channel_id: add v2 channel_id derivtion helper

v2 of channel open uses the channel revocation basepoints to calculate
the channel_id, instead of the funding_txid + outnum

Moving away from the funding_txid opens the way for splicing + rbf

common/channel_id.c

@@ -1,3 +1,4 @@
+#include <bitcoin/pubkey.h>
 #include <bitcoin/tx.h>
 #include <common/channel_id.h>
 #include <common/type_to_string.h>
@@ -12,6 +13,39 @@ void derive_channel_id(struct channel_id *channel_id,
 	channel_id->id[sizeof(*channel_id)-1] ^= txout;
 }
 
+void derive_channel_id_v2(struct channel_id *channel_id,
+			  const struct pubkey *basepoint_1,
+			  const struct pubkey *basepoint_2)
+{
+	/* BOLT-df8bb5994d99e4c78053f7cb57694795f8393dc5 #2:
+	 * `channel_id`, v2
+	 *  For channels established using the v2 protocol, the
+	 *  `channel_id` is the
+	 *      SHA256(lesser-revocation-basepoint ||
+	 *             greater-revocation-basepoint),
+	 *  where the lesser and greater is based off the order of
+	 *  the basepoint. The basepoints are compact
+	 *  DER-encoded public keys.
+	 */
+	u8 der_keys[PUBKEY_CMPR_LEN * 2];
+	struct sha256 sha;
+	int offset_1, offset_2;
+
+	/* basepoint_1 is first? */
+	if (pubkey_idx(basepoint_1, basepoint_2) == 0) {
+		offset_1 = 0;
+		offset_2 = PUBKEY_CMPR_LEN;
+	} else {
+		offset_1 = PUBKEY_CMPR_LEN;
+		offset_2 = 0;
+	}
+	pubkey_to_der(der_keys + offset_1, basepoint_1);
+	pubkey_to_der(der_keys + offset_2, basepoint_2);
+	sha256(&sha, der_keys, sizeof(der_keys));
+	BUILD_ASSERT(sizeof(*channel_id) == sizeof(sha));
+	memcpy(channel_id, &sha, sizeof(*channel_id));
+}
+
 void towire_channel_id(u8 **pptr, const struct channel_id *channel_id)
 {
 	towire(pptr, channel_id, sizeof(*channel_id));

common/channel_id.h

@@ -5,6 +5,7 @@
 #include <ccan/structeq/structeq.h>
 
 struct bitcoin_txid;
+struct pubkey;
 
 /* BOLT #2:
  *
@@ -22,6 +23,9 @@ STRUCTEQ_DEF(channel_id, 0, id);
 void derive_channel_id(struct channel_id *channel_id,
 		       const struct bitcoin_txid *txid, u16 txout);
 
+void derive_channel_id_v2(struct channel_id *channel_id,
+			  const struct pubkey *basepoint_1,
+			  const struct pubkey *basepoint_2);
 /* Marshalling/unmarshalling functions */
 void towire_channel_id(u8 **pptr, const struct channel_id *channel_id);
 void fromwire_channel_id(const u8 **cursor, size_t *max,