Security for Scalable Registration, Authentication, and Authorization
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## Business-to-Business

Page standards status: Trial-use

This guide supports B2B client applications using either the client credentials or authorization code grant types. The B2B transactions in this guide occur between a requesting organization (the Requestor operating the client application) and a responding organization (the Responder operating the OAuth Server and Resource Server holding the data of interest to the Requestor). In some cases, the Requestor’s client app operates in an automated manner. In other cases, there will also be a local user from the requesting organization (the User interacting with the Requestor’s client application). The client credentials grant type is always used for automated (aka “headless”) client apps. However, when a User is involved, either the client credentials or authorization code grant may be used, as discussed below.

For client credentials flow, any necessary User authentication and authorization is performed by the Requestor as a prerequisite, before the Requestor’s client app interacts with the Responder’s servers, i.e. the Requestor is responsible for ensuring that only its authorized Users access the client app and only make requests allowed by the Requestor. How the Requestor performs this is out of scope for this guide but will typically rely on internal user authentication and access controls.

Examples of automated client apps that use the client credentials grant type include SMART App Launch Backend Services and certain IUA Authorization Clients.

For authorization code flow, the User is expected to be interacting with the Requestor’s client app in real time, at least during the initial authorization of the client app with the Responder’s OAuth Server. Typically, the User must authenticate to the Responder’s system at the time of initial authorization. If the local user has been issued credentials by the Responder to use for this purpose, the authorization code flow will typically involve use of those credentials. However, it is anticipated that in some workflows, the local user will not have their own credentials on the Responder’s system, but will instead have credentials on their “home” system. In these cases, the UDAP Tiered OAuth workflow is used so that the Responder’s OAuth Server can interact with the Requestor’s OIDC Server in an automated manner to authenticate the User, as described in Section 6.

Thus, this guide provides two different paths (client credentials grants and authorization code grants with Tiered OAuth) that a user affiliated with the Requestor without credentials on the Responder’s system may use to obtain access to data held by the Responder.

B2B client applications registered to use the authorization code grant SHALL obtain an access token for access to FHIR resources by following the OAuth 2.0 authorization code grant flow described in Section 4.1 of RFC 6749, with the additional options and constraints discussed below.

Client applications registered to use the client credentials grant SHALL obtain an access token for access to FHIR resources by following the OAuth 2.0 client credentials grant flow described in Section 4.4 of RFC 6749, and with the additional options and constraints discussed below. As noted in Section 3, the Requestor is responsible for ensuring that the Requestor’s User, if applicable, is using the app only as authorized by the Requestor.

### Obtaining an authorization code

The section does not apply to client applications registered to use the client credentials grant.

The workflow for obtaining an authorization code is summarized in the following diagram:

Client applications registered to use the authorization code grant SHALL request an authorization code as per Section 4.1.1 of RFC 6749, with the following additional constraints. Client applications and servers MAY optionally support UDAP Tiered OAuth for User Authentication to allow for cross-organizational or third party user authentication as described in Section 6.

Servers SHALL handle and respond to authorization code requests as per Section 4.1.2 of RFC 6749.

Client applications and Authorization Servers using the authorization code flow SHALL conform to the additional constraints for authorization code flow found in Section 7.2 of this guide.

### Obtaining an access token

The workflow for obtaining an access token is summarized in the following diagram:

Client applications using the authorization code flow SHALL exchange authorization codes for access tokens as per Section 4.1.3 of RFC 6749, with the following additional options and constraints. Client applications using the client credentials flow do not use authorization codes when requesting an access token.

Client applications using the authorization code flow SHALL include a `code_verifier` parameter and value in the token request as per Section 4.5 of RFC 7636.

#### Constructing Authentication Token

Client apps following this guide will have registered to authenticate using a private key rather than a shared `client_secret`. Thus, the client SHALL use its private key to sign an Authentication Token as described in this section, and include this JWT in the `client_assertion` parameter of its token request as described in Section 5.1 of UDAP JWT-Based Client Authentication and detailed further in Section 5.2.2 of this guide.

