| Internet-Draft | TLS 1.3 PAKE | January 2025 |
| Bauman, et al. | Expires 4 August 2025 | [Page] |
The pre-shared key mechanism available in TLS 1.3 is not suitable for usage with low-entropy keys, such as passwords entered by users. This document describes an extension that enables the use of password-authenticated key exchange protocols with TLS 1.3.¶
This note is to be removed before publishing as an RFC.¶
Discussion of this document takes place on the Transport Layer Security Working Group mailing list (tls@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/tls/.¶
Source for this draft and an issue tracker can be found at https://github.com/chris-wood/draft-bmw-tls-pake13.¶
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This Internet-Draft will expire on 4 August 2025.¶
Copyright (c) 2025 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. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
DISCLAIMER: Much of this text is copied from [FIRST-DRAFT] and is in the process of being updated.¶
DISCLAIMER: This is a work-in-progress draft and has not yet seen significant security analysis. See Section 8 and Section 8.1 for more information.¶
In some applications, it is desirable to enable a client and server to authenticate to one another using a low-entropy pre-shared value, such as a user-entered password.¶
In prior versions of TLS, this functionality has been provided by the integration of the Secure Remote Password PAKE protocol (SRP) [RFC5054]. The specific SRP integration described in RFC 5054 does not immediately extend to TLS 1.3 because it relies on the Client Key Exchange and Server Key Exchange messages, which no longer exist in 1.3.¶
TLS 1.3 itself provides a mechanism for authentication with pre-shared keys (PSKs). However, PSKs used with this protocol need to be "full-entropy", because the binder values used for authentication can be used to mount a dictionary attack on the PSK. So while the TLS 1.3 PSK mechanism is suitable for the session resumption cases for which it is specified, it cannot be used when the client and server share only a low-entropy secret.¶
Enabling TLS to address this use case effectively requires the TLS
handshake to execute a password-authenticated key establishment
(PAKE) protocol. This document describes a TLS extension pake
that can carry data necessary to execute a PAKE.¶
This extension is generic, in that it can be used to carry key exchange information for multiple different PAKEs. We assume that prior to the TLS handshake the client and server will both have knowledge of the password or PAKE-specific values derived from the password (e.g. augmented PAKEs only require one party to know the actual password). The choice of PAKE and any required parameters will be explicitly specified using IANA assigned values. As a first case, this document defines a concrete protocol for executing the SPAKE2+ PAKE protocol [RFC9383].¶
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.¶
The mechanisms described in this document also apply to DTLS 1.3 [RFC9147], but for brevity, we will refer only to TLS throughout.¶
In order to use the extension specified in this document, a TLS client and server need to have pre-provisioned a password (or derived values as described by the desired PAKE protocol(s)). The details of this pre-provisioned information are specific to each PAKE algorithm and are not specified here.¶
Servers will of course have multiple instances of this configuration information for different clients. Clients may also have multiple identities, even within a given server.¶
This section describes how the PAKE protocol is integrated and executed in the TLS handshake.¶
To offer support for a PAKE protocol, the client sends a pake extension
in the ClientHello carrying a PAKEClientHello value:¶
enum {
pake(0xTODO), (65535)
} ExtensionType;
¶
The payload of the client extension has the following PAKEClientHello
structure:¶
enum {
SPAKE2PLUS_V1 (0xXXXX),
} PAKEScheme;
struct {
PAKEScheme pake_scheme;
opaque pake_message<1..2^16-1>;
} PAKEShare;
struct {
opaque client_identity<0..2^16-1>;
opaque server_identity<0..2^16-1>;
PAKEShare client_shares<0..2^16-1>;
} PAKEClientHello;
¶
The PAKEClientHello structure consists of an identity pair under which the
client can authenticate alongside a list of PAKE algorithms and the
client's first message for each underlying PAKE protocol.
