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Keywords: transport layer security

Independent Submission Y. Sheffer
Request for Comments: 8672 Intuit
Category: Experimental D. Migault
ISSN: 2070-1721 Ericsson
October 2019
TLS Server Identity Pinning with Tickets
Abstract
Misissued public-key certificates can prevent TLS clients from
appropriately authenticating the TLS server. Several alternatives
have been proposed to detect this situation and prevent a client from
establishing a TLS session with a TLS end point authenticated with an
illegitimate public-key certificate. These mechanisms are either not
widely deployed or limited to public web browsing.
This document proposes experimental extensions to TLS with opaque
pinning tickets as a way to pin the server's identity. During an
initial TLS session, the server provides an original encrypted
pinning ticket. In subsequent TLS session establishment, upon
receipt of the pinning ticket, the server proves its ability to
decrypt the pinning ticket and thus the ownership of the pinning
protection key. The client can now safely conclude that the TLS
session is established with the same TLS server as the original TLS
session. One of the important properties of this proposal is that no
manual management actions are required.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This is a contribution to the RFC Series, independently
of any other RFC stream. The RFC Editor has chosen to publish this
document at its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see 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/rfc8672.
Copyright Notice
Copyright (c) 2019 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
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.
Table of Contents
1. Introduction
1.1. Conventions Used in This Document
1.2. Scope of Experimentation
2. Protocol Overview
2.1. Initial Connection
2.2. Subsequent Connections
2.3. Indexing the Pins
3. Message Definitions
4. Cryptographic Operations
4.1. Pinning Secret
4.2. Pinning Ticket
4.3. Pinning Protection Key
4.4. Pinning Proof
5. Operational Considerations
5.1. Protection Key Synchronization
5.2. Ticket Lifetime
5.3. Certificate Renewal
5.4. Certificate Revocation
5.5. Disabling Pinning
5.6. Server Compromise
5.7. Disaster Recovery
6. Security Considerations
6.1. Trust-on-First-Use (TOFU) and MITM Attacks
6.2. Pervasive Monitoring
6.3. Server-Side Error Detection
6.4. Client Policy and SSL Proxies
6.5. Client-Side Error Behavior
6.6. Stolen and Forged Tickets
6.7. Client Privacy
6.8. Ticket Protection Key Management
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Previous Work
A.1. Comparison: HPKP
A.2. Comparison: TACK
Acknowledgments
Authors' Addresses
1. Introduction
Misissued public-key certificates can prevent TLS [RFC8446] clients
from appropriately authenticating the TLS server. This is a
significant risk in the context of the global public key
infrastructure (PKI), and similarly for large-scale deployments of
certificates within enterprises.
This document proposes experimental extensions to TLS with opaque
pinning tickets as a way to pin the server's identity. The approach
is intended to be easy to implement and deploy, and reuses some of
the ideas behind TLS session resumption [RFC5077].
Ticket pinning is a second-factor server authentication method and is
not proposed as a substitute for the authentication method provided
in the TLS key exchange. More specifically, the client only uses the
pinning identity method after the TLS key exchange is successfully
completed. In other words, the pinning identity method is only
performed over an authenticated TLS session. Note that ticket
pinning does not pin certificate information and therefore is truly
an independent second-factor authentication.
Ticket pinning is a trust-on-first-use (TOFU) mechanism, in that the
first server authentication is only based on PKI certificate
validation, but for any follow-on sessions, the client is further
ensuring the server's identity based on the server's ability to
decrypt the ticket, in addition to normal PKI certificate
authentication.
During initial TLS session establishment, the client requests a
pinning ticket from the server. Upon receiving the request the
server generates a pinning secret that is expected to be
unpredictable for peers other than the client or the server. In our
case, the pinning secret is generated from parameters exchanged
during the TLS key exchange, so client and server can generate it
locally and independently. The server constructs the pinning ticket
with the necessary information to retrieve the pinning secret. The
server then encrypts the ticket and returns the pinning ticket to the
client with an associated pinning lifetime.
The pinning lifetime value indicates for how long the server promises
to retain the server-side ticket-encryption key, which allows it to
complete the protocol exchange correctly and prove its identity. The
server commitment (and ticket lifetime) is typically on the order of
weeks.
Once the key exchange is completed, and the server is deemed
authenticated, the client generates locally the pinning secret and
caches the server's identifiers to index the pinning secret as well
as the pinning ticket and its associated lifetime.
When the client reestablishes a new TLS session with the server, it
sends the pinning ticket to the server. Upon receiving it, the
server returns a proof of knowledge of the pinning secret. Once the
key exchange is completed, and the server has been authenticated, the
client checks the pinning proof returned by the server using the
client's stored pinning secret. If the proof matches, the client can
conclude that the server to which it is currently connecting is, in
fact, the correct server.
