draft-ietf-tls-subcerts-01.txt   draft-ietf-tls-subcerts-02.txt 
Network Working Group R. Barnes Network Working Group R. Barnes
Internet-Draft Mozilla Internet-Draft Mozilla
Intended status: Standards Track S. Iyengar Intended status: Standards Track S. Iyengar
Expires: January 3, 2019 Facebook Expires: February 18, 2019 Facebook
N. Sullivan N. Sullivan
Cloudflare Cloudflare
E. Rescorla E. Rescorla
RTFM, Inc. RTFM, Inc.
July 02, 2018 August 17, 2018
Delegated Credentials for TLS Delegated Credentials for TLS
draft-ietf-tls-subcerts-01 draft-ietf-tls-subcerts-02
Abstract Abstract
The organizational separation between the operator of a TLS server The organizational separation between the operator of a TLS server
and the certificate authority that provides it credentials can cause and the certification authority can create limitations. For example,
problems, for example when it comes to reducing the lifetime of the lifetime of certificates, how they may be used, and the
certificates or supporting new cryptographic algorithms. This algorithms they support are ultimately determined by the
document describes a mechanism to allow TLS server operators to certification authority. This document describes a mechanism by
create their own credential delegations without breaking which operators may delegate their own credentials for use in TLS,
compatibility with clients that do not support this specification. without breaking compatibility with clients that do not support this
specification.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 3, 2019. This Internet-Draft will expire on February 18, 2019.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Change Log . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Related Work . . . . . . . . . . . . . . . . . . . . . . 5 2.2. Related Work . . . . . . . . . . . . . . . . . . . . . . 5
3. Client and Server behavior . . . . . . . . . . . . . . . . . 6 3. Delegated Credentials . . . . . . . . . . . . . . . . . . . . 6
4. Delegated Credentials . . . . . . . . . . . . . . . . . . . . 7 3.1. Client and Server behavior . . . . . . . . . . . . . . . 8
4.1. Certificate Requirements . . . . . . . . . . . . . . . . 8 3.2. Certificate Requirements . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6.1. Security of delegated private key . . . . . . . . . . . . 9 5.1. Security of delegated private key . . . . . . . . . . . . 10
6.2. Revocation of delegated credentials . . . . . . . . . . . 9 5.2. Revocation of delegated credentials . . . . . . . . . . . 10
6.3. Privacy considerations . . . . . . . . . . . . . . . . . 9 5.3. Privacy considerations . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction 1. Introduction
Typically, a TLS server uses a certificate provided by some entity Typically, a TLS server uses a certificate provided by some entity
other than the operator of the server (a "Certification Authority" or other than the operator of the server (a "Certification Authority" or
CA) [RFC5246] [RFC5280]. This organizational separation makes the CA) [RFC8446] [RFC5280]. This organizational separation makes the
TLS server operator dependent on the CA for some aspects of its TLS server operator dependent on the CA for some aspects of its
operations, for example: operations, for example:
o Whenever the server operator wants to deploy a new certificate, it o Whenever the server operator wants to deploy a new certificate, it
has to interact with the CA. has to interact with the CA.
o The server operator can only use TLS authentication schemes for o The server operator can only use TLS authentication schemes for
which the CA will issue credentials. which the CA will issue credentials.
These dependencies cause problems in practice. Server operators These dependencies cause problems in practice. Server operators
often want to create short-lived certificates for servers in low- often want to create short-lived certificates for servers in low-
trust zones such as CDNs or remote data centers. This allows server trust zones such as CDNs or remote data centers. This allows server
operators to limit the exposure of keys in cases that they do not operators to limit the exposure of keys in cases that they do not
realize a compromise has occurred. The risk inherent in cross- realize a compromise has occurred. The risk inherent in cross-
organizational transactions makes it operationally infeasible to rely organizational transactions makes it operationally infeasible to rely
on an external CA for such short-lived credentials. In OCSP on an external CA for such short-lived credentials. In OCSP stapling
stapling, if an operator chooses to talk frequently to the CA to (i.e., using the Certificate Status extension types ocsp [RFC6066] or
obtain stapled responses, then failure to fetch an OCSP stapled ocsp_multi [RFC6961]), if an operator chooses to talk frequently to
response results only in degraded performance. On the other hand, the CA to obtain stapled responses, then failure to fetch an OCSP
failure to fetch a potentially large number of short lived stapled response results only in degraded performance. On the other
certificates would result in the service not being available which 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. creates greater operational risk.
