Delegated Credentials for (D)TLS
draft-ietf-tls-subcerts-15

Network Working Group                                          R. Barnes
Internet-Draft                                                     Cisco
Intended status: Standards Track                              S. Iyengar
Expires: 17 December 2022                                       Facebook
                                                             N. Sullivan
                                                              Cloudflare
                                                             E. Rescorla
                                                                 Mozilla
                                                            15 June 2022

                    Delegated Credentials for (D)TLS
                       draft-ietf-tls-subcerts-15

Abstract

   The organizational separation between operators of TLS and DTLS
   endpoints and the certification authority can create limitations.
   For example, the lifetime of certificates, how they may be used, and
   the algorithms they support are ultimately determined by the
   certification authority.  This document describes a mechanism to to
   overcome some of these limitations by enabling operators to delegate
   their own credentials for use in TLS and DTLS without breaking
   compatibility with peers that do not support this specification.

Discussion Venues

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

   Source for this draft and an issue tracker can be found at
   https://github.com/tlswg/tls-subcerts (https://github.com/tlswg/tls-
   subcerts).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 17 December 2022.

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Table of Contents

   1.  Introduction
   2.  Conventions and Terminology
     2.1.  Change Log
   3.  Solution Overview
     3.1.  Rationale
     3.2.  Related Work
   4.  Delegated Credentials
     4.1.  Client and Server Behavior
       4.1.1.  Server Authentication
       4.1.2.  Client Authentication
       4.1.3.  Validating a Delegated Credential
     4.2.  Certificate Requirements
   5.  Operational Considerations
     5.1.  Client Clock Skew
   6.  IANA Considerations
   7.  Security Considerations
     7.1.  Security of Delegated Credential's Private Key
     7.2.  Re-use of Delegated Credentials in Multiple Contexts
     7.3.  Revocation of Delegated Credentials
     7.4.  Interactions with Session Resumption
     7.5.  Privacy Considerations
     7.6.  The Impact of Signature Forgery Attacks
   8.  Acknowledgements
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  ASN.1 Module
   Appendix B.  Example Certificate
   Authors' Addresses

1.  Introduction

   Server operators often deploy (D)TLS termination to act as the server
   for inbound TLS connections.  These termination services can be in
   locations such as remote data centers or Content Delivery Networks
   (CDNs) where it may be difficult to detect compromises of private key
   material corresponding to TLS certificates.  Short-lived certificates
   may be used to limit the exposure of keys in these cases.

   However, short-lived certificates need to be renewed more frequently
   than long-lived certificates.  If an external Certification Authority
   (CA) is unable to issue a certificate in time to replace a deployed
   certificate, the server would no longer be able to present a valid
   certificate to clients.  With short-lived certificates, there is a
   smaller window of time to renew a certificates and therefore a higher
   risk that an outage at a CA will negatively affect the uptime of the
   TLS-fronted service.

   Typically, a (D)TLS server uses a certificate provided by some entity
   other than the operator of the server (a CA) [RFC8446] [RFC5280].
   This organizational separation makes the (D)TLS server operator
   dependent on the CA for some aspects of its operations, for example:

   *  Whenever the server operator wants to deploy a new certificate, it
      has to interact with the CA.

   *  The CA might only issue credentials containing certain types of
      public key, which can limit the set of (D)TLS signature schemes
      usable by the server operator.

   To reduce the dependency on external CAs, this document specifies a
   limited delegation mechanism that allows a (D)TLS peer to issue its
   own credentials within the scope of a certificate issued by an
   external CA.  These credentials only enable the recipient of the
   delegation to terminate connections for names that the CA has
   authorized.  Furthermore, this mechanism allows the server to use
   modern signature algorithms such as Ed25519 [RFC8032] even if their
   CA does not support them.

   This document refers to the certificate issued by the CA as a
   "certificate", or "delegation certificate", and the one issued by the
   operator as a "delegated credential" or "DC".

Client            Front-End            Back-End
  |                   |<--DC distribution->|
  |----ClientHello--->|                    |
  |<---ServerHello----|                    |
  |<---Certificate----|                    |
  |<---CertVerify-----|                    |
  |        ...        |                    |

Legend:
Client: (D)TLS client
Front-End: (D)TLS server (could be a TLS-termination service like a CDN)
Back-End: Service with access to private key

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

2.1.  Change Log

   RFC EDITOR PLEASE DELETE THIS SECTION.

