Network Working Group                                          R. Barnes
Internet-Draft                                                     Cisco
Intended status: Standards Track                              S. Iyengar
Expires: May 6, 8 August 2020                                          Facebook
                                                             N. Sullivan
                                                             E. Rescorla
                                                       November 03, 2019
                                                         5 February 2020

                     Delegated Credentials for TLS


   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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
     2.1.  Change Log  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Related Work  . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Delegated Credentials . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Client and Server behavior  . . . . . . . . . . . . . . .   8
       4.1.1.  Server authentication . . . . . . . . . . . . . . . .   8
       3.1.2.   9
       4.1.2.  Client authentication . . . . . . . . . . . . . . . .   9
       4.1.3.  Validating a Delegated Credential . . . . . . . . . .   9
     3.2.  10
     4.2.  Certificate Requirements  . . . . . . . . . . . . . . . .  10
   4.  11
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
     6.1.  Security of delegated private key . . . . . . . . . . . .  11
     5.2.  12
     6.2.  Re-use of delegated credentials in multiple
           contexts  . .  11
     5.3. . . . . . . . . . . . . . . . . . . . . . .  12
     6.3.  Revocation of delegated credentials . . . . . . . . . . .  11
     5.4.  12
     6.4.  Interactions with session resumption  . . . . . . . . . .  12
     6.5.  Privacy considerations  . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   7.  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  13  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14  15

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 authentication schemes for
      which the CA will issue credentials.

   These dependencies cause problems in practice.  Server operators
   often want to create short-lived certificates for servers in low-
   trust zones such as Content Delivery Network (CDNs) or remote data
   centers.  This allows server operators to limit the exposure of keys
   in cases that they do not realize a compromise has occurred.  The
   risk inherent in cross-organizational transactions makes it
   operationally infeasible to rely on an external CA for such short-
   lived credentials.  In Online Certiicate Certificate Status Protocol (OCSP)
   stapling (i.e., using the Certificate Status extension types type ocsp
   [RFC6066] or ocsp_multi [RFC6961]),
   [RFC8446], if an operator chooses to talk frequently to the CA to
   obtain stapled responses, then failure to fetch an OCSP stapled
   response results only in degraded performance.  On the other hand,
   failure to fetch a potentially large number of short lived
   certificates would result in the service not being available, which
   creates greater operational risk.

   To remove these dependencies, 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.  Because the above problems do not relate to the CA's inherent
   function of validating possession of names, it is safe to make such
   delegations as long as they 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",
   "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

   (*) indicates changes to the wire protocol.


   *  Modified IANA section, fixed nits


   *  Removed support for PKCS 1.5 RSA signature algorithms.


   *  Additional security considerations.



   *  Add support for client certificates.



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


   *  Change public key type. (*)


   *  Change DelegationUsage extension to be NULL and define its object


   *  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


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 secret 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 normal
   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 intitiator 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's working key for the TLS hadshake

   As detailed in Section 3, 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 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 a 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 compromised,
   peers may not set to 7 days.  Peers MUST NOT issue
   credentials with a validity period longer than 7
   days. the maximum validity
   period.  This mechanism is described in detail in Section 3.1. 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  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 3.2.)

2.1. 4.2.)

3.1.  Rationale

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


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


   *  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]
      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.

   It is possible to address the short-lived certificate concerns above
   by automating certificate issuance, e.g., with Automated Certificate
   Managmeent Encvironment (ACME) [RFC8555].  In addition to requiring
   frequent operationally-critical interactions with an external party,
   this makes the server operator dependent on the CA's willingness to
   issue certificates with sufficiently short lifetimes.  It also fails
   to address the issues with algorithm support.  Nonetheless, existing
   automated issuance APIs like ACME may be useful for provisioning
   credentials within an operator network.


4.  Delegated Credentials

   While X.509 forbids end-entity certificates from being used as
   issuers for other certificates, it is perfectly fine to use them to
   issue other signed objects as long as the certificate contains the
   digitalSignature KeyUsage ([RFC5280] section  We define a
   new signed object format that would encode only the semantics that
   are needed for this application.  The credential has the following

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

   valid_time:  Relative time in seconds from the beginning of the
      delegation certificate's notBefore value after which the delegated
      credential is no longer valid.

   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 [X690] [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

   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

   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.  Minimizing their semantics in
   this way is intended to mitigate the risk of cross protocol attacks
   involving delegated credentials.

   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 {
      } ExtensionType;


4.1.1.  Server authentication

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

      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"

   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

   The algorithm and expected_cert_verify_algorithm fields MUST be of a
   type advertised by the client in the "signature_algorithms" extension SignatureSchemeList and are
   considered invalid otherwise.  Clients that receive invalid delegated
   credentials MUST terminate the connection with an "illegal_parameter"


4.1.2.  Client authentication

   A server which supports this specification SHALL send an empty
   "delegated_credential" extension in the CertificateRequest message
   when requesting client authentication.  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

   The algorithm and expected_cert_verify_algorithm fields MUST be of a
   type advertised by the server in the "signature_algorithms" extension
   and are considered invalid otherwise.  Servers that receive invalid
   delegated 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 in the usual way.  It
   also takes the following steps:

   1.  Verify that the current time is within the validity interval of
       the credential and that the credential's time to live is no more
       than 7 days. 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.

   2.  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.

   3.  Verify that the end-entity certificate satisfies the conditions
       in Section 3.2. 4.2.

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

   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 [X680] [X.680] for the
   DelegationUsage certificate extension.

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

       DelegationUsage ::= NULL

       id-ce-delegationUsage OBJECT IDENTIFIER ::= {
           1 3 6 1 4 1 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]).


5.  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 TBD. 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
   an Object Identfiier (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.


6.  Security Considerations

6.1.  Security of delegated private key

   Delegated credentials limit the exposure of the TLS private key by
   limiting its validity.  An attacker who compromises the private key
   of a delegated credential can act as a man-in-the-middle until the
   delegate 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 private key is thus important and several
   access control mechanisms SHOULD be used to protect it, including
   file system controls, physical security, or hardware security


6.2.  Re-use of delegated credentials in multiple contexts

   It is possible to use the same delegated credential for both client
   and server authentication if the Certificate allows it.  This is safe
   because the context string used for delegated credentials is distinct
   in both contexts.


6.3.  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.


6.4.  Interactions with session resumption

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


6.5.  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.  Acknowledgements

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


8.  References


8.1.  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,

   [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,

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Infrastructure Using X.509 (PKIX)", Words", BCP 14, RFC 5912, 8174, DOI 10.17487/RFC5912, June 2010,
              <>. 10.17487/RFC8174,
              May 2017, <>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,


   [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.


8.2.  Informative References

   [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,

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension",

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 6961, 5912,
              DOI 10.17487/RFC6961, 10.17487/RFC5912, June 2013,
              <>. 2010,

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,

   [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 { joint-iso-itu-t(2) country(16) us(840)
   organization(1) gov(101) dod(2) infosec(1) modules(0) iso(1) identified-organization(3) dod(6)
   internet(1) security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-
   delegated-credential-extn(TBD) }





   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) } ;

   - OIDS

   id-cloudflare OBJECT IDENTIFIER ::= { 1 3 6 1 4 1 44363 }


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

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


Authors' Addresses

   Richard Barnes


   Subodh Iyengar


   Nick Sullivan


   Eric Rescorla