add spec draft to repo

solanaceae/ngc_hs2/spec_ngc_hs2.md

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+# [NGC] Group-History-Sync (v2) [PoC] [Draft]
+
+Simple group history sync that uses `peer public key` + `message_id` + `timestamp` (`ppk+mid+ts`) to, mostly, uniquely identify messages and deliver them.
+
+## Requirements
+
+TODO
+
+### File transfers
+
+For sending packs of messages. A single message can be larger than a single custom packet, so this is a must-have.
+
+## Procedure
+
+Peer A can request `ppk+mid+ts` list for a given time range from peer B.
+
+Peer B then sends a filetransfer (with special file type) of list of `ppk+mid+ts`.
+Optionally compressed. (Delta-coding / zstd)
+
+Peer A keeps doing that until the desired time span is covered.
+
+After that or simultaniously, Peer A requests messages from peer B, either indivitually, or packed? in ranges?.
+Optionally compressed.
+
+During all that, peer B usually does the same thing to peer A.
+
+## Traffic savings
+
+It is recomended to remember if a range has been requested and answered from a given peer, to reduce traffic.
+
+While compression is optional, it is recommended.
+
+## Message uniqueness
+
+This protocol relies on the randomness of `message_id` and the clocks to be more or less synchronized.
+
+However, `message_id` can be manipulated freely by any peer, this can make messages appear as duplicates.
+
+This can be used here, if you don't wish your messages to be syncronized (to an extent).
+
+## Security
+
+Only sync publicly sent/recieved messages.
+
+Only allow sync or extended time ranges from peers you trust (enough).
+
+The default shall be to not offer any messages.
+
+Indirect messages shall be low in credibility, while direct synced (by author), with mid credibility.
+
+Either only high or mid credibility shall be sent.
+
+
+Manual exceptions to all can be made at the users discretion, eg for other self owned devices.
+
+## File transfer requests
+
+TODO: is reusing the ft request api a good idea for this?
+
+| fttype | name | content (ft id) |
+|------|------|---------------------|
+| 0x01 | time range | - ts start </br> - ts end </br> - supported compression? |
+|      | TODO: id range based request? | |
+| 0x01 | single message | - ppk </br> - mid </br> - ts |
+
+## File transfers
+
+| fttype | name | content |
+|--------|------|---------------------|
+| 0x01   | time range | - feature bitset (1byte? different compressions?) </br> - ts start </br> - ts end </br> - list size </br> \\+ entry `ppk` </br> \\+ entry `mid` </br> \\+ entry `ts` |
+| 0x01   | single message | - message type (text/textaction/file) </br> - text if text or action, file type and file id if file |
+
+## TODO
+
+- [ ] figure out a pro-active approach (instead of waiting for a range request)
+- [ ] compression in the ft layer? (would make it reusable) hint/autodetect?
+