Concretely, these structure fields are defined as follows:¶
A list of PAKEShare values, each one with a distinct PAKEScheme algorithm.¶
The client identity used for the PAKE. It may be empty.¶
The server identity used for the PAKE. It may be empty.¶
The 2-byte identifier of the PAKE algorithm.¶
The client PAKE message used to initialize the protocol.¶
The client and server identity fields are common to all PAKEShares to prevent client enumeration attacks; see Section 8.¶
The PAKEScheme field in the PAKEShare allows implementations to
support multiple PAKEs and negotiate which to use in the context of
the handshake. For instance, if a client knows a password but not which
PAKE the server supports it could send corresponding PAKEShares for each
PAKE.¶
Section 9.2 of [TLS13] specifies that a valid ClientHello
must include either a pre_shared_key extension or both
a signature_algorithms and supported_groups extension. With the
addition of the pake extension specified here, the new requirement
is that a valid ClientHello must satisfy at least one of the
following options:¶
includes a pre_shared_key extension¶
includes both a signature_algorithms and supported_groups extensions¶
includes a pake extension¶
If a client sends the pake extension, then it MAY also send the
key_share and pre_shared_key extensions, to allow the server to
choose an authentication mode. Unlike PSK-based authentication,
however, authentication with PAKE cannot be combined with the
normal TLS key exchange mechanism. Forward secrecy is provided by
the PAKE itself.¶
The server identity value provided in the PAKEClientHello structure are disjoint from that which the client may provide in the ServerNameIndication (SNI) field.¶
A server that receives a pake extension examines its contents to determine
if it is well-formed. In particular, if there are duplicate PAKEShare values
in the PAKEClientHello structure for the same PAKEScheme, the server aborts the
handshake with an "illegal_parameter" alert.¶
If the list of PAKEShare values is well-formed, the server then scans the list of PAKEShare values to determine if there is one corresponding to a server supported PAKEScheme. If the server does not support any of the offered PAKESchemes in the client PAKEShares then the server MUST abort the protocol with an "illegal_parameter" alert.¶
If the server has a PAKEScheme in common with the client then the server uses the client_identity and server_identity alongside its local database of PAKE registration information to determine if the request corresponds to a legitimate client registration record. If one does not exist, the server simulates a PAKE response as described in Section 4.4. Simulating a response prevents client enumeration attacks on the server's PAKE database; see Section 8.¶
If there exists a valid PAKE registration, the server indicates its selection
by including a pake extension in its ServerHello. The content of this extension
is a PAKEServerHello value, specifying the PAKE the server has selected, and the
server's first message in the PAKE protocol. The format of this structure is as follows:¶
struct {
PAKEShare server_share;
} PAKEServerHello;
¶
The server_share value of this structure is a PAKEShare, which echoes
back the PAKE algorithm chosen and the server's PAKE message generated
in response to the client's PAKE message.¶
If a server uses PAKE authentication, then it MUST NOT send an
extension of type key_share, pre_shared_key, or early_data.¶
Use of PAKE authentication is not compatible with standard certificate-based authentication of both clients and servers. If use of a PAKE is negotiated, then servers MUST NOT include a Certificate or CertificateRequest message in the handshake.¶
When the client and server agree on a PAKE to use, a shared secret derived
from the PAKE protocol is used as the ECDH(E) input to the TLS 1.3
key schedule. Details for the shared secret computation are left to the
specific PAKE algorithm. See Section 6 for information about how
the SPAKE2+ variant operates.¶
As with client authentication via certificates, the server has not authenticated the client until after it has received the client's Finished message. When a server negotiates the use of this mechanism for authentication, it SHOULD NOT send application data before it has received the client's Finished message, as it would otherwise be sending data to an unauthenticated client.¶
To simulate a fake PAKE response, the server does the following:¶
Select a random PAKEScheme supported by the client and server.¶
Include the pake extension in its ServerHello, containing a PAKEShare value with
the randomly selected PAKEScheme and corresponding pake_message. To generate the pake_message
for this PAKEShare value, the server should select a value uniformly at random from
the set of possible values of the PAKE algorithm shares. For example, for SPAKE2+,
this would be a random point on the elliptic curve group.¶
Perform the rest of the protocol as normal.¶
Because the server's share was selected uniformly at random, the server will reject the client's Finished message with overwhelming probability.¶
A server that performs the simulation of the protocol acts only as an all-or-nothing oracle for whether a given (identity, password) pair is correct. If an attacker does not supply a correct pair, they do not learn anything beyond this fact.¶
In order to be usable with the pake extension, a PAKE protocol
must specify some syntax for its messages, and the protocol itself
must be compatible with the message flow described above. A
specification describing the use of a particular PAKE protocol with
TLS must provide the following details:¶
A PAKEScheme registered value indicating pre-provisioned parameters;¶
Content of the pake_message field in a ClientHello;¶
Content of the pake_message field in a ServerHello;¶
How the PAKE protocol is executed based on those messages; and¶
How the outputs of the PAKE protocol are used to populate the (EC)DHE input to the TLS key schedule.¶
In addition, to be compatible with the security requirements of TLS 1.3, PAKE protocols defined for use with TLS 1.3 MUST provide forward secrecy.¶
Several current PAKE protocols satisfy these requirements, for example:¶
This section describes the SPAKE2+ instantiation of the pake extension for TLS.
The SPAKE2+ protocol is described in [SPAKE2PLUS].
Section 6.1 describes the setup required before the protocol runs,
and Section 6.2 describes the protocol execution in TLS.¶
The TLS client and server roles map to the Prover and Verifier roles in the
SPAKE2+ specification, respectively. Clients are configured with a client
identity, server identity, and password verifier (w0 and w1 according to [SPAKE2PLUS]).
