Federated TLS Authentication

Internet-Draft	FedTLS	January 2025
Schlyter & Halén	Expires 3 August 2025	[Page]

Abstract

This document describes the Federated TLS Authentication (FedTLS) protocol, enabling secure machine-to-machine communication within a federation. Both clients and servers perform mutual TLS authentication, establishing trust based on a centrally managed trust anchor published by the federation. Additionally, FedTLS ensures unambiguous identification of entities, as only authorized members within the federation can publish metadata, further mitigating risks associated with unauthorized entities impersonating legitimate participants. This framework promotes seamless and secure interoperability across different trust domains adhering to common policies and standards within the federation.¶

3. Trust Model

The FedTLS framework operates on a trust model that is central to its design and functionality. This section outlines the key components of this trust model and its implications for federation members and the federation operator.¶

3.1. Role of the Federation Operator

The federation operator plays a critical role in the FedTLS framework. This entity is responsible for:¶

Managing the central trust anchor, which is used to establish trust across different domains within the federation.¶
Vetting federation members to ensure they meet the required standards and policies.¶
Maintaining and securing the federation metadata, which includes public key pins [RFC7469], issuer certificates, and other essential information.¶

Additionally, the federation operator SHOULD develop their own threat models to proactively identify potential risks and threats. This process involves examining the operating environment, evaluating both internal and external threats, and understanding how vulnerabilities can be exploited. The goal of the threat model is to enable the federation operator to establish mitigation strategies that address the identified risks.¶

The security and stability of the federation rely on the integrity and competence of the federation operator. Members must be able to fully trust this central authority, as its role is essential to maintaining the federation's reliability and security.¶

3.2. Federation Members' Responsibilities

Federation members share the responsibility of maintaining trust and security within the federation. Their responsibilities include:¶

Adhering to the federation's security policies and procedures.¶
Ensuring the accuracy and timeliness of their metadata submissions.¶
Cooperating with the federation operator's vetting and security measures.¶

By fulfilling these responsibilities, federation members help sustain the overall trust framework that enables secure and reliable communication within the federation. Federation members submit member metadata to the federation. Both the authenticity of the submitted member metadata and the submitting member need to be ensured by the federation.¶

3.3. Chain of Trust

Each federation operates within a trust framework that encompasses its own security policies and procedures. This framework is designed to ensure the integrity, authenticity, and confidentiality of communications within the federation. Key components of this framework include:¶

Public key pinning [RFC7469] and preloading to thwart man-in-the-middle attacks by ensuring validated certificates.¶
Regular updates and verification of federation metadata to prevent the use of outdated or compromised information.¶

The federation operator aggregates, signs, and publishes the federation metadata, which combines all members' member metadata along with additional federation-specific information. By placing trust in the federation and its associated signing key, federation members trust the information contained within the federation metadata.¶

The trust anchor for the federation is established through the federation's signing key, a critical component requiring secure distribution and verification. To achieve this, the federation's signing key is distributed using a JSON Web Key Set (JWKS) [RFC7517], providing a flexible framework for exposing multiple keys, including the signing key and keys for rollover. This structured approach ensures members can readily access the necessary keys for verification purposes.¶

An additional layer of security is introduced through thumbprint verification [RFC7638], where federation members can independently verify the key's authenticity. This involves comparing the calculated cryptographic thumbprint of the key with a trusted value, ensuring its integrity. Importantly, this verification process can be conducted through channels separate from the JWKS itself, enhancing security by eliminating reliance on a single distribution mechanism.¶

This trust framework is essential for enabling seamless and secure interoperability across different trust domains within the federation.¶

3.4. Member Vetting

To ensure the security and integrity of the FedTLS framework, a member vetting process is essential. Detailed vetting processes are beyond the scope of this document but can be guided by established frameworks such as eIDAS and eduGAIN.¶

The following are non-normative references to established frameworks:¶

eIDAS: The eIDAS regulation establishes a framework for electronic identification and trust services within the European Union. It ensures secure and standardized electronic interactions across member states, facilitating mutual recognition of electronic IDs. Operators can refer to the eIDAS framework for guidance on robust authentication and identity verification processes. See [eIDAS].¶
eduGAIN: eduGAIN is an interfederation service connecting identity federations worldwide, primarily within the research and education sectors. It ensures high standards of security and interoperability, allowing institutions to collaborate seamlessly. eduGAIN's processes for vetting can serve as a useful reference. See [eduGAIN].¶

3.5. Metadata Authenticity

Ensuring the authenticity of metadata is crucial for maintaining the security and trustworthiness of the FedTLS framework. The specific mechanisms for ensuring metadata authenticity are beyond the scope of this document and must be defined by the federation or regulatory bodies.¶

