TLS 1.3 Extension for
Certificate-Based Authentication with an External Pre-Shared Key
Vigil Security, LLC
516 Dranesville Road
Herndon
VA
20170
United States of America
housley@vigilsec.com
cryptography
This document specifies a TLS 1.3 extension that allows a server to
authenticate with a combination of a certificate and an external
pre-shared key (PSK).
Introduction
The TLS 1.3 handshake
protocol provides two mutually exclusive forms of server
authentication. First, the server can be authenticated by
providing a signature certificate and creating a valid digital
signature to demonstrate that it possesses the corresponding
private key. Second, the server can be authenticated
by demonstrating that it possesses a pre-shared key (PSK) that
was established by a previous handshake. A PSK that
is established in this fashion is called a resumption PSK. A
PSK that is established by any other means is called an external
PSK. This document specifies a TLS 1.3 extension permitting
certificate-based server authentication to be combined with
an external PSK as an input to the TLS 1.3 key schedule.
Several implementors wanted to gain more experience with this
specification before producing a Standards Track RFC. As a
result, this specification is being published as an Experimental
RFC to enable interoperable implementations and gain deployment
and operational experience.
Terminology
The key words "MUST", "MUST NOT",
"REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14
when, and only when, they appear in all capitals, as shown here.
Motivation and Design Rationale
The development of a large-scale quantum computer would pose a serious
challenge for the cryptographic algorithms that are widely deployed
today, including the digital signature algorithms that are used
to authenticate the server in the TLS 1.3 handshake protocol. It
is an open question whether or not it is feasible to build
a large-scale quantum computer, and if so, when that might
happen. However, if such a quantum computer is invented, many
of the cryptographic algorithms and the security protocols that
use them would become vulnerable.
The TLS 1.3 handshake protocol employs key agreement algorithms
and digital signature algorithms that could be broken by the
development of a large-scale quantum computer
. The key agreement algorithms
include Diffie-Hellman (DH) and
Elliptic Curve Diffie-Hellman (ECDH) ;
the digital signature algorithms include RSA
and the Elliptic Curve Digital Signature Algorithm (ECDSA)
. As a result, an adversary that
stores a TLS 1.3 handshake protocol exchange today could
decrypt the associated encrypted communications in the
future when a large-scale quantum computer becomes
available.
In the near term, this document describes a TLS 1.3 extension to protect
today's communications from the future invention of a large-scale
quantum computer by providing a strong external PSK as an input to
the TLS 1.3 key schedule while preserving the authentication provided
by the existing certificate and digital signature mechanisms.
Extension Overview
This section provides a brief overview of the
"tls_cert_with_extern_psk" extension.
The client includes the "tls_cert_with_extern_psk" extension in the
ClientHello message. The "tls_cert_with_extern_psk" extension MUST
be accompanied by the "key_share", "psk_key_exchange_modes", and
"pre_shared_key" extensions. The client MAY also find it useful
to include the "supported_groups" extension. Since the
"tls_cert_with_extern_psk" extension is intended to be used only
with initial handshakes, it MUST NOT be sent alongside the
"early_data" extension. These extensions are all described in
, which also requires
the "pre_shared_key" extension to be the last extension in the
ClientHello message.
If the client includes both the "tls_cert_with_extern_psk" extension
and the "early_data" extension, then the server MUST terminate the
connection with an "illegal_parameter" alert.
If the server is willing to use one of the external PSKs listed in the
"pre_shared_key" extension and perform certificate-based authentication,
then the server includes the "tls_cert_with_extern_psk" extension in the
ServerHello message. The "tls_cert_with_extern_psk" extension MUST be
accompanied by the "key_share" and "pre_shared_key" extensions. If none
of the external PSKs in the list provided by the client is acceptable
to the server, then the "tls_cert_with_extern_psk" extension is
omitted from the ServerHello message.
When the "tls_cert_with_extern_psk" extension is successfully
negotiated, the TLS 1.3 key schedule processing includes
both the selected external PSK and the (EC)DHE shared secret
value. (EC)DHE refers to Diffie-Hellman over either finite fields
or elliptic curves. As a result, the Early Secret, Handshake
Secret, and Master Secret values all depend upon the value of the
selected external PSK. Of course, the Early Secret does not
depend upon the (EC)DHE shared secret.
