Network Working Group R. Barnes
Internet-Draft O. Friel
Intended status: Informational Cisco
Expires: October 15, 2018 April 13, 2018
Usage of SPAKE with TLS 1.3
draft-barnes-tls-pake-01
Abstract
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 how the SPAKE password-authenticated key
exchange can be used with TLS 1.3.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 15, 2018.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. TLS Extensions . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
DISCLAIMER: This is a work-in-progress draft of MLS and has not yet
seen significant security analysis. It should not be used as a basis
for building production systems.
In some applications, it is desireable 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 becauseit relies on the Client Key
Exchange and Server Key Exchange messages, which no longer exist in
1.3. At a more fundamental level, the messaging patterns required by
SRP do not map cleanly to the standard TLS 1.3 handshake, which has
fewer round-trips than prior versions.
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 perform a password-authenticated key establishment
(PAKE) protocol. This document describes an embedding of the SPAKE2
PAKE protocol in TLS 1.3 [I-D.irtf-cfrg-spake2] [I-D.ietf-tls-tls13].
This mechanism also applies to DTLS 1.3 [I-D.ietf-tls-dtls13], but
for brevity, we will refer only to TLS throughout.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Setup
In order to use this protocol, a TLS client and server need to have
pre-provisioned the values required to execute the SPAKE2 protocol
(see Section 3.1 of [I-D.irtf-cfrg-spake2]):
o A DH group "G" of order "p*h", with "p" a large prime
o Fixed elements "M" and "N" for the group
o A hash function "H"
o A password "p"
Note that the hash function "H" might be different from the hash
function associated with the ciphersuite negotiated by the two
parties. The hash function "H" MUST be a hash function suitable for
hashing passwords, e.g., Argon2 or scrypt [I-D.irtf-cfrg-argon2]
[RFC7914].
The TLS client and server roles map to the "A" and "B" roles in the
SPAKE specification, respectively. The identity of the server is the
domain name sent in the "server_name" extension of the ClientHello
message. The identity of the client is an opaque octet string,
specified in the "spake2" ClientHello extension, defined below.
From the shared password, each party computes a shared integer "w" in
the following way:
struct {
opaque client_identity<0..255>;
opaque server_name<0..255>;
opaque password<0..255>;
} PasswordInput;
struct {
opaque salt<0..255>;
opaque idpass[H.length];
} PasswordWithContext;
o Encode the following values into a "PasswordInput" structure:
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* "client_identity": The client's identity, in the same form that
is presented in the "identity" field of the "SPAKE2ClientHello"
structure.
* "server_name": The server's identity, in the same form
presented in the "server_name" extension sent by the client.
* "password": The password itself
o Use the hash function "H" with the encoded "PasswordInput"
structure as input to derive an "n"-byte string, where "n" is the
byte-length of "p".
o Interpret the "n"-bit string as an integer in network byte order
and let "w" be the result of reducing this integer mod "p".
Servers SHOULD store only the product "w*N" of "w" with the fixed
element "N" of the group. Clients MAY compute "w" dynamically, based
on the password and client and server identities for a given session.
4. TLS Extensions
A client offers to authenticate with SPAKE2 by including an "spake2"
extension in its ClientHello. The content of this exension is an
"SPAKE2ClientHello" value, specifying the client's identity, where
the identity matches that used in 'IdentifierAndPassword', and a key
share "T". The value "T" is computed as specified in
[I-D.irtf-cfrg-spake2], as "T = w*M + X", where "M" is a fixed value
for the DH group and "X" is the public key of a fresh DH key pair.
The format of the key share "T" is the same as for a "KeyShareEntry"
value from the same group.
If a client sends the "spake2" 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 SPAKE2 cannot be combined with the
normal TLS ECDH mechanism.
struct {
opaque identity<0..2^16-1>;
opaque key_exchange<1..2^16-1>;
} SPAKE2Share;
struct {
SPAKE2Share client_shares<0..2^16-1>;
} SPAKE2ClientHello;
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A server that receives an "spake2" extension examines the list of
client shares to see if there is one with an identity the server
recognizes. If so, the server may indicate its use of SPAKE2
authentication by including an "spake2" extension in its ServerHello.