4. Metadata Repository

The FedTLS metadata repository acts as a central vault, securely storing all information about all participating federation members and their respective entities. This information, known as federation metadata, is presented as a JWS [RFC7515]to ensure its authenticity and integrity.¶

The metadata repository is subject to stringent security measures to safeguard the integrity of the stored information. This MAY involve:¶

Member Management: The federation operator can centrally enforce security policies and vet new members before they are added to the repository.¶
Access Controls: Only authorized members within the federation should have access to the repository.¶
Regular Backups: Robust backup procedures ensure data recovery in case of unforeseen circumstances.¶

Before member metadata is added to the federation's repository, the submitted metadata MUST undergo a validation process. This process aims to verify the accuracy, completeness, and validity of the information provided by a member. The validation process MUST include, at a minimum but not limited to, the following checks:¶

Format Validation: The system checks if the submitted metadata adheres to the defined schema and format specifications.¶
Unique Entity ID: Checks are performed to ensure that the entity_id in the submitted metadata is not already registered by another member. Each entity within the federation must have a unique identifier.¶
Unique Public Key Pins: Public key pins [RFC7469] are used to identify client entities within the federation metadata during the connection validation process. When a server validates a client's TLS connection, it extracts the pin from the client's TLS certificate and matches it against entries in the federation metadata. The requirements for pin uniqueness and usage are detailed in Section Section 6.1.1.1.¶
Certificate Verification: The issuer certificates listed in the metadata are validated to ensure that the algorithms used in the certificates are well-known and secure, and that the certificates are currently valid and have not expired¶
Tag Validation: Ensures that tags (see Section 6.1.1.1) in the metadata adhere to the defined tag structure, verifying both mandatory and optional tags. This process is crucial for maintaining consistency and preventing unauthorized tags within a federation.¶

The FedTLS metadata repository serves as the vital foundation for establishing trust and enabling secure communication within a FedTLS environment. By providing a central, secure, and controlled repository for critical information, the metadata repository empowers members to confidently discover other trusted entities, and establish secure connections for seamless interaction.¶

4.1. Metadata Submission

It is up to the federation to determine which channels should be provided to members for submitting their metadata to the metadata repository. Members typically have the option to either upload the metadata directly to the repository, provided such functionality exists, or to send it to the federation operator through a designated secure channel. If an insecure channel is used, additional measures MUST be taken to verify the authenticity and integrity of the metadata. Such measures may include verifying the checksum of the metadata through another channel. The choice of submission channel may depend on factors such as the federation's guidelines and the preferences of the member.¶

4.2. Maintaining Up-to-Date Metadata

In a FedTLS federation, accurate and current metadata is essential for ensuring secure and reliable communication between members. This necessitates maintaining up-to-date metadata accessible by all members.¶

Federation Metadata: The federation operator publishes a JWS containing an aggregate of all entity metadata. This JWS serves as the source of truth for information about all members within the federation. Outdated information in the JWS can lead to issues like failed connections, discovery challenges, and potential security risks.¶
Local Metadata: Each member maintains a local metadata store containing information about other members within the federation. This information is retrieved from the federation's publicly accessible JWS. Outdated data in the local store can hinder a member's ability to discover and connect with other relevant entities.¶

The following outlines the procedures for keeping metadata up-to-date:¶

Federation Operator Role: The federation operator plays a crucial role in maintaining data integrity within the federation. Their responsibilities include:¶
- Defining regulations for metadata management that MUST include, at a minimum but not limited to, expiration and cache time management.¶
- Implementing mechanisms to update the published federation metadata, ensuring it adheres to the expiration time (exp, see Section 6.4) and cache TTL (cache_ttl, see Section 6.1) specifications.¶
Member Responsibility: Members must follow the federation's metadata management regulations and refresh their local metadata store according to the defined expiration and cache regulations.¶

By adhering to these responsibilities, the Federation ensures that information remains valid for the defined timeframe and that caching mechanisms utilize up-to-date data effectively.¶

5. Authentication

All communication established within the federation leverages mutual TLS authentication, as defined in [RFC8446]. This mechanism ensures the authenticity of both communicating parties, establishing a robust foundation for secure data exchange.¶

5.1. Public Key Pinning

FedTLS implements public key pinning as specified in [RFC7469]. Public key pinning associates one or more unique public keys with each endpoint within the federation, stored in the federation metadata. During a connection, clients and servers extract the public key from the received certificate and validate it against the pre-configured public key pins retrieved from the federation metadata.¶

5.1.1. Benefits of Public Key Pinning

The decision to utilize public key pinning in the FedTLS framework was driven by several critical factors aimed at enhancing security and ensuring trust:¶

