Network Working Group E. Rescorla
Internet-Draft RTFM, Inc.
Intended status: Standards Track R. Barnes
Expires: January 8, 2017 Mozilla
S. Iyengar
Facebook
N. Sullivan
CloudFlare Inc.
July 7, 2016
Delegated Credentials for TLS
draft-rescorla-tls-subcerts-00
Abstract
The organizational separation between the operator of a TLS server
and the certificate authority that provides it credentials can cause
problems, for example when it comes to reducing the lifetime of
certificates or supporting new cryptographic algorithms. This
document describes a mechanism to allow TLS server operators to
create their own credential delegations without breaking
compatibility with clients that do not support this specification.
Status of This Memo
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This Internet-Draft will expire on January 8, 2017.
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Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. Related Work . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Client Behavior . . . . . . . . . . . . . . . . . . . . . . . 5
5. Delegated Credentials . . . . . . . . . . . . . . . . . . . . 6
5.1. Option 1a. Name Constraints . . . . . . . . . . . . . . . 7
5.2. Option 1b. End Entities as Issuers . . . . . . . . . . . 7
5.3. Option 2. Define a New Structure . . . . . . . . . . . . 8
5.4. Re-Use of the Master Certificate . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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) [RFC5246] [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. The risk inherent
in cross-organizational transactions makes it infeasible to rely on
an external CA for such short-lived credentials.
To remove these dependencies, this document proposes a limited
delegation mechanism that allows a TLS server operator to issue its
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own credentials within the scope of a certificate issued by an
external CA. Because the above problems do not relate to the CAs
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" and the one issued by the operator as a "delegated
credential".
[[ Ed. - We use the phrase "credential" for the sub-certificates
since it's an open issue whether they will be certificates or not. ]]
[[ Ed. - This document is framed as a single solution, because it
would be best for the WG to ultimately settle on one solution to this
problem. However, to facilitate discussion, we outline several
possible options for how credential can be realized. Once the WG
agrees on an overall approach, this draft will be revised to provide
more details of that approach. ]]
2. Solution Overview
A delegated credential is a digitally signed data structure with the
following semantic fields:
o A validity interval
o A public key (with its associated algorithm)
o A list of valid server names
The signature on the credential indicates a delegation from the
certificate which is issued to the TLS server operator. The key pair
used to sign a credential is presumed to be one whose 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
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 credential.
o The client uses information in the server's certificate to verify
the signature on the credential and verify that the server is
asserting an expected identity.
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o The client uses the public key in the credential as the server's
working key for the TLS handshake.
o The client uses the list of valid server names in the credential
to verify that the credential is valid for the server.
The credential signature is subject to the negotiated signature
algorithms. A credential cannot be used if the client advertises
support for credentials however a server does not have a certificate
which is compatible with any of the negotiated signature algorithms.
It was noted by [J"{a}ger et al.] 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, clients MUST NOT accept connections from
certificates with the keyEncipherment KeyUsage.
[[ Nick - This is a much less stringent requirement than a new flag,
since it means that all existing ECDSA certificates can be re-used.]]
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. [[
TODO: which alert should the client send? ]]
[[ Ed. - The specifics of how credentials are structured and provided
by the server are still to be determined; see below. ]]
3. 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].
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LURK:
Client Front-End Back-End
|----ClientHello--->| |
|<---ServerHello----| |
|<---Certificate----| |
| |<-------LURK------->|
|<---CertVerify-----| |
| ... | |
Delegated credentials:
Client Front-End Back-End
| |<---Cred Provision--|
|----ClientHello--->| |
|<---ServerHello----| |
|<---Certificate----| |
|<---CertVerify-----| |
These two classes of mechanism 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.
4. Client Behavior
This document defines the following extension code point.
enum {
...
delegated_credential(TBD),
(65535)
} ExtensionType;
A client which supports this document SHALL send an empty
"delegated_credential" extension in its ClientHello.
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[[Option 1]] A server MUST NOT send this extension. If the extension
is present, the server MAY send a credential. If the extension is
not present, the server MUST NOT send a credential. A credential
MUST NOT be provided unless a Certificate message is also sent.
[[Option 2]] If the extension is present, the server MAY send a
"delegated credential" extension containing the credential in the
response. If the extension is not present, the server MUST NOT send
a credential. A credential MUST NOT be provided unless a Certificate
message is also sent.
On receiving a 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 then takes the following additional steps:
o Verify that the current time is within the validity interval of
the credential
o Verify that the server name is included in the credential
o Use the public key in the server's end-entity certificate to
verify the signature on the credential
o Use the public key in the credential to verify the
CertificateVerify message provided in the handshake
[[ Ed. - This will need to be updated if the sub-certificate can be
restricted to a subset of the names in the master certificate. ]]
5. Delegated Credentials
[[ Ed. - This section is currently a sketch, intended to lay out the
design space to facilitate discussion ]]
Credentials obviously need to have some defined structure. It is
possible to re-use X.509, but it may be better to define something
new.
