Remote attestation over EDHOC

Remote attestation over EDHOC Inria

yuxuan.song@inria.fr

Security Lightweight Authenticated Key Exchange Internet-Draft This document specifies how to perform remote attestation as part of the lightweight authenticated Diffie-Hellman key exchange protocol EDHOC (Ephemeral Diffie-Hellman Over COSE), based on the Remote ATtestation procedureS (RATS) architecture. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Lightweight Authenticated Key Exchange Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Remote attestation is a security process which verifies and confirms the integrity and trustworthiness of a remote device or system in the network. This process helps establish a level of trust in the remote system before allowing the device to e.g. join the network or get the access to some sensitive information and resources. The use cases that require remote attestation include secure boot and firmware management, cloud computing, network access control, etc. The IETF working group Remote ATtestation procedureS (RATS) has defined an architecture for remote attestation. The three main roles in the RATS architecture are the Attester, the Verifier and the Relying Party. The Attester generates the evidence concerning its identity and integrity, which must be appraised by the Verifier for its validity. Then, the Verifier produces the attestation result, which is consequently used by the Relying Party for the purposes of reliably applying application-specific actions. One type of interaction model defined in the RATS architecture is called the background-check model. It resembles the procedure of how employers perform background checks to determine the prospective employee's trustworthiness, by contacting the respective organization that issues a report. In this case, the employer acts as the Relying Party, the employee acts as the Attester and the organization acts as the Verifier. The Attester conveys evidence directly to the Relying Party and the Relying Party forwards the evidence to the Verifier for appraisal. Once the attestation result is computed by the Verifier, it is sent back to the Relying Party to decide what action to take based on the attestation result. Another model is called passport model, where the Attester communicates directly with the Verifier. The Attester presents the evidence to the Verifier and gets an attestation result from the Verifier. THen the Attester conveys the attestation result to the Relying Party. This specification employs both the RATS background-check model and the passport model. One way of conveying attestation evidence/ attestation result is the Entity Attestation Token (EAT) . It provides an attested claims set which can be used to determine a level of trustworthiness. This specification relies on the EAT as the format for attestation evidence and the attestation result. Ephemeral Diffie-Hellman over COSE (EDHOC) is a lightweight authenticated key exchange protocol for highly constrained networks. In EDHOC, the two parties involved in the key exchange are referred to as the Initiator (I) and the Responder (R). EDHOC supports the transport of external authorization data, through the dedicated EAD fields. This specification delivers EAT through EDHOC. Specifically, EAT is transported as an EAD item. There are also some new EAD items defined in . For the generation of evidence, the Attester incorporates an internal attestation service, including a specific trusted element known as the "root of trust". Root of trust serves as the starting point for establishing and validating the trustworthiness appraisals of other components on the system. The measurements signed by the attestation service are referred to as the Evidence. The signing is requested through an attestation API. How the components are separated between the secure and non-secure worlds on a device is out of scope of this specification.

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. The reader is assumed to be familiar with the terms and concepts defined in EDHOC and RATS architecture.

Problem Description This specification describes how to perform remote attestation over the EDHOC protocol according to the RATS architecture. Remote attestation protocol elements are carried within EDHOC's External Authorization Data (EAD) fields. More specifically, this specification supports both the RATS background-check model and passport model. It considers three cases: 1.Forward remote attestation with the EDHOC Initiator as an Attester and the EDHOC Responder as a Relying Party. 2.Reverse attestation over reverse EDHOC message flow (see ), in both background-check model and passport model. 3.Mutual attestation, one using background-check -- background-check model, and the other one using background-check -- passport model. The specification describes how the Attester EDHOC Initiator and EDHOC Responder complete the EDHOC handshake complemented with remote attestation protocol elements in the above cases.