The authentication of the server and optional authentication of
the client depend upon the ability to generate a signature that
can be validated with the public key in their certificates. The
authentication processing is not changed in any way by the
selected external PSK.
Each external PSK is associated with a single hash algorithm, which
is required by . The
hash algorithm MUST be set when the PSK is established, with a
default of SHA-256.
Certificate with External PSK Extension
This section specifies the "tls_cert_with_extern_psk" extension,
which MAY appear in the ClientHello message and ServerHello message. It
MUST NOT appear in any other messages. The "tls_cert_with_extern_psk"
extension MUST NOT appear in the ServerHello message unless the
"tls_cert_with_extern_psk" extension appeared in the preceding
ClientHello message. If an implementation recognizes the
"tls_cert_with_extern_psk" extension and receives it in any other
message, then the implementation MUST abort the handshake with an
"illegal_parameter" alert.
The general extension mechanisms enable clients and servers to
negotiate the use of specific extensions. Clients request
extended functionality from servers with the extensions field
in the ClientHello message. If the server responds with a
HelloRetryRequest message, then the client sends another
ClientHello message as described in , including the same
"tls_cert_with_extern_psk" extension as the original
ClientHello message, or aborts the handshake.
Many server extensions are carried in the EncryptedExtensions
message; however, the "tls_cert_with_extern_psk" extension is
carried in the ServerHello message. Successful negotiation of
the "tls_cert_with_extern_psk" extension affects the key used for
encryption, so it cannot be carried in the EncryptedExtensions
message. Therefore, the "tls_cert_with_extern_psk" extension
is only present in the ServerHello message if the server
recognizes the "tls_cert_with_extern_psk" extension and the
server possesses one of the external PSKs offered by the client
in the "pre_shared_key" extension in the ClientHello message.
The Extension structure is defined in ;
it is repeated here for convenience.
struct {
ExtensionType extension_type;
opaque extension_data<0..2^16-1>;
} Extension;
The "extension_type" identifies the particular extension type,
and the "extension_data" contains information specific to the
particular extension type.
This document specifies the "tls_cert_with_extern_psk" extension,
adding one new type to ExtensionType:
enum {
tls_cert_with_extern_psk(33), (65535)
} ExtensionType;
The "tls_cert_with_extern_psk" extension is relevant when the
client and server possess an external PSK in common that can be
used as an input to the TLS 1.3 key schedule. The
"tls_cert_with_extern_psk" extension is essentially a flag to
use the external PSK in the key schedule, and it has the
following syntax:
struct {
select (Handshake.msg_type) {
case client_hello: Empty;
case server_hello: Empty;
};
} CertWithExternPSK;
Companion Extensions
lists the extensions that are required to accompany the
"tls_cert_with_extern_psk" extension. Most of those extensions
are not impacted in any way by this specification. However, this
section discusses the extensions that require additional consideration.
The "psk_key_exchange_modes" extension is defined in
of . The
"psk_key_exchange_modes"
extension restricts the use of both the PSKs offered in this
ClientHello and those that the server might supply via a subsequent
NewSessionTicket. As a result, when the "psk_key_exchange_modes"
extension is included in the ClientHello message, clients MUST
include psk_dhe_ke mode. In addition, clients MAY also include
psk_ke mode to support a subsequent NewSessionTicket. When the
"psk_key_exchange_modes" extension is included in the ServerHello
message, servers MUST select the psk_dhe_ke mode for the initial
handshake. Servers MUST select a key exchange mode that is listed
by the client for subsequent handshakes that include the resumption
PSK from the initial handshake.
The "pre_shared_key" extension is defined in . The
syntax is repeated below for
convenience. All of the listed PSKs MUST be external PSKs. If a
resumption PSK is listed along with the "tls_cert_with_extern_psk"
extension, the server MUST abort the handshake with an
"illegal_parameter" alert.
struct {
opaque identity<1..2^16-1>;
uint32 obfuscated_ticket_age;
} PskIdentity;
opaque PskBinderEntry<32..255>;
struct {
PskIdentity identities<7..2^16-1>;
PskBinderEntry binders<33..2^16-1>;
} OfferedPsks;
struct {
select (Handshake.msg_type) {
case client_hello: OfferedPsks;
case server_hello: uint16 selected_identity;
};
} PreSharedKeyExtension;
"OfferedPsks" contains the list of PSK identities and
associated binders for the external PSKs that the client is
willing to use with the server.
