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Improve the server key exchange portion of the s2s specification

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Most of the text has been shuffled into the swagger definitions to bring it closer to where it matters.

This also attempts to clarify what is out in the wild. Most importantly, the first version of the key exchange is outright removed from the specification. Other research points/questions are:

* What is a "Key ID"?
  * 1241156c82/synapse/rest/key/v2/local_key_resource.py (L81-L83)
  * 1241156c82/synapse/rest/key/v2/local_key_resource.py (L88-L91)
* Returning a cached response if the server throws a 400, 500, or otherwise not-offline status code
  * 1241156c82/synapse/rest/key/v2/remote_key_resource.py (L227-L229)
* `minimum_valid_until_ts` default
  * This branch of the ladder: 1241156c82/synapse/rest/key/v2/remote_key_resource.py (L192)
* Returning empty arrays when querying offline/no servers
  * Queried by hand against matrix.org as a notary server with a fake domain name to query
* Returning all keys even when querying for specific keys
  * Queried by hand using matrix.org as a notary server against a server publishing multiple keys.

The examples and descriptions were also improved as part of this commit.
This commit is contained in:
Travis Ralston 2018-07-18 10:51:28 -06:00
parent b0744aa1e9
commit 8e97b0ca81
7 changed files with 143 additions and 149 deletions
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46
api/server-server/definitions/keys.yaml
View file

@ -20,50 +20,60 @@ properties:
server_name:
type: string
description: DNS name of the homeserver.
required: true # TODO: Verify
required: true
example: "example.org"
verify_keys:
type: object
description: Public keys of the homeserver for verifying digital signatures.
required: true # TODO: Verify
description: |-
Public keys of the homeserver for verifying digital signatures.
The object's key is the algorithm and version combined (``ed25519`` being the
algorithm and ``abc123`` being the version in the example below). Together,
this forms the Key ID.
required: true
additionalProperties:
type: object
title: Verify Key
example: {
"ed25519:auto2": {
"key": "Base+64+Encoded+Signature+Verification+Key"
"ed25519:abc123": {
"key": "VGhpcyBzaG91bGQgYmUgYSByZWFsIGVkMjU1MTkgcGF5bG9hZA=="
}
}
properties:
key:
type: string
description: The key
description: The `Unpadded Base64`_ encoded key.
required: true
example: "Base+64+Encoded+Signature+Verification+Key"
example: "VGhpcyBzaG91bGQgYmUgYSByZWFsIGVkMjU1MTkgcGF5bG9hZA=="
old_verify_keys:
type: object
description: The public keys that the server used to use and when it stopped using them.
description: |-
The public keys that the server used to use and when it stopped using them.
The object's key is the algorithm and version combined (``ed25519`` being the
algorithm and ``0ldK3y`` being the version in the example below). Together,
this forms the Key ID.
additionalProperties:
type: object
title: Old Verify Key
example: {
"ed25519:auto1": {
"ed25519:0ldK3y": {
"expired_ts": 922834800000,
"key": "Base+64+Encoded+Signature+Verification+Key"
"key": "VGhpcyBzaG91bGQgYmUgeW91ciBvbGQga2V5J3MgZWQyNTUxOSBwYXlsb2FkLg=="
}
}
properties:
expired_ts:
type: integer
format: int64
description: The expiration time.
description: POSIX timestamp for when this key expired.
required: true
example: 922834800000
key:
type: string
description: The key.
description: The `Unpadded Base64`_ encoded key.
required: true
example: "Base+64+Encoded+Signature+Verification+Key"
example: "VGhpcyBzaG91bGQgYmUgeW91ciBvbGQga2V5J3MgZWQyNTUxOSBwYXlsb2FkLg=="
signatures:
type: object
description: Digital signatures for this object signed using the ``verify_keys``.
@ -72,7 +82,7 @@ properties:
title: Signed Server
example: {
"example.org": {
"ad25519:auto2": "Base+64+Encoded+Signature+Verification+Key"
"ad25519:abc123": "VGhpcyBzaG91bGQgYWN0dWFsbHkgYmUgYSBzaWduYXR1cmU="
}
}
additionalProperties:
@ -87,10 +97,12 @@ properties:
properties:
sha256:
type: string
description: The encoded fingerprint.
example: Base+64+Encoded+SHA-256-Fingerprint
description: The `Unpadded Base64`_ encoded fingerprint
example: "VGhpcyBpcyBoYXNoIHdoaWNoIHNob3VsZCBiZSBieXRlcw=="
valid_until_ts:
type: integer
format: int64
description: POSIX timestamp when the list of valid keys should be refreshed.
description: |-
POSIX timestamp when the list of valid keys should be refreshed. Keys used beyond this
timestamp are no longer valid.
example: 1052262000000