5.1.1.1. Interfederation Trust

In interfederation environments, where multiple federations need to trust each other, public key pinning remains effective. Each federation can pin the public keys of entities in other federations, ensuring trust across boundaries. Unlike private certificate chains, which can become complex and difficult to manage across multiple federations, public key pinning provides a straightforward mechanism for establishing trust. FedTLS interfederation addresses this challenge by aggregating metadata from all participating federations into a unified metadata repository. This shared metadata enables secure communication between entities in different federations, ensuring consistent key validation and robust cross-federation trust and security.¶

5.1.1.2. Fortifying Security Against Threats

Public key pinning provides a robust defense mechanism by directly binding a peer to a specific public key. This ensures that only the designated key is trusted, preventing attackers from exploiting fraudulent certificates. By eliminating reliance on external trust intermediaries, this approach significantly enhances resilience against potential threats.¶

5.1.1.3. Use of Self-Signed Certificates

The use of self-signed certificates within the federation leverages public key pinning to establish trust. By bypassing external CAs, servers and clients rely on the federation's mechanisms to validate trust. Public key pinning ensures that only the specific self-signed public keys, identified by key pins in the metadata, are trusted.¶

5.1.1.4. Revocation

If any certificate in a certificate chain is compromised, the revocation process can be complex and slow. This complexity arises because not only the compromised certificate but potentially multiple certificates within the chain might need to be revoked and reissued. Public key pinning mitigates this complexity by allowing clients to explicitly trust a specific public key, thereby reducing dependency on the entire certificate chain's integrity.¶

If a leaf certificate is compromised within a FedTLS federation, the revocation process involves removing the pin associated with the compromised certificate and updating the metadata with a pin from a new certificate. This eliminates the need for traditional revocation mechanisms and focuses the trust relationship on the specific, updated public key.¶

5.2. Pin Discovery and Preloading

Peers in the federation retrieve these unique public key pins, serving as pre-configured trust parameters, from the federation metadata. The federation MUST facilitate the discovery process, allowing peers to identify the relevant pins for each endpoint. Information such as organization, tags, and descriptions within the federation metadata supports this discovery.¶

Before initiating any connection, clients and servers MUST preload the designated pins from the federation metadata. This aligns with the principle described in Section 2.7 of [RFC7469], which introduces optional sources for pinning information, with the federation metadata serving as one such source. Preloading pins restricts connections to endpoints with matching public keys, mitigating the risks posed by fraudulent certificates.¶

5.3. Verification of Received Certificates

Upon connection establishment, both endpoints, client and server, must either leverage public key pinning or validate the received certificate against the published pins. Additionally, the federation metadata contains issuer information, which implementations MAY optionally use to verify certificate issuers. This step remains at the discretion of each individual implementation.¶

In scenarios where a TLS session terminates independent of the application (e.g., via a reverse proxy), the termination point can utilize optional untrusted TLS client certificate authentication or validate the certificate issuer itself. Depending on the specific implementation, pin validation can then be deferred to the application itself, assuming the peer certificate is appropriately transferred (e.g., via an HTTP header).¶

5.4. Failure to Validate

A received certificate that fails validation MUST result in the immediate termination of the connection. This strict enforcement ensures that only authorized and secure communication channels are established within the federation.¶

5.5. Certificate Rotation:

To replace a certificate, whether due to expiration or other reasons, the following procedure must be followed:¶

Publishing New Metadata: When a certificate needs to be changed, federation members publish new metadata containing the pin (SHA256 thumbprint) of the new public key. This ensures that the new pin is available to all federation members.¶
Propagation Period: Allow time for the updated metadata to propagate throughout the federation before switching to the new certificate. This overlap period ensures that all nodes recognize the new pin and avoid connection issues.¶
Switching to the New Certificate: After ensuring the new metadata has propagated, members switch to the new certificate in their TLS stack.¶
Removing Old Pin: After successfully switching to the new certificate, members must publish updated metadata that excludes the old pin. This final step ensures that only the current public keys are trusted.¶

5.6. Implementation Guidelines

Public key validation MUST always be enforced, either through direct pinning or by deferring validation to the application.¶

For clients, public key validation typically occurs within the application handling the TLS session, either by enforcing direct pinning or by extracting and validating the public key against the published pins.¶

For servers, validation depends on deployment. If the application terminates the TLS session, it performs direct pinning or extracts and validates the public key. If a reverse proxy terminates the TLS session, it can enforce direct pinning or forward the certificate to the application (e.g., via an HTTP header) for validation.¶

