Network Working Group R. Barnes
Internet-Draft Mozilla
Intended status: Standards Track S. Iyengar
Expires: February 18, 2019 Facebook
N. Sullivan
Cloudflare
E. Rescorla
RTFM, Inc.
August 17, 2018
Delegated Credentials for TLS
draft-ietf-tls-subcerts-02
Abstract
The organizational separation between the operator of a TLS server
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 clients 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.
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This Internet-Draft will expire on February 18, 2019.
Copyright Notice
Copyright (c) 2018 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 . . . . . . . . . . . . . . . . . . . . . . 5
3. Delegated Credentials . . . . . . . . . . . . . . . . . . . . 6
3.1. Client and Server behavior . . . . . . . . . . . . . . . 8
3.2. Certificate Requirements . . . . . . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
5.1. Security of delegated private key . . . . . . . . . . . . 10
5.2. Revocation of delegated credentials . . . . . . . . . . . 10
5.3. Privacy considerations . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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.
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 CDNs or remote data centers. This allows server
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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 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 server operator 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".
1.1. Change Log
(*) indicates changes to the wire protocol.
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.
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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
support for this mechanism.
o The server provides both the certificate chain terminating in its
certificate as well as the delegated credential.
o The client uses information in the server's certificate to verify
the delegated credential and that the server is asserting an
expected identity.
o The client uses the public key in the credential as the server's
working key for the TLS handshake.
As detailed in Section 3, the delegated credential is
cryptographically bound to the end-entity certificate and the
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
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.)
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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 which 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 version of TLS
and signature algorithm. This prevents them from being used for
other protocols or with other signature algorithms than service
owner allows.
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., LURK [I-D.mglt-lurk-tls-requirements]). 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 [I-D.ietf-tls-tls13], where DC is delegated credentials.
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LURK:
Client Front-End Back-End
|----ClientHello--->| |
|<---ServerHello----| |
|<---Certificate----| |
| |<-------LURK------->|
|<---CertVerify-----| |
| ... | |
Delegated credentials:
Client Front-End Back-End
| |<----DC minting---->|
|----ClientHello--->| |
|<---ServerHello----| |
|<---Certificate----| |
|<---CertVerify-----| |
| ... | |
These two mechanisms can be complementary. A server could use
credentials for clients that support them, while using LURK to
support legacy clients.
It is possible to address the short-lived certificate concerns above
by automating certificate issuance, e.g., with ACME
[I-D.ietf-acme-acme]. 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.
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:
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struct {
uint32 valid_time;
SignatureScheme expected_cert_verify_algorithm;
ProtocolVersion expected_version;
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.
expected_version: The version of TLS in which the credential will be
used, where the type ProtocolVersion is as defined in [RFC8446].
This is expected to match the protocol version that is negotiated
by the client and server.
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 signature over the credential with the end-entity
certificate's public key, using the scheme.
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".
3. A single 0 byte, which serves as the separator.
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4. The DER-encoded X.509 end-entity certificate used to sign the
DelegatedCredential.
5. DelegatedCredential.algorithm.
6. DelegatedCredential.scheme.
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, 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.
The code changes to create and verify delegated credentials would be
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).
3.1. Client and Server behavior
This document defines the following extension code point.
enum {
...
delegated_credential(TBD),
(65535)
} ExtensionType;
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
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.
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"
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extension. A delegated credential MUST NOT be negotiated otherwise,
even if the client advertises support for delegated credentials.
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:
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.
2. Verify that expected_cert_verify_algorithm matches the scheme
indicated in the server's CertificateVerify message.
3. Verify that expected_version matches the protocol version
indicated in the server's "supported_versions" extension.
4. Verify that the end-entity certificate satisfies the conditions
specified in Section 3.2.
5. Use the public key in the server'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 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.
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.
id-ce-delegationUsage OBJECT IDENTIFIER ::= { 1.3.6.1.4.1.44363.44 }
DelegationUsage ::= NULL
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
TBD
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 such as file
system controls, physical security or hardware security modules.
5.2. 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.
5.3. 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 Christopher Patton, Kyle Nekritz, Anirudh Ramachandran,
Benjamin Kaduk, Kazuho Oku, Daniel Kahn Gillmor for their
discussions, ideas, and bugs they have found.
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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>.
[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]
Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
Kasten, "Automatic Certificate Management Environment
(ACME)", draft-ietf-acme-acme-14 (work in progress),
August 2018.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
[I-D.mglt-lurk-tls-requirements]
Migault, D. and K. Ma, "Authentication Model and Security
Requirements for the TLS/DTLS Content Provider Edge Server
Split Use Case", draft-mglt-lurk-tls-requirements-00 (work
in progress), January 2016.
[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>.
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[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
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.
Authors' Addresses
Richard Barnes
Mozilla
Email: rlb@ipv.sx
Subodh Iyengar
Facebook
Email: subodh@fb.com
Nick Sullivan
Cloudflare
Email: nick@cloudflare.com
Eric Rescorla
RTFM, Inc.
Email: ekr@rtfm.com
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