filter channel: accept a #hashtag name, derive its key

Hashtag channels key off the first 16 bytes of SHA256("#name") (same as
MeshCore clients), so `filter channel #selftest` derives the key directly
instead of needing a pasted PSK.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

docs/cli_commands.md

@@ -1139,11 +1139,13 @@ Repeater only. Lets a repeater decrypt channels it holds the key for, inspect th
 
 #### Add or remove a channel to decrypt
 **Usage:**
+- `filter channel <#name>`
 - `filter channel <psk>`
 - `filter channel public`
 - `filter channel clear`
 
 **Parameters:**
+- `#name`: A hashtag channel, given by its name (e.g. `#selftest`). The key is derived the same way MeshCore clients derive it — the first 16 bytes of `SHA256("#name")` — so you don't need to paste a key. Only works for hashtag channels (named channels like `public` use a random key, not a name-derived one).
 - `psk`: Channel pre-shared key, as either Base64 (the form MeshCore clients share, e.g. `izOH6cXN6mrJ5e26oRXNcg==`) or raw hex (`62be0e1267c401381f4ea6f44217d7a9`). Either way it must decode to 16 or 32 bytes. The literal `public` is a shortcut for the well-known public channel key.
 
 **Note:** Adding a channel only enables decryption/inspection of that channel; messages still forward normally unless they match a blocked keyword or sender. `filter channel clear` removes all channel keys (the repeater stops decrypting and forwards everything blind again).

examples/simple_repeater/MyMesh.cpp

@@ -687,6 +687,11 @@ bool MyMesh::addFilterChannel(const char *psk) {
   if (strcmp(psk, "public") == 0) {
     memcpy(ch->secret, PUBLIC_CHANNEL_SECRET, sizeof(PUBLIC_CHANNEL_SECRET));
     len = sizeof(PUBLIC_CHANNEL_SECRET);
+  } else if (psk[0] == '#') {   // hashtag channel: key = first 16 bytes of SHA256(name)
+    uint8_t full[32];
+    mesh::Utils::sha256(full, sizeof(full), (const uint8_t *)psk, strlen(psk));
+    memcpy(ch->secret, full, 16);
+    len = 16;
   } else if (isHexKey(psk)) {
     len = strlen(psk) / 2;
     if (!mesh::Utils::fromHex(ch->secret, len, psk)) return false;
@@ -913,7 +918,7 @@ void MyMesh::handleFilterCommand(char *command, char *reply) {
     strcpy(reply, "OK - filter reset");
     return;
   }
-  strcpy(reply, "Err - usage: filter [list|stats [reset]|channel <b64|hex|public|clear>|block <kw>|sender <name>|clear|reset]");
+  strcpy(reply, "Err - usage: filter [list|stats [reset]|channel <#name|b64|hex|public|clear>|block <kw>|sender <name>|clear|reset]");
 }
 #endif  // WITH_CHANNEL_FILTER