Internet DRAFT - draft-halen-fed-tls-auth
draft-halen-fed-tls-auth
Network Working Group J. Schlyter
Internet-Draft Kirei AB
Intended status: Informational S. Halen
Expires: 19 August 2024 The Swedish Internet Foundation
16 February 2024
Federated TLS Authentication
draft-halen-fed-tls-auth-08
Abstract
This document describes the Federated TLS Authentication (FedTLS)
protocol, enabling secure end-to-end communication within a federated
environment. 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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 19 August 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
Schlyter & Halen Expires 19 August 2024 [Page 1]
Internet-Draft FedTLS February 2024
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Reserved Words . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Federation Chain of Trust . . . . . . . . . . . . . . . . . . 4
3. Authentication . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Public Key Pinning . . . . . . . . . . . . . . . . . . . 5
3.2. Pin Discovery and Preloading . . . . . . . . . . . . . . 5
3.3. Verification of Received Certificates . . . . . . . . . . 5
3.4. Failure to Validate . . . . . . . . . . . . . . . . . . . 6
4. Federation Metadata . . . . . . . . . . . . . . . . . . . . . 6
4.1. Federation Metadata claims . . . . . . . . . . . . . . . 6
4.1.1. Entities . . . . . . . . . . . . . . . . . . . . . . 6
4.2. Metadata Schema . . . . . . . . . . . . . . . . . . . . . 8
4.3. Metadata Signing . . . . . . . . . . . . . . . . . . . . 8
4.4. Metadata Example . . . . . . . . . . . . . . . . . . . . 9
5. Usage Examples . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3. SPKI Generation . . . . . . . . . . . . . . . . . . . . . 12
5.4. Curl and Public Key Pinning . . . . . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
6.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. Federation Metadata Updates . . . . . . . . . . . . . . . 13
6.3. Verifying the Federation Metadata Signature . . . . . . . 13
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
9. Normative References . . . . . . . . . . . . . . . . . . . . 14
10. Informative References . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
This document outlines the Federated TLS Authentication (FedTLS)
protocol, which facilitates secure end-to-end communication between
two parties within a federation. Both the client and server undergo
mutual TLS authentication (as defined in [RFC8446]), establishing a
robust foundation of trust. This trust relies on a central trust
anchor held and published by the federation, acting as a trusted
third party connecting distinct trust domains under a common set of
policies and standards.
Schlyter & Halen Expires 19 August 2024 [Page 2]
Internet-Draft FedTLS February 2024
The FedTLS framework leverages a centralized repository of federation
metadata to ensure secure communication between servers and clients
within the federation. This repository includes information about
public keys, certificate issuers, and additional entity details, such
as organizational information and service descriptions. This
centralized approach simplifies certificate management, promotes
interoperability, and establishes trust within the federation. By
directly accessing the federation metadata, efficient connections are
established, eliminating manual configuration even for new
interactions.
Without a FedTLS federation, implementing mutual TLS authentication
often requires organizations to establish their own PKI
infrastructure (or rely on third-party CAs) to issue and validate
client certificates, leading to complexity and administrative burden.
FedTLS allows the use of self-signed certificates, potentially
reducing costs and administrative overhead. While self-signed
certificates inherently lack the trust level of certificates issued
by trusted CAs, the strong trust within the FedTLS framework is
established through several mechanisms, including public key pinning
[RFC7469] and member vetting procedures. This ensures the validity
and authenticity of self-signed certificates within the federation,
fostering secure communication without compromising trust.
The Swedish education sector illustrates the value of FedTLS by
securing user lifecycle management endpoints through this framework.
This successful collaboration between school authorities and service
providers highlights FedTLS's ability to enable trust, simplify
operations, and strengthen security within federated environments.
1.1. Reserved Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
1.2. Terminology
Federation: A trusted network of entities that adhere to common
security policies and standards,using FedTLS for secure
communication.
Federation Metadata: A centralized repository storing critical
information about all entities within the federation.
Member Metadata: Information about entities associated with a
specific member within the federation.
Schlyter & Halen Expires 19 August 2024 [Page 3]
Internet-Draft FedTLS February 2024
Federation Member: An entity that has been approved to join the
federation and can leverage FedTLS for secure communication with
other members.
Federation Operator: The entity responsible for the overall
operation and management of the federation, including managing the
federation metadata, enforcing security policies, and onboarding
new members.
Member Vetting: The process of verifying and approving
applicants to join the federation, ensuring they meet security and
trustworthiness requirements.
Trust Anchor: The federation's root of trust is established by
the federation metadata signing certificate, which verifies the
federation metadata and allows participants to confidently rely on
the information it contains.
2. Federation Chain of Trust
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. The specific methods
for achieving this are beyond the scope of this document.