   (*) indicates changes to the wire protocol.

   draft-11

   *  Editorial changes based on AD comments

   *  Add support for DTLS

   *  Address address ambiguity in cert expiry

   draft-10

   *  Address superficial comments

   *  Add example certificate

   draft-09

   *  Address case nits

   *  Fix section bullets in 4.1.3.

   *  Add operational considerations section for clock skew

   *  Add text around using an oracle to forge DCs in the future and
      past

   *  Add text about certificate extension vs EKU

   draft-08

   *  Include details about the impact of signature forgery attacks

   *  Copy edits

   *  Fix section about DC reuse

   *  Incorporate feedback from Jonathan Hammell and Kevin Jacobs on the
      list

   draft-07

   *  Minor text improvements

   draft-06

   *  Modified IANA section, fixed nits

   draft-05

   *  Removed support for PKCS 1.5 RSA signature algorithms.

   *  Additional security considerations.

   draft-04

   *  Add support for client certificates.

   draft-03

   *  Remove protocol version from the Credential structure. (*)

   draft-02

   *  Change public key type. (*)

   *  Change DelegationUsage extension to be NULL and define its object
      identifier.

   *  Drop support for TLS 1.2.

   *  Add the protocol version and credential signature algorithm to the
      Credential structure. (*)

   *  Specify undefined behavior in a few cases: when the client
      receives a DC without indicated support; when the client indicates
      the extension in an non-valid protocol version; and when DCs are
      sent as extensions to certificates other than the end-entity
      certificate.

3.  Solution Overview

   A delegated credential (DC) is a digitally signed data structure with
   two semantic fields: a validity interval and a public key (along with
   its associated signature algorithm).  The signature on the delegated
   credential indicates a delegation from the certificate that is issued
   to the peer.  The private key used to sign a credential corresponds
   to the public key of the peer's X.509 end-entity certificate
   [RFC5280].

   A (D)TLS handshake that uses delegated credentials differs from a
   standard handshake in a few important ways:

   *  The initiating peer provides an extension in its ClientHello or
      CertificateRequest that indicates support for this mechanism.

   *  The peer sending the Certificate message provides both the
      certificate chain terminating in its certificate as well as the
      delegated credential.

   *  The initiator uses information from the peer's certificate to
      verify the delegated credential and that the peer is asserting an
      expected identity, determining an authentication result for the
      peer.

   *  Peers accepting the delegated credential use it as the certificate
      key for the (D)TLS handshake.

   As detailed in Section 4, the delegated credential is
   cryptographically bound to the end-entity certificate with which the
   credential may be used.  This document specifies the use of delegated
   credentials in (D)TLS 1.3 or later; their use in prior versions of
   the protocol is not allowed.

   Delegated credentials allow a peer to terminate (D)TLS connections on
   behalf of the certificate owner.  If a credential is stolen, there is
   no mechanism for revoking it without revoking the certificate itself.
   To limit exposure in case of the compromise of a delegated
   credential's private key, delegated credentials have a maximum
   validity period.  In the absence of an application profile standard
   specifying otherwise, the maximum validity period is set to 7 days.
   Peers MUST NOT issue credentials with a validity period longer than
   the maximum validity period or that extends beyond the validity
   period of the delegation certificate.  This mechanism is described in
   detail in Section 4.1.

   It was noted in [XPROT] that certificates in use by servers that
   support outdated protocols such as SSLv2 can be used to forge
   signatures for certificates that contain the keyEncipherment KeyUsage
   ([RFC5280] section 4.2.1.3).  In order to reduce the risk of cross-
   protocol attacks on certificates that are not intended to be used
   with DC-capable TLS stacks, we define a new DelegationUsage extension
   to X.509 that permits use of delegated credentials.  (See
   Section 4.2.)

3.1.  Rationale

   Delegated credentials present a better alternative than other
   delegation mechanisms like proxy certificates [RFC3820] for several
   reasons:

   *  There is no change needed to certificate validation at the PKI
      layer.

