Internet DRAFT - draft-reitzenstein-kitten-opaque
draft-reitzenstein-kitten-opaque
WG Working Group N. von Reitzenstein Čerpnjak
Internet-Draft 16 January 2023
Intended status: Informational
Expires: 20 July 2023
A SASL and GSS-API Mechanism using the asymmetric password-authenticated
key agreement OPAQUE
draft-reitzenstein-kitten-opaque-02
Abstract
This specification describes a Simple Authentication and Security
Layer (SASL, RFC4422) authentication mechanisms based on the OPAQUE
asymmetric password-authenticated key agreement (PAKE) protocol.
The mechanism offers two distinct advantages over the SCRAM family of
mechanisms. The underlying OPAQUE protocol provides the ability for
clients to register without the server having to have access to the
clear text password of an user, preventing password exfiltration at
registration. Secondly a successful authentication produces a long-
term secret key specific to the user that can be used to access
encrypted server-side data without needing to share keys between
clients via side-band mechanisms.
When used in combination with TLS or an equivalent security layer
these mechanisms allow for secure channel binding.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the Common Authentication
Technology Next Generation Working Group mailing list
(kitten@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/kitten/.
Source for this draft and an issue tracker can be found at
https://github.com/dequbed/draft-reitzenstein-auth-opaque.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 20 July 2023.
Copyright Notice
Copyright (c) 2023 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
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Table of Contents
1. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. OPAQUE Algorithm Overview . . . . . . . . . . . . . . . . . . 4
4. OPAQUE Mechanism Name . . . . . . . . . . . . . . . . . . . . 5
5. OPAQUE-3DH configuration for OPAQUE-A255SHA(-PLUS) . . . . . 5
6. OPAQUE Authentication Exchange . . . . . . . . . . . . . . . 5
6.1. OPAQUE Attributes . . . . . . . . . . . . . . . . . . . . 6
6.1.1. KSF parameter encoding . . . . . . . . . . . . . . . 7
6.2. SASL Mechanism Requirements . . . . . . . . . . . . . . . 8
7. Channel Binding . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Default Channel Binding . . . . . . . . . . . . . . . . . 9
8. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 12
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
12.1. Normative References . . . . . . . . . . . . . . . . . . 12
12.2. Informative References . . . . . . . . . . . . . . . . . 14
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Conventions and Definitions
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.
2. Introduction
This specification describes an authentication mechanism called
OPAQUE, based on the asymmetric PAKE of the same name. The
mechanisms provide strong mutual authentication and allow binding the
authentication to an pre-existing underlying encrypted transport.
The mechanism specified in this document is a Simple Authentication
and Security Layer (SASL) mechanism compatible to the bridge between
SASL and the Generic Security Services Application Programming
Interface (GSS-API) called "GS2" [RFC5801]. This means that the
mechanism can be used as either a SASL mechanism or a GSS-API
mechanism.
The OPAQUE algorithm provides the following features which this
mechanism makes use of:
* The authentication information stored in an authentication
database on the server is not sufficient to impersonate the
client. It is additionally salted and bound to a private key of
the server, making pre-stored dictionary attack impossible.
* Successful authentication does not grant the server enough
information to impersonate the client.
* Mutual authentication is implicit and required. A successful
authentication always strongly authenticates both sides of the
exchange.
* A successful authentication provides both parties with an
ephemeral shared secret. This secret has high entropy and can be
used to establish a trusted encrypted channel without deriving
trust from a 3rd party.
* A successful authentication additionally provides the client with
a constant secret. This secret is only known to the client and
the same for every authentication. It can be used to e.g. store
encrypted data on the server without having to manage keys
locally.
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3. OPAQUE Algorithm Overview
The Authenticated Key Exchange defined by OPAQUE consists of three
messages -- KE1, KE2 and KE3 -- send by the client (KE1, KE3) and
server (KE2) respectively. A client knows the outcome of the
authentication after receiving KE2, the server after receiving KE3.
