hsmd: HSM_VERSION 6: get_per_commitment_point never returns secret

Changelog-Changed: hsmd: HSM_VERSION 6: get_per_commitment_point does not imply revocation of index - 2, makes it safe to call on any index.

Author: ksedgwic

channeld/channeld.c

@@ -1430,10 +1430,8 @@ static void start_commit_timer(struct peer *peer)
 }
 
 /* Fetch the requested point. The secret is no longer returned, use
- * revoke_commitment.
- *
- * NOTE - Because the internals of this call also implicitly revoke
- * index - 2 it is an error to call this past the next commitment.
+ * revoke_commitment.  It is leagal to call this on any commitment
+ * (including distant future).
  */
 static void get_per_commitment_point(u64 index, struct pubkey *point)
 {

common/hsm_version.h

@@ -23,7 +23,8 @@
  * v5 with hsmd_revoke_commitment_tx: 5742538f87ef5d5bf55b66dc19e52c8683cfeb1b887d3e64ba530ba9a4d8e638
  * v5 with sign_any_cannouncement: 5fdb9068c43a21887dc03f7dce410d2e3eeff6277f0d49b4fc56595a798fd4a4
  * v5 drop init v2: 5024454532fe5a78bb7558000cb344190888b9915360d3d56ddca22eaba9b872
+ * v6  no secret from get_per_commitment_point: 5024454532fe5a78bb7558000cb344190888b9915360d3d56ddca22eaba9b872
 */
 #define HSM_MIN_VERSION 5
-#define HSM_MAX_VERSION 5
+#define HSM_MAX_VERSION 6
 #endif /* LIGHTNING_COMMON_HSM_VERSION_H */

hsmd/hsmd_wire.csv

@@ -294,10 +294,12 @@ msgdata,hsmd_sign_splice_tx,input_index,u32,
 msgtype,hsmd_sign_tx_reply,112
 msgdata,hsmd_sign_tx_reply,sig,bitcoin_signature,
 
-# Openingd/channeld/onchaind asks for Nth per_commitment_point, if > 2, gets N-2 secret.
+# Openingd/channeld/onchaind asks for Nth per_commitment_point
+# Prior to HSM_VERSION 6 we will return an old_commitment_secret
 msgtype,hsmd_get_per_commitment_point,18
 msgdata,hsmd_get_per_commitment_point,n,u64,
 
+# IMPORTANT - Beginning HSM_VERSION 6 we never return an old_commitment_secret
 msgtype,hsmd_get_per_commitment_point_reply,118
 msgdata,hsmd_get_per_commitment_point_reply,per_commitment_point,pubkey,
 msgdata,hsmd_get_per_commitment_point_reply,old_commitment_secret,?secret,