The federation operator aggregates, signs, and publishes the
federation metadata, which combines all members' member metadata with
some additional information added by the federation. By trusting the
federation and its certificate, federation members trust the
information contained within the federation metadata.
The trust anchor for the federation is established through the
federation's signing certificate, which in turn needs to be securely
distributed and verified. The distribution and verification methods
for the federation's certificate are outside the scope of this
document.
3. Authentication
All communication established within the federation leverages mutual
Transport Layer Security (TLS) authentication, as defined in
[RFC8446]. This mechanism ensures the authenticity of both
communicating parties, establishing a robust foundation for secure
data exchange.
Schlyter & Halen Expires 19 August 2024 [Page 4]
Internet-Draft FedTLS February 2024
3.1. Public Key Pinning
To further fortify this trust and mitigate risks associated with
fraudulent certificates issued by unauthorized entities, the
federation implements public key pinning as specified in [RFC7469].
Public key pinning associates a unique public key with each endpoint
within the federation, stored in the federation metadata. During
connection establishment, clients and servers validate the received
certificate against the pre-configured public key pins retrieved from
the federation metadata. This effectively thwarts attempts to
utilize fraudulent certificates impersonating legitimate endpoints.
3.2. 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,
enabling peers to identify the relevant pins for each endpoint.
Information such as organization, tags, and descriptions within the
federation metadata aids in this discovery.
Before initiating any connection, both clients and servers preload
the chosen pins in strict adherence to the guidelines outlined in
section 2.7 of [RFC7469]. This preloading ensures connections only
occur with endpoints possessing matching public keys, effectively
blocking attempts to use fraudulent certificates.
3.3. 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).
Schlyter & Halen Expires 19 August 2024 [Page 5]
Internet-Draft FedTLS February 2024
3.4. Failure to Validate
It is crucial to note that failure to validate a received certificate
against the established parameters, whether through pinning or issuer
verification, results in immediate termination of the connection.
This strict approach ensures only authorized and secure communication
channels are established within the federation.
4. 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
select 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.
4.1. Federation Metadata claims
This section defines the set of claims that can be included in
metadata.
* version (REQUIRED)
Schema version follows semantic versioning (https://semver.org
(https://semver.org))
* cache_ttl (OPTIONAL)
The duration (in seconds) to cache the downloaded federation
metadata.
* Entities (REQUIRED)
List of entities (see Section 4.1.1)
4.1.1. 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:
Schlyter & Halen Expires 19 August 2024 [Page 6]
Internet-Draft FedTLS February 2024
* entity_id (REQUIRED)
URI that identifies the entity. It MUST be globally unique.
Example: "https://example.com" (https://example.com")
* organization (OPTIONAL)
A name identifying the organization that the entity's metadata
represents.
Example: “Example Org”.
* issuers (REQUIRED)
A list of certificate issuers that are allowed to issue
certificates for the entity's endpoints. For each issuer, the
issuer's root CA certificate is included in the x509certificate
property (PEM-encoded).
* servers (OPTIONAL)
List of the entity's servers (see Section 4.1.1.1).
* clients (OPTIONAL)
List of the entity's clients (see Section 4.1.1.1).
4.1.1.1. Servers / Clients
A list of the entity's servers and clients.
* description (OPTIONAL)
A human readable text describing the server or client.
Example: "SCIM Server 1"
* base_uri (OPTIONAL)
The base URL of the server (hence required for endpoints
describing servers).
Example: "https://scim.example.com/" (https://scim.example.com/")
* pins (REQUIRED)
Schlyter & Halen Expires 19 August 2024 [Page 7]
Internet-Draft FedTLS February 2024
A list of Public Key Pins [RFC7469]. Each pin has the following
properties:
- alg (REQUIRED)
The name of the cryptographic hash algorithm. The only allowed
value is "sha256".
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.
Example: "+hcmCjJEtLq4BRPhrILyhgn98Lhy6DaWdpmsBAgOLCQ="
* tags (OPTIONAL)
A list of strings that describe the endpoint's capabilities.
Pattern: ^[a-z0-9]{1,64}$
Example: ["scim", "xyzzy"]
4.2. Metadata Schema
The metadata JSON schema can be found at https://www.fedtls.se/schema
(https://www.fedtls.se/schema).
4.3. 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]. It is RECOMMENDED that federation
metadata signatures are created with algorithm _ECDSA using P-256 and
SHA-256_ ("ES256") as defined in [RFC7518].
The following federation metadata signature protected headers are
REQUIRED:
* alg (Algorithm)
Schlyter & Halen Expires 19 August 2024 [Page 8]
Internet-Draft FedTLS February 2024
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.
* 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.