The following is a description of a full SASL OPAQUE-A255SHA
authentication exchange. Nothing in OPAQUE-A255SHA prevents sending
the first client response with the SASL authentication request as
defined by an application protocol ("initial client response"). See
[RFC4422] for more details.
The OPAQUE client starts by being in possession of an username and
password. It uses the password to generate a KE1, and sends this
message and the username to the server.
The server retrieves the corresponding authentication information,
i.e. registration record, OPRF seed, server private key, and the key-
stretching function (KSF) parameters that were used at registration.
It uses the first three to generate a KE2 message as per [OPAQUE] and
sends that, channel binding data (if any) and the KSF parameters to
the client.
The client authenticates the server using KE2 and the KSF parameters,
also showing the integrity of the channel binding data in the
process, and generates a final KE3 message it can return to the
server.
The three messages KE1, KE2 and KE3 are generated using the following
functions specified in [OPAQUE] with the configuration specified in
Section 5:
KE1 := ClientInit(password)
KE2 := ServerInit(
server_identity, server_private_key, server_public_key,
record, credential_identifier, oprf_seed, KE1, client_identity
)
KE3 := ClientFinish(client_identity, server_identity, KE2)
The values of client_identity and server_identity are set to the byte
sequences:
client_identity := client-first-message + "," + client_public_key
server_identity := server-message-bare + "," + server_public_key
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With the values and encodings of the remaining parameters per the
OPAQUE specification, client_- and server_public_key being encoded as
raw bytes, and + indicating concatenation.
Upon receipt of KE3 the server can validate the authentication
exchange including integrity of the channel binding data it sent
previously, and extract a session key that strongly authenticates the
client to the server.
4. OPAQUE Mechanism Name
The name of the mechanism specified in this document is "OPAQUE-
A255SHA" or "OPAQUE-A255SHA-PLUS" respectively. The "-PLUS" suffix
is only used when the authenticating parties support and intent to
use channel binding. If the server supports channel binding it
SHOULD advertise both the bare and the plus version of this
mechanism. If the server does not it will only advertise the bare
version.
5. OPAQUE-3DH configuration for OPAQUE-A255SHA(-PLUS)
The OPAQUE-3DH configuration according to Section 7 of [OPAQUE] used
by the OPAQUE-A255SHA mechanism is made up of the following
cryptographic primitives:
* OPRF(ristretto255, SHA-512) as specified in Section 4.1 of
[I-D.irtf-cfrg-voprf-17]
* HKDF [RFC5869] using SHA-512 as KDF
* HMAC [RFC2104] using SHA-512 as MAC
* SHA-512 as Hash
* Argon2id [RFC9106] as KSF, with the remaining parameters being set
during an authentication exchange
* The same ristretto255 group used by the OPRF as Group
* The ASCII-String "SASL-OPAQUE-A255SHA" as Context
Implementations of this mechanism SHOULD default to Argon2id
parameters of (t=1, p=4, m=2^21).
6. OPAQUE Authentication Exchange
An example of an OPAQUE-A255SHA authentication exchange consisting of
three messages, send by the client, server and client respectively:
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C: n,,n=user,r=<ke1>
S: c=biws,i=bT0yMDk3MTUyLHQ9MSxwPTQ=,v=<ke2>
C: p=<ke3>
First, the client sends the "client-first-message" containing:
* A GS2 header consisting of a flag indicating channel binding
support and usage, and an optional SASL authorization identity.
* The authentication ID (AuthID) of the user.
* OPAQUE KE1, containing the OPRF credential request, a nonce, and
an ephemeral public key.
In response the server sends the "server-message" containing:
* An encoding of requested channel binding data
* Parameters for the KSF that needs to be used by the client
* OPAQUE KE2, containing the OPRF credential response, a nonce, and
an ephemeral public key.
* A MAC proving the integrity of the exchange so far and
cryptographically authenticating the server to the client (also
contained in KE2)
The client then recovers a client-only export key and a shared secret
specific to this session from the OPRF response using the defined KSF
with the user-provided password and parameters sent by the server.
To finalize the authentication a client sends a "client-final-
message" containing itself a MAC over the exchange (in KE3), thus
cryptographically authenticating the client to the server.
