   Many of the security considerations for this document closely follow
   those of [RFC5280].  However, the use of the extensions introduced in
   this document does bring rise to additional considerations.

   The motivation behind this document is to provide a method of
   upgrading PKIs and dependent systems to achieve quantum-safe state.
   However, state-of-the-art quantum-safe signature schemes tend to have
   large signature or key sizes.  As such, their inclusion on CSRs,
   certificates, or CRLs means that the sizes of these data structures
   will significantly increase.  This could potentially cause problems
   in protocols or implementations expecting more reasonable sizes.
   Even if enterprises choose instead to upgrade their PKI to new, but
   still classically secure signature algorithms, these algorithms can
   also be expected to have large signature or key sizes; often a by-
   product of an increased level of security is larger signatures or key
   sizes.

   There is a great deal of flexibility inherent to the use of the
   extensions introduced in this document.  Their design is such that a
   clean separation is made between the old and new signatures.  The new
   signatures have no dependency on the old signatures and no
   understanding of the new signatures is required to compute or verify
   the old signature.  As such, one could rely on the conventional
   signature only, the alternative signature only, or both, depending on
   the policies of the entity.

Truskovsky, et al.      Expires September 1, 2018              [Page 19]
Internet-Draft                    MPKAC                    February 2018

   It is paramount that all private keying material be kept secret; a
   subject covered in the Security Considerations section of [RFC5280].
   If the PKI is upgraded to use quantum-safe technologies, then it is
   of key importance to ensure that all private materials are protected
   against quantum-enabled adversaries as well.  How such a feat is
   accomplished is outside the scope of this document.  Additionally,
   issues such as re-keying or key management are outside the scope of
   this document.

   Typically, the SubjectAltPublicKeyInfoExt, AltSignatureAlgorithmExt
   and AltSignatureValueExt extensions will be marked as non-critical so
   that a non-upgraded system could treat a multiple public-key
   algorithm certificate or CSR as a conventional certificate.  However,
   a PKI could choose to enforce the usage of both conventional and
   alternative public-key algorithms, in which case it MAY mark these
   extensions as critical.  The reasons why a PKI may want to do this
   are outside the scope of this document.

8.1.  Post-Quantum Security Considerations

   While this document is intended to facilitate transitioning a PKI
   from a classical public-key algorithm to a quantum-safe public-key
   algorithm, with the transition completing before the development of
   quantum computers capable of breaking classical public-key
   algorithms, it is worth discussing security considerations if
   multiple public-key algorithm certificates are used in the presence
   of a quantum-enabled adversary.

   A quantum-enabled adversary is expected to be able to forge
   signatures for certificates and CRLs using classically secure
   signature algorithms.  Thus, a CA SHOULD add an alternative signature
   to any certificate it issues if the issuing certificate contains a
   SubjectAltPublicKeyInfoExt extension.  If the trust anchor is not a
   certificate, the alternative signature SHOULD be added if the trust
   anchor has an associated alternative public key which could be used
   for verification.  Similarly, when verifying certificates or CRLs an
   application SHOULD reject certificates or CRLs if they don't contain
   an alternative signature but the issuer certificate does contain a
   SubjectAltPublicKeyInfoExt or the trust anchor has an alternative
   public key.  If the CA does not add the alternative signature in
   these cases, and an upgraded application does not take this
   precaution when verifying, then a quantum-enabled adversary could
   create a certificate or CRL without an alternative signature, and
   forge the conventional signature of any issuer, causing upgraded
   applications to accept forged credentials.

   If an upgraded relying party processing a non-multiple public-key
   algorithm CRL encounters a multiple public-key algorithm certificate

Truskovsky, et al.      Expires September 1, 2018              [Page 20]
Internet-Draft                    MPKAC                    February 2018

   (containing an AltSignatureValueExt extension) in the list of revoked
   certificates, it SHOULD NOT treat that certificate as revoked.  If
   the conventional signature of the CRL uses a non-quantum-safe
   signature algorithm (e.g.  RSA or ECDSA), a quantum-enabled attacker
   may have forged the CRL, thereby revoking certificates that the CA
   didn't intend to revoke.  If one of those certificates has the
   multiple public-key algorithm extension then it was intended to be
   processed using the alternative public-key algorithm and should not
   be revoked based on only the results of the conventional public-key
   algorithm.

