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AREA WG Working Group Internet-Draft This specification describes a family of Simple Authentication and Security Layer (SASL, RFC4422) authentication mechanisms based on the OPAQUE asymmetric password-authenticated key agreement (PAKE) algorithm. They offer two distinct advantages over the SCRAM family of mechanisms. The underlying OPAQUE algorithm provides the ability for clients to register without the servers getting access to the clear text password of an user, preventing password exfiltration at registration. Secondly a successful authentication produces a long-term secret key only known to the client that can be used to access encrypted server-side data without needing to share keys between clients via a side-band mechanism. When used in combination with TLS or an equivalent security layer these mechanisms allow for secure channel binding. Discussion Venues Discussion of this document takes place on the Common Authentication Technology Next Generation Working Group mailing list (kitten@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

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 when, and only when, they appear in all capitals, as shown here.

Introduction This specification describes a family of authentication mechanisms 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. OPAQUE as 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" . 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.
• Successfull authentication does not grant the server enough information to impersonate the client.
• Mutual authentication is implicit and required. A successfull authentication always strongly authenticates both sides of the exchange.
• A successfull authentication provides both parties with an emphemeral 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 successfull 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.

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 authentication exchange. Nothing in OPAQUE prevents sending the first client response with the SASL authentication request as defined by an application protocol ("initial client response"). See for more details. The OPAQUE client starts by being in posession of an username and password. It uses the password to generate a KE1 structure as per OPAQUE , and sends it 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 used at registration. It uses the first three to generate a KE2 structure 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 it can return to the server. The three messages KE1, KE2 and KE3 are generated using the following functions specified in OPAQUE : The values of client_identity and server_identity are set to: With the values and encodings of the remaining parameters per the OPAQUE specification, 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.

OPAQUE Mechanism Names An OPAQUE mechanism name is the string "OPAQUE-" followed by an uppercase identifier for the cryptographic primitives being used. The identifier is limited to 7 octets (20 - len("OPAQUE-") - len("-PLUS")) which is too short to contain the full names of all cryptographic primitives used. Thus OPAQUE mechanisms using new groups of primitives SHOULD be registered with IANA to allow implementers to identify all required primitives. The PLUS suffix is only used when the authenticating parties support channel binding. If the server supports channel binding it SHOULD advertise both the "bare" and "plus" version of whichever OPAQUE variant it support. If the server does not it will only advertise the "bare" version.

OPAQUE Authentication Exchange S: c=,i=,v= C: p= ]]> 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, containting 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 long term private key and client-only export key 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.

OPAQUE Attributes This section details all attributes permissible in messages, their use and their value format. All Attributes a single US-ASCII letters and case-sensitive. The selection of letters used for attributes is based on SCRAM to make it easier to adapt extensions defined for SCRAM to this mechanism. Note that similar to SCRAM 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.

• a: This is an optional attribute and is part of the GS2 bridge between GSS-API and SASL. Its specification and usage is the same as defined in .
• n: This attribute specifies the name of the user whose password is used for authentication (aka "authentication identity" ). Its encoding, preparation, and usage is the same as defined in .
• 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 .
• c: This REQUIRED attribute specifies the base64-encoded GS2 header and channel binding data. Its specification is the same as defined in , 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 to the KSF in use.
• v: This attribute specifies a base64-encoded serialization of the KE2 message as specified by .
• p: This attribute specifies a base64-encoded serialization of the KE3 message as specified by .
• 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.

SASL Mechanism Requirements This section describes the required information for SASL mechanisms as laid out in . 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.

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 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.

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' channel binding type if they implement any channel binding and use TLS. Clients SHOULD implement the 'tls-exporter' channel binding type if they implement any and use TLS. Servers MUST use the channel binding type indicated by the client, or fail authentication if they do not support it.

OPAQUE-A255SHA(-PLUS) TODO: Define one set of primitives; probably OPAQUE-A255SHA(-PLUS), using HKDF, HMAC, ristretto255, SHA-512 and Argon2i(d?)

Formal Syntax The following syntax specification is written in Augmented Backus-Naur Form (ABNF) notation as specified in . The non-terminals "UTF8-2", "UTF8-3" and "UTF8-4" are defined in . The syntax is based in large parts on , which may be referenced for clarification. If this specification and are in conflict, this speification takes priority. Used definitions from are reproduced here for convenience: DIGIT = UTF8-2 = UTF8-3 = UTF8-4 = 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 . 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:

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. 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 algorithm, so all security considerations applying to that specification also apply to this one.

Open Issues

• OPAQUE allows for a static context to be bound into generated keys to prevent cross-protocol and downgrade attacks. We should probably set that to the mechanism name (e.g. 'OPAQUE-A255SHA')
• 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.

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.

Normative References This document describes how to use a Generic Security Service Application Program Interface (GSS-API) mechanism in the Simple Authentication and Security Layer (SASL) framework. This is done by defining a new SASL mechanism family, called GS2. This mechanism family offers a number of improvements over the previous "SASL/ GSSAPI" mechanism: it is more general, uses fewer messages for the authentication phase in some cases, and supports negotiable use of channel binding. Only GSS-API mechanisms that support channel binding and mutual authentication are supported. [STANDARDS-TRACK] The secure authentication mechanism most widely deployed and used by Internet application protocols is the transmission of clear-text passwords over a channel protected by Transport Layer Security (TLS). There are some significant security concerns with that mechanism, which could be addressed by the use of a challenge response authentication mechanism protected by TLS. Unfortunately, the challenge response mechanisms presently on the standards track all fail to meet requirements necessary for widespread deployment, and have had success only in limited use. This specification describes a family of Simple Authentication and Security Layer (SASL; RFC 4422) authentication mechanisms called the Salted Challenge Response Authentication Mechanism (SCRAM), which addresses the security concerns and meets the deployability requirements. When used in combination with TLS or an equivalent security layer, a mechanism from this family could improve the status quo for application protocol authentication and provide a suitable choice for a mandatory-to-implement mechanism for future application protocol standards. [STANDARDS-TRACK] Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [STANDARDS-TRACK] ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. This document defines a channel binding type, tls-exporter, that is compatible with TLS 1.3 in accordance with RFC 5056, "On the Use of Channel Bindings to Secure Channels". Furthermore, it updates the default channel binding to the new binding for versions of TLS greater than 1.2. This document updates RFCs 5801, 5802, 5929, and 7677. n.d. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. The Simple Authentication and Security Layer (SASL) is a framework for providing authentication and data security services in connection-oriented protocols via replaceable mechanisms. It provides a structured interface between protocols and mechanisms. The resulting framework allows new protocols to reuse existing mechanisms and allows old protocols to make use of new mechanisms. The framework also provides a protocol for securing subsequent protocol exchanges within a data security layer. This document describes how a SASL mechanism is structured, describes how protocols include support for SASL, and defines the protocol for carrying a data security layer over a connection. In addition, this document defines one SASL mechanism, the EXTERNAL mechanism. This document obsoletes RFC 2222. [STANDARDS-TRACK] Algorand Foundation Novi Research Cloudflare, Inc. This document describes the OPAQUE protocol, a secure asymmetric password-authenticated key exchange (aPAKE) that supports mutual authentication in a client-server setting without reliance on PKI and with security against pre-computation attacks upon server compromise. In addition, the protocol provides forward secrecy and the ability to hide the password from the server, even during password registration. This document specifies the core OPAQUE protocol and one instantiation based on 3DH.

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.