The content of this exension is an "SPAKE2ServerHello" value,
specifying the identity value for the password the server has
selected, and the server's key share "S". The value "S" is computed
as specified in [I-D.irtf-cfrg-spake2], as "S = w*N + Y", where "N"
is a fixed value for the DH group and "Y" is the public key of a
fresh DH key pair. The format of the key share "S" is the same as
for a "KeyShareEntry" value from the same group.
Use of SPAKE2 authenication is not inconsistent with standard
certificate-based authentication of both clients and servers.
authentication are not mutually exclusive. If a server includes an
"spake2" extension in its ServerHello, it may still send the
Certificate and CertificateVerify messages, and/or send a
CertificateRequest message to the client.
If a server uses SPAKE2 authentication, then it MUST NOT send an
extension of type "key_share", "pre_shared_key", or "early_data".
struct {
SPAKE2Share server_share;
} SPAKE2ServerHello;
Based on these messages, both the client and server can compute the
shared key "K = x*(S-w*N) = y*(T-w*M)", as specified in
[I-D.irtf-cfrg-spake2]. The value "K" is then used as the "(EC)DHE"
input to the TLS key schedule. The integer "w" is used as the PSK
input, encoded as an integer in network byte order, using the
smallest number of octets possible.
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 MUST NOT send application data before it has
received the client's Finished message.
5. Security Considerations
For the most part, the security properties of the password-based
authentication described in this document are the same as those
described in the Security Considerations of [I-D.irtf-cfrg-spake2].
The TLS Finished MAC provides the key confirmation required for the
security of the protocol. Note that all of the elements covered by
the example confirmation hash listed in that document are also
covered by the Finished MAC:
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o "A", "B", and "T" are included via the ClientHello
o "S" via the ServerHello
o "K", and "w" via the TLS key schedule
The "x" and "y" values used in the SPAKE2 protocol MUST have the same
ephemerality properties as the key shares sent in the "key_shares"
extension. This ensures that TLS sessions using SPAKE2 have the same
forward secrecy properties as sessions using the normal TLS (EC)DH
mechanism.
The mechanism described above does not provide protection for the
client's identity, in contrast to TLS client authentication with
certificates.
[[ XXX(rlb@ipv.sx): Or maybe there's some HRR dance we could do. For
example: Server provides a key share in HRR, client does ECIES on
identity. ]]
6. IANA Considerations
This document requests that IANA add a value to the TLS ExtensionType
Registry with the following contents:
+-------+----------------+---------+-----------+
| Value | Extension Name | TLS 1.3 | Reference |
+-------+----------------+---------+-----------+
| TBD | spake2 | CH, SH | RFC XXXX |
+-------+----------------+---------+-----------+
[[ RFC EDITOR: Please replace "TBD" in the above table with the value
assigned by IANA, and replace "XXXX" with the RFC number assigned to
this document. ]]
7. References
7.1. Normative References
[I-D.ietf-tls-dtls13]
Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", draft-ietf-tls-dtls13-26 (work in progress), March
2018.
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[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
March 2018.
[I-D.irtf-cfrg-spake2]
Ladd, W. and B. Kaduk, "SPAKE2, a PAKE", draft-irtf-cfrg-
spake2-05 (work in progress), February 2018.
[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>.
7.2. Informative References
[I-D.irtf-cfrg-argon2]
Biryukov, A., Dinu, D., Khovratovich, D., and S.
Josefsson, "The memory-hard Argon2 password hash and
proof-of-work function", draft-irtf-cfrg-argon2-03 (work
in progress), August 2017.
[RFC5054] Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
"Using the Secure Remote Password (SRP) Protocol for TLS
Authentication", RFC 5054, DOI 10.17487/RFC5054, November
2007, <https://www.rfc-editor.org/info/rfc5054>.
[RFC7914] Percival, C. and S. Josefsson, "The scrypt Password-Based
Key Derivation Function", RFC 7914, DOI 10.17487/RFC7914,
August 2016, <https://www.rfc-editor.org/info/rfc7914>.
Authors' Addresses
Richard Barnes
Cisco
Email: rlb@ipv.sx
Owen Friel
Cisco
Email: ofriel@cisco.com
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