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Frauschi
commented
Oct 14, 2026
The supported groups extension stored its named groups as a linked list with one node per two byte group identifier. A client advertising 16 groups made 16 allocations for 32 bytes of wire data, and a server made a second set of nodes for the intersection with the client's list. Every insert also walked the list to check for a duplicate. Replace the list with a single block holding a count, a capacity and a flat array of names. The block starts with room for four names and doubles when a build advertises more, so a handshake normally makes one allocation per list. Inserts and lookups index the array instead of chasing pointers, and freeing the extension is a single XFREE.
A server with its own supported groups list built a second list holding the groups it has in common with the client, swapped that into the extension and freed the original. Now that the groups are a flat array, the common ones are moved to the front of the server's own list instead, so the intersection needs no allocation. The kept groups stay in the client's order, as before. A name that has already been moved sits outside the range still searched, so a group repeated in the client list is added once. Adds a test for the resulting list and for the TLS 1.2 handshake failure when no group is common.
Before sending EncryptedExtensions a TLS 1.3 server checked whether the client offered the group the server prefers most. It answered that by populating a whole temporary extension list with every group this build offers, reading the first name out of it, and freeing the list again. The TLS 1.2 FFDHE selection did the same to find the highest priority group both sides have. Both cost a TLSX node, a groups block and a walk of the full preference chain on every handshake. Turn the preference chain into TLSX_WalkSupportedGroups(), which calls a callback for each group in the same order. Populating an extension list is now one caller of the walk, finding the preferred group is another that stops at the first group, and the FFDHE selection is a third that stops at the first group the client also has. The server builds a list only in the case that sends it back to the client.
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Description
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Fixes zd#
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