6.1. OPAQUE Attributes
This section details all attributes permissible in messages, their
use and their value format. All Attribute keys are a single US-ASCII
letter and case-sensitive. The selection of letters used for
attribute keys is based on SCRAM [RFC5802] to make it easier to adapt
extensions defined for SCRAM to this mechanism.
The order of attributes is fixed for all messages, except for
extension attributes which are limited to designated positions but
may appear in any order. Implementations MUST NOT assume a specific
ordering of extensions.
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* a: This is an optional attribute and is part of the GS2 [RFC5801]
bridge between GSS-API and SASL. Its specification and usage is
the same as defined in [RFC5802], Section 5.1.
* n: This attribute specifies the name of the user whose password is
used for authentication (aka "authentication identity" [RFC4422]).
Its encoding, preparation, and usage is the same as defined in
[RFC5802], Section 5.1.
* m: This attribute is reserved for future extensibility. In this
version of OPAQUE its presence in a client or server message MUST
cause authentication failure when the attribute is parsed by the
other end.
* r: This attribute specifies a base64-encoded serialization of the
KE1 message as specified by [OPAQUE].
* c: This REQUIRED attribute specifies the base64-encoded GS2 header
and channel binding data. Its specification is the same as
defined in [RFC5802], Section 5.1, however it is sent by the
server to the client instead of the other way around as in SCRAM.
* i: This attribute specifies base64-encoded parameters for the KSF
to be used. The format of the parameters is specific in
Section 6.1.1.
* v: This attribute specifies a base64-encoded serialization of the
KE2 message as specified by [OPAQUE].
* p: This attribute specifies a base64-encoded serialization of the
KE3 message as specified by [OPAQUE].
* Further as of now unspecified mandatory and optional extensions.
Mandatory extensions are encoded using the "m" attribute, optional
attributes may use any unassigned attribute name. Unknown
optional attributes MUST be ignored upon receipt.
6.1.1. KSF parameter encoding
The Argon2id [RFC9106] algorithm as used by OPAQUE-A255SHA requires
the three parameters t, p, and m to be additionally transferred from
server to client for an authentication exchange. The values for
these parameters are fixed at registration time, but may be different
for each user.
// Note: Argon2 may get a PKCS#5 parameter encoding, e.g.
// https://github.com/P-H-C/phc-winner-argon2/issues/348 ; should we
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// wait on that or specify our own format?
//
// -- Nadja
The limits and interpretation of the parameters set in [RFC9106]
apply. Parameters are encoded as a sequence of ASCII key-value pairs
separated by ASCII commas. The key and value in each pair are
separated by a single ASCII equals sign ('='). The keys for the
parameters are the single letter identifiers assigned by [RFC9106],
the values are encoded as decimal numbers with no digit delimiters or
separators.
Example of the encoding of the parameters number of passes = 1,
degree of parallelism = 4 and memory size = 2 GiB (i.e. 2^21 KiB)
using the above rules:
m=2097152,t=1,p=4
6.2. SASL Mechanism Requirements
This section describes the required information for SASL mechanisms
as laid out in [RFC4422], Section 5.
1) "OPAQUE-A255SHA" and "OPAQUE-A255SHA-PLUS"
2a) OPAQUE is a client-first mechanism
2b) OPAQUE does not send any additional data to indicate a successful
outcome. All authentication exchanges take 3 messages regardless of
success.
3) OPAQUE can transfer authorization identities from the client to
the server.
4) OPAQUE does not offer security layers but allows channel binding.
5) OPAQUE uses a MAC to protect the integrity of the entire
authentication exchange including the authzid.
7. Channel Binding
OPAQUE supports binding the authentication to an underlying secure
transport. Support for channel binding is optional, therefore the
usage of channel binding is negotiable.
The negotiation of channel binding is performed as defined in
[RFC5802], Section 6 with the following differences:
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* The non-PLUS and PLUS variants of the mechanism are instead named
OPAQUE-<variant> and OPAQUE-<variant>-PLUS respectively.