   *  X.509 semantics are very rich.  This can cause unintended
      consequences if a service owner creates a proxy certificate where
      the properties differ from the leaf certificate.  Proxy
      certificates can be useful in controlled environments, but remain
      a risk in scenarios where the additional flexibility they provide
      is not necessary.  For this reason, delegated credentials have
      very restricted semantics that should not conflict with X.509
      semantics.

   *  Proxy certificates rely on the certificate path building process
      to establish a binding between the proxy certificate and the end-
      entity certificate.  Since the certificate path building process
      is not cryptographically protected, it is possible that a proxy
      certificate could be bound to another certificate with the same
      public key, with different X.509 parameters.  Delegated
      credentials, which rely on a cryptographic binding between the
      entire certificate and the delegated credential, cannot.

   *  Each delegated credential is bound to a specific signature
      algorithm for use in the (D)TLS handshake ([RFC8446] section
      4.2.3).  This prevents them from being used with other, perhaps
      unintended, signature algorithms.  The signature algorithm bound
      to the delegated credential can be chosen independently of the set
      of signature algorithms supported by the end-entity certificate.

3.2.  Related Work

   Many of the use cases for delegated credentials can also be addressed
   using purely server-side mechanisms that do not require changes to
   client behavior (e.g., a PKCS#11 interface or a remote signing
   mechanism, [KEYLESS] being one example).  These mechanisms, however,
   incur per-transaction latency, since the front-end server has to
   interact with a back-end server that holds a private key.  The
   mechanism proposed in this document allows the delegation to be done
   off-line, with no per-transaction latency.  The figure below compares
   the message flows for these two mechanisms with (D)TLS 1.3 [RFC8446]
   [I-D.ietf-tls-dtls13].

Remote key signing:

Client            Front-End            Back-End
  |----ClientHello--->|                    |
  |<---ServerHello----|                    |
  |<---Certificate----|                    |
  |                   |<---remote sign---->|
  |<---CertVerify-----|                    |
  |        ...        |                    |

Delegated Credential:

Client            Front-End            Back-End
  |                   |<--DC distribution->|
  |----ClientHello--->|                    |
  |<---ServerHello----|                    |
  |<---Certificate----|                    |
  |<---CertVerify-----|                    |
  |        ...        |                    |

Legend:
Client: (D)TLS client
Front-End: (D)TLS server (could be a TLS-termination service like a CDN)
Back-End: Service with access to private key

   These two mechanisms can be complementary.  A server could use
   delegated credentials for clients that support them, while using a
   server-side mechanism to support legacy clients.  Both mechanisms
   require a trusted relationship between the Front-End and Back-End --
   the delegated credential can be used in place of a certificate
   private key.

   Use of short-lived certificates with automated certificate issuance,
   e.g., with Automated Certificate Management Environment (ACME)
   [RFC8555], reduces the risk of key compromise, but has several
   limitations.  Specifically, it introduces an operationally-critical
   dependency on an external party (the CA).  It also limits the types
   of algorithms supported for (D)TLS authentication to those the CA is
   willing to issue a certificate for.  Nonetheless, existing automated
   issuance APIs like ACME may be useful for provisioning delegated
   credentials.

4.  Delegated Credentials

   While X.509 forbids end-entity certificates from being used as
   issuers for other certificates, it is valid to use them to issue
   other signed objects as long as the certificate contains the
   digitalSignature KeyUsage ([RFC5280] section 4.2.1.3).  (All
   certificates compatible with TLS 1.3 are required to contain the
   digitalSignature KeyUsage.)  This document defines a new signed
   object format that would encode only the semantics that are needed
   for this application.  The Credential has the following structure:

      struct {
        uint32 valid_time;
        SignatureScheme dc_cert_verify_algorithm;
        opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
      } Credential;

   valid_time:  Time, in seconds relative to the delegation
      certificate's notBefore value, after which the delegated
      credential is no longer valid.  By default, unless set to an
      alternative value by an application profile (see
      Section Section 3), endpoints will reject delegated credentials
      that expire more than 7 days from the current time (as described
      in Section 4.1.3).

   dc_cert_verify_algorithm:  The signature algorithm of the Credential
      key pair, where the type SignatureScheme is as defined in
      [RFC8446].  This is expected to be the same as the sender's
      CertificateVerify.algorithm (as described in Section 4.1.3).  Only
      signature algorithms allowed for use in CertificateVerify messages
      are allowed (as described in [RFC8446] Section 11).  When using
      RSA, the public key MUST NOT use the rsaEncryption OID.  As a
      result, the following algorithms are not allowed for use with
      delegated credentials: rsa_pss_rsae_sha256, rsa_pss_rsae_sha384,
      rsa_pss_rsae_sha512.

