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Minor clarifications for AUTH48 (#878)
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* Remove contradictory definitions of the info parameter

* Note that GREASE has to be applied consistently

* Allow GREASE for leaf node extensions

* Clarify that only non-default extension types need to be listed

* Restrict the extensions consistency to non-default extensions

* Be explicit about defaults; re-add consistent GREASE
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@bifurcation
bifurcation authored May 30, 2023
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Showing 1 changed file with 48 additions and 24 deletions.
72 changes: 48 additions & 24 deletions draft-ietf-mls-protocol.md
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Expand Up @@ -2319,14 +2319,36 @@ member's public signing key. The `credential` field contains information
authenticating both the member's identity and the provided signing key, as
described in {{credentials}}.

The `capabilities` field indicates what protocol versions, ciphersuites,
extensions, credential types, and non-default proposal types are supported by a client.
Proposal and extension types defined in this document are considered "default" and thus need
not be listed, while any credential types the application wishes to use MUST
be listed. Extensions that appear in the `extensions` field of a LeafNode
The `capabilities` field indicates the protocol features that the client
supports, including protocol versions, ciphersuites, credential types,
non-default proposal types, and non-default extension types. The following
proposal and extension types are considered "default" and MUST NOT be
listed:

* Proposal types:
* 0x0001 - `add`
* 0x0002 - `update`
* 0x0003 - `remove`
* 0x0004 - `psk`
* 0x0005 - `reinit`
* 0x0006 - `external_init`
* 0x0007 - `group_context_extensions`
* Extension types:
* 0x0001 - `application_id`
* 0x0002 - `ratchet_tree`
* 0x0003 - `required_capabilities`
* 0x0004 - `external_pub`
* 0x0005 - `external_senders`

There are no default values for the other fields of a capabilities object. The
client MUST list all values for the respective parameters that it supports.

The types of any non-default extensions that appear in the `extensions` field of a LeafNode
MUST be included in the `extensions` field of the `capabilities` field, and the
credential type used in the LeafNode MUST be included in the `credentials` field
of the `capabilities` field. As discussed in {{extensibility}}, unknown values
of the `capabilities` field.

As discussed in {{extensibility}}, unknown values
in `capabilities` MUST be ignored, and the creator of a `capabilities` field
SHOULD include some random GREASE values to help ensure that other clients correctly
ignore unknown values.
Expand Down Expand Up @@ -3284,9 +3306,6 @@ context = SetupBaseR(kem_output, external_priv, "")
init_secret = context.export("MLS 1.0 external init secret", KDF.Nh)
~~~

In both cases, the `info` input to HPKE is set to the GroupInfo for the
previous epoch, encoded using the TLS serialization.

## Pre-Shared Keys

Groups that already have an out-of-band mechanism to generate
Expand Down Expand Up @@ -5164,6 +5183,11 @@ extensions MAY have any contents selected by the sender, since they will be
ignored by a correctly-implemented receiver. For example, a sender might
populate these extensions with a randomly-sized amount of random data.

Note that any GREASE values added to `LeafNode.extensions` need to be reflected
in `LeafNode.capabilities.extensions`, since the LeafNode validation process
described in {{leaf-node-validation}} requires that these two fields be
consistent.

GREASE values MUST NOT be sent in the following fields, because an unsupported
value in one these fields (including a GREASE value), will cause the enclosing
message to be rejected:
Expand Down Expand Up @@ -5879,21 +5903,21 @@ Initial contents:
| 0x0003 | required_capabilities | GC | Y | RFC XXXX |
| 0x0004 | external_pub | GI | Y | RFC XXXX |
| 0x0005 | external_senders | GC | Y | RFC XXXX |
| 0x0A0A | GREASE | KP, GI | Y | RFC XXXX |
| 0x1A1A | GREASE | KP, GI | Y | RFC XXXX |
| 0x2A2A | GREASE | KP, GI | Y | RFC XXXX |
| 0x3A3A | GREASE | KP, GI | Y | RFC XXXX |
| 0x4A4A | GREASE | KP, GI | Y | RFC XXXX |
| 0x5A5A | GREASE | KP, GI | Y | RFC XXXX |
| 0x6A6A | GREASE | KP, GI | Y | RFC XXXX |
| 0x7A7A | GREASE | KP, GI | Y | RFC XXXX |
| 0x8A8A | GREASE | KP, GI | Y | RFC XXXX |
| 0x9A9A | GREASE | KP, GI | Y | RFC XXXX |
| 0xAAAA | GREASE | KP, GI | Y | RFC XXXX |
| 0xBABA | GREASE | KP, GI | Y | RFC XXXX |
| 0xCACA | GREASE | KP, GI | Y | RFC XXXX |
| 0xDADA | GREASE | KP, GI | Y | RFC XXXX |
| 0xEAEA | GREASE | KP, GI | Y | RFC XXXX |
| 0x0A0A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0x1A1A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0x2A2A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0x3A3A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0x4A4A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0x5A5A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0x6A6A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0x7A7A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0x8A8A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0x9A9A | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0xAAAA | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0xBABA | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0xCACA | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0xDADA | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0xEAEA | GREASE | KP, GI, LN | Y | RFC XXXX |
| 0xF000 - 0xFFFF | Reserved for Private Use | N/A | - | RFC XXXX |

## MLS Proposal Types
Expand Down

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