Delegated Credentials for TLS
draft-ietf-tls-subcerts-09

Versions: (draft-rescorla-tls-subcerts) 00 01 02         Standards Track
          03 04 05 06 07 08 09                                          
Network Working Group                                          R. Barnes
Internet-Draft                                                     Cisco
Intended status: Standards Track                              S. Iyengar
Expires: 28 December 2020                                       Facebook
                                                             N. Sullivan
                                                              Cloudflare
                                                             E. Rescorla
                                                                 Mozilla
                                                            26 June 2020


                     Delegated Credentials for TLS
                       draft-ietf-tls-subcerts-09

Abstract

   The organizational separation between the operator of a TLS endpoint
   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 by
   which operators may delegate their own credentials for use in TLS,
   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
   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."




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   This Internet-Draft will expire on 28 December 2020.

Copyright Notice

   Copyright (c) 2020 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
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   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
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
     2.1.  Change Log  . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Related Work  . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Delegated Credentials . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Client and Server Behavior  . . . . . . . . . . . . . . .   9
       4.1.1.  Server Authentication . . . . . . . . . . . . . . . .   9
       4.1.2.  Client Authentication . . . . . . . . . . . . . . . .  10
       4.1.3.  Validating a Delegated Credential . . . . . . . . . .  11
     4.2.  Certificate Requirements  . . . . . . . . . . . . . . . .  11
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  12
     5.1.  Client Clock Skew . . . . . . . . . . . . . . . . . . . .  12
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
     7.1.  Security of Delegated Credential's Private Key  . . . . .  13
     7.2.  Re-use of Delegated Credentials in Multiple
           Contexts  . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.3.  Revocation of Delegated Credentials . . . . . . . . . . .  13
     7.4.  Interactions with Session Resumption  . . . . . . . . . .  13
     7.5.  Privacy Considerations  . . . . . . . . . . . . . . . . .  14
     7.6.  The Impact of Signature Forgery Attacks . . . . . . . . .  14
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17




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1.  Introduction

   Typically, a TLS server uses a certificate provided by some entity
   other than the operator of the server (a "Certification Authority" or
   CA) [RFC8446] [RFC5280].  This organizational separation makes the
   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 server operator can only use TLS signature schemes for which
      the CA will issue credentials.

   These dependencies cause problems in practice.  Server operators
   often deploy TLS termination services in locations such as remote
   data centers or Content Delivery Networks (CDNs) where it may be
   difficult to detect key compromises.  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 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 service.

   To reduce the dependency on external CAs, this document proposes a
   limited delegation mechanism that allows a 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
   speak for names that the CA has authorized.  For clarity, we will
   refer 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".

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.







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2.1.  Change Log

   (*) indicates changes to the wire protocol.

   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




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   *  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 invalid protocol version; and when DCs are
      sent as extensions to certificates other than the end-entity
      certificate.

3.  Solution Overview

   A delegated credential 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 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 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 authenticating initiator uses information from the peer's
      certificate to verify the delegated credential and that the peer
      is asserting an expected identity.

   *  Peers accepting the delegated credential use it as the certificate
      key for the TLS handshake

   As detailed in Section 4, the delegated credential is
   cryptographically bound to the end-entity certificate with which the



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   credential may be used.  This document specifies the use of delegated
   credentials in TLS 1.3 or later; their use in prior versions of the
   protocol is not allowed.

   Delegated credentials allow a peer to terminate 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 delegated credential private key
   compromise, 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.  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 prevent this type of cross-
   protocol attack, 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.  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
      server 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 that may be used to sign the TLS handshake ([RFC8446]




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      section 4.2.3).  This prevents them from being used with other,
      perhaps unintended signature algorithms.

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]).  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 TLS 1.3 [RFC8446].

   Remote key signing:

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


   Delegated credentials:

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

   These two mechanisms can be complementary.  A server could use
   credentials for clients that support them, while using [KEYLESS] to
   support legacy clients.  The private key for a delegated credential
   can be used in place of a certificate private key, so it is important
   that the Front-End and Back-End are parties that have a trusted
   relationship.

   Use of short-lived certificates with automated certificate issuance,
   e.g., with Automated Certificate Managment Environment (ACME)
   [RFC8555], reduces the risk of key compromise, but has several
   limitations.  Specifically, it introduces an operationally-critical
   dependency on an external party.  It also limits the types of



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   algorithms supported for 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).  We define 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 expected_cert_verify_algorithm;
        opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
      } Credential;

   valid_time:  Time in seconds relative to the beginning of the
      delegation certificate's notBefore value after which the delegated
      credential is no longer valid.  This MUST NOT exceed 7 days.

   expected_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
      CertificateVerify.algorithm sent by the server.  Only signature
      algorithms allowed for use in CertificateVerify messages are
      allowed.  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 delegated credential has the following structure:

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

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



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   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.  A string that consists of octet 32 (0x20) repeated 64 times.