   ASN1_subjectPublicKeyInfo:  The Credential's public key, a DER-
      encoded [X.690] SubjectPublicKeyInfo as defined in [RFC5280].

   The DelegatedCredential has the following structure:

      struct {
        Credential cred;
        SignatureScheme algorithm;
        opaque signature<0..2^16-1>;
      } DelegatedCredential;

   cred:  The Credential structure as previously defined.

   algorithm:  The signature algorithm used to create
      DelegatedCredential.signature.

   signature:  The delegation, a signature that binds the credential to
      the end-entity certificate's public key as specified below.  The
      signature scheme is specified by DelegatedCredential.algorithm.

   The signature of the DelegatedCredential is computed over the
   concatenation of:

   1.  An octet stream that consists of octet 32 (0x20) repeated 64
       times.

   2.  The non-null terminated context string "TLS, server delegated
       credentials" for server authentication and "TLS, client delegated
       credentials" for client authentication.

   3.  A single octet 0x00, which serves as the separator.

   4.  The DER-encoded X.509 end-entity certificate used to sign the
       DelegatedCredential.

   5.  DelegatedCredential.cred.

   6.  DelegatedCredential.algorithm.

   The signature is computed by using the private key of the peer's end-
   entity certificate, with the algorithm indicated by
   DelegatedCredential.algorithm.

   The signature effectively binds the credential to the parameters of
   the handshake in which it is used.  In particular, it ensures that
   credentials are only used with the certificate and signature
   algorithm chosen by the delegator.

   The code changes required in order to create and verify delegated
   credentials, and the implementation complexity this entails, are
   localized to the (D)TLS stack.  This has the advantage of avoiding
   changes to the often-delicate security-critical PKI code.

4.1.  Client and Server Behavior

   This document defines the following (D)TLS extension code point.

      enum {
        ...
        delegated_credential(34),
        (65535)
      } ExtensionType;

4.1.1.  Server Authentication

   A client that is willing to use delegated credentials in a connection
   SHALL send a "delegated_credential" extension in its ClientHello.
   The body of the extension consists of a SignatureSchemeList (defined
   in [RFC8446]):

      struct {
        SignatureScheme supported_signature_algorithm<2..2^16-2>;
      } SignatureSchemeList;

   If the client receives a delegated credential without having
   indicated support in its ClientHello, then the client MUST abort the
   handshake with an "unexpected_message" alert.

   If the extension is present, the server MAY send a delegated
   credential; if the extension is not present, the server MUST NOT send
   a delegated credential.  When a (D)TLS version negotiated is less
   than 1.3, the server MUST ignore this extension.  An example of when
   a server could choose not to send a delegated credential is when the
   SignatureSchemes listed only contain signature schemes for which a
   corresponding delegated credential does not exist or are otherwise
   unsuitable for the connection.

   The server MUST send the delegated credential as an extension in the
   CertificateEntry of its end-entity certificate; the client SHOULD
   ignore delegated credentials sent as extensions to any other
   certificate.

   The algorithm field MUST be of a type advertised by the client in the
   "signature_algorithms" extension of the ClientHello message and the
   dc_cert_verify_algorithm field MUST be of a type advertised by the
   client in the SignatureSchemeList and is considered not valid
   otherwise.  Clients that receive non-valid delegated credentials MUST
   terminate the connection with an "illegal_parameter" alert.

4.1.2.  Client Authentication

   A server that supports this specification SHALL send a
   "delegated_credential" extension in the CertificateRequest message
   when requesting client authentication.  The body of the extension
   consists of a SignatureSchemeList.  If the server receives a
   delegated credential without having indicated support in its
   CertificateRequest, then the server MUST abort with an
   "unexpected_message" alert.

   If the extension is present, the client MAY send a delegated
   credential; if the extension is not present, the client MUST NOT send
   a delegated credential.  When a (D)TLS version negotiated is less
   than 1.3, the client MUST ignore this extension.

