elements: Fix fee estimation for commitment transactions

Signed-off-by: Christian Decker <decker.christian@gmail.com>

common/initial_commit_tx.c

@@ -6,7 +6,6 @@
 #include <common/permute_tx.h>
 #include <common/status.h>
 #include <common/type_to_string.h>
-#include <common/utils.h>
 #include <inttypes.h>
 
 /* BOLT #3:

common/initial_commit_tx.h

@@ -6,6 +6,7 @@
 #include <bitcoin/pubkey.h>
 #include <common/amount.h>
 #include <common/htlc.h>
+#include <common/utils.h>
 
 struct bitcoin_txid;
 struct keyset;
@@ -42,6 +43,27 @@ static inline struct amount_sat commit_tx_base_fee(u32 feerate_per_kw,
 	 */
 	weight += 172 * num_untrimmed_htlcs;
 
+	if (is_elements) {
+		/* Each transaction has surjection and rangeproof (both empty
+		 * for us as long as we use unblinded L-BTC transactions). */
+		weight += 2 * 4;
+
+		/* Inputs have 6 bytes of blank proofs attached. This TX only
+		 * has a single input. */
+		weight += 6;
+
+		/* Each direct output has a bit more weight to it */
+		weight += (32 + 1 + 1 + 1) * 4 * 2; /* Elements added fields */
+
+		/* Each HTLC output also carries a bit more weight */
+		weight += (32 + 1 + 1 + 1) * 4 * num_untrimmed_htlcs;
+
+		/* For elements we also need to add the fee output and the
+		 * overhead for rangeproofs into the mix. */
+		weight += (8 + 1) * 4; /* Bitcoin style output */
+		weight += (32 + 1 + 1 + 1) * 4; /* Elements added fields */
+	}
+
 	/* BOLT #3:
 	 *
 	 *    3. Multiply `feerate_per_kw` by `weight`, divide by 1000 (rounding