This document only applies to TLS 1.3. We believe that the idea can
also be retrofitted into earlier versions of the protocol, but this
would require significant changes. One example is that TLS 1.2
[RFC5246] and earlier versions do not provide a generic facility of
encrypted handshake extensions, such as is used here to transport the
ticket.
The main advantages of this protocol over earlier pinning solutions
are the following:
* The protocol is at the TLS level, and as a result is not
restricted to HTTP at the application level.
* The protocol is robust to changes in server IP address,
certification authority (CA), and public key. The server is
characterized by the ownership of the pinning protection key,
which is never provided to the client. Server configuration
parameters such as the CA and the public key may change without
affecting the pinning ticket protocol.
* Once a single parameter is configured (the ticket's lifetime),
operation is fully automated. The server administrator need not
bother with the management of backup certificates or explicit
pins.
* For server clusters, we reuse the existing infrastructure
[RFC5077] where it exists.
* Pinning errors, presumably resulting from man-in-the-middle (MITM)
attacks, can be detected both by the client and the server. This
allows for server-side detection of MITM attacks using large-scale
analytics, and with no need to rely on clients to explicitly
report the error.
A note on terminology: unlike other solutions in this space, we do
not do "certificate pinning" (or "public key pinning"), since the
protocol is oblivious to the server's certificate. We prefer the
term "server identity pinning" for this new solution. In our
solution, the server proves its identity by generating a proof that
it can read and decrypt an encrypted ticket. As a result, the
identity proof relies on proof of ownership of the pinning protection
key. However, this key is never exchanged with the client or known
by it, and so cannot itself be pinned.
1.1. Conventions Used in This Document
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.
1.2. Scope of Experimentation
This document describes an experimental extension to the TLS
protocol. This section defines constraints on this experiment and
how it can yield useful information, potentially resulting in a
standard.
The protocol is designed so that if the server does not support it,
the client and server fall back to a normal TLS exchange, with the
exception of a single PinningTicket extension being initially sent by
the client. In addition, the protocol is designed only to strengthen
the validation of the server's identity ("second factor"). As a
result, implementation or even protocol errors should not result in
weakened security compared to the normal TLS exchange. Given these
two points, experimentation can be run on the open Internet between
consenting client and server implementations.
The goal of the experiment is to prove that:
* Non-supporting clients and servers are unaffected.
* Connectivity between supporting clients and servers is retained
under normal circumstances, whether the client connects to the
server frequently (relative to the ticket's lifetime) or very
rarely.
* Enterprise middleboxes do not interrupt such connectivity.
* Misissued certificates and rogue TLS-aware middleboxes do result
in broken connectivity, and these cases are detected on the client
and/or server side. Clients and servers can be recovered even
after such events and the normal connectivity restored.
Following two years of successful deployment, the authors will
publish a document that summarizes the experiment's findings and will
resubmit the protocol for consideration as a Proposed Standard.
2. Protocol Overview
The protocol consists of two phases: the first time a particular
client connects to a server, and subsequent connections.
This protocol supports full TLS handshakes, as well as 0-RTT
handshakes. Below we present it in the context of a full handshake,
but behavior in 0-RTT handshakes should be identical.
The document presents some similarities with the ticket resumption
mechanism described in [RFC5077]. However the scope of this document
differs from session resumption mechanisms implemented with [RFC5077]
or with other mechanisms. Specifically, the pinning ticket does not
carry any state associated with a TLS session and thus cannot be used
for session resumption or client authentication. Instead, the
pinning ticket only contains the encrypted pinning secret. The
pinning ticket is used by the server to prove its ability to decrypt
it, which implies ownership of the pinning protection key.
[RFC5077] has been obsoleted by [RFC8446], and ticket resumption is
now defined by Section 2.2 of [RFC8446]. This document references
[RFC5077] as an informational document since it contains a more
thorough discussion of stateless ticket resumption, and because
ticket resumption benefits from significant operational experience
with TLS 1.2 that is still widely deployed at the time of writing.
This experience, as well as deployment experience, can easily be re-
used for identity pinning.
With TLS 1.3, session resumption is based on a Pre-Shared Key (PSK).
This is orthogonal to this protocol. With TLS 1.3, a TLS session can
be established using PKI and a pinning ticket, and later resumed with
PSK.
However, the protocol described in this document addresses the
problem of misissued certificates. Thus, it is not expected to be
used outside a certificate-based TLS key exchange, such as in PSK.
As a result, PSK handshakes MUST NOT include the extension defined
here.
2.1. Initial Connection
When a client first connects to a server, it requests a pinning
ticket by sending an empty PinningTicket extension, and receives it
as part of the server's first response, in the returned PinningTicket
extension.
Client Server
ClientHello
+ key_share
+ signature_algorithms
+ PinningTicket -------->
ServerHello
+ key_share
{EncryptedExtensions
+ PinningTicket}
{CertificateRequest*}
{Certificate*}
{CertificateVerify*}
<-------- {Finished}
{Certificate*}
{CertificateVerify*}
{Finished} -------->
[Application Data] <-------> [Application Data]
* Indicates optional or situation-dependent
messages that are not always sent.
