A SASL and GSS-API Mechanism using the asymmetric password-authenticated key agreement OPAQUE

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Table of Contents

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.¶

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

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:¶

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.¶

m=2097152,t=1,p=4

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:¶

• 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.¶

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
                 ;; 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 ] ","
                  ;; 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

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.¶

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.¶

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