Similarly, servers are configured with a list of client identity, server identity,
and password registration values (w0 and L according to [SPAKE2PLUS]). Servers
use this list when completing the SPAKE2+ protocol. The values for the password
verifiers and registration records (w0, w1, and L) are not specified here; see
Section 3.2 of [SPAKE2PLUS] for more information.¶
The PAKEScheme value for SPAKE2+ fully defines the parameters associated with
the protocol, including the prime-order group G, cryptographic hash function Hash,
key derivation function KDF, and message authentication code MAC. Additionally,
the PAKEScheme value for SPAKE2+ fully defines the constants for M and N
as needed for the protocol; see Section 4 of [SPAKE2PLUS].¶
The content of one PAKEShare value in the PAKEClientHello structure consists
of the PAKEScheme value SPAKE2PLUS_V1 and the value shareP as computed in
Section 3.3 of [SPAKE2PLUS].¶
The content of the server PAKEShare value in the PAKEServerHello structure
consists of the PAKEScheme value SPAKE2PLUS_V1 and the value shareV || confirmV,
i.e., shareV and confirmV concatenated, as computed in Section 3.3 of [SPAKE2PLUS].¶
Given shareP and shareV, the client and server can then both compute
K_main, the root secret in the protocol as described in Section 3.4 of [SPAKE2PLUS].
The "Context" value for SPAKE2+ is "TLS-SPAKE2PLUS_V1". The rest of the values
needed for the transcript derivation are as configured in Section 6.1,
exchanged over the wire, or computed by client and server.¶
Using K_main, the client and server both compute K_shared which is used as
input to the TLS 1.3 key schedule. Specifically, K_shared is used
as the (EC)DHE input to the key schedule in Section 7.1 of [TLS13], as shown below.¶
0
|
v
PSK -> HKDF-Extract = Early Secret
|
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
|
v
Derive-Secret(., "derived", "")
|
v
K_shared -> HKDF-Extract = Handshake Secret
^^^^^^^^ |
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
|
v
Derive-Secret(., "derived", "")
|
v
0 -> HKDF-Extract = Master Secret
|
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
+-----> Derive-Secret(...)
¶
Note that the client and server do not additionally compute or verify the key confirmation messages as described in Section 3.4 of [SPAKE2PLUS]. See Section 8.1 for more information about the safety of this approach.¶
Client and server identities are sent in the clear in the PAKEClientHello extension. While normally the TLS server identity is already in the clear -- carried in the SNI extension -- TLS client identities are encrypted under the TLS handshake secrets. Thus, the PAKEClientHello extension reveals more information to a passive network attacker than normal, mutually-authenticated TLS handshakes.¶
The implications of leaking the client identity to a passive network attacker vary. For instance, a successful TLS handshake after negotiating use of a PAKE indicates that the chosen client identity is valid. This is relevant in settings where client enumeration may be a concern.¶
Applications for which this leak is a problem can use the TLS Encrypted ClientHello (ECH) extension to encrypt the PAKEClientHello extension in transit to the server [ECH].¶
Many of the security properties of this protocol will derive from the PAKE protocol being used. Security considerations for PAKE protocols are noted in Section 5.¶
If a server doesn't recognize the identity supplied by the
client in the ClientHello pake extension, the server MAY abort the handshake with an
"illegal_parameter" alert. In this case, the server acts as an oracle
for identities, in which each handshake allows an attacker
to learn whether the server recognizes any of the identities in a set.¶
Alternatively, if the server wishes to hide the fact that these client identities are unrecognized, the server MAY simulate the protocol as if an identity was recognized, but then reject the client's Finished message with a "decrypt_error" alert, as if the password was incorrect. This is similar to the procedure outlined in [RFC5054]. The simulation mechanism is described in Section 4.4.¶
Section 6 describes how to integrate SPAKE2+ into TLS using the pake
extension in this document. This integration deviates from the SPAKE2+
protocol in [SPAKE2PLUS] in one important way: the explicit key confirmation
checks required in [SPAKE2PLUS] are replaced with the TLS Finished messages.
This is because the TLS Finished messages compute a MAC over the TLS transcript,
which includes both the shareP and shareV values exchanged for SPAKE2+.¶
[[OPEN ISSUE: this requires formal analysis to confirm.]]¶
This document requests that IANA add a value to the TLS ExtensionType Registry with the following contents:¶
| Value | Extension Name | TLS 1.3 | Reference |
|---|---|---|---|
| 0xTODO | pake | CH, SH | (this document) |
[[ RFC EDITOR: Please replace "TODO" in the above table with the value assigned by IANA, and replace "(this document)" with the RFC number assigned to this document. ]]¶
This document requests that IANA create a new registry called "PAKE Schemes" with the following contents:¶
| Value | PAKEScheme | Reference | Notes |
|---|---|---|---|
| 0xTODO | SPAKE2PLUS_V1 | (this document) | N/A |
The SPAKE2PLUS_V1 PAKEScheme variant has the following parameters associated with it:¶
Additionally, it uses the M and N values from Section 4 of [SPAKE2PLUS], included below, as compressed points on the P-256 curve, for completeness.¶
M = 02886e2f97ace46e55ba9dd7242579f2993b64e16ef3dcab95afd497333d8fa12f N = 03d8bbd6c639c62937b04d997f38c3770719c629d7014d49a24b4f98baa1292b49¶
The authors would like to thank the original authors of [FIRST-DRAFT] for providing a firm basis for the extension mechanism specified in this document.¶