{} Indicates messages protected using keys
derived from the ephemeral secret.
[] Indicates messages protected using keys
derived from the master secret.
If a client supports the PinningTicket extension and does not have
any pinning ticket associated with the server, the exchange is
considered as an initial connection. Other reasons the client may
not have a pinning ticket include the client having flushed its
pinning ticket store, or the committed lifetime of the pinning ticket
having expired.
Upon receipt of the PinningTicket extension, the server computes a
pinning secret (Section 4.1) and sends the pinning ticket
(Section 4.2) encrypted with the pinning protection key
(Section 4.3). The pinning ticket is associated with a lifetime
value by which the server assumes the responsibility of retaining the
pinning protection key and being able to decrypt incoming pinning
tickets during the period indicated by the committed lifetime.
Once the pinning ticket has been generated, the server returns the
pinning ticket and the committed lifetime in a PinningTicket
extension embedded in the EncryptedExtensions message. We note that
a PinningTicket extension MUST NOT be sent as part of a
HelloRetryRequest.
Upon receiving the pinning ticket, the client MUST NOT accept it
until the key exchange is completed and the server authenticated. If
the key exchange is not completed successfully, the client MUST
ignore the received pinning ticket. Otherwise, the client computes
the pinning secret and SHOULD cache the pinning secret and the
pinning ticket for the duration indicated by the pinning ticket
lifetime. The client SHOULD clean up the cached values at the end of
the indicated lifetime.
2.2. Subsequent Connections
When the client initiates a connection to a server it has previously
seen (see Section 2.3 on identifying servers), it SHOULD send the
pinning ticket for that server. The pinning ticket, pinning secret,
and pinning ticket lifetime computed during the establishment of the
previous TLS session are designated in this document as the
"original" ones, to distinguish them from a new ticket that may be
generated during the current session.
The server MUST extract the original pinning_secret value from the
ticket and MUST respond with a PinningTicket extension, which
includes:
* A proof that the server can understand the ticket that was sent by
the client; this proof also binds the pinning ticket to the
server's (current) public key, as well as the ongoing TLS session.
The proof is mandatory and MUST be included if a pinning ticket
was sent by the client.
* A fresh pinning ticket. The main reason for refreshing the ticket
on each connection is privacy: to avoid the ticket serving as a
fixed client identifier. While a fresh pinning ticket might be of
zero length, it is RECOMMENDED to include a fresh ticket with a
nonzero length with each response.
If the server cannot validate the received ticket, that might
indicate an earlier MITM attack on this client. The server MUST then
abort the connection with a handshake_failure alert and SHOULD log
this failure.
The client MUST verify the proof, and if it fails to do so, the
client MUST issue a handshake_failure alert and abort the connection
(see also Section 6.5). It is important that the client does not
attempt to "fall back" by omitting the PinningTicket extension.
When the connection is successfully set up, i.e., after the Finished
message is verified, the client SHOULD store the new ticket along
with the corresponding pinning_secret, replacing the original ticket.
Although this is an extension, if the client already has a ticket for
a server, the client MUST interpret a missing PinningTicket extension
in the server's response as an attack, because of the server's prior
commitment to respect the ticket. The client MUST abort the
connection in this case. See also Section 5.5 on ramping down
support for this extension.
2.3. Indexing the Pins
Each pin is associated with a set of identifiers that include, among
others, hostname, protocol (TLS or DTLS), and port number. In other
words, the pin for port TCP/443 may be different from that for DTLS,
or from the pin for port TCP/8443. These identifiers are expected to
be relevant to characterize the identity of the server as well as the
establishing TLS session. When a hostname is used, it MUST be the
value sent inside the Server Name Indication (SNI) extension. This
definition is similar to the concept of a Web Origin [RFC6454], but
does not assume the existence of a URL.
The purpose of ticket pinning is to pin the server identity. As a
result, any information orthogonal to the server's identity MUST NOT
be considered in indexing. More particularly, IP addresses are
ephemeral and forbidden in SNI, and therefore pins MUST NOT be
associated with IP addresses. Similarly, CA names or public keys
associated with server MUST NOT be used for indexing as they may
change over time.
3. Message Definitions
This section defines the format of the PinningTicket extension. We
follow the message notation of [RFC8446].
opaque pinning_ticket<0..2^16-1>;
opaque pinning_proof<0..2^8-1>;
struct {
select (Role) {
case client:
pinning_ticket ticket<0..2^16-1>; //omitted on 1st connection
case server:
pinning_proof proof<0..2^8-1>; //no proof on 1st connection
pinning_ticket ticket<0..2^16-1>; //omitted on ramp down
uint32 lifetime;
}
} PinningTicketExtension;
ticket a pinning ticket sent by the client or returned by the
server. The ticket is opaque to the client. The extension
MUST contain exactly 0 or 1 tickets.
proof a demonstration by the server that it understands the
received ticket and therefore that it is in possession of
the secret that was used to generate it originally. The
extension MUST contain exactly 0 or 1 proofs.
lifetime the duration (in seconds) that the server commits to accept
offered tickets in the future.