Assumptions The details of the protocol between Relying Party and Verifier in background-check model, and the protocol between the Attester and the Verifier in passport model are out of the scope. It could be an EDHOC protocol, TLS protocol or other security protocols. In background-check model, one assumption is that the Verifier outputs a fresh nonce and that same nonce is passed on to the EDHOC session. That is where the link between the two protocols comes in. The remainder, such as the evidence type selection is just the negotiation. The Verifier is supposed to know how to verify more than one format of the evidence type. Therefore, the Verifier MUST send back at least one format to the Relying Party. We assume in this specification, the Relying Party also has knowledge about the Attester, so it can narrow down the type selection and send to the Attester only one format of evidence type. The Attester should have an explicit relation with the Verifier, such as from device manufacuture, so that the Verifier can evaluate the Evidence that is produced by the Attester. In passport model, the credential of the Verifier is assumed to be stored at the Attester and the Relying Party, which means the Verifier is trusted by the Attester and the Relying Party to obtain the attestation result.

The Protocol

Forward remote attestation A common use case for forward remote attestation is to attest an IoT device to a network server. For example, doing remote attestation to verify that the latest version of firmware is running on the IoT device before the network server allows it to join the network (see ).

Background-check model An overview of doing forward remote attestation over EDHOC is established in . EDHOC session is between the Attester and the Relying Party in background-check model. EDHOC Initiator plays the role of the RATS Attester. EDHOC Responder plays the role of the RATS Relying Party. The Attester and the Relying Party communicate by transporting messages within EDHOC's External Authorization Data (EAD) fields. An external entity, out of scope of this specification, plays the role of the RATS Verifier. The EAD items for background-check model are defined in . Remote attestation starts with an Attestation proposal in EAD_1 by providing the supported evidence types from the Attester. The Relying Party generates an Attestation request in EAD_2 based on the selected evidence type and the nonce from the Verifier, then sends it to the Attester. The Attester calls its attestation service to generate the evidence according to the Attestation request. The Evidence is sent as an EAT in EAD_3 from the Attester to the Relying Party. The Relying Party treats the Evidence as an opaque data and sends it to the Verifier. The Verifier consumes the Evidence and generates an attestation result, which is then sent back to the Relying Party.

Reverse attestation One use case for reverse attestation is when a network server needs to attest itself to a client (e.g., an IoT device). For example, the client needs to send some sensitive data to the network server, which requires the network server to be attested first. In reverse attestation, the network server acts as an Attester and the client acts as a Relying Party.

Background-check model In this section, the reverse attestation in background-check model is performed over reverse EDHOC message flow (see ). EDHOC session is between the Relying Party and the Attester. An overview of the message flow is shown in . The Relying Party triggers a new EDHOC session with a Uri-Path: "/.well-known/edhoc". EDHOC message_1 is then sent from the Attester with an Attestation proposal carried in EAD_1. The Attestation request is then carried in EAD_2 and the Evidence is carried in EAD_3.

Passport model This section details the reverse attestation process in the passport model over reverse EDHOC message flow (see ). The EDHOC session is between the Relying Party and the Attester. An overview of the message flow is illustrated in . The EAD items specific to the passport model are defined in . During the EDHOC session using passport model, the Attester provides a list of the Verifier identities from which it can obtain the Attestation result. The Relying Party then selects a Verifier identity it trusts for the Attester to obtain the Attestation result from. The Attester sends EDHOC message_1 containing a Result proposal carried in EAD_1. The Result proposal includes a list of trusted Verifier identities by the Attester. In EAD_2, a Result request is sent including a selected Verifier identity from the Relying Party. Upon receiving the Result request, the Attester contacts the designated Verifier to obtain the Attestation result. The attestation result is then carried in EAD_3. An open discussion here is the expected freshness of the Attestation result at the Verifier, considering two perspectives: 1. Fresh Remote Attestation: This approach invloves performing a fresh remote attestation when the Attester sends a Result request to the Verifier. The attestation result is generated with a nonce included from the Relying Party in Result request. 2. Pre-stored Attestation with Timestamp: Alternatively, the remote attestation may finish when the Result request is sent, and the attestation result is stored at the Verifier with a timestamp indicating its validation period. In this case, the Relying Party needs to have the capability of time measurement and see whether the attestation result is still validated or not. We seek input from the working group on the expected freshness in the passport model.