   2.  The context string "TLS, server delegated credentials" for
       servers and "TLS, client delegated credentials" for clients.

   3.  A single 0 byte, 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 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 TLS stack.  This has the advantage of avoiding
   changes to security-critical and often delicate PKI code.

4.1.  Client and Server Behavior

   This document defines the following TLS extension code point.

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

4.1.1.  Server Authentication

   A client which supports this specification SHALL send a
   "delegated_credential" extension in its ClientHello.  The body of the
   extension consists of a SignatureSchemeList:





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

   If the client receives a delegated credential without indicating
   support, then the client MUST abort 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.  The server MUST ignore the extension unless
   TLS 1.3 or a later version is negotiated.

   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 expected_cert_verify_algorithm field MUST be of a type advertised
   by the client in the SignatureSchemeList and is considered invalid
   otherwise.  Clients that receive invalid 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 indicating 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.  The client MUST ignore the extension unless
   TLS 1.3 or a later version is negotiated.

   The client MUST send the delegated credential 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 expected_cert_verify_algorithm field MUST be of a type
   advertised by the server in the SignatureSchemeList and considered
   invalid otherwise.  Servers that receive invalid delegated



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   credentials MUST terminate the connection with an "illegal_parameter"
   alert.

4.1.3.  Validating a Delegated Credential

   On receiving a delegated credential and a certificate chain, the peer
   validates the certificate chain and matches the end-entity
   certificate to the peer's expected identity.  It also takes the
   following steps:

   1.  Verify that the current time is within the validity interval of
       the credential.  This is done by asserting that the current time
       is no more than the delegation certificate's notBefore value plus
       DelegatedCredential.cred.valid_time.

   2.  Verify that the credential's remaining validity time is no more
       than the maximum validity period.  This is done by asserting that
       the current time is no more than the delegation certificate's
       notBefore value plus DelegatedCredential.cred.valid_time plus the
       maximum validity period.

   3.  Verify that expected_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 invalid.  Clients and servers that receive invalid delegated
   credentials MUST terminate the connection with an "illegal_parameter"
   alert.  If successful, the participant receiving the Certificate
   message uses the public key in the credential to verify the signature
   in the peer's CertificateVerify message.

4.2.  Certificate Requirements

   We define 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.






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

       DelegationUsage ::= NULL

       id-ce-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].)  The client MUST NOT accept a delegated credential unless
   the server'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 TLS and PKI software stacks
   without requiring CAs to issue new intermediate certificates.

5.  Operational Considerations

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 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_credentials" extension in the
   "TLS ExtensionType Values" registry.  The "delegated_credentials"
   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 TLS 1.3 [RFC8446].

   This document also defines an ASN.1 module for the DelegationUsage
   certificate extension in Appendix A.  IANA is requested to register



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   an Object Identfier (OID) for the ASN.1 in "SMI Security for PKIX
   Module Identifier" arc.  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

7.1.  Security of Delegated Credential's Private Key

   Delegated credentials limit the exposure of the private key used in a
   TLS connection by limiting its validity period.  An attacker who
   compromises the private key of a delegated credential can act as a
   man-in-the-middle until the delegated credential expires.  However,
   they cannot create new delegated credentials.  Thus, delegated
   credentials should not be used to send a delegation to an untrusted
   party, but is 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 would revoke the credential.  Revocation of the long term
   private key that signs the delegated credential also implicitly
   revokes the delegated credential.

7.4.  Interactions with Session Resumption

   If a client decides to cache the certificate chain and re-validate it
   when resuming a connection, the client SHOULD also cache the
   associated delegated credential and re-validate it.










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7.5.  Privacy Considerations

   Delegated credentials can be valid for 7 days and it is much easier
   for a service to create delegated credential 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

   When 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
   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
   credential could then be used by the attacker as the equivalent of a
   man-in-the-middle certificate, valid for 7 days.

   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 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.








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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

   [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>.

   [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>.

   [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/info/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/info/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.



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   [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/info/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/info/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/info/rfc5912>.

   [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/info/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].
















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

   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-ce-delegationUsage }

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

   DelegationUsage ::= NULL

   END

Authors' Addresses

   Richard Barnes
   Cisco

   Email: rlb@ipv.sx


   Subodh Iyengar
   Facebook

   Email: subodh@fb.com



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   Nick Sullivan
   Cloudflare

   Email: nick@cloudflare.com


   Eric Rescorla
   Mozilla

   Email: ekr@rtfm.com









































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