4. Cryptographic Operations
This section provides details on the cryptographic operations
performed by the protocol peers.
4.1. Pinning Secret
The pinning secret is generated locally by the client and the server,
which means they must use the same inputs to generate it. This value
must be generated before the ServerHello message is sent, as the
server includes the corresponding pinning ticket in the same flight
as the ServerHello message. In addition, the pinning secret must be
unpredictable to any party other than the client and the server.
The pinning secret is derived using the Derive-Secret function
provided by TLS 1.3, described in Section 7.1 of [RFC8446].
pinning secret = Derive-Secret(Handshake Secret, "pinning secret",
ClientHello...ServerHello)
4.2. Pinning Ticket
The pinning ticket contains the pinning secret. The pinning ticket
is provided by the client to the server, which decrypts it in order
to extract the pinning secret and responds with a pinning proof. As
a result, the characteristics of the pinning ticket are:
* Pinning tickets MUST be encrypted and integrity-protected using
strong cryptographic algorithms.
* Pinning tickets MUST be protected with a long-term pinning
protection key.
* Pinning tickets MUST include a pinning protection key ID or serial
number as to enable the pinning protection key to be refreshed.
* The pinning ticket MAY include other information, in addition to
the pinning secret. When additional information is included, a
careful review needs to be performed to evaluate its impact on
privacy.
The pinning ticket's format is not specified by this document, but a
format similar to the one proposed by [RFC5077] is RECOMMENDED.
4.3. Pinning Protection Key
The pinning protection key is used only by the server and so remains
server implementation specific. [RFC5077] recommends the use of two
keys, but when using Authenticated Encryption with Associated Data
(AEAD) algorithms, only a single key is required.
When a single server terminates TLS for multiple virtual servers
using the SNI mechanism, it is strongly RECOMMENDED that the server
use a separate protection key for each one of them, in order to allow
migrating virtual servers between different servers while keeping
pinning active.
As noted in Section 5.1, if the server is actually a cluster of
machines, the protection key MUST be synchronized between all the
nodes that accept TLS connections to the same server name. When
[RFC5077] is deployed, an easy way to do it is to derive the
protection key from the session-ticket protection key, which is
already synchronized. For example:
pinning_protection_key = HKDF-Expand(resumption_protection_key,
"pinning protection", L)
Where resumption_protection_key is the ticket protection key defined
in [RFC5077]. Both resumption_protection_key and
pinning_protection_key are only used by the server.
The above solution attempts to minimize code changes related to
management of the resumption_protection_key. The drawback is that
this key would be used both to directly encrypt session tickets and
to derive the pinning_protection_key, and such mixed usage of a
single key is not in line with cryptographic best practices. Where
possible, it is RECOMMENDED that the resumption_protection_key be
unrelated to the pinning_protection_key and that they are separately
shared among the relevant servers.
4.4. Pinning Proof
The pinning proof is sent by the server to demonstrate that it has
been able to decrypt the pinning ticket and to retrieve the pinning
secret. The proof must be unpredictable and must not be replayed.
Similarly to the pinning ticket, the pinning proof is sent by the
server in the ServerHello message. In addition, it must not be
possible for a MITM server with a fake certificate to obtain a
pinning proof from the original server.
In order to address these requirements, the pinning proof is bound to
the TLS session as well as the public key of the server:
pinning_proof_secret=Derive-Secret(Handshake Secret,
"pinning proof 1", ClientHello...ServerHello)
proof = HMAC(original_pinning_secret, "pinning proof 2" +
pinning_proof_secret + Hash(server_public_key))
where HMAC [RFC2104] uses the Hash algorithm that was negotiated in
the handshake, and the same hash is also used over the server's
public key. The original_pinning_secret value refers to the secret
value extracted from the ticket sent by the client, to distinguish it
from a new pinning secret value that is possibly computed in the
current exchange. The server_public_key value is the DER
representation of the public key, specifically the
SubjectPublicKeyInfo structure as-is.
5. Operational Considerations
The main motivation behind the current protocol is to enable identity
pinning without the need for manual operations. Manual operations
are susceptible to human error, and in the case of public key
pinning, can easily result in "server bricking": the server becoming
inaccessible to some or all of its users. To achieve this goal,
operations described in identity pinning are only performed within
the current TLS session, and there is no dependence on any TLS
configuration parameters such as CA identity or public keys. As a
result, configuration changes are unlikely to lead to desynchronized
state between the client and the server.