Mutual attestation When both entities (e.g., an IoT device and a network server) need to attest to each other, they can perform a mutual attestation to establish a bidirectional trust. This ensures that both the IoT device and the network server are trusted before exchanging sensitive information. This specification outlines two options for mutual attestation, depending on the different computing environments. The first option is mutual attestation using the background-check model -- background-check model, as detailed in . The second option is mutual attestation using a hybrid of the background-check model and passport model, as described in .

Background-check model -- Background-check model In this section, both the EDHOC Initiator and the EDHOC Responder perform remote attestation using the background-check model. This approach is suitable for devices with no connectivity constraints. The process is as follows:

The EDHOC Initiator starts the remote attestation by sending an Attestation proposal in EAD_1.
The EDHOC Responder initiates the second remote attestation by sending an Attestation proposal in EAD_2, along with an Attestation request in response to the EDHOC Initiator's proposal.
In EAD_3, the EDHOC Initiator sends two EAD items: the Evidence of the first remote attestation and an Attestation request for the second remote attestation.
A forth EDHOC message is required to send the Evidence of the second remote attestation from the EDHOC Responder to the EDHOC Initiator.

Background-check model -- Passport model This use case is applicable when the EDHOC Initiator is constrained in terms of connectivity. In this case, the mutual attestation begins with a forward background-check attestation, followed by a reverse passport attestation.

EDHOC Initiator sends the first EDHOC message with an Attestation proposal in EAD_1.
In EAD_2, two EAD items are included: an Attestation request responding to the EDHOC Initiator's proposal and a Result proposal to initiate a reverse passport attestation. The EDHOC Responder SHOULD connect to a Verifier to select the evidence type for the forward attestation in background-check model.
In EAD_3, two EAD items are included: an Evidence and a Result request. The EDHOC Responder SHOULD connect to the Verfier in the passport model(VP) indicated in the Result request to obtain the Result from that Verifier. Simultaneously, the EDHOC Responder sends the received Evidence to the Verifier in the background-check model(VB) to get the attestation result of the first forward attestation.
The EDHOC Responder sends the Result that is obtained from VP to the EDHOC Initiator in EAD_4, and consumes the Attetation result from VB.

Overview of mutual attestation of Background-check model -- Passport model. EDHOC is used between A and RP. | Relying | EvidenceTypes | Verifier | | Verifier | | | | +-------------->| | | | | | Attestation | Party |<--------------+Background| | Passport | | (A) | request | (RP) | Selected | check | | model | | |<-------------+ |EvidenceType(s)| model | | | + / | Result | / | | | | | | | proposal | | | | | | | Relying | | Attester | | (VB) | | (VP) | | | Evidence | | Evidence | | Result request| | | Party +------------->| +-------------->| ---------+-------------->| | | (RP) | Result | (A) |<--------------+ | | | | | request | | Attestation | | | | | | | | result | | | | + |<-------------+ |<--------------+----------+---------------+ | | | Result | | | | Result | | | | | | | | | | +-----------+ +-----------+ +----------+ +----------+ <--------------> EDHOC session ]]>

External Authorization Data (EAD) items EDHOC supports one or more EAD items in each EAD field. EAD item is a CBOR sequence of an ead_label and an optional ead_value.

Attestation_proposal To start a remote attestation in background-check model, the Attester transports the Proposed_EvidenceType object. It signals to the Relying Party the proposal to do remote attestation, as well as which attestation claims the Attester supports. The supported attestation claims are encoded in CBOR in the form of a sequence. The EAD item for an attestation proposal is:

ead_label = TBD1
ead_value = Attestation_proposal, which is a CBOR byte string:

where

content-format is an array that contains all the supported evidence types by the Attester.
There MUST be at least one item in the array.
content-format is an indicator of the format type (e.g., application/eat+cwt with an appropriate eat_profile parameter set), from .