The identities are a list of external PSK identities that the
client is willing to negotiate with the server. Each external
PSK has an associated identity that is known to the client
and the server; the associated identities may be known to other
parties as well. In addition, the binder validation (see below)
confirms that the client and server have the same key associated
with the identity.
The "obfuscated_ticket_age" is not used for external PSKs. As
stated in , clients
SHOULD set this value to 0, and servers MUST ignore the value.
The binders are a series of HMAC values, one
for each external PSK offered by the client, in the same order as the
identities list. The HMAC value is computed using the binder_key, which
is derived from the external PSK, and a partial transcript of the current
handshake. Generation of the binder_key from the external PSK is
described in . The
partial transcript of the current handshake includes a partial
ClientHello up to and including the PreSharedKeyExtension.identities
field, as described in .
The "selected_identity" contains the index of the external PSK
identity that the server selected from the list offered by the
client. As described in ,
the server MUST validate the binder value that corresponds to the
selected external PSK, and if the binder does not validate, the
server MUST abort the handshake with an "illegal_parameter" alert.
Authentication
When the "tls_cert_with_extern_psk" extension is successfully
negotiated, authentication of the server depends upon the ability to
generate a signature that can be validated with the public key in
the server's certificate. This is accomplished by the server
sending the Certificate and CertificateVerify messages, as described
in Sections and of .
TLS 1.3 does not permit the server to send a CertificateRequest message
when a PSK is being used. This restriction is removed when the
"tls_cert_with_extern_psk" extension is negotiated, allowing
certificate-based authentication for both the client and the server. If
certificate-based client authentication is desired, this is accomplished
by the client sending the Certificate and CertificateVerify messages as
described in Sections and of .
Keying Material
specifies the
TLS 1.3 key schedule. The successful negotiation of the
"tls_cert_with_extern_psk" extension requires the key schedule
processing to include both the external PSK and the (EC)DHE
shared secret value.
If the client and the server have different values associated
with the selected external PSK identifier, then the client and
the server will compute different values for every entry in the
key schedule, which will lead to the client aborting the
handshake with a "decrypt_error" alert.
IANA Considerations
IANA has updated the "TLS ExtensionType Values" registry
to include "tls_cert_with_extern_psk" with a value of 33 and the list of
messages "CH, SH" in which the "tls_cert_with_extern_psk" extension may
appear.
Security Considerations
The Security Considerations in
remain relevant.
TLS 1.3 does not permit
the server to send a CertificateRequest message when a PSK
is being used. This restriction is removed when the
"tls_cert_with_extern_psk" extension is offered by the client
and accepted by the server. However, TLS 1.3 does not
permit an external PSK to be used in the same fashion as a
resumption PSK, and this extension does not alter those
restrictions. Thus, a certificate MUST NOT be used with
a resumption PSK.
Implementations must protect the external pre-shared key (PSK).
Compromise of the external PSK will make the encrypted session
content vulnerable to the future development of a large-scale
quantum computer. However, the generation, distribution, and
management of the external PSKs is out of scope for this
specification.
Implementers should not transmit the same content on a connection
that is protected with an external PSK and a connection that is
not. Doing so may allow an eavesdropper to correlate the
connections, making the content vulnerable to the future
invention of a large-scale quantum computer.
Implementations must generate external PSKs with a secure key-management
technique, such as pseudorandom generation of the key or derivation of
the key from one or more other secure keys. The use of inadequate
pseudorandom number generators (PRNGs) to generate external PSKs can
result in little or no security. An attacker may find it much easier
to reproduce the PRNG environment that produced the external PSKs and
search the resulting small set of possibilities, rather than brute-force
searching the whole key space. The generation of quality random
numbers is difficult. offers important
guidance in this area.
If the external PSK is known to any party other than the client and
the server, then the external PSK MUST NOT be the sole basis for
authentication. The reasoning is explained in Section 4.2 of
. When this extension is used, authentication
is based on certificates, not the external PSK.