5.1. Protection Key Synchronization
The only operational requirement when deploying this protocol is
that, if the server is part of a cluster, protection keys (the keys
used to encrypt tickets) MUST be synchronized between all cluster
members. The protocol is designed so that if resumption ticket
protection keys [RFC5077] are already synchronized between cluster
members, nothing more needs to be done.
Moreover, synchronization does not need to be instantaneous, e.g.,
protection keys can be distributed a few minutes or hours in advance
of their rollover. In such scenarios, each cluster member MUST be
able to accept tickets protected with a new version of the protection
key, even while it is still using an old version to generate keys.
This ensures that, when a client receives a "new" ticket, it does not
next hit a cluster member that still rejects this ticket.
Misconfiguration can lead to the server's clock being off by a large
amount of time. Consider a case where a server's clock is
misconfigured, for example, to be 1 year in the future, and the
system is allowed to delete expired keys automatically. The server
will then delete many outstanding keys because they are now long
expired and will end up rejecting valid tickets that are stored by
clients. Such a scenario could make the server inaccessible to a
large number of clients.
The decision to delete a key should at least consider the largest
value of the ticket lifetime as well as the expected time
desynchronization between the servers of the cluster and the time
difference for distributing the new key among the different servers
in the cluster.
5.2. Ticket Lifetime
The lifetime of the ticket is a commitment by the server to retain
the ticket's corresponding protection key for this duration, so that
the server can prove to the client that it knows the secret embedded
in the ticket. For production systems, the lifetime SHOULD be
between 7 and 31 days.
5.3. Certificate Renewal
The protocol ensures that the client will continue speaking to the
correct server even when the server's certificate is renewed. In
this sense, pinning is not associated with certificates, which is the
reason we designate the protocol described in this document as
"server identity pinning".
Note that this property is not impacted by the use of the server's
public key in the pinning proof because the scope of the public key
used is only the current TLS session.
5.4. Certificate Revocation
The protocol is orthogonal to certificate validation in the sense
that, if the server's certificate has been revoked or is invalid for
some other reason, the client MUST refuse to connect to it regardless
of any ticket-related behavior.
5.5. Disabling Pinning
A server implementing this protocol MUST have a "ramp down" mode of
operation where:
* The server continues to accept valid pinning tickets and responds
correctly with a proof.
* The server does not send back a new pinning ticket.
After a while, no clients will hold valid tickets, and the feature
may be disabled. Note that clients that do not receive a new pinning
ticket do not necessarily need to remove the original ticket.
Instead, the client may keep using the ticket until its lifetime
expires. However, as detailed in Section 6.7, re-use of a ticket by
the client may result in privacy concerns as the ticket value may be
used to correlate TLS sessions.
Issuing a new pinning ticket with a shorter lifetime would only delay
the ramp down process, as the shorter lifetime can only affect
clients that actually initiated a new connection. Other clients
would still see the original lifetime for their pinning tickets.
5.6. Server Compromise
If a server compromise is detected, the pinning protection key MUST
be rotated immediately, but the server MUST still accept valid
tickets that use the old, compromised key. Clients that still hold
old pinning tickets will remain vulnerable to MITM attacks, but those
that connect to the correct server will immediately receive new
tickets protected with the newly generated pinning protection key.
The same procedure applies if the pinning protection key is
compromised directly, e.g., if a backup copy is inadvertently made
public.
5.7. Disaster Recovery
All web servers in production need to be backed up, so that they can
be recovered if a disaster (including a malicious activity) ever
wipes them out. Backup often includes the certificate and its
private key, which must be backed up securely. The pinning secret,
including earlier versions that are still being accepted, must be
backed up regularly. However since it is only used as an
authentication second factor, it does not require the same level of
confidentiality as the server's private key.
Readers should note that [RFC5077] session resumption keys are more
security sensitive and should normally not be backed up, but rather
treated as ephemeral keys. Even when servers derive pinning secrets
from resumption keys (Section 4.1), they MUST NOT back up resumption
keys.
6. Security Considerations
This section reviews several security aspects related to the proposed
extension.
6.1. Trust-on-First-Use (TOFU) and MITM Attacks
This protocol is a trust-on-first-use protocol. If a client
initially connects to the "right" server, it will be protected
against MITM attackers for the lifetime of each received ticket. If
it connects regularly (depending, of course, on the server-selected
lifetime), it will stay constantly protected against fake
certificates.
However if it initially connects to an attacker, subsequent
connections to the "right" server will fail. Server operators might
want to advise clients on how to remove corrupted pins, once such
large-scale attacks are detected and remediated.
The protocol is designed so that it is not vulnerable to an active
MITM attacker who has real-time access to the original server. The
pinning proof includes a hash of the server's public key to ensure
the client that the proof was in fact generated by the server with
which it is initiating the connection.
6.2. Pervasive Monitoring
Some organizations, and even some countries, perform pervasive
monitoring on their constituents [RFC7258]. This often takes the
form of always-active SSL proxies. Because of the TOFU property,
this protocol does not provide any security in such cases.