The format question also mostly decides the question of how the
credential will be signed and delivered to the client. If the
credential is an X.509 certificate, then it will be signed in that
format, and probably provided in the TLS Certificate message as the
end-entity certificate. If some new structure is devised, then it
will need to define a signature method, and it will probably make
more sense to carry it in a TLS extension.
The delivery mechanism is mostly a trivial question, but given that
the server is switching between a normal certificate chain and one
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including a credential based on a ClientHello extension, there could
be some impact on the ease of implementation. For example, it may be
easier to change the extensions in the ServerHello than to switch the
certificate chain, or alternately it may be easier to simply let the
server operator provide a whole chain terminating in the credential,
depending on how much sanity checking the server does.
5.1. Option 1a. Name Constraints
It would be consistent with the requirements above to realize
credentials as sub-certificates by having the CA issue a subordinate
CA certificate to the TLS server operator, with a nameConstraints
extension encoding the names the server operator is authorized for.
Then the credentials would simply be normal end-entity certificates
issued under this subordinate.
In order for this solution to be safe the subordinate CA certificate
needs to have a critical nameConstraints extension. Historically,
this solution has been unworkable due to legacy clients that could
not process name constraints. However, since in this case we require
the client to indicate support, it may be possible to have critical
name constraints without compatibility impact.
Pro:
o Re-use existing issuance and validation code
o No change to client certificate validation and CertificateVerify
processing
Con:
o Requires server operator to get a name-constrained subordinate CA
certificate
o Name constraints are not universally recognized
o X.509 provides much richer semantics than required
5.2. Option 1b. End Entities as Issuers
One could also imagine a scheme in which the server could use an end-
entity certificate as the issuer for a sub-certificate. Since
servers are typically issued end-entity certificates by CAs, this
could align better with CA issuance practices.
It's important to note that this would not enable existing end-entity
certificates to be used to issue sub-certificates. That would create
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risks such as those noted in [J"{a}ger et al.]. So there would be a
need to define some marker that would be inserted into an end-entity
certificate to indicate that it could be used to issue sub-
certificates. It may be enough to require the certificate to not
contain a keyEncipherment KeyUsage.
Pro:
o No change to client CertificateVerify processing (still uses last
cert in the chain)
Con:
o Violates the semantics of the CA bit in basicConstraints
o Requires change to X.509 validation logic to allow sub-
certificates
o X.509 provides much richer semantics than required
5.3. Option 2. Define a New Structure
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 key usage (RFC5280 section 4.2.1.3). We could
define a new signed object format that would encode only the
semantics that are needed for this application. For example, the TLS
"digitally-signed" structure could be used:
digitally-signed struct {
uint64 notBefore;
uint64 notAfter;
SignatureScheme algorithm;
ServerName serverName;
opaque publicKey<0..2^24-1>;
} DelegatedCredential;
The ServerNameList can be defined as follows (similar to RFC 6066
section 3): ~~~~~~~~~~ struct { NameType name_type; select
(name_type) { case host_name: HostName; } name; } ServerName;
enum { host_name(0), (255) } NameType;
opaque HostName<1..2^16-1>; ~~~~~~~~~~
This would avoid any mis-match in semantics with X.509, and would
likely require more processing code in the client. The code changes
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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).
Pro:
o No change to client certificate validation
o No risk of conflict with X.509 semantics
Con:
o Requires new logic for generating and verifying credentials
o Requires changes to client CertificateVerify processing
5.4. Re-Use of the Master Certificate
[[ OPEN ISSUE ]]
The certificate can be configured so that it is usable directly as a
TLS end-entity certificate (this is the natural design for Option 2)
or alternately can be configured so that it is not acceptable as a
TLS server certificate but rather can only be used for signing sub-
certificates. In the former case, the server operator need only have
one certificate, but with the risk that if the TLS server is
compromised the attacker could issue themselves an arbitrary number
of sub-certificates. Conversely, the master certificate may be
configured so that it is not directly usable, thus requiring the
name-holder to get two certificates, one for signing credentials and
one for use in its TLS server. This adds additional complexity for
the operator but allows the master certificate to be offline.
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
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[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,
<http://www.rfc-editor.org/info/rfc5280>.
8.2. Informative References
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf-
acme-acme-02 (work in progress), March 2016.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-13 (work in progress),
May 2016.
[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.
Authors' Addresses
Eric Rescorla
RTFM, Inc.
Email: ekr@rtfm.com
Richard Barnes
Mozilla
Email: rlb@ipv.sx
Subodh Iyengar
Facebook
Email: subodh@fb.com
Nick Sullivan
CloudFlare Inc.
Email: nick@cloudflare.com
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