To remove these dependencies, this document proposes a limited To remove these dependencies, this document proposes a limited
delegation mechanism that allows a TLS server operator to issue its delegation mechanism that allows a TLS server operator to issue its
own credentials within the scope of a certificate issued by an own credentials within the scope of a certificate issued by an
external CA. Because the above problems do not relate to the CAs external CA. Because the above problems do not relate to the CA's
inherent function of validating possession of names, it is safe to inherent function of validating possession of names, it is safe to
make such delegations as long as they only enable the recipient of make such delegations as long as they only enable the recipient of
the delegation to speak for names that the CA has authorized. For 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 clarity, we will refer to the certificate issued by the CA as a
"certificate" and the one issued by the operator as a "delegated "certificate", or "delegation certificate", and the one issued by the
credential". operator as a "delegated credential".
2. Solution Overview 1.1. Change Log
A delegated credential is a digitally signed data structure with the (*) indicates changes to the wire protocol.
following semantic fields:
o A validity interval draft-02
o A public key (with its associated algorithm) o Change public key type. (*)
The signature on the credential indicates a delegation from the o Change DelegationUsage extension to be NULL and define its object
certificate that is issued to the TLS server operator. The secret identifier.
key used to sign a credential is presumed to be one whose
corresponding public key is contained in an X.509 certificate that
associates one or more names to the credential.
A TLS handshake that uses credentials differs from a normal handshake o Drop support for TLS 1.2.
in a few important ways:
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 TLS
server operator. The secret key used to sign a credential
corresponds to the public key of the TLS server's X.509 end-entity
certificate.
A TLS handshake that uses delegated credentials differs from a normal
handshake in a few important ways:
o The client provides an extension in its ClientHello that indicates o The client provides an extension in its ClientHello that indicates
support for this mechanism. support for this mechanism.
o The server provides both the certificate chain terminating in its o The server provides both the certificate chain terminating in its
certificate as well as the credential. certificate as well as the delegated credential.
o The client uses information in the server's certificate to verify o The client uses information in the server's certificate to verify
the signature on the credential and verify that the server is the delegated credential and that the server is asserting an
asserting an expected identity. expected identity.
o The client uses the public key in the credential as the server's o The client uses the public key in the credential as the server's
working key for the TLS handshake. working key for the TLS handshake.
Delegated credentials can be used either in TLS 1.3 or TLS 1.2. As detailed in Section 3, the delegated credential is
Differences between the use of delegated credentials in the protocols cryptographically bound to the end-entity certificate and the
are explicitly stated. protocol in 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 explicitly disallowed.
Delegated credentials allow the server 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, servers 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 It was noted in [XPROT] that certificates in use by servers that
support outdated protocols such as SSLv2 can be used to forge support outdated protocols such as SSLv2 can be used to forge
signatures for certificates that contain the keyEncipherment KeyUsage signatures for certificates that contain the keyEncipherment KeyUsage
([RFC5280] section 4.2.1.3) In order to prevent this type of cross- ([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 protocol attack, we define a new DelegationUsage extension to X.509
that permits use of delegated credentials. Clients MUST NOT accept that permits use of delegated credentials. (See Section 3.2.)
delegated credentials associated with certificates without this
extension.
Credentials allow the server 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 the exposure of a delegation credential compromise, servers
MUST NOT issue credentials with a validity period longer than 7 days.
Clients MUST NOT accept credentials with longer validity periods.
2.1. Rationale 2.1. Rationale
Delegated credentials present a better alternative than other Delegated credentials present a better alternative than other
delegation mechanisms like proxy certificates [RFC3820] for several delegation mechanisms like proxy certificates [RFC3820] for several
reasons: reasons:
o There is no change needed to certificate validation at the PKI o There is no change needed to certificate validation at the PKI
layer. layer.
o X.509 semantics are very rich. This can cause unintended o X.509 semantics are very rich. This can cause unintended
consequences if a service owner creates a proxy cert where the consequences if a service owner creates a proxy certificate where
properties differ from the leaf certificate. the properties differ from the leaf certificate. For this reason,
delegated credentials have very restricted semantics which should
o Delegated credentials have very restricted semantics which should
not conflict with X.509 semantics. not conflict with X.509 semantics.
