diff --git a/bitcoin_tx.c b/bitcoin_tx.c
index 02df29a9a..5a0ca0a98 100644
--- a/bitcoin_tx.c
+++ b/bitcoin_tx.c
@@ -1,8 +1,10 @@
 #include "bitcoin_tx.h"
 #include <ccan/crypto/sha256/sha256.h>
+#include <ccan/endian/endian.h>
 #include <assert.h>
 
-static void sha256_varint(struct sha256_ctx *ctx, varint_t v)
+static void add_varint(varint_t v,
+		       void (*add)(const void *, size_t, void *), void *addp)
 {
 	u8 buf[9], *p = buf;
 
@@ -29,39 +31,81 @@ static void sha256_varint(struct sha256_ctx *ctx, varint_t v)
 		(*p++) = v >> 8;
 		(*p++) = v;
 	}
-	sha256_update(ctx, buf, p - buf);
+	add(buf, p - buf, addp);
 }
 
-static void sha256_tx_input(struct sha256_ctx *ctx,
-			    const struct bitcoin_tx_input *input)
+static void add_le32(u32 v,
+		     void (*add)(const void *, size_t, void *), void *addp)
 {
-	sha256_update(ctx, &input->txid, sizeof(input->txid));
-	sha256_le32(ctx, input->index);
-	sha256_varint(ctx, input->script_length);
-	sha256_update(ctx, input->script, input->script_length);
-	sha256_le32(ctx, input->sequence_number);
+	le32 l = cpu_to_le32(v);
+	add(&l, sizeof(l), addp);
 }
 
-static void sha256_tx_output(struct sha256_ctx *ctx,
-			     const struct bitcoin_tx_output *output)
+static void add_le64(u64 v,
+		     void (*add)(const void *, size_t, void *), void *addp)
 {
-	sha256_le64(ctx, output->amount);
-	sha256_varint(ctx, output->script_length);
-	sha256_update(ctx, output->script, output->script_length);
+	le64 l = cpu_to_le64(v);
+	add(&l, sizeof(l), addp);
 }
 
-void sha256_tx(struct sha256_ctx *ctx, const struct bitcoin_tx *tx)
+static void add_tx_input(const struct bitcoin_tx_input *input,
+			 void (*add)(const void *, size_t, void *), void *addp)
+{
+	add(&input->txid, sizeof(input->txid), addp);
+	add_le32(input->index, add, addp);
+	add_varint(input->script_length, add, addp);
+	add(input->script, input->script_length, addp);
+	add_le32(input->sequence_number, add, addp);
+}
+
+static void add_tx_output(const struct bitcoin_tx_output *output,
+			  void (*add)(const void *, size_t, void *), void *addp)
+{
+	add_le64(output->amount, add, addp);
+	add_varint(output->script_length, add, addp);
+	add(output->script, output->script_length, addp);
+}
+
+static void add_tx(const struct bitcoin_tx *tx,
+		   void (*add)(const void *, size_t, void *), void *addp)
 {
 	varint_t i;
 
-	sha256_le32(ctx, tx->version);
-	sha256_varint(ctx, tx->input_count);
+	add_le32(tx->version, add, addp);
+	add_varint(tx->input_count, add, addp);
 	for (i = 0; i < tx->input_count; i++)
-		sha256_tx_input(ctx, &tx->input[i]);
-	sha256_varint(ctx, tx->output_count);
+		add_tx_input(&tx->input[i], add, addp);
+	add_varint(tx->output_count, add, addp);
 	for (i = 0; i < tx->output_count; i++)
-		sha256_tx_output(ctx, &tx->output[i]);
-	sha256_le32(ctx, tx->lock_time);
+		add_tx_output(&tx->output[i], add, addp);
+	add_le32(tx->lock_time, add, addp);
+}
+
+static void add_sha(const void *data, size_t len, void *shactx_)
+{
+	struct sha256_ctx *ctx = shactx_;
+	sha256_update(ctx, data, len);
+}
+
+void sha256_tx(struct sha256_ctx *ctx, const struct bitcoin_tx *tx)
+{
+	add_tx(tx, add_sha, ctx);
+}
+
+static void add_linearize(const void *data, size_t len, void *pptr_)
+{
+	u8 **pptr = pptr_;
+	size_t oldsize = tal_count(*pptr);
+
+	tal_resize(pptr, oldsize + len);
+	memcpy(*pptr + oldsize, data, len);
+}
+
+u8 *linearize_tx(const tal_t *ctx, const struct bitcoin_tx *tx)
+{
+	u8 *arr = tal_arr(ctx, u8, 0);
+	add_tx(tx, add_linearize, &arr);
+	return arr;
 }
 
 void bitcoin_txid(const struct bitcoin_tx *tx, struct sha256_double *txid)
diff --git a/bitcoin_tx.h b/bitcoin_tx.h
index 79d3baf09..2c0d4dd53 100644
--- a/bitcoin_tx.h
+++ b/bitcoin_tx.h
@@ -39,6 +39,9 @@ void bitcoin_txid(const struct bitcoin_tx *tx, struct sha256_double *txid);
 /* Useful for signature code. */
 void sha256_tx(struct sha256_ctx *ctx, const struct bitcoin_tx *tx);
 
+/* Linear bytes of tx. */
+u8 *linearize_tx(const tal_t *ctx, const struct bitcoin_tx *tx);
+
 /* Allocate a tx: you just need to fill in inputs and outputs (they're
  * zeroed with inputs' sequence_number set to FFFFFFFF) */
 struct bitcoin_tx *bitcoin_tx(const tal_t *ctx, varint_t input_count,