   The client MUST send the DC as an extension in the CertificateEntry
   of its end-entity certificate; the server SHOULD ignore delegated
   credentials sent as extensions to any other certificate.

   The algorithm field MUST be of a type advertised by the server in the
   "signature_algorithms" extension of the CertificateRequest message
   and the dc_cert_verify_algorithm field MUST be of a type advertised
   by the server in the SignatureSchemeList and is considered not valid
   otherwise.  Servers that receive non-valid delegated credentials MUST
   terminate the connection with an "illegal_parameter" alert.

4.1.3.  Validating a Delegated Credential

   On receiving a delegated credential and certificate chain, the peer
   validates the certificate chain and matches the end-entity
   certificate to the peer's expected identity in the same way that it
   is done when delegated credentials are not in use.  It then performs
   the following checks with expiry time set to the delegation
   certificate's notBefore value plus
   DelegatedCredential.cred.valid_time:

   1.  Verify that the current time is within the validity interval of
       the credential.  This is done by asserting that the current time
       does not exceed the expiry time.  (The start time of the
       credential is implicitly validated as part of certificate
       validation.)

   2.  Verify that the delegated credential's remaining validity period
       is no more than the maximum validity period.  This is done by
       asserting that the expiry time does not exceed the current time
       plus the maximum validity period (7 days by default).

   3.  Verify that dc_cert_verify_algorithm matches the scheme indicated
       in the peer's CertificateVerify message and that the algorithm is
       allowed for use with delegated credentials.

   4.  Verify that the end-entity certificate satisfies the conditions
       in Section 4.2.

   5.  Use the public key in the peer's end-entity certificate to verify
       the signature of the credential using the algorithm indicated by
       DelegatedCredential.algorithm.

   If one or more of these checks fail, then the delegated credential is
   deemed not valid.  Clients and servers that receive non-valid
   delegated credentials MUST terminate the connection with an
   "illegal_parameter" alert.

   If successful, the participant receiving the Certificate message uses
   the public key in DelegatedCredential.cred to verify the signature in
   the peer's CertificateVerify message.

4.2.  Certificate Requirements

   This document defines a new X.509 extension, DelegationUsage, to be
   used in the certificate when the certificate permits the usage of
   delegated credentials.  What follows is the ASN.1 [X.680] for the
   DelegationUsage certificate extension.

       ext-delegationUsage EXTENSION  ::= {
           SYNTAX DelegationUsage IDENTIFIED BY id-pe-delegationUsage
       }

       DelegationUsage ::= NULL

       id-pe-delegationUsage OBJECT IDENTIFIER ::=
           { iso(1) identified-organization(3) dod(6) internet(1)
             private(4) enterprise(1) id-cloudflare(44363) 44 }

   The extension MUST be marked non-critical.  (See Section 4.2 of
   [RFC5280].)  An endpoint MUST NOT accept a delegated credential
   unless the peer's end-entity certificate satisfies the following
   criteria:

   *  It has the DelegationUsage extension.

   *  It has the digitalSignature KeyUsage (see the KeyUsage extension
      defined in [RFC5280]).

   A new extension was chosen instead of adding a new Extended Key Usage
   (EKU) to be compatible with deployed (D)TLS and PKI software stacks
   without requiring CAs to issue new intermediate certificates.

5.  Operational Considerations

   The operational consideration documented in this section should be
   taken into consideration when using Delegated Certificates.

5.1.  Client Clock Skew

   One of the risks of deploying a short-lived credential system based
   on absolute time is client clock skew.  If a client's clock is
   sufficiently ahead or behind of the server's clock, then clients will
   reject delegated credentials that are valid from the server's
   perspective.  Clock skew also affects the validity of the original
   certificates.  The lifetime of the delegated credential should be set
   taking clock skew into account.  Clock skew may affect a delegated
   credential at the beginning and end of its validity periods, which
   should also be taken into account.

6.  IANA Considerations

   This document registers the "delegated_credential" extension in the
   "TLS ExtensionType Values" registry.  The "delegated_credential"
   extension has been assigned a code point of 34.  The IANA registry
   lists this extension as "Recommended" (i.e., "Y") and indicates that
   it may appear in the ClientHello (CH), CertificateRequest (CR), or
   Certificate (CT) messages in (D)TLS 1.3 [RFC8446]
   [I-D.ietf-tls-dtls13].  Additionally, the "DTLS-Only" column is
   assigned the value "N".