Implementations SHOULD, when possible, rely on libraries with native support for pinning. Libcurl, for example, supports pinning via the PINNEDPUBLICKEY option. In Python, the cryptography library can extract public keys, while the requests package together with urllib3 can intercept certificates. Go provides crypto/tls and crypto/x509 for certificate inspection and public key extraction. In Java, java.security.cert.X509Certificate enables public key extraction, while java.net.http.HttpClient allows pinning enforcement using a custom SSLContext and TrustManager. The choice of library is left to the discretion of each implementation.¶

If bypassing standard CA validation is possible, it SHOULD be done. If not, the issuers listed in the federation metadata MUST be used as the trust store to validate certificate issuers while still enforcing key pinning. Without issuer validation against issuers in metadata, self-signed certificates would not be accepted. These mechanisms ensure compatibility with existing TLS infrastructure while maintaining strict security guarantees.¶

6. Federation Metadata

Federation metadata is published as a JWS [RFC7515]. The payload contains statements about federation members entities.¶

Metadata is used for authentication and service discovery. A client selects a server based on metadata claims (e.g., organization, tags). The client then use the selected server claims base_uri, pins and if needed issuers to establish a connection.¶

Upon receiving a connection, a server validates the received client certificate using the client's published pins. Server MAY also check other claims such as organization and tags to determine if the connections is accepted or terminated.¶

6.1. Federation Metadata claims

This section defines the set of claims that can be included in metadata.¶

version (REQUIRED)¶

Indicates the schema version of the federation metadata. This ensures compatibility between members of the federation by defining a clear versioning mechanism for interpreting metadata.¶
- Data Type: String¶
- Syntax: Must adhere to Semantic Versioning (https://semver.org).¶
- Example: 1.0.0¶
cache_ttl (OPTIONAL)¶

Specifies the duration in seconds for caching downloaded federation metadata, allowing for independent caching outside of specific HTTP configurations, particularly useful when the communication mechanism isn't HTTP-based. In the event of a metadata publication outage, members can rely on cached metadata until it expires, as indicated by the exp claim in the JWS header (see Section 6.4). Once expired, metadata MUST no longer be trusted. If omitted, a mechanism to refresh metadata MUST still exist to ensure the metadata remains valid.¶
- Data Type: Integer¶
- Syntax: Integer representing the duration in seconds.¶
- Example: 3600¶
Entities (REQUIRED)¶

Contains the list of entities within the federation.¶
- Data Type: Array of Objects¶
- Syntax: Each object MUST conform the entity definition (see Section Section 6.1.1).¶

6.1.1. Entities

Metadata contains a list of entities that may be used for communication within the federation. Each entity describes one or more endpoints owned by a member. An entity has the following properties:¶

entity_id (REQUIRED)¶

A URI that uniquely identifies the entity. This identifier MUST NOT collide with any other entity_id within the federation or within any other federation that the entity interacts with.¶
- Data Type: URI¶
- Syntax: A valid URI.¶
- Example: "https://example.com"¶
organization (OPTIONAL)¶

A name identifying the organization that the entity's metadata represents. The federation operator MUST ensure a mechanism is in place to verify that the organization claim corresponds to the rightful owner of the information exchanged between nodes. This is crucial for the trust model, ensuring certainty about the identities of the involved parties. The federation operator SHOULD choose an approach that best suits the specific needs and trust model of the federation.¶
- Data Type: String¶
- Syntax: A name identifying the organization represented by the entity.¶
- Example: "Example Org"¶
issuers (REQUIRED)¶

A list of certificate issuers allowed to issue certificates for the entity's endpoints. For each issuer, the issuer's root CA certificate MUST be included in the x509certificate property, PEM-encoded. Certificate verification relies on public key pinning, with the list of allowed issuers used only when a certificate chain validation mechanism is unavoidable. For self-signed certificates, the certificate itself acts as its own issuer and MUST be listed as such in the metadata.¶
- Data Type: List of Objects¶
- Syntax: Each object contains a issuer certificate, PEM-encoded.¶
- Example: Issuer truncated for readability.¶
```
"issuers": [{
  "x509certificate": "-----BEGIN CERTIFICATE-----\nMIIDDD"
}]
```
  ¶
servers (OPTIONAL)¶

Contains the list of servers within the entity.¶
- Data Type: Array of Objects¶
- Syntax: Each object MUST conform to the server definition (see Section 6.1.1.1).¶
clients (OPTIONAL)¶

Contains the list of clients within the entity.¶
- Data Type: Array of Objects¶
- Syntax: Each object MUST conform to the client definition (see Section 6.1.1.1).¶

6.1.1.1. Servers / Clients

A list of the entity's servers and clients.¶

description (OPTIONAL)¶

A human readable text describing the server or client.¶
- Data Type: String¶
- Syntax: Free-form text describing the server or client.¶
- Example: "SCIM Server 1"¶
base_uri (OPTIONAL)¶