* 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, along with the corresponding issuer's certificate, enables
relying parties to confidently determine information origin and
establish trust with entities within the identified federation.
This ensures secure communication and mitigates potential security
risks.
* kid (Key Identifier)
The key ID is used to identify the signing key in the key set used
to sign the JWS.
4.4. Metadata Example
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
Schlyter & Halen Expires 19 August 2024 [Page 9]
Internet-Draft FedTLS February 2024
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="
}]
}]
}]
}
5. Usage Examples
The examples in this section are non-normative.
Schlyter & Halen Expires 19 August 2024 [Page 10]
Internet-Draft FedTLS February 2024
The example below is from the federation called "Skolfederation"
where federated TLS authentication is already in use. Clients and
servers are registered in the federation. The clients intend 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)-->+ |
| | | | | |
| | | | | |
+--------+ +--------+ +--------+
A. Entities collect member metadata from the federation metadata.
B. The client pins the server's public key pins.
C. The reverse proxy trust anchor is setup with the clients’
certificate issuers.
D. The client establishes a connection with the server using the
base_uri from the federation metadata.
E. The reverse proxy forwards the client certificate to the
application.
F. 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.
Schlyter & Halen Expires 19 August 2024 [Page 11]
Internet-Draft FedTLS February 2024
5.1. 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:
1. Find possible server candidates by filtering the remote entity's
list of servers based on tags.
2. 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.
3. If pinning was not used, validate the received server certificate
using the entity's published pins.
4. Commence transactions.
5.2. 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:
1. 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).
2. Once a connection has been accepted, validate the received client
certificate using the client's published pins.
3. Commence transactions.
5.3. SPKI Generation
Example of how to use OpenSSL to generate a SPKI fingerprint from a
PEM-encoded certificate.
Schlyter & Halen Expires 19 August 2024 [Page 12]
Internet-Draft FedTLS February 2024
openssl x509 -in <certificate.pem> -pubkey -noout | \
openssl pkey -pubin -outform der | \
openssl dgst -sha256 -binary | \
openssl enc -base64
5.4. Curl and Public Key Pinning
Example of public key pinning with curl. Line breaks are for
readability only.
curl --cert client.pem --client.key --pinnedpubkey 'sha256//0Ok2aNf
crCNDMhC2uXIdxBFOvMfEVtzlNVUT5pur0Dk=' https://host.example.com
6. Security Considerations
6.1. TLS
The security considerations for TLS 1.3 [RFC8446] are detailed in
Section 10, along with Appendices C, D, and E of RFC 8446.
6.2. Federation Metadata Updates
Regularly updating the local copy of federation metadata is essential
for accessing the latest information about active entities, current
public key pins, and valid certificates. The use of outdated
metadata may expose systems to security risks, such as interaction
with revoked entities or acceptance of manipulated data. If
specified in the federation metadata, cache_ttl values SHOULD be
respected.
6.3. Verifying the Federation Metadata Signature
Ensuring data integrity and security within the FedTLS framework
relies on verifying the signature of downloaded federation metadata.
This process confirms the data's origin, validating that it comes
from the intended source and has not been altered by unauthorized
parties. Through the process of verifying the metadata's
authenticity, trust is established in the information it contains,
including valid member certificates and public key pins.
7. IANA Considerations
This document has no IANA actions.
Schlyter & Halen Expires 19 August 2024 [Page 13]
Internet-Draft FedTLS February 2024
8. Acknowledgements
This project was funded through the NGI0 PET Fund, a fund established
by NLnet with financial support from the European Commission’s Next
Generation Internet programme, under the aegis of DG Communications
Networks, Content and Technology under grant agreement No 825310.
The authors would like to thank the following people for the detailed
review and suggestions:
* Rasmus Larsson
* Mats Dufberg
* Joe Siltberg
* Stefan Norberg
* Petter Blomberg
The authors would also like to thank participants in the EGIL working
group for their comments on this specification.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7469] Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
2015, <https://www.rfc-editor.org/info/rfc7469>.
[RFC7515] Jones, M., Bradley, J., and N. Sakimura, "JSON Web
Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
10. Informative References
[RFC7518] Jones, M., "JSON Web Algorithms (JWA)", RFC 7518,
DOI 10.17487/RFC7518, May 2015,
<https://www.rfc-editor.org/info/rfc7518>.
Schlyter & Halen Expires 19 August 2024 [Page 14]
Internet-Draft FedTLS February 2024
Authors' Addresses
Jakob Schlyter
Kirei AB
Email: jakob@kirei.se
Stefan Halen
The Swedish Internet Foundation
Email: stefan.halen@internetstiftelsen.se
Schlyter & Halen Expires 19 August 2024 [Page 15]