4
api/server-server/definitions/keys_query_response.yaml
View file

@ -15,13 +15,13 @@ type: object
description: Server keys
example: {
"server_keys": [{
$ref: "../examples/server_key.json"
$ref: "../examples/server_key_notary_signed.json"
}]
}
properties:
server_keys:
type: array
title: Server Keys
description: The server keys.
description: The queried server's keys, signed by the notary server.
items:
$ref: "keys.yaml"

12
api/server-server/examples/server_key.json
View file

@ -1,23 +1,23 @@
{
"server_name": "example.org",
"verify_keys": {
"ed25519:auto2": {
"key": "Base+64+Encoded+Signature+Verification+Key"
"ed25519:abc123": {
"key": "VGhpcyBzaG91bGQgYmUgYSByZWFsIGVkMjU1MTkgcGF5bG9hZA=="
}
},
"old_verify_keys": {
"ed25519:auto1": {
"ed25519:0ldk3y": {
"expired_ts": 922834800000,
"key": "Base+64+Encoded+Old+Verify+Key"
"key": "VGhpcyBzaG91bGQgYmUgeW91ciBvbGQga2V5J3MgZWQyNTUxOSBwYXlsb2FkLg=="
}
},
"signatures": {
"example.org": {
"ed25519:auto2": "Base+64+Encoded+Signature"
"ed25519:auto2": "VGhpcyBzaG91bGQgYWN0dWFsbHkgYmUgYSBzaWduYXR1cmU="
}
},
"tls_fingerprints": [{
"sha256": "Base+64+Encoded+SHA-256-Fingerprint"
"sha256": "VGhpcyBpcyBoYXNoIHdoaWNoIHNob3VsZCBiZSBieXRlcw=="
}],
"valid_until_ts": 1052262000000
}

11
api/server-server/examples/server_key_notary_signed.json Normal file
View file

@ -0,0 +1,11 @@
{
"$ref": "server_key.json",
"signatures": {
"example.org": {
"ed25519:abc123": "VGhpcyBzaG91bGQgYWN0dWFsbHkgYmUgYSBzaWduYXR1cmU="
},
"notary.server.com": {
"ed25519:010203": "VGhpcyBpcyBhbm90aGVyIHNpZ25hdHVyZQ=="
}
}
}

69
api/server-server/keys_query.yaml
View file

@ -25,49 +25,60 @@ produces:
paths:
"/query/{serverName}/{keyId}":
get:
summary: Retrieve a server key.
description: Retrieve a server key.
operationId: perspectivesKeyQuery
summary: Query for another server's keys
description: |-
Query for another server's keys. The receiving (notary) server must
sign the keys returned by the queried server.
operationId: getQueryKeys
parameters:
- in: path
name: serverName
type: string
description: Server name.
description: The server's DNS name to query
required: true
x-example: matrix.org
- in: path
name: keyId
type: string
description: Key ID.
required: true
x-example: TODO # No examples in spec so far
description: |-
The key ID to look up. If omitted or empty, all the server's keys
are to be queried for.
required: false
x-example: "ed25519:abc123"
- in: query
name: minimum_valid_until_ts
type: integer
format: int64
description: Minimum Valid Until Milliseconds.
required: true # TODO: Verify
description: |-
A millisecond POSIX timestamp indicating when the returned certificates
will need to be valid until to be useful to the requesting server.
If not supplied, the current time as determined by the notary server is used.
required: false
x-example: 1234567890
responses:
200:
description: The keys for the server
description: |-
The keys for the server, or an empty array if the server could not be reached
and no cached keys were available.
schema:
$ref: "definitions/keys_query_response.yaml"
"/query":
post:
summary: Retrieve a server key
description: Retrieve a server key.
operationId: bulkPerspectivesKeyQuery
summary: Query for several server's keys
description: |-
Query for keys from multiple servers in a batch format. The receiving (notary)
server must sign the keys returned by the queried servers.
operationId: postQueryKeys
parameters:
- in: body
name: body
schema:
type: object
# TODO: Improve example
example: {
"server_keys": {
"{server_name}": {
"{key_id}": {
"example.org": {
"ed25519:abc123": {
"minimum_valid_until_ts": 1234567890
}
}
@ -76,7 +87,16 @@ paths:
properties:
server_keys:
type: object
description: The query criteria.
description: |-
The query criteria. The outer ``string`` key on the object is the
server name (eg: ``matrix.org``). The inner ``string`` key is the
Key ID to query for the particular server. If no key IDs are given
to be queried, the notary server should query for all keys. If no
servers are given, the notary server must return an empty ``server_keys``
array in the response.
The notary server may return multiple keys regardless of the Key IDs
given.
additionalProperties:
type: object
name: ServerName
@ -84,16 +104,25 @@ paths:
additionalProperties:
type: object
title: Query Criteria
description: The server keys to query.
description: The server key IDs to query.
properties:
minimum_valid_until_ts:
type: integer
format: int64
description: Minimum Valid Until MS.
description: |-
A millisecond POSIX timestamp indicating when the returned
certificates will need to be valid until to be useful to the
requesting server.
If not supplied, the current time as determined by the notary
server is used.
example: 1234567890
required: ['server_keys']
responses:
200:
description: The keys for the server.
description: |-
The keys for the queried servers, signed by the notary server. Servers which
are offline and have no cached keys will not be included in the result. This
may result in an empty array.
schema:
$ref: "definitions/keys_query_response.yaml"