The base URL of the server, which is required for endpoints that describe server.¶
- Data Type: URI¶
- Syntax: A valid URL.¶
- Example: "https://scim.example.com/"¶
pins (REQUIRED)¶

A list of objects representing Public Key Pins [RFC7469].¶
- Data Type: Array of Objects¶
- Syntax: A list of objects, where each object represents a single public key pin with the following properties:¶
  - alg (REQUIRED)¶
    
    The name of the cryptographic hash algorithm. Currently, the RECOMMENDED value is 'sha256'. As more secure algorithms are developed over time, federations should be ready to adopt these newer options for enhanced security.¶
    - Data Type: String¶
    - Syntax: The name of the algorithm.¶
    - Example: "sha256"¶
  - digest (REQUIRED)¶
    
    The public key of the end-entity certificate converted to a Subject Public Key Information (SPKI) fingerprint, as specified in section 2.4 of [RFC7469]. For clients, the digest MUST be globally unique for unambiguous identification. However, within the same entity_id object, the same digest MAY be assigned to multiple clients.¶
    - Data Type: String¶
    - Syntax: SPKI fingerprint.¶
    - Example: "+hcmCjJEtLq4BRPhrILyhgn98Lhy6DaWdpmsBAgOLCQ="¶
- Example:¶
```
"pins": [{
  "alg": "sha256",
  "digest": "+hcmCjJEtLq4BRPhrILyhgn98Lhy6DaWdpmsBAgOLCQ="
}]
```
  ¶
tags (OPTIONAL)¶

A list of strings that describe the endpoint's capabilities.¶
- Data Type: Array of Strings¶
- Syntax: Strings describing endpoint capabilities.¶
- Pattern: ^[a-z0-9]{1,64}$¶
- Example: ["scim", "xyzzy"]¶
Tags are fundamental for discovery within a federation, aiding both servers and clients in identifying appropriate connections.¶
- Server Tags: Tags associated with servers are used by clients to discover servers offering the services they require. Clients can search for servers based on tags that indicate supported protocols or the type of data they handle, enabling discovery of compatible servers.¶
- Client Tags: Tags associated with clients are used by servers to identify clients with specific characteristics or capabilities. For instance, a server might only accept connections from clients that support particular protocols. By filtering incoming requests based on these tags, servers can identify suitable clients.¶
Federation-Specific Considerations¶

While tags are tied to individual federations and serve distinct purposes within each, several key considerations are crucial to ensure clarity and promote consistent tag usage:¶
- Well-Defined Scope: Each federation MUST establish a clear scope for its tags, detailing their intended use, allowed tag values, associated meanings, and any relevant restrictions. Maintaining a well-defined and readily accessible registry of approved tags is essential for the federation.¶
- Validation Mechanisms: Implementing validation mechanisms for tags is highly recommended. This may involve a dedicated operation or service verifying tag validity and compliance with the federation's regulations. Such validation ensures consistency within the federation by preventing the use of unauthorized or irrelevant tags.¶

6.2. Metadata Schema

The FedTLS metadata schema is defined in Appendix A. This schema specifies the format for describing entities involved in FedTLS and their associated information.¶

Note: The schema in Appendix A is folded due to line length limitations as specified in [RFC8792].¶

6.3. Example Metadata

The following is a non-normative example of a metadata statement. Line breaks within the issuers' claim is for readability only.¶