Pervasive monitoring may also result in privacy concerns detailed in
Section 6.7.
6.3. Server-Side Error Detection
Uniquely, this protocol allows the server to detect clients that
present incorrect tickets and therefore can be assumed to be victims
of a MITM attack. Server operators can use such cases as indications
of ongoing attacks, similarly to fake certificate attacks that took
place in a few countries in the past.
6.4. Client Policy and SSL Proxies
Like it or not, some clients are normally deployed behind an SSL
proxy. Similar to [RFC7469], it is acceptable to allow pinning to be
disabled for some hosts according to local policy. For example, a
User Agent (UA) MAY disable pinning for hosts whose validated
certificate chain terminates at a user-defined trust anchor, rather
than a trust anchor built into the UA (or underlying platform).
Moreover, a client MAY accept an empty PinningTicket extension from
such hosts as a valid response.
6.5. Client-Side Error Behavior
When a client receives a malformed or empty PinningTicket extension
from a pinned server, it MUST abort the handshake. If the client
retries the request, it MUST NOT omit the PinningTicket in the retry
message. Doing otherwise would expose the client to trivial fallback
attacks, similar to those described in [RFC7507].
However, this rule can negatively impact clients that move from
behind SSL proxies into the open Internet, and vice versa, if the
advice in Section 6.4 is not followed. Therefore, it is RECOMMENDED
that browser and library vendors provide a documented way to remove
stored pins.
6.6. Stolen and Forged Tickets
An attacker gains no benefit from stealing pinning tickets, even in
conjunction with other pinning parameters such as the associated
pinning secret, since pinning tickets are used to secure the client
rather than the server. Similarly, it is useless to forge a ticket
for a particular server.
6.7. Client Privacy
This protocol is designed so that an external attacker cannot link
different requests to a single client, provided the client requests
and receives a fresh ticket upon each connection. This may be of
concern particularly during ramp down, if the server does not provide
a new ticket, and the client reuses the same ticket. To reduce or
avoid such privacy concerns, it is RECOMMENDED for the server to
issue a fresh ticket with a reduced lifetime. This would at least
reduce the time period in which the TLS sessions of the client can be
linked. The server MAY also issue tickets with a zero-second
lifetime until it is confident all tickets are expired.
On the other hand, the server to which the client is connecting can
easily track the client. This may be an issue when the client
expects to connect to the server (e.g., a mail server) with multiple
identities. Implementations SHOULD allow the user to opt out of
pinning, either in general or for particular servers.
This document does not define the exact content of tickets.
Including client-specific information in tickets would raise privacy
concerns and is NOT RECOMMENDED.
6.8. Ticket Protection Key Management
While the ticket format is not mandated by this document, protecting
the ticket using authenticated encryption is RECOMMENDED. Some of
the algorithms commonly used for authenticated encryption, e.g.,
Galois/Counter Mode (GCM), are highly vulnerable to nonce reuse, and
this problem is magnified in a cluster setting. Therefore,
implementations that choose AES-GCM or any AEAD equivalent MUST adopt
one of these three alternatives:
* Partition the nonce namespace between cluster members and use
monotonic counters on each member, e.g., by setting the nonce to
the concatenation of the cluster member ID and an incremental
counter.
* Generate random nonces but avoid the so-called birthday bound,
i.e., never generate more than the maximum allowed number of
encrypted tickets (2**64 for AES-128-GCM) for the same ticket
pinning protection key.
* An alternative design that has been attributed to Karthik
Bhargavan is as follows. Start with a 128-bit master key K_master
and then for each encryption, generate a 256-bit random nonce and
compute: K = HKDF(K_master, Nonce || "key"), then N =
HKDF(K_master, Nonce || "nonce"). Use these values to encrypt the
ticket, AES-GCM(K, N, data). This nonce should then be stored and
transmitted with the ticket.
7. IANA Considerations
The IANA has allocated a TicketPinning extension value in the "TLS
ExtensionType Values" registry.
[RFC8447] defines the procedure, requirements, and the necessary
information for the IANA to update the "TLS ExtensionType Values"
registry [TLS-EXT]. The registration procedure is "Specification
Required" [RFC8126].
The TicketPinning extension is registered as follows. (The extension
is not limited to Private Use, and as such has its first byte in the
range 0-254.)
Value: 32
Name: ticket_pinning
Recommended: No
TLS 1.3: CH, EE (to indicate that the extension is present in
ClientHello and EncryptedExtensions messages)
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>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
[RFC8447] Salowey, J. and S. Turner, "IANA Registry Updates for TLS
and DTLS", RFC 8447, DOI 10.17487/RFC8447, August 2018,
<https://www.rfc-editor.org/info/rfc8447>.