30
api/server-server/keys_server.yaml
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@ -25,18 +25,38 @@ produces:
paths:
"/server/{keyId}":
get:
summary: Get the server's key
description: Get the server's key.
summary: Get the homeserver's public key(s)
description: |-
Gets the homeserver's published TLS fingerprints and signing keys.
The homeserver may have any number of active keys and may have a
number of old keys. Homeservers SHOULD return a single JSON object
listing all of its keys, regardless of the ``keyId`` path argument.
This is to reduce the number of round trips needed to discover the
relevant keys for a homeserver.
Intermediate notary servers should cache a response for half of its
lifetime to avoid serving a stale response. Originating servers should
avoid returning responses that expire in less than an hour to avoid
repeated reqests for a certificate that is about to expire. Requesting
servers should limit how frequently they query for certificates to
avoid flooding a server with requests.
If the server fails to respond to this request, intermediate notary
servers should continue to return the last response they received
from the server so that the signatures of old events can still be
checked.
operationId: getServerKey
parameters:
- in: path
name: keyId
type: string
description: Key ID
description: |-
The key ID to look up. If omitted or empty, all server keys are
to be returned.
required: false
x-example: TODO # No examples in the spec so far
x-example: "ed25519:abc123"
responses:
200:
description: The server's keys.
description: The homeserver's keys
schema:
$ref: "definitions/keys.yaml"

