Forbid the sender from sending duplicate supported groups entries. #1354
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This seems reasonable, but it's a normative change, so the chairs need to tell us how to proceed. |
martinthomson
suggested changes
Apr 16, 2024
| @@ -2245,7 +2245,8 @@ Finite Field Groups (DHE): | |||
| {:br } | |||
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| Items in "named_group_list" are ordered according to the sender's | |||
| preferences (most preferred choice first). | |||
| preferences (most preferred choice first). The "named_group_list" | |||
| MUST NOT contain any duplicate entries. | |||
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When you specify a requirement on the sender, it always helps to specify how the receiver might react if the sender misbehaves. In this case, I think it best to only allow the receiver to reject the handshake, rather than require it. After all, I believe that some implementations are tolerant of that.
Suggested change
| MUST NOT contain any duplicate entries. | |
| MUST NOT contain any duplicate entries. A sender MAY abort a connection | |
| with a fatal illegal_parameter alert if it detects a duplicate entry. |
(I'm guessing the alert type here.)
More generally, do we want to do this sort of thing with all such lists, not just groups? key exchange mode? certificate type? etc...
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yes, probably "MAY abort a connection with an illegal_parameter Alert"
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Supported Groups is intimately tied to Key Share, where Key Share must be sent in the preference order specified by Supported Groups.
Duplicate Key Shares for a group are already forbidden from being sent, but allowing duplicates in Supported Groups makes this a bit muddled if supported groups is permitted to be, for example, ABA and the client sends key share B then A.
Now, nothing sane should actually be sending duplicate supported groups in a preference order, but it's still not forbidden to do so today. I'm suggesting we just not allow this so server side implementations can reject attempts to do so.