* As it is the server who sends the channel binding data the client
is responsible to verify this data by constructing the expected
value of the "c=" attribute and comparing it to the received one.
This comparison SHOULD be implemented to be constant-time.
7.1. Default Channel Binding
'tls-exporter' is the default channel binding type for any
application that do not specify one.
Servers MUST implement the 'tls-exporter' [RFC9266] channel binding
type if they implement any channel binding and make use of TLS-1.3
[RFC8446]. Clients SHOULD implement the 'tls-exporter' [RFC9266]
channel binding type if they implement any channel binding and make
use of TLS-1.3.
Server and clients SHOULD implement the 'tls-unique' [RFC5929]
channel binding if they implement channel binding and make use of
TLS-1.2. If a server or client implements 'tls-unique' they MUST
ensure appropriate protection from the [TripleHandshake]
vulnerability using e.g. the Extended Master Secret Extension
[RFC7627].
Servers MUST use the channel binding type indicated by the client, or
fail authentication if they do not support it.
8. Formal Syntax
The following syntax specification is written in Augmented Backus-
Naur Form (ABNF) notation as specified in [RFC5234]. The non-
terminals "UTF8-2", "UTF8-3" and "UTF8-4" are defined in [RFC3629].
The syntax is based in large parts on [RFC5802], Section 7, which may
be referenced for clarification. If this specification and [RFC5802]
are in conflict, this specification takes priority.
Used definitions from [RFC5802] are reproduced here for convenience:
ALPHA = <as defined in RFC 5234 appendix B.1>
DIGIT = <as defined in RFC 5234 appendix B.1>
UTF8-2 = <as defined in RFC 3629 (STD 63)>
UTF8-3 = <as defined in RFC 3629 (STD 63)>
UTF8-4 = <as defined in RFC 3629 (STD 63)>
attr-val = ALPHA "=" value
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;; Generic syntax of any attribute sent
;; by server or client
value = 1*value-char
value-safe-char = %x01-2B / %x2D-3C / %x3E-7F /
UTF8-2 / UTF8-3 / UTF8-4
;; UTF8-char except NUL, "=", and ",".
value-char = value-safe-char / "="
printable = %x21-2B / %x2D-7E
;; Printable ASCII except ",".
;; Note that any "printable" is also
;; a valid "value".
base64-char = ALPHA / DIGIT / "/" / "+"
base64-4 = 4base64-char
base64-3 = 3base64-char "="
base64-2 = 2base64-char "=="
base64 = *base64-4 [base64-3 / base64-2]
posit-number = %x31-39 *DIGIT
;; A positive number.
saslname = 1*(value-safe-char / "=2C" / "=3D")
;; Conforms to <value>.
authzid = "a=" saslname
;; Protocol specific.
cb-name = 1*(ALPHA / DIGIT / "." / "-")
;; See RFC 5056, Section 7.
;; E.g., "tls-server-end-point" or
;; "tls-unique".
gs2-cbind-flag = ("p=" cb-name) / "n" / "y"
;; "n" -> client doesn't support channel binding.
;; "y" -> client does support channel binding
;; but thinks the server does not.
;; "p" -> client requires channel binding.
;; The selected channel binding follows "p=".
gs2-header = gs2-cbind-flag "," [ authzid ] ","
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;; GS2 header for OPAQUE
username = "n=" saslname
;; Usernames are prepared using SASLprep.
reserved-mext = "m=" 1*(value-char)
;; Reserved for signaling mandatory extensions.
;; The exact syntax will be defined in
;; the future.
channel-binding = "c=" base64
;; base64 encoding of cbind-input.
cbind-data = 1*OCTET
cbind-input = gs2-header [ cbind-data ]
;; cbind-data MUST be present for
;; gs2-cbind-flag of "p" and MUST be absent
;; for "y" or "n".
