Internet Engineering Task Force (IETF) R. Barnes

Internet Engineering Task Force (IETF) R. Barnes
Request for Comments: 9345 Cisco
Category: Standards Track S. Iyengar
ISSN: 2070-1721 Facebook
N. Sullivan
Cloudflare
E. Rescorla
Windy Hill Systems, LLC
July 2023

Delegated Credentials for TLS and DTLS

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 (CA). This document describes a mechanism 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.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9345.

Copyright Notice

Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Table of Contents

1. Introduction
2. Conventions and Terminology
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. References
8.1. Normative References
8.2. Informative References
Appendix A. ASN.1 Module
Appendix B. Example Certificate
Acknowledgements
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 certificate 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 keys, 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".

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.

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]. Figure 1 shows the intended deployment architecture.

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 a private key

Figure 1: Delegated Credentials Deployment Architecture

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 and 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
offline, with no per-transaction latency. The figure below compares
the message flows for these two mechanisms with (D)TLS 1.3 [RFC8446]
[RFC9147].

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 a 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.

The use of short-lived certificates with automated certificate
issuance, e.g., with the 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 encodes 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 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).
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,
and 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<1..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_algorithms<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 MUST NOT
use delegated credentials sent as extensions to any other
certificate, and SHOULD ignore them, but MAY abort the handshake with
an "illegal_parameter" alert. If the server sends multiple delegated
credentials extensions in a single CertificateEntry, the client MUST
abort the handshake with an "illegal_parameter" alert.

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; otherwise, the credential is
considered not valid. 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 delegated credential as an extension in the
CertificateEntry of its end-entity certificate; the server MUST NOT
use delegated credentials sent as extensions to any other
certificate, and SHOULD ignore them, but MAY abort the handshake with
an "illegal_parameter" alert. If the client sends multiple delegated
credentials extensions in a single CertificateEntry, the server MUST
abort the handshake with an "illegal_parameter" alert.

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; otherwise, the credential
is considered not valid. 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) and that the
expiry time is less than the certificate's expiry_time.

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
described 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 considerations documented in this section should be
taken into consideration when using delegated credentials.

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 of or behind 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 the ExtensionType value 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] [RFC9147]. 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" (1.3.6.1.5.5.7.0) 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 considerations documented in this section should be
taken into consideration when using delegated credentials.

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. Rather, they 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 delegated credential 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 which credentials the
client accepts. Since client time can be unique to a particular
client, privacy-sensitive clients who do not trust the service, such
as browsers in incognito mode, might not want to advertise support
for delegated credentials, or might 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]. The TLS 1.2 specification describes a
strategy for preventing these attacks that requires careful
implementation of timing-resistant countermeasures. (See
Section 7.4.7.1 of [RFC5246].)

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 an 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. 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,
<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>.

[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.

[X.680] ITU-T, "Information technology - Abstract Syntax Notation
One (ASN.1): Specification of basic notation", ISO/
IEC 8824-1:2021, February 2021,
<https://www.itu.int/rec/T-REC-X.680>.

[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 8825-1:2021, February 2021,
<https://www.itu.int/rec/T-REC-X.690>.

8.2. 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,
<https://link.springer.com/chapter/10.1007/BFb0055716>.

[KEYLESS] Stebila, D. and N. Sullivan, "An Analysis of TLS Handshake
Proxying", IEEE Trustcom/BigDataSE/ISPA 2015, 2015,
<https://ieeexplore.ieee.org/document/7345293>.

[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>.

[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/info/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/info/rfc8555>.

[ROBOT] Boeck, H., Somorovsky, J., and C. Young, "Return Of
Bleichenbacher's Oracle Threat (ROBOT)", 27th USENIX
Security Symposium, 2018,
<https://www.usenix.org/conference/usenixsecurity18/
presentation/bock>.

[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,
<https://dl.acm.org/doi/10.1145/2810103.2813657>.

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 that
satisfies the requirements described in Section 4.2 (i.e., uses the
DelegationUsage extension and has the digitalSignature KeyUsage).

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

Acknowledgements

Thanks to David Benjamin, Christopher Patton, Kyle Nekritz, Anirudh
Ramachandran, Benjamin Kaduk, 奥一穂 (Kazuho Oku), Daniel Kahn Gillmor,
Watson Ladd, Robert Merget, Juraj Somorovsky, and Nimrod Aviram for
their discussions, ideas, and bugs they have found.

Authors' Addresses

Richard Barnes
Cisco
Email: [email protected]

Subodh Iyengar
Facebook
Email: [email protected]

Nick Sullivan
Cloudflare
Email: [email protected]

Eric Rescorla
Windy Hill Systems, LLC
Email: [email protected]