Authentication Tokens submitted by client apps SHALL conform to the general JWT header requirements in Section 7.1 of this guide and SHALL include the following parameters in the JWT claims, as defined in Section 4 of UDAP JWT-Based Client Authentication and Section 4 of UDAP Client Authorization Grants using JSON Web Tokens:

Authentication JWT Claims
`iss` | required | The application's `client_id` as assigned by the Authorization Server during the registration process
`sub` | required | The application's `client_id` as assigned by the Authorization Server during the registration process
`aud` | required | The FHIR Authorization Server's token endpoint URL
`exp` | required | Expiration time integer for this authentication JWT, expressed in seconds since the "Epoch" (1970-01-01T00:00:00Z UTC)
`iat` | required | Issued time integer for this authentication JWT, expressed in seconds since the "Epoch"
`jti` | required | A nonce string value that uniquely identifies this authentication JWT. See Section 7.1.4 for additional requirements regarding reuse of values.
`extensions` | conditional | A JSON object containing one or more extensions. The HL7 B2B Authorization Extension Object defined below is required for B2B client apps using the `client_credentials` flow; omit for client apps using the `authorization_code` flow

The maximum lifetime for an Authentication Token SHALL be 5 minutes, i.e. the value of `exp` minus the value of `iat` SHALL NOT exceed 300 seconds. The Authorization Server MAY ignore any unrecognized claims in the Authentication Token. The Authentication Token SHALL be signed and serialized using the JSON compact serialization method.

##### B2B Authorization Extension Object

The B2B Authorization Extension Object is used by client apps following the `client_credentials` flow to provide additional information regarding the context under which the request for data would be authorized. The client app constructs a JSON object containing the following keys and values and includes this object in the `extensions` object of the Authentication JWT as the value associated with the key name `hl7-b2b`.

B2B Authorization Extension Object Key Name: "hl7-b2b"
`version` | required | String with fixed value: `"1"`
`subject_name` | conditional | String containing the human readable name of the human or non-human requestor; required if known.
`subject_id` | conditional | String containing a unique identifier for the requestor; required if known for human requestors when the `subject_name` parameter is present and the human requestor has been assigned an applicable identifier. Omit for non-human requestors and for human requestors who have not been assigned an applicable identifier. See Section 5.2.1.3 below for the preferred format of the identifier value string.
`subject_role` | conditional | String containing a code identifying the role of the requestor; required if known for human requestors when the `subject_name` parameter is present. See Section 5.2.1.3 below for the preferred format of the code value string.
`organization_name` | optional | String containing the human readable name of the organizational requestor. If a subject is named, the organizational requestor is the organization represented by the subject.
`organization_id` | required | String containing a unique identifier for the organizational requestor. If a subject is named, the organizational requestor is the organization represented by the subject. The identifier SHALL be a Uniform Resource Identifier (URI). Trust communities SHALL define the allowed URI scheme(s). If a URL is used, the issuer SHALL include a URL that is resolvable by the receiving party.
`purpose_of_use` | required | An array of one or more strings, each containing a code identifying a purpose for which the data is being requested. See Section 5.2.1.3 below for the preferred format of each code value string array element.
`consent_policy` | optional | An array of one or more strings, each containing a URI identifiying a privacy consent directive policy or other policy consistent with the value of the `purpose_of_use` parameter.
`consent_reference` | conditional | An array of one or more strings, each containing an absolute URL consistent with a literal reference to a FHIR Consent or DocumentReference resource containing or referencing a privacy consent directive relevant to a purpose identified by the `purpose_of_use` parameter and the policy or policies identified by the `consent_policy` parameter. The issuer of this Authorization Extension Object SHALL only include URLs that are resolvable by the receiving party. If a referenced resource does not include the raw document data inline in the resource or as a contained resource, then it SHALL include a URL to the attachment data that is resolvable by the receiving party. Omit if `consent_policy` is not present.

Servers that support the B2B client credentials flow described in this guide SHALL support this B2B Authorization Extension Object. Other implementation guides MAY define additional Authorization Extension Objects to use together with this object for B2B client credentials workflows.

##### Preferred code systems and naming systems for US Realm

For `subject_id`, trust communities SHALL constrain the allowed naming system or systems, and are encouraged to require the individual National Provider Identifier (NPI) when known for human requestors who have been assigned an individual NPI.

For `subject_role`, trust communities SHOULD constrain the allowed values and formats, and are encouraged to draw from the National Uniform Claim Committee (NUCC) Provider Taxonomy Code Set, but are not required to do so to be considered conformant.

For `purpose_of_use`, trust communities SHOULD constrain the allowed values, and are encouraged to draw from the HL7 PurposeOfUse value set, but are not required to do so to be considered conformant.

