Disclaim replay protection #271
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As [discussed on the MLS mailing list](https://mailarchive.ietf.org/arch/msg/mls/SRAV33J5Ka9A4R-Ymv8a_T_TXhE/), an attacker with access to the groups symmetric secrets can create a new PrivateMessage containing the content of a previously sent message, and a different generation value. This PR clarifies that this sort of replay is not protected against by the protocol.
ekr
approved these changes
Feb 24, 2025
Co-authored-by: Eric Rescorla <[email protected]>
beurdouche
approved these changes
Feb 26, 2025
Bren2010
reviewed
Feb 27, 2025
Comment on lines
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| MLS does not provide protections against replay of group messages by members | ||
| of the group. Applications for whom replay is an important risk should apply | ||
| mitigations at the application layer, as discussed below. |
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| MLS does not provide protections against replay of group messages by members | |
| of the group. Applications for whom replay is an important risk should apply | |
| mitigations at the application layer, as discussed below. | |
| MLS does not protect against one group member replaying a PrivateMessage sent by another | |
| group member within the same epoch that the message was originally sent. Similarly, MLS | |
| does not protect against the replay (by a group member or otherwise) of a PublicMessage | |
| within the same epoch that the message was originally sent. Applications for | |
| whom replay is an important risk should apply mitigations at the application layer, as | |
| discussed below. |
|
I feel it's worth saying that replay is only possible within an epoch, and also that PublicMessages can be replayed by anyone (not just a group member) |
rohanmahy
reviewed
Feb 27, 2025
| ratchet. If the other members of the group interpret a message with a new | ||
| generation as a fresh message, then this message will appear fresh. (This is | ||
| possible because the message signature does not cover the `generation` field | ||
| of the message.) Messages sent as PublicMessages objects similarly lack replay |
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Suggested change
| of the message.) Messages sent as PublicMessages objects similarly lack replay | |
| of the message.) Messages sent as PublicMessage objects similarly lack replay |
rohanmahy
approved these changes
Feb 27, 2025
|
In WGLC; see this. |
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As discussed on the MLS mailing list, an attacker with access to the groups symmetric secrets can create a new PrivateMessage containing the content of a previously sent message, and a different generation value. This PR clarifies that this sort of replay is not protected against by the protocol.