o Proxy certificates rely on the certificate path building process o Proxy certificates rely on the certificate path building process
to establish a binding between the proxy certificate and the to establish a binding between the proxy certificate and the
server certificate. Since the cert path building process is not server certificate. Since the certificate path building process
cryptographically protected, it is possible that a proxy is not cryptographically protected, it is possible that a proxy
certificate could be bound to another certificate with the same certificate could be bound to another certificate with the same
public key, with different X.509 parameters. Delegated public key, with different X.509 parameters. Delegated
credentials, which rely on a cryptographic binding between the credentials, which rely on a cryptographic binding between the
entire certificate and the delegated credential, cannot. entire certificate and the delegated credential, cannot.
o Delegated credentials are bound to specific versions of TLS. This o Each delegated credential is bound to a specific version of TLS
prevents them from being used for other protocols if a service and signature algorithm. This prevents them from being used for
owner allows multiple versions of TLS. other protocols or with other signature algorithms than service
owner allows.
2.2. Related Work 2.2. Related Work
Many of the use cases for delegated credentials can also be addressed Many of the use cases for delegated credentials can also be addressed
using purely server-side mechanisms that do not require changes to using purely server-side mechanisms that do not require changes to
client behavior (e.g., LURK [I-D.mglt-lurk-tls-requirements]). These client behavior (e.g., LURK [I-D.mglt-lurk-tls-requirements]). These
mechanisms, however, incur per-transaction latency, since the front- mechanisms, however, incur per-transaction latency, since the front-
end server has to interact with a back-end server that holds a end server has to interact with a back-end server that holds a
private key. The mechanism proposed in this document allows the private key. The mechanism proposed in this document allows the
delegation to be done off-line, with no per-transaction latency. The delegation to be done off-line, with no per-transaction latency. The
figure below compares the message flows for these two mechanisms with figure below compares the message flows for these two mechanisms with
TLS 1.3 [I-D.ietf-tls-tls13]. TLS 1.3 [I-D.ietf-tls-tls13], where DC is delegated credentials.
LURK: LURK:
Client Front-End Back-End Client Front-End Back-End
|----ClientHello--->| | |----ClientHello--->| |
|<---ServerHello----| | |<---ServerHello----| |
|<---Certificate----| | |<---Certificate----| |
| |<-------LURK------->| | |<-------LURK------->|
|<---CertVerify-----| | |<---CertVerify-----| |
| ... | | | ... | |
Delegated credentials: Delegated credentials:
Client Front-End Back-End Client Front-End Back-End
| |<--Cred Provision-->| | |<----DC minting---->|
|----ClientHello--->| | |----ClientHello--->| |
|<---ServerHello----| | |<---ServerHello----| |
|<---Certificate----| | |<---Certificate----| |
|<---CertVerify-----| | |<---CertVerify-----| |
| ... | |
These two mechanisms can be complementary. A server could use These two mechanisms can be complementary. A server could use
credentials for clients that support them, while using LURK to credentials for clients that support them, while using LURK to
support legacy clients. support legacy clients.
It is possible to address the short-lived certificate concerns above It is possible to address the short-lived certificate concerns above
by automating certificate issuance, e.g., with ACME by automating certificate issuance, e.g., with ACME
[I-D.ietf-acme-acme]. In addition to requiring frequent [I-D.ietf-acme-acme]. In addition to requiring frequent
operationally-critical interactions with an external party, this operationally-critical interactions with an external party, this
makes the server operator dependent on the CA's willingness to issue makes the server operator dependent on the CA's willingness to issue
certificates with sufficiently short lifetimes. It also fails to certificates with sufficiently short lifetimes. It also fails to
address the issues with algorithm support. Nonetheless, existing address the issues with algorithm support. Nonetheless, existing
automated issuance APIs like ACME may be useful for provisioning automated issuance APIs like ACME may be useful for provisioning
credentials, within an operator network. credentials, within an operator network.
3. Client and Server behavior 3. Delegated Credentials
This document defines the following extension code point. 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:
enum { struct {
... uint32 valid_time;
delegated_credential(TBD), SignatureScheme expected_cert_verify_algorithm;
(65535) ProtocolVersion expected_version;
} ExtensionType; opaque ASN1_subjectPublicKeyInfo<1..2^24-1>;
} Credential;
A client which supports this document SHALL send an empty valid_time: Relative time in seconds from the beginning of the
"delegated_credential" extension in its ClientHello. delegation certificate's notBefore value after which the delegated
credential is no longer valid.