##### Preferred format for identifiers and codes

The preferred format to represent an identifier or code as a string value within an authorization extension object is as a Uniform Resource Identifier (URI) as defined in RFC 3986. Trust communities are encouraged to use this preferred format, but are not required to do so to be considered conformant with this guide.

If the identifier or code is itself a URI, then the native representation is preferred. Otherwise, the preferred method to construct a URI is as follows:

For identifiers, concatenate a URI identifying the namespace, the ‘#’ character, and the unique identifier assigned within the namespace.

For codes, concatenate a URI identifying the code system, the ‘#’ character, and a code taken from the code system.

For example, the U.S. NPI number 1234567890 can be represented as `urn:oid:2.16.840.1.113883.4.6#1234567890` and the purpose of use treatment can be represented as `urn:oid:2.16.840.1.113883.5.8#TREAT`.

#### Submitting a token request

##### Authorization code grant

Client applications using the authorization code grant and authenticating with a private key and Authentication Token as per Section 5.2.1 SHALL submit a POST request to the Authorization Server’s token endpoint containing the following parameters as per Section 5.1 of UDAP JWT-Based Client Authentication. Client apps authenticating in this manner SHALL NOT include an HTTP Authorization header or client secret in its token endpoint request. The token request SHALL include the following parameters:

Token request parameters
`grant_type` | required | Fixed value: `authorization_code`
`code` | required | The code that the app received from the Authorization Server
`redirect_uri` | conditional | The client application's redirection URI. This parameter SHALL be present only if the `redirect_uri` parameter was included in the authorization request in Section 5.1, and their values SHALL be identical.
`client_assertion_type` | required | Fixed value: `urn:ietf:params:oauth:client-assertion-type:jwt-bearer`
`client_assertion` | required | The signed Authentication Token JWT
`udap` | required | Fixed value: `1`

##### Client credentials grant

Client applications using the client credentials grant and authenticating with a private key and Authentication Token as per Section 5.2.1 SHALL submit a POST request to the Authorization Server’s token endpoint containing the following parameters as per Section 5.2 of UDAP JWT-Based Client Authentication. Client apps authenticating in this manner SHALL NOT include an HTTP Authorization header or client secret in its token endpoint request. The token request SHALL include the following parameters:

Token request parameters
`grant_type` | required | Fixed value: `client_credentials`
`scope` | required | Space-delimited list of requested scopes of access.
`client_assertion_type` | required | Fixed value: `urn:ietf:params:oauth:client-assertion-type:jwt-bearer`
`client_assertion` | required | The signed Authentication Token JWT
`udap` | required | Fixed value: `1`

#### Server processing of token requests

An Authorization Server receiving token requests containing Authentication Tokens as above SHALL validate and respond to the request as per Sections 6 and 7 of UDAP JWT-Based Client Authentication.

For client applications using an authorization code grant, the Authorization Server SHALL return an error as per Section 4.6 of RFC 7636 if the client included a `code_challenge` in its authorization request but did not include the correct `code_verfier` value in the corresponding token request.

For all successful token requests, the Authorization Server SHALL issue access tokens with a lifetime no longer than 60 minutes.

This guide does not currently constrain the type or format of access tokens issued by Authorization Servers. Note that other implementation guides (e.g. SMART App Launch, IUA, etc.), when used together with this guide, may limit the allowed access token types (e.g. Bearer) and/or formats (e.g. JWT).

#### Client application use of access tokens

A client application SHALL only use an access token in a manner consistent with any assertions made when requesting that token. For example, if a client asserted a `subject_id` and `purpose_of_use` in the B2B Authorization Extension Object included in its token request, then the access token granted in response to that request can only be used in that authorization context, i.e. for that requestor and for that purpose. If the same client application subsequently needs to retrieve a resource for a different requestor and/or for a different purpose from the same resource server, it cannot reuse the same access token. Instead, it must obtain a new access token by submitting another token request with an updated B2B Authorization Extension Object asserting the new authorization context.

### Refresh tokens

This guide supports the use of refresh tokens, as described in Section 1.5 of RFC 6749. Authorization Servers MAY issue refresh tokens to B2B client applications that use the authorization code grant type as per Section 5 of RFC 6749. Refresh tokens are not used with the client credentials grant type. Client apps that have been issued refresh tokens MAY make refresh requests to the token endpoint as per Section 6 of RFC 6749. Client apps authenticate to the Authorization Server for refresh requests by constructing and including an Authentication Token in the same manner as for initial token requests.