8.2. Informative References
[Netcraft] Mutton, P., "HTTP Public Key Pinning: You're doing it
wrong!", March 2016,
<https://news.netcraft.com/archives/2016/03/30/http-
public-key-pinning-youre-doing-it-wrong.html>.
[Oreo] Berkman, O., Pinkas, B., and M. Yung, "Firm Grip
Handshakes: A Tool for Bidirectional Vouching", Cryptology
and Network Security pp. 142-157, 2012.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[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>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454,
DOI 10.17487/RFC6454, December 2011,
<https://www.rfc-editor.org/info/rfc6454>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <https://www.rfc-editor.org/info/rfc7469>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<https://www.rfc-editor.org/info/rfc7507>.
[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>.
[TLS-EXT] IANA, "TLS Extension Type Value",
<https://www.iana.org/assignments/tls-extensiontype-
values/>.
[TLS-TACK] Marlinspike, M., "Trust Assertions for Certificate Keys",
Work in Progress, Internet-Draft, draft-perrin-tls-tack-
02, 7 January 2013,
<https://tools.ietf.org/html/draft-perrin-tls-tack-02>.
Appendix A. Previous Work
The global PKI system relies on the trust of a CA issuing
certificates. As a result, a corrupted trusted CA may issue a
certificate for any organization without the organization's approval
(a misissued or "fake" certificate), and use the certificate to
impersonate the organization. There are many attempts to resolve
these weaknesses, including the Certificate Transparency (CT)
protocol [RFC6962], HTTP Public Key Pinning (HPKP) [RFC7469], and
Trust Assertions for Certificate Keys (TACK) [TLS-TACK].
CT requires cooperation of a large portion of the hundreds of extant
certificate authorities (CAs) before it can be used "for real", in
enforcing mode. It is noted that the relevant industry forum (CA/
Browser Forum) is indeed pushing for such extensive adoption.
However the public nature of CT often makes it inappropriate for
enterprise use because many organizations are not willing to expose
their internal infrastructure publicly.
TACK has some similarities to the current proposal, but work on it
seems to have stalled. Appendix A.2 compares our proposal to TACK.
HPKP is an IETF standard, but so far has proven hard to deploy. HPKP
pins (fixes) a public key, one of the public keys listed in the
certificate chain. As a result, HPKP needs to be coordinated with
the certificate management process. Certificate management impacts
HPKP and thus increases the probability of HPKP failures. This risk
is made even higher given the fact that, even though work has been
done in the Automated Certificate Management Environment (ACME)
working group to automate certificate management, in many or even
most cases, certificates are still managed manually. As a result,
HPKP cannot be completely automated, resulting in error-prone manual
configuration. Such errors could prevent the web server from being
accessed by some clients. In addition, HPKP uses an HTTP header,
which makes this solution HTTPS specific and not generic to TLS. On
the other hand, the current document provides a solution that is
independent of the server's certificate management, and that can be
entirely and easily automated. Appendix A.1 compares HPKP to the
current document in more detail.
The ticket pinning proposal augments these mechanisms with a much
easier to implement and deploy solution for server identity pinning,
by reusing some of the ideas behind TLS session resumption.
This section compares ticket pinning to two earlier proposals, HPKP
and TACK.
A.1. Comparison: HPKP
The current IETF standard for pinning the identity of web servers is
HPKP [RFC7469].
The main differences between HPKP and the current document are the
following:
* HPKP limits its scope to HTTPS, while the current document
considers all application above TLS.
* HPKP pins the public key of the server (or another public key
along the certificate chain), and as such, is highly dependent on
the management of certificates. Such dependency increases the
potential error surface, especially as certificate management is
not yet largely automated. The current proposal, on the other
hand, is independent of certificate management.
* HPKP pins public keys that are public and used for the standard
TLS authentication. Identity pinning relies on the ownership of
the pinning key, which is not disclosed to the public and not
involved in the standard TLS authentication. As a result,
identity pinning is a completely independent, second-factor
authentication mechanism.
* HPKP relies on a backup key to recover the misissuance of a key.
We believe such backup mechanisms add excessive complexity and
cost. Reliability of the current mechanism is primarily based on
its being highly automated.
* HPKP relies on the client to report errors to the report-uri. The
current document does not need any out-of-band mechanism, and the
server is informed automatically. This provides an easier and
more reliable health monitoring.
On the other hand, HPKP shares the following aspects with identity
pinning:
* Both mechanisms provide hard failure. With HPKP, only the client
is aware of the failure, while with the current proposal both
client and server are informed of the failure. This provides room
for further mechanisms to automatically recover from such
failures.
* Both mechanisms are subject to a server compromise in which users
are provided with an invalid ticket (e.g., a random one) or HTTP
header with a very long lifetime. For identity pinning, this
lifetime SHOULD NOT be longer than 31 days. In both cases,
clients will not be able to reconnect the server during this
lifetime. With the current proposal, an attacker needs to
compromise the TLS layer, while with HPKP, the attacker needs to
compromise the HTTP server. Arguably, the TLS-level compromise is
typically more difficult for the attacker.