If the extension is present, the server MAY send a expected_cert_verify_algorithm: The signature algorithm of the
DelegatedCredential extension. If the extension is not present, the credential key pair, where the type SignatureScheme is as defined
server MUST NOT send a credential. A credential MUST NOT be provided in [RFC8446]. This is expected to be the same as
unless a Certificate message is also sent. CertificateVerify.algorithm sent by the server.
When negotiating TLS 1.3, and using Delegated credentials, the server expected_version: The version of TLS in which the credential will be
MUST send the DelegatedCredential as an extension in the used, where the type ProtocolVersion is as defined in [RFC8446].
CertificateEntry of its end-entity certificate. When negotiating TLS This is expected to match the protocol version that is negotiated
1.2, the DelegatedCredential MUST be sent as an extension in the by the client and server.
ServerHello.
The DelegatedCredential contains a signature from the public key in ASN1_subjectPublicKeyInfo: The credential's public key, a DER-
the end-entity certificate using a signature algorithm advertised by encoded [X690] SubjectPublicKeyInfo as defined in [RFC5280].
the client in the "signature_algorithms" extension. Additionally,
the credential's public key MUST be of a type that enables at least
one of the supported signature algorithms. A delegated credential
MUST NOT be negotiated by the server if its signature is not
compatible with any of the supported signature algorithms or the
credential's public key is not usable with the supported signature
algorithms of the client, even if the client advertises support for
delegated credentials.
On receiving a credential and a certificate chain, the client The delegated credential has the following structure:
validates the certificate chain and matches the end-entity
certificate to the server's expected identity following its normal
procedures. It then takes the following steps:
o Verify that the current time is within the validity interval of struct {
the credential and that the credential's time to live is no more Credential cred;
than 7 days. SignatureScheme algorithm;
opaque signature<0..2^16-1>;
} DelegatedCredential;
o Verify that the certificate has the DelegationUsage extension, algorithm: The signature algorithm used to verify
which permits the use of Delegated credentials. DelegatedCredential.signature.
o Use the public key in the server's end-entity certificate to signature: The signature over the credential with the end-entity
verify the signature on the credential. certificate's public key, using the scheme.
If one or more of these checks fail, then the delegated credential is The signature of the DelegatedCredential is computed over the
deemed invalid. Clients that receive invalid delegated credentials concatenation of:
MUST terminate the connection with an "illegal_parameter" alert. If
successful, the client uses the public key in the credential to
verify a signature provided in the handshake: in particular, the
CertificateVerify message in TLS 1.3 and the ServerKeyExchange in
1.2.
4. Delegated Credentials 1. A string that consists of octet 32 (0x20) repeated 64 times.
While X.509 forbids end-entity certificates from being used as 2. The context string "TLS, server delegated credentials".
issuers for other certificates, it is perfectly fine to use them to
issue other signed objects as long as the certificate contains the
digitalSignature key usage (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.
struct { 3. A single 0 byte, which serves as the separator.
uint32 valid_time;
opaque public_key<0..2^16-1>;
} Credential;
struct { 4. The DER-encoded X.509 end-entity certificate used to sign the
Credential cred; DelegatedCredential.
SignatureScheme scheme;
opaque signature<0..2^16-1>;
} DelegatedCredential;
valid_time: Relative time in seconds from the beginning of the 5. DelegatedCredential.algorithm.
certificate's notBefore value after which the delegated credential
is no longer valid.
public_key: The delegated credential's public key, which is an 6. DelegatedCredential.scheme.
encoded SubjectPublicKeyInfo [RFC5280].
scheme: The signature algorithm used to sign the delegated The signature effectively binds the credential to the parameters of
credential. the handshake in which it is used. In particular, it ensures that
credentials are only used with the certificate, protocol, 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.
signature: The signature over the credential with the end-entity The code changes to create and verify delegated credentials would be
certificate's public key, using the scheme. localized to the TLS stack, which has the advantage of avoiding
changes to security-critical and often delicate PKI code (though of
course moves that complexity to the TLS stack).
The DelegatedCredential structure is similar to the CertificateVerify 3.1. Client and Server behavior
structure in TLS 1.3. Since the SignatureScheme is defined in TLS
1.3, TLS 1.2 clients should translate the scheme into an appropriate
group and signature algorithm to perform validation.