9.  Normative References

   [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/info/rfc2119>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [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/info/rfc5280>.

Appendix A.  ASN.1 Structures and OIDs

   This appendix includes all of the ASN.1 type and value definitions
   introduced in this document.

   DEFINITIONS IMPLICIT TAGS ::= BEGIN

   -- EXPORTS All --
   -- IMPORTS NONE --

   -- Object Identifiers for the certificate extensions introduced in
   -- Section 4.

   id-subjectAltPublicKeyInfo OBJECT IDENTIFIER  ::=  { TBD }

   id-altSignatureAlgorithm OBJECT IDENTIFIER  ::=  { TBD }

   id-altSignatureValue OBJECT IDENTIFIER  ::=  { TBD }

Truskovsky, et al.      Expires September 1, 2018              [Page 21]
Internet-Draft                    MPKAC                    February 2018

   -- X.509 Certificate extensions

   SubjectAltPublicKeyInfoExt  ::=  SEQUENCE  {
        algorithm                  AlgorithmIdentifier,
        subjectAltPublicKey        BIT STRING }

   AltSignatureAlgorithmExt  ::=  AlgorithmIdentifier

   AltSignatureValueExt  ::=  BIT STRING

   -- attribute data types

   subjectAltPublicKeyInfoAttr ATTRIBUTE  ::=  {
        WITH SYNTAX SubjectPublicKeyInfo
        ID id-subjectAltPublicKeyInfo }

   altSignatureAlgorithmAttr ATTRIBUTE  ::=  {
        WITH SYNTAX AlgorithmIdentifier
        ID id-altSignatureAlgorithm }

   altSignatureValueAttr ATTRIBUTE  ::=  {
        WITH SYNTAX BIT STRING
        EQUALITY MATCHING RULE bitStringMatch
        ID id-altSignatureValue }

   END

Appendix B.  Upgrading PKI and Dependent Systems

   One way to upgrade these systems is to employ a "top down" approach:
   First the root CA is upgraded, then the same is done for any
   subordinate CAs, and finally for end entities.  The dependent
   applications can then be upgraded in phases, where the upgraded
   applications can switch to using the new public-key algorithms while
   non-upgraded systems can continue using the old public-key
   algorithms.

Appendix C.  Options for Alternative Algorithms

   Out of all branches of mathematics thought to be suitable for
   quantum-safe cryptographic algorithm development, the theory of hash
   functions, specifically hash-based signatures are currently the most
   trusted in regard to their quantum security assurances.  While the
   private key state management makes using them challenging in some
   high-frequency use cases, they are very well suited for roots of
   trust and code signing; hash-based algorithms can already be used to
   upgrade CA certificates.  Furthermore, the option will be available

Truskovsky, et al.      Expires September 1, 2018              [Page 22]
Internet-Draft                    MPKAC                    February 2018

   to use stateless digital signatures in end-entity certificates when
   they become available.

Authors' Addresses

   Alexander Truskovsky
   ISARA Corporation
   560 Westmount Rd N
   Waterloo, Ontario  N2L 2Y6
   Canada

   Email: alexander.truskovsky@isara.com

   Philip Lafrance
   ISARA Corporation
   560 Westmount Rd N
   Waterloo, Ontario  N2L 0A9
   Canada

   Email: philip.lafrance@isara.com

   Daniel Van Geest
   ISARA Corporation
   560 Westmount Rd N
   Waterloo, Ontario  N2L 0A9
   Canada

   Email: daniel.vangeest@isara.com

   Scott Fluhrer
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: sfluhrer@cisco.com

   Panos Kampanakis
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: pkampana@cisco.com

Truskovsky, et al.      Expires September 1, 2018              [Page 23]
Internet-Draft                    MPKAC                    February 2018

   Mike Ounsworth
   Entrust Datacard, Ltd
   1000 Innovation Drive
   Kanata, Ontario  K2K 3E7
   Canada

   Email: mike.ounsworth@entrustdatacard.com

   Serge Mister
   Entrust Datacard, Ltd
   1000 Innovation Drive
   Kanata, Ontario  K2K 3E7
   Canada

   Email: serge.mister@entrustdatacard.com

Truskovsky, et al.      Expires September 1, 2018              [Page 24]