   This document also defines an ASN.1 module for the DelegationUsage
   certificate extension in Appendix A.  IANA has registered value 95
   for "id-mod-delegated-credential-extn" in the "SMI Security for PKIX
   Module Identifier" (1.3.5.1.5.5.7.0) registry.  An OID for the
   DelegationUsage certificate extension is not needed as it is already
   assigned to the extension from Cloudflare's IANA Private Enterprise
   Number (PEN) arc.

7.  Security Considerations

   The security consideration documented in this section should be taken
   into consideration when using Delegated Certificates.

7.1.  Security of Delegated Credential's Private Key

   Delegated credentials limit the exposure of the private key used in a
   (D)TLS connection by limiting its validity period.  An attacker who
   compromises the private key of a delegated credential cannot create
   new delegated credentials, but they can impersonate the compromised
   party in new TLS connections until the delegated credential expires.

   Thus, delegated credentials should not be used to send a delegation
   to an untrusted party, but are meant to be used between parties that
   have some trust relationship with each other.  The secrecy of the
   delegated credential's private key is thus important, and access
   control mechanisms SHOULD be used to protect it, including file
   system controls, physical security, or hardware security modules.

7.2.  Re-use of Delegated Credentials in Multiple Contexts

   It is not possible to use the same delegated credential for both
   client and server authentication because issuing parties compute the
   corresponding signature using a context string unique to the intended
   role (client or server).

7.3.  Revocation of Delegated Credentials

   Delegated credentials do not provide any additional form of early
   revocation.  Since it is short-lived, the expiry of the delegated
   credential revokes the credential.  Revocation of the long term
   private key that signs the delegated credential (from the end-entity
   certificate) also implicitly revokes the delegated credential.

7.4.  Interactions with Session Resumption

   If a peer decides to cache the certificate chain and re-validate it
   when resuming a connection, they SHOULD also cache the associated
   delegated credential and re-validate it.  Failing to do so may result
   in resuming connections for which the DC has expired.

7.5.  Privacy Considerations

   Delegated credentials can be valid for 7 days (by default) and it is
   much easier for a service to create delegated credentials than a
   certificate signed by a CA.  A service could determine the client
   time and clock skew by creating several delegated credentials with
   different expiry timestamps and observing whether the client would
   accept it.  Client time could be unique and thus privacy-sensitive
   clients, such as browsers in incognito mode, who do not trust the
   service might not want to advertise support for delegated credentials
   or limit the number of probes that a server can perform.

7.6.  The Impact of Signature Forgery Attacks

   Delegated credentials are only used in (D)TLS 1.3 connections.
   However, the certificate that signs a delegated credential may be
   used in other contexts such as (D)TLS 1.2.  Using a certificate in
   multiple contexts opens up a potential cross-protocol attack against
   delegated credentials in (D)TLS 1.3.

   When (D)TLS 1.2 servers support RSA key exchange, they may be
   vulnerable to attacks that allow forging an RSA signature over an
   arbitrary message [BLEI].  TLS 1.2 [RFC5246] (Section 7.4.7.1.)
   describes a mitigation strategy requiring careful implementation of
   timing resistant countermeasures for preventing these attacks.
   Experience shows that in practice, server implementations may fail to
   fully stop these attacks due to the complexity of this mitigation
   [ROBOT].  For (D)TLS 1.2 servers that support RSA key exchange using
   a DC-enabled end-entity certificate, a hypothetical signature forgery
   attack would allow forging a signature over a delegated credential.
   The forged delegated credential could then be used by the attacker as
   the equivalent of a on-path-attacker, valid for a maximum of 7 days
   (if the default valid_time is used).

   Server operators should therefore minimize the risk of using DC-
   enabled end-entity certificates where a signature forgery oracle may
   be present.  If possible, server operators may choose to use DC-
   enabled certificates only for signing credentials, and not for
   serving non-DC (D)TLS traffic.  Furthermore, server operators may use
   elliptic curve certificates for DC-enabled traffic, while using RSA
   certificates without the DelegationUsage certificate extension for
   non-DC traffic; this completely prevents such attacks.