The sign of ead_label TBD1 MUST be negative to indicate that the EAT item is critical. If the receiver cannot recognize the critical EAD item, or cannot process the information in the critical EAD item, then the receiver MUST send an EDHOC error message back.

Attestation_request As a response to the attestation proposal, the Relying Party signals to the Attester the supported and requested evidence type. In case none of the evidence types is supported, the Relying Party rejects the first message_1 with an error indicating support for another evidence type. The EAD item for an attestation request is:

ead_label = TBD2
ead_value = Attestation_request, which is a CBOR byte string:

where

content-format is the selected evidence type by the Relying Party and supported by the Verifier.
nonce is generated by the Verifier and forwarded by the Relying Party.

The sign of ead_label TBD2 MUST be negative to indicate that the EAT item is critical. If the receiver cannot recognize the critical EAD item, or cannot process the information in the critical EAD item, then the receiver MUST send an EDHOC error message back.

Evidence As a response to the attestation request, the Attester calls its local attestation service to generate and return the serialized EAT as Evidence. The EAD item is:

ead_label = TBD3
ead_value is a serialized EAT.

Result_proposal In passport model, the attestation result is transfered from the Attester to the Relying Party. Before sending the attestation result, the Attester needs to negotiate with the Relying Party the Verifier identities from which it should get the attestation result. The Attester firstly sends an attestation result proposal which contains the identification of the credentials of the Verifiers to indicate Verifiers' indentities. The identification of credentials relies on COSE header parameters , with a header lable and credential value. The EAD item for an attestation result proposal is:

ead_label = TBD4
ead_value = Result_proposal, which is a CBOR byte string:

values } ]]> where

Proposed_VerifierIdentity is defined as a list of one or more VerifierIdentity elements.
Each VerifierIdentity within the list is a map defined in that:
- label = int / tstr
- values = any

Result_request As a response to the attestation result proposal, the Relying Party signals to the Attester the trusted Verifier. In case none of the Verifiers can be trusted by the Relying Party, the session is aborted. Relying Party generates a nonce to ensure the freshness of the attestation result from the Verifier. The EAD item for an attestation result request is:

ead_label = TBD5
ead_value = Result_request, which is a CBOR byte string:

Result The attestation result is generated and signed by the Verifier as a serialized EAT . The Relying Party can decide what action to take with regards to the Attester based on the information elements in attetation result. The EAD item is:

ead_label = TBD6
ead_value is a serialized EAT.

Error Handling This section specifies a new EDHOC error code and how it is used in the proposed protocol.

EDHOC Error "Attestation failed" This section specifies a new EDHOC error "Attetation failed". The format of the error message follows the one in EDHOC protocol(see ).

EDHOC error code and error information for Attestation failed. Error code TBD7 indicates to the receiver that the remote attestation is failed after the evidence is sent. This can occur in two cases:

The Verifier evaluates the attestation evidence and returns a negative result based on the Verifier's appraisal policy.
The Verifier provides a positive attestation result to the Relying Party, but the Relying Party can not establish a sufficient level of trust to proceed decision-specific actions based on its appraisal policy.

In case 1, the Verifier signals the error to the Relying Party, which then generates an EDHOC "Attestation failed" error and send it to the Attester. In case 2, the Relying Party directly generates and sends the "Attestation failed" error to the Attester. The application decides how to handle the error message.

Security Considerations This specification is performed over EDHOC by using EDHOC's EAD fields. The privacy considerations of EADs in EDHOC apply to this specification. EAD_1 is not resistant to either active attackers or passive attackers, because neither the Initiator nor the Responder has been authenticated. Although EAD_2 is encrypted, the Initiator has not been authenticated, rendering EAD_2 vulnerable against the active attackers. The ead items in EAD_1 and EAD_2 MAY be very specific and potentially reveal sensitive information about the device. The leaking of the data in EAD_1 and/or EAD_2 MAY risk to be used by the attackers for malicious purposes. Data in EAD_3 and EAD_4 are protected between the Initiator and the Responder in EDHOC. Mutual attestation carries a lower risk for EAD items when the Responder is the Attester. For the mutual attestation at the EDHOC Responder, only the Attestation_proposal/Result_proposal in EAD_2 is not protected to active attackers. Both the Attestation_request/Result_request in EAD_3 and the Evidence/Result in EAD_4 are protected.