{
  "version": "1.0.0",
  "cache_ttl": 3600,
  "entities": [{
    "entity_id": "https://example.com",
    "organization": "Example Org",
    "issuers": [{
      "x509certificate": "-----BEGIN CERTIFICATE-----\nMIIDDDCCAf
      SgAwIBAgIJAIOsfJBStJQhMA0GCSqGSIb3DQEBCwUAMBsxGTAXBgNV\nBAM
      MEHNjaW0uZXhhbXBsZS5jb20wHhcNMTcwNDA2MDc1MzE3WhcNMTcwNTA2MD
      c1\nMzE3WjAbMRkwFwYDVQQDDBBzY2ltLmV4YW1wbGUuY29tMIIBIjANBgk
      qhkiG9w0B\nAQEFAAOCAQ8AMIIBCgKCAQEAyr+3dXTC8YXoi0LDJTH0lTfv
      8omQivWFOr3+/PBE\n6hmpLSNXK/EZJBD6ZT4Q+tY8dPhyhzT5RFZCVlrDs
      e/kY00F4yoflKiqx9WSuCrq\nZFr1AUtIfGR/LvRUvDFtuHo1MzFttiK8Wr
      wskMYZrw1zLHTIVwBkfMw1qr2XzxFK\njt0CcDmFxNdY5Q8kuBojH9+xt5s
      ZbrJ9AVH/OI8JamSqDjk9ODyGg+GrEZFClP/B\nxa4Fsl04En/9GfaJnCU1
      NpU0cqvWbVUlLOy8DaQMN14HIdkTdmegEsg2LR/XrJkt\nho16diAXrgS25
      3xbkdD3T5d6lHiZCL6UxkBh4ZHRcoftSwIDAQABo1MwUTAdBgNV\nHQ4EFg
      QUs1dXuhGhGc2UNb7ikn3t6cBuU34wHwYDVR0jBBgwFoAUs1dXuhGhGc2U\
      nNb7ikn3t6cBuU34wDwYDVR0TAQH/BAUwAwEB/zANBgkqhkiG9w0BAQsFAA
      OCAQEA\nrR9wxPhUa2XfQ0agAC0oC8TFf8wbTYb0ElP5Ej834xMMW/wWTSA
      N8/3WqOWNQJ23\nf0vEeYQwfvbD2fjLvYTyM2tSPOWrtQpKuvulIrxV7Zz8
      A61NIjblE3rfea1eC8my\nTkDOlMKV+wlXXgUxirride+6ubOWRGf92fgze
      DGJWkmm/a9tj0L/3e0xIXeujxC7\nMIt3p99teHjvnZQ7FiIBlvGc1o8FD1
      FKmFYd74s7RxrAusBEAAmBo3xyB89cFU0d\nKB2fkH2lkqiqkyOtjrlHPoy
      6ws6g1S6U/Jx9n0NEeEqCfzXnh9jEpxisSO+fBZER\npCwj2LMNPQxZBqBF
      oxbFPw==\n-----END CERTIFICATE-----"
    }],
    "servers": [{
      "description": "SCIM Server 1",
      "base_uri": "https://scim.example.com/",
      "pins": [{
        "alg": "sha256",
        "digest": "+hcmCjJEtLq4BRPhrILyhgn98Lhy6DaWdpmsBAgOLCQ="
      }],
      "tags": [
        "scim"
      ]
    }],
    "clients": [{
      "description": "SCIM Client 1",
      "pins": [{
        "alg": "sha256",
        "digest": "+hcmCjJEtLq4BRPhrILyhgn98Lhy6DaWdpmsBAgOLCQ="
      }]
    }]
  }]
}

6.4. Metadata Signing

The federation metadata is signed with JWS and published using JWS JSON Serialization according to the General JWS JSON Serialization Syntax defined in [RFC7515]. The Federation metadata signatures are RECOMMENDED to be created using the algorithm ECDSA using P-256 and SHA-256 ("ES256") as defined in [RFC7518]. However, to accommodate evolving cryptographic standards, alternative algorithms MAY be used, provided they meet the security requirements of the federation.¶

The following federation metadata signature protected headers are REQUIRED:¶

alg (Algorithm)¶

Identifies the algorithm used to generate the JWS signature [RFC7515], section 4.1.1.¶
iat (Issued At)¶

Identifies the time on which the signature was issued. Its value MUST be a number containing a NumericDate value [RFC7519], section 4.1.6.¶
exp (Expiration Time)¶

Identifies the expiration time on and after which the signature and federation metadata are no longer valid. The expiration time of the federation metadata MUST be set to the value of exp. Its value MUST be a number containing a NumericDate value [RFC7519], section 4.1.4.¶
iss (Issuer)¶

A URI uniquely identifying the issuing federation, playing a critical role in establishing trust and securing interactions within the FedTLS framework. The iss claim differentiates federations, preventing ambiguity and ensuring entities are recognized within their intended context. Verification of the iss claim enables determining the origin of information and establishing trust with entities within the identified federation [RFC7519], section 4.1.1.¶
kid (Key Identifier)¶

The key ID is used to identify the signing key in the key set used to sign the JWS [RFC7515], section 4.1.4.¶

6.5. Example Signature Protected Header

The following is a non-normative example of a signature protected header.¶

{
    "alg": "ES256",
    "exp": 1707739718,
    "iat": 1706875718,
    "iss": "https://fedtls.example.com",
    "kid": "c2fb760e-f4b6-4f7e-b17a-7115d2826d51"
}

7. Example Usage Scenarios

The examples in this section are non-normative.¶

The following example describes a scenario within the federation "Skolfederation" where FedTLS is already established. Both clients and servers are registered members of the federation. In this scenario, clients aim to manage cross-domain user accounts within the service. The standard used for account management is SS 12000:2018 (i.e., a SCIM extension).¶