The signature of the DelegatedCredential is computed over the This document defines the following extension code point.
concatenation of:
1. A string that consists of octet 32 (0x20) repeated 64 times. enum {
...
delegated_credential(TBD),
(65535)
} ExtensionType;
2. The context string "TLS, server delegated credentials". 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.
3. A single 0 byte which serves as the separator. 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. A delegated credential MUST NOT be provided
unless a Certificate message is also sent. The server MUST ignore
the extension unless TLS 1.3 or a later version is negotiated.
4. Big endian serialized 2 bytes ProtocolVersion of the negotiated The server MUST send the delegated credential as an extension in the
TLS version, defined by TLS. CertificateEntry of its end-entity certificate; the client SHOULD
ignore delegated credentials sent as extensions to any other
certificate.
5. DER encoded X.509 certificate used to sign the The algorithm and expected_cert_verify_algorithm fields MUST be of a
DelegatedCredential. type advertised by the client in the "signature_algorithms"
extension. A delegated credential MUST NOT be negotiated otherwise,
even if the client advertises support for delegated credentials.
6. Big endian serialized 2 byte SignatureScheme scheme. On receiving a delegated credential and a certificate chain, the
client validates the certificate chain and matches the end-entity
certificate to the server's expected identity following its normal
procedures. It also takes the following steps:
7. The Credential structure. 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.
This signature has a few desirable properties: 2. Verify that expected_cert_verify_algorithm matches the scheme
indicated in the server's CertificateVerify message.
o It is bound to the certificate that signed it. 3. Verify that expected_version matches the protocol version
indicated in the server's "supported_versions" extension.
o It is bound to the protocol version that is negotiated. This is 4. Verify that the end-entity certificate satisfies the conditions
intended to avoid cross-protocol attacks with signing oracles. specified in Section 3.2.
The code changes to create and verify delegated credentials would be 5. Use the public key in the server's end-entity certificate to
localized to the TLS stack, which has the advantage of avoiding verify the signature of the credential using the algorithm
changes to security-critical and often delicate PKI code (though of indicated by DelegatedCredential.algorithm.
course moves that complexity to the TLS stack).
4.1. Certificate Requirements If one or more of these checks fail, then the delegated credential is
deemed invalid. Clients that receive invalid delegated credentials
MUST terminate the connection with an "illegal_parameter" alert. If
successful, the client uses the public key in the credential to
verify the signature in the server's CertificateVerify message.
We define a new X.509 extension, DelegationUsage to be used in the 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 certificate when the certificate permits the usage of delegated
credentials. When this extension is not present the client MUST not credentials.
accept a delegated credential even if it is negotiated by the server.
When it is present, the client MUST follow the validation procedure.
id-ce-delegationUsage OBJECT IDENTIFIER ::= { TBD } id-ce-delegationUsage OBJECT IDENTIFIER ::= { 1.3.6.1.4.1.44363.44 }
DelegationUsage ::= NULL
DelegationUsage ::= BIT STRING { allowed (0) } The extension MUST be marked non-critical. (See Section 4.2 of
Conforming CAs MUST mark this extension as non-critical. This allows [RFC5280].) The client MUST NOT accept a delegated credential unless
the certificate to be used by service owners for clients that do not the server's end-entity certificate satisfies the following criteria:
support certificate delegation as well and not need to obtain two
certificates.
5. IANA Considerations o It has the DelegationUsage extension.
o It has the digitalSignature KeyUsage (see the KeyUsage extension
defined in [RFC5280]).
4. IANA Considerations
TBD TBD
6. Security Considerations 5. Security Considerations
6.1. Security of delegated private key 5.1. Security of delegated private key
Delegated credentials limit the exposure of the TLS private key by Delegated credentials limit the exposure of the TLS private key by
limiting its validity. An attacker who compromises the private key limiting its validity. An attacker who compromises the private key
of a delegated credential can act as a man in the middle until the of a delegated credential can act as a man in the middle until the
delegate credential expires, however they cannot create new delegated delegate credential expires, however they cannot create new delegated
credentials. Thus delegated credentials should not be used to send a credentials. Thus delegated credentials should not be used to send a
delegation to an untrusted party, but is meant to be used between delegation to an untrusted party, but is meant to be used between
parties that have some trust relationship with each other. The parties that have some trust relationship with each other. The
secrecy of the delegated private key is thus important and several secrecy of the delegated private key is thus important and several
access control mechanisms SHOULD be used to protect it such as file access control mechanisms SHOULD be used to protect it such as file
system controls, physical security or hardware security modules. system controls, physical security or hardware security modules.
