Federated TLS Authentication
draft-halen-fed-tls-auth-01
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Jakob Schlyter
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=?utf-8?q?Stefan_Hal=C3=A9n?=
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2021-04-12
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Network Working Group J. Schlyter
Internet-Draft Kirei AB
Intended status: Informational S. Halen
Expires: 14 October 2021 The Swedish Internet Foundation
12 April 2021
Federated TLS Authentication
draft-halen-fed-tls-auth-01
Abstract
This document describes how to establish a secure end-to-end channel
between two parties within a federation, where both client and server
are mutually authenticated. The trust relationship is based upon a
trust anchor held and published by the federation. A federation is a
trusted third party that inter-connect different trust domains with a
common set of policies and standards.
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
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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 14 October 2021.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Reserved Words . . . . . . . . . . . . . . . . . . . . . 3
2. Federation Chain of Trust . . . . . . . . . . . . . . . . . . 3
3. Authentication . . . . . . . . . . . . . . . . . . . . . . . 3
4. Federation Metadata . . . . . . . . . . . . . . . . . . . . . 4
4.1. Federation Metadata claims . . . . . . . . . . . . . . . 4
4.1.1. Entities . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Metadata Schema . . . . . . . . . . . . . . . . . . . . . 6
4.3. Metadata Signing . . . . . . . . . . . . . . . . . . . . 6
4.4. Metadata Example . . . . . . . . . . . . . . . . . . . . 7
5. Usage Examples . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Client . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Server . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.3. SPKI Generation . . . . . . . . . . . . . . . . . . . . . 10
5.4. Curl and Public Key Pinning . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6.1. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.2. Federation Metadata Updates . . . . . . . . . . . . . . . 11
6.3. Federation Metadata Signing Key . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
9. Normative References . . . . . . . . . . . . . . . . . . . . 12
10. Informative References . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
This document describes how to establish a secure end-to-end channel
between two parties within a federation, where both client and server
are mutually authenticated (TLS [RFC8446]). The trust relationship
is based upon a trust anchor held and published by the federation. A
federation is a trusted third party that inter-connect different
trust domains with a common set of policies and standards. The
federation aggregates and publishes information ("federation
metadata") about all the federated entities including certificate
issuers and public key information. When the term "federation
metadata" is used in this document, it always refer to the aggregated
information published by a federation in the sense of this document.
The federation provides a common framework for providing endpoint
information. When two parties establish a connection, the
information of the other endpoint is retrieved from the metadata.
The parties leverage this every time they commence a transaction with
a new entity in the federation.
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Mutual TLS authentication involves provisioning of key material.
This key exchange is performed through the publication of the
federation metadata by the federation and the use of that federation
metadata by its members. Without a federation, the server side is
often required to create a public key infrastructure (PKI) in order
to distribute client certificates.The Swedish education sector uses
federated TLS authentication to secure endpoints used for user
lifecycle management . That Federation is a collaboration between
school authorities and service providers. If the certificate
distribution would be handled without a federated framework, it would
mean a higher administrative burden and a higher risk of compromised
security.
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].
2. Federation Chain of Trust
The members of a federation submit their issuer certificates and
other requested data (in this document called "member metadata") to
the federation. Both the authenticity of the submitted member
metadata and the submitting member MUST be assured by the federation.
How this is achieved is out of scope for this document. The
federation operator aggregates, signs and publishes the federation
metadata, i.e., an aggregation of all members' member metadata and
some additional information added by the federation. By trusting the
federation and its certificate, federation members trust the
federation metadata content.
The root of the chain of trust is the federation metadata signature
and the trust anchor is the federation's signing certificate. That
certificate needs to be securely distributed, there MUST be an out-
of-band function to verify the certificate. The distribution of the
federation's certificate is out-of-scope of this document.
3. Authentication
All TLS sessions between clients and servers are authenticated via
mutual TLS authentication. Trust is limited to the set of public key
pins published for each endpoint in the federation metadata. Public
key pinning associates a public key with an endpoint to reduce the
risk of attacks with rogue certificates.
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Upon connection, the endpoints (client and server) MUST validate the
other endpoint's certificate against the published matching public
key pin. Issuers in metadata are only used to help validate the
server and client certificate. It is up to each implementation to
decide if these are needed. Failure to validate triggers termination
of the connection.
If a TLS session is terminated separately from the application (e.g.,
when using a reverse proxy), the TLS session termination point can
validate the certificate issuer and defer public key pin matching to
the application given that the peer certificate is transferred to the
application (e.g., via a HTTP header).
4. Federation Metadata
Federation metadata is published as an JWS [RFC7519]. Entities have
an organization claim that is used for identification. Server
endpoints include a base URI to connect to the endpoint. Servers and
clients also have a list of public key pins used to limit valid
endpoint certificates.
Public key pinning is defined in [RFC7469]. Clients and servers
preloads pins as defined in [RFC7469], section 2.7
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)
How long (in seconds) to cache metadata. Effective maximum TTL is
the minimum of HTTP Expire and TTL
* Entities (REQUIRED)
List of entities (see Section 4.1.1)
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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:
* 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.
Example: "SCIM Server 1"
* base_uri (OPTIONAL)
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The base URL of the server (hence required for endpoints
describing servers).
Example: "https://scim.example.com/" (https://scim.example.com/")
* pins (REQUIRED)
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)
End-entity certificate base64 encoded Subject Public Key
Information (SPKI) fingerprint [RFC7469], for client the digest
MUST be globally unique. MAY, locally in the same entity_id
object, 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 federation metadata JSON schema (in YAML format) can be found at
https://github.com/dotse/tls-fed-auth (https://github.com/dotse/tls-
fed-auth/blob/master/tls-fed-metadata.yaml).
4.3. Metadata Signing
The federation metadata is signed with JWS and published using JWS
JSON Serialization. It is RECOMMENDED that federation metadata
signatures are created with algorithm _ECDSA using P-256 and SHA-256_
("ES256") as defined in [RFC7518].
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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.
* "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)
URI that identifies the publisher of federation metadata. The
issuer claim MUST be used to prevent conflicts of entities of the
same name from different federations.
* "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 federation 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
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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.
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).
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+---------------------------------------------+
| |
| 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.
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
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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.
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.
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curl --cert client.pem --client.key --pinnedpubkey 'sha256//0Ok2aNf
crCNDMhC2uXIdxBFOvMfEVtzlNVUT5pur0Dk=' https://host.example.com
6. Security Considerations
6.1. TLS
Security considerations for TLS 1.3 [RFC8446] are described in
Section 10 and Appendices C, D and E of RFC 8446.
6.2. Federation Metadata Updates
Frequent check of the federation metadata aggregate to use the most
recent version is required. It is required to check the federation
metadata periodically to find out if entities, pins and issuers are
still active.
6.3. Federation Metadata Signing Key
To ensure the validity of the federation metadata the refresh process
must verify the signature on each and every federation metadata
fetch. The federation's public key authenticity must be assured and
verified in a secure way.
7. IANA Considerations
This document has no IANA actions.
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
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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>.
[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<https://www.rfc-editor.org/info/rfc7519>.
[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>.
Authors' Addresses
Jakob Schlyter
Kirei AB
Email: jakob@kirei.se
Stefan Halen
The Swedish Internet Foundation
Email: stefan.halen@internetstiftelsen.se
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