   Note that if a signature can be forged over an arbitrary credential,
   the attacker can choose any value for the valid_time field.  Repeated
   signature forgeries therefore allow the attacker to create multiple
   delegated credentials that can cover the entire validity period of
   the certificate.  Temporary exposure of the key or a signing oracle
   may allow the attacker to impersonate a server for the lifetime of
   the certificate.

8.  Acknowledgements

   Thanks to David Benjamin, Christopher Patton, Kyle Nekritz, Anirudh
   Ramachandran, Benjamin Kaduk, Kazuho Oku, Daniel Kahn Gillmor, Watson
   Ladd, Robert Merget, Juraj Somorovsky, Nimrod Aviram for their
   discussions, ideas, and bugs they have found.

9.  References

9.1.  Normative References

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              dtls13-43>.

   [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>.

   [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>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [X.680]    ITU-T, "Information technology - Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ISO/
              IEC 8824-1:2015, November 2015.

   [X.690]    ITU-T, "Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ISO/IEC 8825-1:2015, November 2015.

9.2.  Informative References

   [BLEI]     Bleichenbacher, D., "Chosen Ciphertext Attacks against
              Protocols Based on RSA Encryption Standard PKCS #1",
              Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462,
              pages: 1-12 , 1998.

   [KEYLESS]  Sullivan, N. and D. Stebila, "An Analysis of TLS Handshake
              Proxying", IEEE Trustcom/BigDataSE/ISPA 2015 , 2015.

   [RFC3820]  Tuecke, S., Welch, V., Engert, D., Pearlman, L., and M.
              Thompson, "Internet X.509 Public Key Infrastructure (PKI)
              Proxy Certificate Profile", RFC 3820,
              DOI 10.17487/RFC3820, June 2004,
              <https://www.rfc-editor.org/rfc/rfc3820>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/rfc/rfc5246>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/rfc/rfc5912>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/rfc/rfc8032>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/rfc/rfc8555>.

   [ROBOT]    Boeck, H., Somorovsky, J., and C. Young, "Return Of
              Bleichenbacher's Oracle Threat (ROBOT)", 27th USENIX
              Security Symposium , 2018.

   [XPROT]    Jager, T., Schwenk, J., and J. Somorovsky, "On the
              Security of TLS 1.3 and QUIC Against Weaknesses in PKCS#1
              v1.5 Encryption", Proceedings of the 22nd ACM SIGSAC
              Conference on Computer and Communications Security , 2015.

Appendix A.  ASN.1 Module

   The following ASN.1 module provides the complete definition of the
   DelegationUsage certificate extension.  The ASN.1 module makes
   imports from [RFC5912].

   DelegatedCredentialExtn
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-delegated-credential-extn(95) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   -- EXPORT ALL

   IMPORTS

   EXTENSION
     FROM PKIX-CommonTypes-2009 -- From RFC 5912
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-pkixCommon-02(57) } ;

   -- OID

   id-cloudflare OBJECT IDENTIFIER ::=
     { iso(1) identified-organization(3) dod(6) internet(1) private(4)
       enterprise(1) 44363 }

   -- EXTENSION

   ext-delegationUsage EXTENSION ::=
     { SYNTAX DelegationUsage
       IDENTIFIED BY id-pe-delegationUsage }

   id-pe-delegationUsage OBJECT IDENTIFIER ::= { id-cloudflare 44 }

   DelegationUsage ::= NULL

   END

Appendix B.  Example Certificate

   The following is an example of a delegation certificate which
   satisfies the requirements described in Section 4.2 (i.e., uses the
   DelegationUsage extension and has the digitalSignature KeyUsage).