In this extension, the external PSK preserves confidentiality if the
(EC)DH key agreement is ever broken by cryptanalysis or the future
invention of a large-scale quantum computer. As long as the attacker
does not know the PSK and the key derivation algorithm remains
unbroken, the attacker cannot derive the session secrets, even if they
are able to compute the (EC)DH shared secret. Should the attacker be
able compute the (EC)DH shared secret, the forward-secrecy advantages
traditionally associated with ephemeral (EC)DH keys will no longer be
relevant. Although the ephemeral private keys used during a given TLS
session are destroyed at the end of a session, preventing the attacker
from later accessing them, these private keys would nevertheless be
recoverable due to the break in the algorithm. However, a more
general notion of "secrecy after key material is destroyed" would still
be achievable using external PSKs, if they are managed in a way that
ensures their destruction when they are no longer needed, and with
the assumption that the algorithms that use the external PSKs remain
quantum-safe.
TLS 1.3 key derivation makes use of the HMAC-based Key Derivation
Function (HKDF) algorithm, which depends
upon the HMAC construction and a hash
function. This extension provides the desired protection for the
session secrets, as long as HMAC with the selected hash function is
a pseudorandom function (PRF) .
This specification does not require that the external PSK is known only by
the client and server. The external PSK may be known to a group. Since
authentication depends on the public key in a certificate, knowledge of
the external PSK by other parties does not enable impersonation. Since
confidentiality depends on the shared secret from (EC)DH, knowledge of
the external PSK by other parties does not enable eavesdropping. However,
group members can record the traffic of other members and then decrypt it
if they ever gain access to a large-scale quantum computer. Also, when
many parties know the external PSK, there are many opportunities for theft
of the external PSK by an attacker. Once an attacker has the external PSK,
they can decrypt stored traffic if they ever gain access to a large-scale
quantum computer, in the same manner as a legitimate group member.
TLS 1.3 takes a conservative approach to PSKs;
they are bound to a specific hash function and KDF. By contrast,
TLS 1.2 allows PSKs to be used with any hash
function and the TLS 1.2 PRF. Thus, the safest approach is to use a PSK
exclusively with TLS 1.2 or exclusively with TLS 1.3. Given one PSK,
one can derive a PSK for exclusive use with TLS 1.2 and derive another
PSK for exclusive use with TLS 1.3 using the mechanism specified in
.
TLS 1.3 has received careful security analysis,
and the following informal reasoning shows that the addition of this
extension does not introduce any security defects. This extension
requires the use of certificates for authentication, but the processing
of certificates is unchanged by this extension. This extension places
an external PSK in the key schedule as part of the computation of the
Early Secret. In the initial handshake without this extension, the
Early Secret is computed as:
Early Secret = HKDF-Extract(0, 0)
With this extension, the Early Secret is computed as:
Early Secret = HKDF-Extract(External PSK, 0)
Any entropy contributed by the external PSK can only make the Early
Secret better; the External PSK cannot make it worse. For these two
reasons, TLS 1.3 continues to meet its security goals when this extension
is used.
Privacy Considerations
discusses identity-exposure
attacks on PSKs. The guidance in this section remains relevant.
This extension makes use of external PSKs to improve resilience against
attackers that gain access to a large-scale quantum computer in the
future. This extension is always accompanied by the "pre_shared_key"
extension to provide the PSK identities in plaintext in the ClientHello
message. Passive observation of the these PSK identities will aid an
attacker in tracking users of this extension.
References
Normative References
Informative References
New Directions in Cryptography
IEEE Transactions on Information Theory
Vol. 22, No. 6
How to construct random functions
Journal of the ACM
Vol. 33, No. 4
pp. 792-807
Digital Signature Standard (DSS)
NIST
TLS ExtensionType Values
IANA
IEEE Standard Specifications for Public-Key Cryptography
IEEE
A Unilateral-to-Mutual Authentication Compiler for Key
Exchange (with Applications to Client Authentication in TLS
1.3)
CCS '16: Proceedings of the 2016 ACM Communications Security
pp. 1438-50
Acknowledgments
Many thanks to
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for their review and comments; their efforts have improved this document.