Internet DRAFT - draft-barnes-tls-pake
draft-barnes-tls-pake
Network Working Group R. Barnes
Internet-Draft O. Friel
Intended status: Informational Cisco
Expires: January 17, 2019 July 16, 2018
Usage of PAKE with TLS 1.3
draft-barnes-tls-pake-04
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 an extension that enables the use of
password-authenticated key exchange protocols with TLS 1.3.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on January 17, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. TLS Extensions . . . . . . . . . . . . . . . . . . . . . . . 3
5. Compatible PAKE Protocols . . . . . . . . . . . . . . . . . . 5
6. SPAKE2+ Implementation . . . . . . . . . . . . . . . . . . . 6
7. Pre-provisioned Parameters . . . . . . . . . . . . . . . . . 6
8. Content of the TLS Extensions . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
9.1. Security when using SPAKE2+ . . . . . . . . . . . . . . . 8
10. Open Items . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. PAKE Algorithm Negotiation . . . . . . . . . . . . . . . 9
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
12.1. Normative References . . . . . . . . . . . . . . . . . . 10
12.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
DISCLAIMER: This is a work-in-progress draft 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 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
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(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
the client and server have pre-negotiated a choice of PAKE (and any
required parameters) in addition to the password itself. As a first
case, this document defines a concrete protocol for executing the
SPAKE2+ PAKE protocol [I-D.irtf-cfrg-spake2].
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].
The mechanisms described in this document also apply to DTLS 1.3
[I-D.ietf-tls-dtls13], but for brevity, we will refer only to TLS
throughout.
3. Setup
In order to use this protocol, a TLS client and server need to have
pre-provisioned the values required to execute the protocol:
o A choice of PAKE protocol
o Any parameters required by the PAKE protocol
o A password (or a derived value as described by the PAKE protocol)
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. We assume that in either
case, a single opaque "identity" value is sufficient to identify the
required parameters.
4. TLS Extensions
A client offers to authenticate with PAKE by including a "pake"
extension in its ClientHello. The content of this exension is a
"PAKEClientHello" value, providing a list of identities under which
the client can authenticate, and for each identity, the client's
first message from the underlying PAKE protocol.
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,
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however, authentication with PAKE cannot be combined with the normal
TLS ECDH mechanism. Forward secrecy is provided by the PAKE itself.
struct {
opaque identity<0..2^16-1>;
opaque pake_message<1..2^16-1>;
} PAKEShare;
struct {
PAKEShare client_shares<0..2^16-1>;
} PAKEClientHello;
A server that receives a "pake" 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 choice of PAKE authentication by
including a "pake" extension in its ServerHello. The content of this
exension is a "PAKEServerHello" value, specifying the identity value
for the password the server has selected, and the server's first
message in the PAKE protocol.
Use of PAKE authenication is compatible with standard certificate-
based authentication of both clients and servers. If a server
includes an "pake" 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 PAKE authentication, then it MUST NOT send an
extension of type "key_share", "pre_shared_key", or "early_data".
struct {
PAKEShare server_share;
} PAKEServerHello;
Based on the messages exchanged in the ClientHello and ServerHello,
the client and server execute the specified PAKE protocol to derive a
shared key. This key is used as the "ECHD(E)" input to the TLS 1.3
key schedule.
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.
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5. Compatible PAKE Protocols
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:
o Parameters that must be pre-provisioned
o Content of the "pake_message" field in a ClientHello
o Content of the "pake_message" field in a ServerHello
o How the PAKE protocol is executed based on those messages
o How the outputs are of the PAKE protocol are used to populate the
"PSK" and "ECDH(E)" inputs to the TLS key schedule.
The underlying cryptographic protocol must be compatible with the
message flow described above:
o It must be possible to execute in one round-trip, with the client
speaking first
o The Finished MAC must provide sufficient key confirmation for the
protocol, taking into account the contents of the handshake
messages
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:
o SPAKE2+ (described below) [I-D.irtf-cfrg-spake2]
o SPEKE and derivatives such as Dragonfly [speke]
[I-D.harkins-tls-dragonfly]
o OPAQUE [opaque]
o SRP [RFC2945]
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6. SPAKE2+ Implementation
7. Pre-provisioned Parameters
In order to use SPAKE2+, 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 of order "p*h", with "p" a large prime, and generator
"G"
o Fixed elements "M" and "N" for the group
o A hash function "H"
o A password "pw"
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
SPAKE2+ 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 two shared integers
"w0" and "w1" by running the following algorithm twice (changing the
"context" value each time):
struct {
uint16 context;
opaque client\_identity<0..255>;
opaque server\_name<0..255>;
opaque password<0..255>;
} PasswordInput;
o Encode the following values into a "PasswordInput" structure:
* "client_identity": The client's identity, as described above.
* "server_name": The server's identity, as described above.
* "password": The password "pw"
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* "context": One of the following values:
+ 0x7730, when generating "w0"
+ 0x7731, when generating "w1"
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 "w" in network byte
order. Return the result "(w % p) * h" of reducing "w" mod p and
multiplying it by "h".