   -----BEGIN CERTIFICATE-----
   MIIFRjCCBMugAwIBAgIQDGevB+lY0o/OecHFSJ6YnTAKBggqhkjOPQQDAzBMMQsw
   CQYDVQQGEwJVUzEVMBMGA1UEChMMRGlnaUNlcnQgSW5jMSYwJAYDVQQDEx1EaWdp
   Q2VydCBFQ0MgU2VjdXJlIFNlcnZlciBDQTAeFw0xOTAzMjYwMDAwMDBaFw0yMTAz
   MzAxMjAwMDBaMGoxCzAJBgNVBAYTAlVTMRMwEQYDVQQIEwpDYWxpZm9ybmlhMRYw
   FAYDVQQHEw1TYW4gRnJhbmNpc2NvMRkwFwYDVQQKExBDbG91ZGZsYXJlLCBJbmMu
   MRMwEQYDVQQDEwprYzJrZG0uY29tMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAE
   d4azI83Bw0fcPgfoeiZpZZnwGuxjBjv++wzE0zAj8vNiUkKxOWSQiGNLn+xlWUpL
   lw9djRN1rLmVmn2gb9GgdKOCA28wggNrMB8GA1UdIwQYMBaAFKOd5h/52jlPwG7o
   kcuVpdox4gqfMB0GA1UdDgQWBBSfcb7fS3fUFAyB91fRcwoDPtgtJjAjBgNVHREE
   HDAaggprYzJrZG0uY29tggwqLmtjMmtkbS5jb20wDgYDVR0PAQH/BAQDAgeAMB0G
   A1UdJQQWMBQGCCsGAQUFBwMBBggrBgEFBQcDAjBpBgNVHR8EYjBgMC6gLKAqhiho
   dHRwOi8vY3JsMy5kaWdpY2VydC5jb20vc3NjYS1lY2MtZzEuY3JsMC6gLKAqhiho
   dHRwOi8vY3JsNC5kaWdpY2VydC5jb20vc3NjYS1lY2MtZzEuY3JsMEwGA1UdIARF
   MEMwNwYJYIZIAYb9bAEBMCowKAYIKwYBBQUHAgEWHGh0dHBzOi8vd3d3LmRpZ2lj
   ZXJ0LmNvbS9DUFMwCAYGZ4EMAQICMHsGCCsGAQUFBwEBBG8wbTAkBggrBgEFBQcw
   AYYYaHR0cDovL29jc3AuZGlnaWNlcnQuY29tMEUGCCsGAQUFBzAChjlodHRwOi8v
   Y2FjZXJ0cy5kaWdpY2VydC5jb20vRGlnaUNlcnRFQ0NTZWN1cmVTZXJ2ZXJDQS5j
   cnQwDAYDVR0TAQH/BAIwADAPBgkrBgEEAYLaSywEAgUAMIIBfgYKKwYBBAHWeQIE
   AgSCAW4EggFqAWgAdgC72d+8H4pxtZOUI5eqkntHOFeVCqtS6BqQlmQ2jh7RhQAA
   AWm5hYJ5AAAEAwBHMEUCICiGfq+hSThRL2m8H0awoDR8OpnEHNkF0nI6nL5yYL/j
   AiEAxwebGs/T6Es0YarPzoQJrVZqk+sHH/t+jrSrKd5TDjcAdgCHdb/nWXz4jEOZ
   X73zbv9WjUdWNv9KtWDBtOr/XqCDDwAAAWm5hYNgAAAEAwBHMEUCIQD9OWA8KGL6
   bxDKfgIleHJWB0iWieRs88VgJyfAg/aFDgIgQ/OsdSF9XOy1foqge0DTDM2FExuw
   0JR0AGZWXoNtJzMAdgBElGUusO7Or8RAB9io/ijA2uaCvtjLMbU/0zOWtbaBqAAA
   AWm5hYHgAAAEAwBHMEUCIQC4vua1n3BqthEqpA/VBTcsNwMtAwpCuac2IhJ9wx6X
   /AIgb+o00k28JQo9TMpP4vzJ3BD3HXWSNc2Zizbq7mkUQYMwCgYIKoZIzj0EAwMD
   aQAwZgIxAJsX7d0SuA8ddf/m7IWfNfs3MQfJyGkEezMJX1t6sRso5z50SS12LpXe
   muGa1FE2ZgIxAL+CDUF5pz7mhrAEIjQ1MqlpF9tH40dJGvYZZQ3W23cMzSkDfvlt
   y5S4RfWHIIPjbw==
   -----END CERTIFICATE-----

Authors' Addresses

   Richard Barnes
   Cisco
   Email: rlb@ipv.sx

   Subodh Iyengar
   Facebook
   Email: subodh@fb.com

   Nick Sullivan
   Cloudflare
   Email: nick@cloudflare.com

   Eric Rescorla
   Mozilla
   Email: ekr@rtfm.com