Unfortunately HPKP has not seen wide deployment yet. As of March
2016, the number of servers using HPKP was less than 3000 [Netcraft].
This may simply be due to inertia, but we believe the main reason is
the interactions between HPKP and manual certificate management that
is needed to implement HPKP for enterprise servers. The penalty for
making mistakes (e.g., being too early or too late to deploy new
pins) is that the server becomes unusable for some of the clients.
To demonstrate this point, we present a list of the steps involved in
deploying HPKP on a security-sensitive web server.
1. Generate two public/private key pairs on a computer that is not
the live server. The second one is the "backup1" key pair.
openssl genrsa -out "example.com.key" 2048;
openssl genrsa -out "example.com.backup1.key" 2048;
2. Generate hashes for both of the public keys. These will be used
in the HPKP header:
openssl rsa -in "example.com.key" -outform der -pubout | \
openssl dgst -sha256 -binary | openssl enc -base64
openssl rsa -in "example.com.backup1.key" -outform der \
-pubout | openssl dgst -sha256 -binary | openssl enc -base64
3. Generate a single CSR (Certificate Signing Request) for the
first key pair, where you include the domain name in the CN
(Common Name) field:
openssl req -new -subj "/C=GB/ST=Area/L=Town/O=Org/ \
CN=example.com" -key "example.com.key" -out "example.com.csr";
4. Send this CSR to the CA and go though the dance to prove you own
the domain. The CA will give you a single certificate that will
typically expire within a year or two.
5. On the live server, upload and set up the first key pair and its
certificate. At this point, you can add the "Public-Key-Pins"
header, using the two hashes you created in step 2.
Note that only the first key pair has been uploaded to the
server so far.
6. Store the second (backup1) key pair somewhere safe, probably
somewhere encrypted like a password manager. It won't expire,
as it's just a key pair; it just needs to be ready for when you
need to get your next certificate.
7. Time passes -- probably just under a year (if waiting for a
certificate to expire), or maybe sooner if you find that your
server has been compromised, and you need to replace the key
pair and certificate.
8. Create a new CSR using the "backup1" key pair, and get a new
certificate from your CA.
9. Generate a new backup key pair (backup2), get its hash, and
store it in a safe place (again, not on the live server).
10. Replace your old certificate and old key pair, update the
"Public-Key-Pins" header to remove the old hash, and add the new
"backup2" key pair.
Note that in the above steps, both the certificate issuance as well
as the storage of the backup key pair involve manual steps. Even
with an automated CA that runs the ACME protocol [RFC8555], key
backup would be a challenge to automate.
A.2. Comparison: TACK
Compared with HPKP, TACK [TLS-TACK] is more similar to the current
document. It can even be argued that this document is a symmetric-
cryptography variant of TACK. That said, there are still a few
significant differences:
* Probably the most important difference is that with TACK,
validation of the server certificate is no longer required, and in
fact TACK specifies it as a "MAY" requirement ([TLS-TACK],
Section 5.3). With ticket pinning, certificate validation by the
client remains a MUST requirement, and the ticket acts only as a
second factor. If the pinning secret is compromised, the server's
security is not immediately at risk.
* Both TACK and the current document are mostly orthogonal to the
server certificate as far as their life cycle, and so both can be
deployed with no manual steps.
* TACK uses Elliptic Curve Digital Signature Algorithm (ECDSA) to
sign the server's public key. This allows cooperating clients to
share server assertions between themselves. This is an optional
TACK feature, and one that cannot be done with pinning tickets.
* TACK allows multiple servers to share its public keys. Such
sharing is disallowed by the current document.
* TACK does not allow the server to track a particular client, and
so has better privacy properties than the current document.
* TACK has an interesting way to determine the pin's lifetime,
setting it to the time period since the pin was first observed,
with a hard upper bound of 30 days. The current document makes
the lifetime explicit, which may be more flexible to deploy. For
example, web sites that are only visited rarely by users may opt
for a longer period than other sites that expect users to visit on
a daily basis.
Acknowledgments
The original idea behind this proposal was published in [Oreo] by
Moti Yung, Benny Pinkas, and Omer Berkman. The current protocol is
but a distant relative of the original Oreo protocol, and any errors
are the responsibility of the authors of this document alone.
We would like to thank Adrian Farrel, Dave Garrett, Daniel Kahn
Gillmor, Alexey Melnikov, Yoav Nir, Eric Rescorla, Benjamin Kaduk,
and Rich Salz for their comments on this document. Special thanks to
Craig Francis for contributing the HPKP deployment script, and to
Ralph Holz for several fruitful discussions.
Authors' Addresses
Yaron Sheffer
Intuit
Email: yaronf.ietf@gmail.com
Daniel Migault
Ericsson
Email: daniel.migault@ericsson.com