IANA Considerations

EDHOC External Authorization Data Registry IANA is requested to register the following entry in the "EDHOC External Authorization Data" registry under the group name "Ephemeral Diffie-Hellman Over Cose (EDHOC)". The ead_label = TBD1 corresponds to the ead_value Attestation_proposal with processing specified in . The ead_label = TBD2 corresponds to the ead_value Attestation_request in . The ead_label = TBD3 corresponds to the ead_value which carries the EAT, as specified in . The ead_lable = TBD4 corresponds to the ead_value Result_proposal in passport model in . The ead_lable = TBD5 corresponds to the ead_value Result_request in passport model in . The ead_lable = TBD6 corresponds to the ead_value Result in passport model in . Addition to the EDHOC EAD registry

Label	Value Type	Description
TBD1	bstr	Attestation Proposal
TBD2	bstr	Attestation Request
TBD3	bstr	Evidence for remote attestation
TBD4	bstr	Result Proposal
TBD5	bstr	Result Proposal
TBD6	bstr	Attestation result

References Normative References The Entity Attestation Token (EAT) Security Theory LLC Mediatek USA Qualcomm Technologies Inc. Red Hound Software, Inc. An Entity Attestation Token (EAT) provides an attested claims set that describes state and characteristics of an entity, a device like a smartphone, IoT device, network equipment or such. This claims set is used by a relying party, server or service to determine the type and degree of trust placed in the entity. An EAT is either a CBOR Web Token (CWT) or JSON Web Token (JWT) with attestation-oriented claims. Ephemeral Diffie-Hellman Over COSE (EDHOC) This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios, and a main use case is to establish an Object Security for Constrained RESTful Environments (OSCORE) security context. By reusing CBOR Object Signing and Encryption (COSE) for cryptography, Concise Binary Object Representation (CBOR) for encoding, and Constrained Application Protocol (CoAP) for transport, the additional code size can be kept very low. Key words for use in RFCs to Indicate Requirement Levels 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. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words 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. Informative References Remote ATtestation procedureS (RATS) Architecture In network protocol exchanges, it is often useful for one end of a communication to know whether the other end is in an intended operating state. This document provides an architectural overview of the entities involved that make such tests possible through the process of generating, conveying, and evaluating evidentiary Claims. It provides a model that is neutral toward processor architectures, the content of Claims, and protocols. Concise Binary Object Representation (CBOR) The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. CBOR Web Token (CWT) CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. CBOR Web Token (CWT) Claims IANA Arm's Platform Security Architecture (PSA) Attestation Token Arm Limited Arm Limited HP Labs Linaro The Arm Platform Security Architecture (PSA) is a family of hardware and firmware security specifications, as well as open-source reference implementations, to help device makers and chip manufacturers build best-practice security into products. Devices that are PSA compliant can produce attestation tokens as described in this memo, which are the basis for many different protocols, including secure provisioning and network access control. This document specifies the PSA attestation token structure and semantics. The PSA attestation token is a profile of the Entity Attestation Token (EAT). This specification describes what claims are used in an attestation token generated by PSA compliant systems, how these claims get serialized to the wire, and how they are cryptographically protected. This informational document is published as an independent submission to improve interoperability with Arm's architecture. It is not a standard nor a product of the IETF. CoAP Content-Formats n.d. Lightweight Authorization using Ephemeral Diffie-Hellman Over COSE Ericsson AB Ericsson AB INRIA INRIA Sandelman Software Works This document describes a procedure for authorizing enrollment of new devices using the lightweight authenticated key exchange protocol Ephemeral Diffie-Hellman Over COSE (EDHOC). The procedure is applicable to zero-touch onboarding of new devices to a constrained network leveraging trust anchors installed at manufacture time. COSE Header Parameters n.d.