The following definitions are specific to OPAQUE:
ke1 = "r=" base64
;; base64 encoding of the OPAQUE KE1 message struct
ke2 = "v=" base64
;; base64 encoding of the OPAQUE KE2 message struct
ke3 = "p=" base64
;; base64 encoding of the OPAQUE KE3 message struct
ksf-params = "i=" base64
;; base64 encoding of KSF parameters
client-first-message-bare =
[reserved-mext ","] username "," ke1 ["," extensions]
client-first-message =
gs2-header client-first-message-bare
server-message-bare =
[reserved-mext ","] channel-binding "," ksf-params
["," extensions]
server-message = server-message-bare "," ke2
client-final-message = ke3
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9. Security Considerations
The KSF parameters and channel bindings aren't authenticated before
KSF usage, allowing a DoS of a client by an malicious actor posing as
the server, as it can send excessively expensive KSF parameters.
If not used with a secure channel providing confidentiality this
mechanism leaks the authid and authzid of an authenticating user to
any passive observer.
The cryptographic security of this mechanism is not increased over
the one provided by the underlying OPAQUE protocol, so all security
considerations listed in the [OPAQUE] specification also apply to
this one.
10. Open Issues
* With the current design the KSF parameters can not be MAC-verified
until after they have been used. This is bad. The only other
option is using the ephemeral keypair to generate a MAC key and
use that. This may impact security.
* This mechanism should be extended to also become a GSS-API
mechanism like SCRAM is.
11. IANA Considerations
A future revision of this document will request a new registry for
the OPAQUE family of SASL mechanism, outlining all required details
on the primitives used by the 'OPAQUE-A255SHA' variant.
12. References
12.1. Normative References
[I-D.irtf-cfrg-voprf-17]
Davidson, A., Faz-Hernandez, A., Sullivan, N., and C. A.
Wood, "Oblivious Pseudorandom Functions (OPRFs) using
Prime-Order Groups", Work in Progress, Internet-Draft,
draft-irtf-cfrg-voprf-17, 9 January 2023,
<https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
voprf-17>.
[OPAQUE] Bourdrez, D., Krawczyk, H., Lewi, K., and C. A. Wood, "The
OPAQUE Asymmetric PAKE Protocol", Work in Progress, I-D
draft-irtf-cfrg-opaque-latest, 12 January 2023,
<https://github.com/cfrg/draft-irtf-cfrg-opaque>.
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[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/rfc/rfc2104>.
[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/rfc/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/rfc/rfc3629>.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<https://www.rfc-editor.org/rfc/rfc4422>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, DOI 10.17487/RFC5801,
July 2010, <https://www.rfc-editor.org/rfc/rfc5801>.
[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
DOI 10.17487/RFC5802, July 2010,
<https://www.rfc-editor.org/rfc/rfc5802>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/rfc/rfc5869>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<https://www.rfc-editor.org/rfc/rfc5929>.
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[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/rfc/rfc7627>.
[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/rfc/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/rfc/rfc8446>.
[RFC9106] Biryukov, A., Dinu, D., Khovratovich, D., and S.
Josefsson, "Argon2 Memory-Hard Function for Password
Hashing and Proof-of-Work Applications", RFC 9106,
DOI 10.17487/RFC9106, September 2021,
<https://www.rfc-editor.org/rfc/rfc9106>.
[RFC9266] Whited, S., "Channel Bindings for TLS 1.3", RFC 9266,
DOI 10.17487/RFC9266, July 2022,
<https://www.rfc-editor.org/rfc/rfc9266>.
12.2. Informative References
[TripleHandshake]
Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti,
A., and P. Strub, "Triple Handshakes and Cookie Cutters:
Breaking and Fixing Authentication over TLS", miTLS, May
2014, <https://www.mitls.org/pages/attacks/3SHAKE>.
Acknowledgments
Thank you to Daniel Bourdrez, Hugo Krawczyk, Kevin Lewi, and C. A.
Wood for their work on the OPAQUE PAKE that this mechanism is based
on. Thank you to Abhijit Menon-Sen, Alexey Melnikov, Nicolas
Williams, and Chris Newman for their work on the SCRAM RFC, most of
which this draft oh so blatanly steals for its own gain.
Author's Address
Nadja von Reitzenstein Čerpnjak
Email: me@dequbed.space
von Reitzenstein Čerpnjak Expires 20 July 2023 [Page 14]
