Network Working Group R. Barnes
Internet-Draft Cisco
Intended status: Standards Track S. Iyengar
Expires: May 6, 2020 Facebook
N. Sullivan
Cloudflare
E. Rescorla
Mozilla
November 03, 2019
Delegated Credentials for TLS
draft-ietf-tls-subcerts-05
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.
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
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 6, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Change Log . . . . . . . . . . . . . . . . . . . . . . . 3
2. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Related Work . . . . . . . . . . . . . . . . . . . . . . 6
3. Delegated Credentials . . . . . . . . . . . . . . . . . . . . 7
3.1. Client and Server behavior . . . . . . . . . . . . . . . 8
3.1.1. Server authentication . . . . . . . . . . . . . . . . 8
3.1.2. Client authentication . . . . . . . . . . . . . . . . 9
3.1.3. Validating a Delegated Credential . . . . . . . . . . 9
3.2. Certificate Requirements . . . . . . . . . . . . . . . . 10
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5.1. Security of delegated private key . . . . . . . . . . . . 11
5.2. Re-use of delegated credentials in multiple contexts . . 11
5.3. Revocation of delegated credentials . . . . . . . . . . . 11
5.4. Interactions with session resumption . . . . . . . . . . 12
5.5. Privacy considerations . . . . . . . . . . . . . . . . . 12
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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:
o Whenever the server operator wants to deploy a new certificate, it
has to interact with the CA.
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o 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 Status Protocol (OCSP)
stapling (i.e., using the Certificate Status extension types ocsp
[RFC6066] or ocsp_multi [RFC6961]), 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".
1.1. Change Log
(*) indicates changes to the wire protocol.
draft-05
o Removed support for PKCS 1.5 RSA signature algorithms.
o Additional security considerations.
draft-04
o Add support for client certificates.
draft-03
o Remove protocol version from the Credential structure. (*)
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draft-02
o Change public key type. (*)
o Change DelegationUsage extension to be NULL and define its object
identifier.
o Drop support for TLS 1.2.
o Add the protocol version and credential signature algorithm to the
Credential structure. (*)
o 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.
2. 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:
o The initiating peer provides an extension in its ClientHello or
CertificateRequest that indicates support for this mechanism.
o The peer sending the Certificate message provides both the
certificate chain terminating in its certificate as well as the
delegated credential.
o The authenticating intitiator uses information from the peer's
certificate to verify the delegated credential and that the peer
is asserting an expected identity.
o Peers accepting the delegated credential use it as the
certificate's working key for the TLS hadshake
As detailed in Section 3, 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
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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 delegated credential is compromised,
peers may not issue credentials with a validity period longer than 7
days. This mechanism is described in detail in Section 3.1.
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 3.2.)
2.1. Rationale
Delegated credentials present a better alternative than other
delegation mechanisms like proxy certificates [RFC3820] for several
reasons:
o There is no change needed to certificate validation at the PKI
layer.
o 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.
o 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.
o 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.
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2.2. 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.
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3. 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 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: 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] 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.
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.
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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. 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.
3.1. Client and Server behavior
This document defines the following TLS extension code point.
enum {
...
delegated_credential(TBD),
(65535)
} ExtensionType;
3.1.1. Server authentication
A client which supports this specification SHALL send an empty
"delegated_credential" extension in its ClientHello. 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
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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 algorithm and expected_cert_verify_algorithm fields MUST be of a
type advertised by the client in the "signature_algorithms" extension
and are considered invalid otherwise. Clients that receive invalid
delegated credentials MUST terminate the connection with an
"illegal_parameter" alert.
3.1.2. 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
certificate.
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.
3.1.3. 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
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than 7 days. 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. 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.
3.2. 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] 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:
o It has the DelegationUsage extension.
o It has the digitalSignature KeyUsage (see the KeyUsage extension
defined in [RFC5280]).
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4. 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. 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.
5. Security Considerations
5.1. 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
modules.
5.2. 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.
5.3. 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.
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5.4. 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.
5.5. 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.
6. 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.
7. References
7.1. Normative References
[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>.
[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>.
[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>.
[X680] ITU-T, "Information technology - Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ISO/
IEC 8824-1:2015, November 2015.
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[X690] 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.
7.2. 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,
<https://www.rfc-editor.org/info/rfc3820>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
DOI 10.17487/RFC6961, June 2013,
<https://www.rfc-editor.org/info/rfc6961>.
[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>.
[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) id-mod-
delegate-credential-extn(TBD) }
DEFINITIONS IMPLICIT TAGS ::=
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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) } ;
- OIDS
id-cloudflare OBJECT IDENTIFIER ::= { 1 3 6 1 4 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
Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
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Eric Rescorla
Mozilla
Email: ekr@rtfm.com
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