Example: Remote Attestation Flow

Example of remote attestation. | | | | | | | | | /newSession | | | +------------------>| | | | | | | | | | | | Body: { | | | | nonce, | | | EDHOC message_2 | types(a,b) | | | {...} | } | | | EAD_2( |<------------------+ | | nonce, | | | | type(a) | | | | ) | | | | Auth_CRED(Sig/MAC) | | | |<-----------------------+ | | Body:{ | | | | nonce, | | | | type(a) | | | | } | | | |<---------------+ | | | Body:{ | | | | nonce, | | | | Evidence | | | | } | | | +--------------->| | | | | EDHOC message_3 | | | | {...} | | | | EAT(nonce,Evidence) | | | | Auth_CRED(sig/MAC) | | | +----------------------->| | | | | | | | | | | | | | | | | Body: { | | | | EAT} | | | +------------------>| | | | Body: { | | | | att-result: AR{} | | | | } | | | |<------------------+ | | +---. | | | | | verify AR{} | | | |<--' | | | | | | | application data | | | |<---------------------->| | | | | | ]]>

Remote attestation in parallel with enrollment authorization This section discusses the possibility of doing remote attestation in parallel with the enrollment authorization procedure defined in . In this case, the message count is much decreased. The detailed procedure is TBD.

Example: Firmware Version The goal in this example is to verify that the firmware running on the device is the latest version, and is neither tampered or compromised. A device acts as the Attester, currently in an untrusted state. The Attester needs to generate the evidence to attest itself. A gateway that can communicate with the Attester and can control its access to the network acts as the Relying Party. The gateway will finally decide whether the device can join the network or not depending on the attestation result. The attestation result is produced by the Verifier, which is a web server that can be seen as the manufacturer of the device. Therefore it can appraise the evidence that is sent by the Attester. The remote attestation session starts with the Attester sending EAD_1 in EDHOC message 1, as specified in . In EAD_1 field, the Attester indicates that the format of EAT is in CWT and the profile of EAT is Platform Security Architecture (PSA) attestation token . PSA attestation token contains the claims relating to the security state of the platform, which are provided by PSA's Initial Attestation API. Therefore, an example of the EAD_1 in EDHOC message_1 could be: According to , IANA is requested to register the Content-Format ID in the "CoAP Content-Formats" registry , for the application/eat+cwt media type witih tihe eat_profile parameter equal to tag:psacertified.org,2023:psa#tfm. We assume the ID that is assigned to this content type is 66. The Media Type equivalent is: If the Verifier and the Relying Party can support this evidence type that is proposed by the Attester, the Relying Party will include in the EAD_2 field the same evidence type, alongside a nonce for message freshness. The Evidence in EAD_3 field is the Platform Security Architecture (PSA) attestation token, which is the attestation of the platform state to assure the firmware integrity. This can be generated from Measured boot, which creates the measurements of loaded code and data during the boot process and make them part of an overall chain of trust. Each stage of the chain of trust stores the measurements in a local root of trust, then the Root of Trust for Report (RTR) of the device can use them as materials to generate the Evidence. The components of the Evidence should at least be: The key for signature is: The resulting COSE object is: which has the following base16 encoding: The Relying Party (co-located with the gateway) then treats the Evidence as opaque and sends it to the Verifier. Once the Verifier sends back the Attestation Result, the Relying Party can be assured on the version of the firmware that the device is running.

Open discussion: remote attestation over EDHOC/ over OSCORE TBD

Acknowledgments The author would like to thank Thomas Fossati, Goran Selander, Malisa Vucinic, Ionut Mihalcea, Muhammad Usama Sardar and Michael Richardson for the provided ideas and feedback.