bitcoin/script: bitcoin_witness_p2wpkh()

For the wallet code to use for change.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

bitcoin/script.c

@@ -281,6 +281,21 @@ void bitcoin_witness_p2sh_p2wpkh(const tal_t *ctx,
 	input->witness[1] = stack_key(input->witness, key);
 }
 
+u8 **bitcoin_witness_p2wpkh(const tal_t *ctx,
+			    const secp256k1_ecdsa_signature *sig,
+			    const struct pubkey *key)
+{
+	u8 **witness;
+
+	/* BIP141: The witness must consist of exactly 2 items (≤ 520
+	 * bytes each). The first one a signature, and the second one
+	 * a public key. */
+	witness = tal_arr(ctx, u8 *, 2);
+	witness[0] = stack_sig(witness, sig);
+	witness[1] = stack_key(witness, key);
+	return witness;
+}
+
 /* Create an output script for a 32-byte witness. */
 u8 *scriptpubkey_p2wsh(const tal_t *ctx, const u8 *witnessscript)
 {

bitcoin/script.h

@@ -85,6 +85,11 @@ u8 **bitcoin_witness_2of2(const tal_t *ctx,
 			  const struct pubkey *key1,
 			  const struct pubkey *key2);
 
+/* Create a witness which spends a p2wpkh. */
+u8 **bitcoin_witness_p2wpkh(const tal_t *ctx,
+			    const secp256k1_ecdsa_signature *sig,
+			    const struct pubkey *key);
+
 /* Create a witness which spends a "secret_or_delay" scriptpubkey */
 u8 **bitcoin_witness_secret(const tal_t *ctx,
 			    const void *secret, size_t secret_len,