+---------------------------------------------+
|                                             |
|             Federation Metadata             |
|                                             |
+---+--------------------------+--------------+
    |                          |
   (A)                        (A)
    |                          |
    v                          v
+---+----+        +------------+--------------+
|Local MD|        |         Local MD          |
+---+----+        +----+------------- ---+----+
    |                  |                 |
   (B)                (C)               (F)
    |                  |                 |
    v                  v                 v
+---+----+        +----+---+        +----+---+
|        |        |        |        |        |
| Client |        | Reverse|        |  App   |
|        +--(D)-->+ Proxy  +--(E)-->+        |
|        |        |        |        |        |
|        |        |        |        |        |
+--------+        +--------+        +--------+

Entities collect member metadata from the federation metadata.¶
The client pins the server's public key pins.¶
The reverse proxy trust anchor is setup with the clients' certificate issuers.¶
The client establishes a connection with the server using the base_uri from the federation metadata.¶
The reverse proxy forwards the client certificate to the application.¶
The application converts the certificate to a public key pin and checks the federation metadata for a matching pin. The entity's entity_id should be used as an identifier.¶

7.1. Client

A certificate is issued for the client and the issuer is published in the federation metadata together with the client's certificate public key pins¶

When the client wants to connect to a remote server (identified by an entity identifier) the following steps need to be taken:¶

Find possible server candidates by filtering the remote entity's list of servers based on tags.¶
Connect to the server URI. Include the entity's list of certificate issuers in the TLS clients list of trusted CAs, or trust the listed pins explicitly.¶
If pinning was not used, validate the received server certificate using the entity's published pins.¶
Commence transactions.¶

7.2. Server

A certificate is issued for the server and the issuer is published in the federation metadata together with the server's name and certificate public key pin.¶

When the server receives a connection from a remote client, the following steps need to be taken:¶

Populate list of trusted CAs using all known entities' published issuers and required TLS client certificate authentication, or configure optional untrusted TLS client certificate authentication (e.g., optional_no_ca).¶
Once a connection has been accepted, validate the received client certificate using the client's published pins.¶
Commence transactions.¶

7.3. SPKI Generation

Example of how to use OpenSSL to generate a SPKI fingerprint from a PEM-encoded certificate.¶

  openssl x509 -in <certificate.pem> -pubkey -noout | \
  openssl pkey -pubin -outform der | \
  openssl dgst -sha256 -binary | \
  openssl enc -base64

7.4. Curl and Public Key Pinning

Example of public key pinning with curl. Line breaks are for readability only.¶

  curl --cert client.pem --key client.key --pinnedpubkey 'sha256//0Ok
  2aNfcrCNDMhC2uXIdxBFOvMfEVtzlNVUT5pur0Dk=' https://host.example.com

8. Deployments of the FedTLS Framework

The FedTLS framework has proven its practical value and robustness through successful deployments in several environments.¶

8.1. Skolfederation Moa

Skolfederation Moa [Moa], is a federation designed to secure communication between digital educational resources and schools. FedTLS is developed to meet Moa's needs and enables secure data exchange for schools, municipalities, educational platforms, and services across Sweden.¶

The community plays a crucial role in this type of federation. Members are active participants, and the FO ensures the federation runs smoothly and serves their needs. Moa's success highlights the importance of collaboration, with members and the FO working together to maintain trust, security, and interoperability in the education sector.¶

The deployment of FedTLS in the Swedish education sector has provided several key insights. Maintaining an accurate registry of metadata ownership with reliable contact information is essential for troubleshooting and ensuring accountability. The deployment also demonstrated the importance of setting reasonable expiration times for metadata. Too short an expiration can hinder the ability to implement contingency plans for publishing new metadata during outages.¶

Metadata validation is necessary to maintain a stable federation. While manual validation may be sufficient in the early stages of a federation, it becomes unmanageable as the federation scales. Without an automated validation process, incorrect metadata uploaded by members is likely to go undetected, leading to publication of incorrect metadata.¶

The signing key is needed to sign metadata. Under fallback scenarios, even if metadata can be retrieved from elsewhere, without access to the signing key, it is impossible to publish metadata. Therefore, secure and redundant management of the signing key is crucial to enable fallback mechanisms and ensure reliable signing and distribution of metadata. If metadata is retrieved from a location other than the official repository, it is mandatory to validate its signature to maintain trust and ensure the authenticity of the metadata.¶

8.2. Swedish National Agency for Education

The Swedish National Agency for Education [SkolverketFedTLS] leverages FedTLS within its digital national test platform to establish a robust authentication mechanism. The platform utilizes an API for client verification prior to secure data transfer to the agency's test service, ensuring the integrity and confidentiality of educational data.¶

8.3. Sambruk's EGIL

Sambruk's EGIL [EGIL], a platform providing digital services to municipalities, has successfully integrated the FedTLS framework. This deployment demonstrates the framework's adaptability to support a wide range of digital service infrastructures.¶