120
specification/server_server_api.rst
View file

@ -106,15 +106,17 @@ Server implementation
Retrieving Server Keys
~~~~~~~~~~~~~~~~~~~~~~
Version 2
+++++++++
.. NOTE::
There was once a "version 1" of the key exchange. It has been removed from the
specification due to lack of significance. It may be reviewed `here
<https://github.com/matrix-org/matrix-doc/blob/51faf8ed2e4a63d4cfd6d23183698ed169956cc0/specification/server_server_api.rst#232version-1>`_.
Each homeserver publishes its public keys under ``/_matrix/key/v2/server/``.
Homeservers query for keys by either getting ``/_matrix/key/v2/server/``
Each homeserver publishes its public keys under ``/_matrix/key/v2/server/{keyId}`.
Homeservers query for keys by either getting ``/_matrix/key/v2/server/{keyId}``
directly or by querying an intermediate notary server using a
``/_matrix/key/v2/query`` API. Intermediate notary servers query the
``/_matrix/key/v2/server/`` API on behalf of another server and sign the
response with their own key. A server may query multiple notary servers to
``/_matrix/key/v2/query/{serverName}/{keyId}`` API. Intermediate notary servers
query the ``/_matrix/key/v2/server/{keyId}`` API on behalf of another server and
sign the response with their own key. A server may query multiple notary servers to
ensure that they all report the same public keys.
This approach is borrowed from the `Perspectives Project`_, but modified to
@ -126,113 +128,33 @@ server by querying other servers.
.. _Perspectives Project: https://web.archive.org/web/20170702024706/https://perspectives-project.org/
Publishing Keys
^^^^^^^^^^^^^^^
+++++++++++++++
Homeservers publish the allowed TLS fingerprints and signing keys in a JSON
object at ``/_matrix/key/v2/server/{key_id}``. The response contains a list of
``verify_keys`` that are valid for signing federation requests made by the
server and for signing events. It contains a list of ``old_verify_keys`` which
homeserver and for signing events. It contains a list of ``old_verify_keys`` which
are only valid for signing events. Finally the response contains a list of TLS
certificate fingerprints to validate any connection made to the server.
A server may have multiple keys active at a given time. A server may have any
number of old keys. It is recommended that servers return a single JSON
response listing all of its keys whenever any ``key_id`` is requested to reduce
the number of round trips needed to discover the relevant keys for a server.
However a server may return different responses for a different ``key_id``.
The ``tls_certificates`` field contains a list of hashes of the X.509 TLS
certificates currently used by the server. The list must include SHA-256 hashes
for every certificate currently in use by the server. These fingerprints are
valid until the millisecond POSIX timestamp in ``valid_until_ts``.
The ``verify_keys`` can be used to sign requests and events made by the server
until the millisecond POSIX timestamp in ``valid_until_ts``. If a homeserver
receives an event with a ``origin_server_ts`` after the ``valid_until_ts`` then
it should request that ``key_id`` for the originating server to check whether
the key has expired.
The ``old_verify_keys`` can be used to sign events with an ``origin_server_ts``
before the ``expired_ts``. The ``expired_ts`` is a millisecond POSIX timestamp
of when the originating server stopped using that key.
Intermediate notary servers should cache a response for half of its remaining
lifetime to avoid serving a stale response. Originating servers should avoid
returning responses that expire in less than an hour to avoid repeated requests
for a certificate that is about to expire. Requesting servers should limit how
frequently they query for certificates to avoid flooding a server with
requests.
If a server goes offline intermediate notary servers should continue to return
the last response they received from that server so that the signatures of old
events sent by that server can still be checked.
certificate fingerprints to validate any connection made to the homeserver.
{{keys_server_ss_http_api}}
Querying Keys Through Another Server
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
++++++++++++++++++++++++++++++++++++
Servers may offer a query API ``/_matrix/key/v2/query/`` for getting the keys
for another server. This API can be used to GET a list of JSON objects for a
given server or to POST a bulk query for a number of keys from a number of
servers. Either way the response is a list of JSON objects containing the
JSON published by the server under ``/_matrix/key/v2/server/`` signed by
both the originating server and by this server.
Servers may query another server's keys through a notary server. The notary
server may be another homeserver. The notary server will retrieve keys from
the queried servers through use of the ``/_matrix/key/v2/server/{keyId}``
API. The notary server will additionally sign the response from the queried
server before returning the results.
The ``minimum_valid_until_ts`` is a millisecond POSIX timestamp indicating
when the returned certificate will need to be valid until to be useful to the
requesting server. This can be set using the maximum ``origin_server_ts`` of
a batch of events that a requesting server is trying to validate. This allows
an intermediate notary server to give a prompt cached response even if the
originating server is offline.
This API can return keys for servers that are offline by using cached responses
taken from when the server was online. Keys can be queried from multiple
servers to mitigate against DNS spoofing.
Notary servers can return keys for servers that are offline or having issues
serving their own keys by using cached responses. Keys can be queried from
multiple servers to mitigate against DNS spoofing.
{{keys_query_ss_http_api}}
Version 1
+++++++++
.. WARNING::
Version 1 of key distribution is obsolete.
Homeservers publish their TLS certificates and signing keys in a JSON object
at ``/_matrix/key/v1``.
==================== =================== ======================================
Key Type Description
==================== =================== ======================================
``server_name`` String DNS name of the homeserver.
``verify_keys`` Object Public keys of the homeserver for
verifying digital signatures.
``signatures`` Object Digital signatures for this object
signed using the ``verify_keys``.
``tls_certificate`` String The X.509 TLS certificate used by this
this server encoded as `Unpadded Base64`_.
==================== =================== ======================================
.. code:: json
{
"server_name": "example.org",
"signatures": {
"example.org": {
"ed25519:auto": "Base+64+Encoded+Signature"
}
},
"tls_certificate": "Base+64+Encoded+DER+Encoded+X509+TLS+Certificate",
"verify_keys": {
"ed25519:auto": "Base+64+Encoded+Signature+Verification+Key"
}
}
When fetching the keys for a server the client should check that the TLS
certificate in the JSON matches the TLS server certificate for the connection
and should check that the JSON signatures are correct for the supplied
``verify_keys``.
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