Servers MUST store only the value "w0" and the product "L = w1*G",
where "G" is the fixed generator of the group. Clients will need to
have access to the values "w0" and "w1" directly, but SHOULD generate
these values dynamically, rather than caching them.
8. Content of the TLS Extensions
The content of a "pake_message" in a ClientHello is the client's 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.key_exchange" value from the same group.
The content of a "pake_message" in a ServerHello is 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.key_exchange" value from the same group.
Based on these messages, both the client and server can compute the
two shared values as specified in [I-D.irtf-cfrg-spake2].
+------+--------+---------------+--------------+
| Name | Value | Client | Server |
+------+--------+---------------+--------------+
| Z | x*y*G | x*(S - w0*N) | x*(T - w0*M) |
| | | | |
| V | w1*y*G | w1*(S - w0*N) | y*L |
+------+--------+---------------+--------------+
The following value is used as the "(EC)DHE" input to the TLS 1.3 key
schedule:
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K = H(Z || V)
Here "H" is the hash function corresponding to the TLS cipher suite
in use and "||" represents concatenation of octet strings.
9. Security Considerations
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.
The mechanism defined in this document does not provide protection
for the client's identity, in contrast to TLS client authentication
with certificates.
TLS servers that offer this mechanism can be used by third party
attackers as an oracle for two questions:
1. Whether the server knows about a given identity
2. Whether the server recognizes a given (identity, password) pair
The former is signaled by whether the server returns a "pake"
extension.
[[TODO: Similar to https://tools.ietf.org/html/rfc5054#section-
2.5.1.3, the server could run through a complete handshake
calculation and fail at the end so that the attacker only knows that
the identity/password pair is incorrect, but does not know if the
identity is recognized or not. This requires that the server can
interpret the pake_message and ascertain the associated PAKE
algorithm, group parameters, etc., which requires a reworking of some
text in this draft as the identity is currently defined as providing
a map to said group parameters. This is related to the discussion in
the Open Items section.]]
The latter is signaled by whether the connection succeeds. These
oracles are all-or-nothing: If the attacker does not have the correct
identity or password, he does not learn anything about the correct
value.
9.1. Security when using SPAKE2+
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
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the example confirmation hash listed in that document are also
covered by the Finished MAC:
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. In particular, "x" and "y" MUST NOT be equal to zero.
This ensures that TLS sessions using SPAKE2 have the same forward
secrecy properties as sessions using the normal TLS (EC)DH mechanism.
10. Open Items
10.1. PAKE Algorithm Negotiation
It is possible that a client may know the password to use, but may
not know in advance which PAKE protocols(s) a particular server
supports. A potential solution to this is similar to TLS1.3
ClientHello "key_share" operation: the client may send an empty
"client_shares" vector in its PAKEClientHello extension. The server
can then send an HelloRetryRequest indicating which PAKE protocol,
and associated group parameters, the client should use. The client
then sends another ClientHello that includes "pake_message" in the
PAKEClientHello extension calculated using the correct algorithm.
This requires definition of a suitable field for transporting PAKE
algorithm and group parameters.
As an optimaisation, similar to TLS1.3 key_share operation, the
client could guess the PAKE protocol and include a "pake_message"
derived from its guess in the initial ClientHello. If the server
does not support the selected PAKE protcol (or protocol group
parameter, etc.), the server can send an HelloRetryRequest indicating
the supported PAKE protocol and group parameters. Note: it is TBD if
sending two different "pake_messages" derived from two different
protocol and/or group parameters in two different ClientHello
messages constitutes a significant attack vector. This needs
cryptographic review.
11. IANA Considerations
This document requests that IANA add a value to the TLS ExtensionType
Registry with the following contents:
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+-------+----------------+---------+-----------+
| Value | Extension Name | TLS 1.3 | Reference |
+-------+----------------+---------+-----------+
| TBD | pake | 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. ]]
12. References
12.1. 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-dtls13-28 (work in progress), July
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>.
12.2. Informative References
[I-D.harkins-tls-dragonfly]
Harkins, D., "Secure Password Ciphersuites for Transport
Layer Security (TLS)", draft-harkins-tls-dragonfly-03
(work in progress), July 2018.
[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.
[opaque] Xu, J., "OPAQUE: An Asymmetric PAKE Protocol Secure
Against Pre-Computation Attacks", 2018.
[RFC2945] Wu, T., "The SRP Authentication and Key Exchange System",
RFC 2945, DOI 10.17487/RFC2945, September 2000,
<https://www.rfc-editor.org/info/rfc2945>.
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[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>.
[speke] Jablon, D., "Extended Password Key Exchange Protocols
Immune to Dictionary Attacks", 1997.
Authors' Addresses
Richard Barnes
Cisco
Email: rlb@ipv.sx
Owen Friel
Cisco
Email: ofriel@cisco.com
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