These deployments highlight the effectiveness of the FedTLS framework in enhancing security and interoperability within the educational sector.¶

Appendix A. JSON Schema for FedTLS Metadata

This JSON schema defines the format of FedTLS metadata.¶

Version: 1.0.0¶

=============== NOTE: '\\' line wrapping per RFC 8792 ===============

{
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "$id": "https://www.fedtls.se/schema/fedtls-metadata-schema.json\
\",
    "title": "JSON Schema for Federated TLS Authentication",
    "description": "Version: 1.0.0",
    "type": "object",
    "additionalProperties": true,
    "required": [
        "version",
        "entities"
    ],
    "properties": {
        "version": {
            "title": "Metadata schema version",
            "description": "Schema version follows semantic versioni\
\ng (https://semver.org)",
            "type": "string",
            "pattern": "^\\d+\\.\\d+\\.\\d+$",
            "examples": [
                "1.0.0"
            ]
        },
        "cache_ttl": {
            "title": "Metadata cache TTL",
            "description": "How long (in seconds) to cache metadata.\
\ Effective maximum TTL is the minimum of HTTP Expire and TTL",
            "type": "integer",
            "minimum": 0,
            "examples": [
                3600
            ]
        },
        "entities": {
            "type": "array",
            "items": {
                "$ref": "#/components/entity"
            }
        }
    },
    "components": {
        "entity": {
            "type": "object",
            "additionalProperties": true,
            "required": [
                "entity_id",
                "issuers"
            ],
            "properties": {
                "entity_id": {
                    "title": "Entity identifier",
                    "description": "Globally unique identifier for t\
\he entity.",
                    "type": "string",
                    "format": "uri",
                    "examples": [
                        "https://example.com"
                    ]
                },
                "organization": {
                    "title": "Name of entity organization",
                    "description": "Name identifying the organizatio\
\n that the entity's metadata represents.",
                    "type": "string",
                    "examples": [
                        "Example Org"
                    ]
                },
                "issuers": {
                    "title": "Entity certificate issuers",
                    "description": "A list of certificate issuers th\
\at are allowed to issue certificates for the entity's endpoints. Fo\
\r each issuer, the issuer's root CA certificate is included in the \
\x509certificate property (PEM-encoded).",
                    "type": "array",
                    "items": {
                        "$ref": "#/components/cert_issuers"
                    }
                },
                "servers": {
                    "type": "array",
                    "items": {
                        "$ref": "#/components/endpoint"
                    }
                },
                "clients": {
                    "type": "array",
                    "items": {
                        "$ref": "#/components/endpoint"
                    }
                }
            }
        },
        "endpoint": {
            "type": "object",
            "additionalProperties": true,
            "required": [
                "pins"
            ],
            "properties": {
                "description": {
                    "title": "Endpoint description",
                    "type": "string",
                    "examples": [
                        "SCIM Server 1"
                    ]
                },
                "tags": {
                    "title": "Endpoint tags",
                    "description": "A list of strings that describe \
\the endpoint's capabilities.",
                    "type": "array",
                    "items": {
                        "type": "string",
                        "pattern": "^[a-z0-9]{1,64}$",
                        "examples": [
                            "xyzzy"
                        ]
                    }
                },
                "base_uri": {
                    "title": "Endpoint base URI",
                    "type": "string",
                    "format": "uri",
                    "examples": [
                        "https://scim.example.com"
                    ]
                },
                "pins": {
                    "title": "Certificate pin set",
                    "type": "array",
                    "items": {
                        "$ref": "#/components/pin_directive"
                    }
                }
            }
        },
        "cert_issuers": {
            "title": "Certificate issuers",
            "type": "object",
            "additionalProperties": false,
            "properties": {
                "x509certificate": {
                    "title": "X.509 Certificate (PEM)",
                    "type": "string"
                }
            }
        },
        "pin_directive": {
            "title": "RFC 7469 pin directive",
            "type": "object",
            "additionalProperties": false,
            "required": [
                "alg",
                "digest"
            ],
            "properties": {
                "alg": {
                    "title": "Directive name",
                    "type": "string",
                    "enum": [
                        "sha256"
                    ],
                    "examples": [
                        "sha256"
                    ]
                },
                "digest": {
                    "title": "Directive value (Base64)",
                    "type": "string",
                    "pattern": "^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+\
\/]{2}==|[A-Za-z0-9+/]{3}=)?$",
                    "examples": [
                        "HiMkrb4phPSP+OvGqmZd6sGvy7AUn4k3XEe8OMBrzt8\
\="
                    ]
                }
            }
        }
    }
}