6.2. Revocation of delegated credentials 5.2. Revocation of delegated credentials
Delegated credentials do not provide any additional form of early Delegated credentials do not provide any additional form of early
revocation. Since it is short lived, the expiry of the delegated revocation. Since it is short lived, the expiry of the delegated
credential would revoke the credential. Revocation of the long term credential would revoke the credential. Revocation of the long term
private key that signs the delegated credential also implictly private key that signs the delegated credential also implicitly
revokes the delegated credential. revokes the delegated credential.
6.3. Privacy considerations 5.3. Privacy considerations
Delegated credentials can be valid for 7 days and it is much easier Delegated credentials can be valid for 7 days and it is much easier
for a service to create delegated credential than a certificate for a service to create delegated credential than a certificate
signed by a CA. A service could determine the client time and clock signed by a CA. A service could determine the client time and clock
skew by creating several delegated credentials with different expiry skew by creating several delegated credentials with different expiry
timestamps and observing whether the client would accept it. Client timestamps and observing whether the client would accept it. Client
time could be unique and thus privacy sensitive clients, such as time could be unique and thus privacy sensitive clients, such as
browsers in incognito mode, who do not trust the service might not browsers in incognito mode, who do not trust the service might not
want to advertise support for delegated credentials or limit the want to advertise support for delegated credentials or limit the
number of probes that a server can perform. number of probes that a server can perform.
7. Acknowledgements 6. Acknowledgements
Thanks to Kyle Nekritz, Anirudh Ramachandran, Benjamin Kaduk, Kazuho
Oku, Daniel Kahn Gillmor for their discussions, ideas, and bugs
they've found.
8. References Thanks to Christopher Patton, Kyle Nekritz, Anirudh Ramachandran,
Benjamin Kaduk, Kazuho Oku, Daniel Kahn Gillmor for their
discussions, ideas, and bugs they have found.
8.1. Normative References 7. References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security 7.1. Normative References
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008, <https://www.rfc-
editor.org/info/rfc5246>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>. <https://www.rfc-editor.org/info/rfc5280>.
8.2. Informative References [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>.
[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 8825-1:2002, 2002.
7.2. Informative References
[I-D.ietf-acme-acme] [I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment Kasten, "Automatic Certificate Management Environment
(ACME)", draft-ietf-acme-acme-12 (work in progress), April (ACME)", draft-ietf-acme-acme-14 (work in progress),
2018. August 2018.
[I-D.ietf-tls-tls13] [I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress), Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018. March 2018.
[I-D.mglt-lurk-tls-requirements] [I-D.mglt-lurk-tls-requirements]
Migault, D. and K. Ma, "Authentication Model and Security Migault, D. and K. Ma, "Authentication Model and Security
Requirements for the TLS/DTLS Content Provider Edge Server Requirements for the TLS/DTLS Content Provider Edge Server
Split Use Case", draft-mglt-lurk-tls-requirements-00 (work Split Use Case", draft-mglt-lurk-tls-requirements-00 (work
in progress), January 2016. in progress), January 2016.
[RFC3820] Tuecke, S., Welch, V., Engert, D., Pearlman, L., and M. [RFC3820] Tuecke, S., Welch, V., Engert, D., Pearlman, L., and M.
Thompson, "Internet X.509 Public Key Infrastructure (PKI) Thompson, "Internet X.509 Public Key Infrastructure (PKI)
Proxy Certificate Profile", RFC 3820, Proxy Certificate Profile", RFC 3820,
DOI 10.17487/RFC3820, June 2004, <https://www.rfc- DOI 10.17487/RFC3820, June 2004,
editor.org/info/rfc3820>. <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>.
[XPROT] Jager, T., Schwenk, J., and J. Somorovsky, "On the [XPROT] Jager, T., Schwenk, J., and J. Somorovsky, "On the
Security of TLS 1.3 and QUIC Against Weaknesses in PKCS#1 Security of TLS 1.3 and QUIC Against Weaknesses in PKCS#1
v1.5 Encryption", Proceedings of the 22nd ACM SIGSAC v1.5 Encryption", Proceedings of the 22nd ACM SIGSAC
Conference on Computer and Communications Security , 2015. Conference on Computer and Communications Security , 2015.
Authors' Addresses Authors' Addresses
Richard Barnes Richard Barnes
Mozilla Mozilla
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