diff --git a/lightningd/Makefile b/lightningd/Makefile
index 334dca5400b0..acb017553595 100644
--- a/lightningd/Makefile
+++ b/lightningd/Makefile
@@ -58,3 +58,4 @@ clean: lightningd-clean
 lightningd-clean:
 	$(RM) $(LIGHTNINGD_OBJS) $(LIGHTNINGD_LIB_OBJS) $(LIGHTNINGD_JSMN_OBJS) 
 
+include lightningd/test/Makefile
diff --git a/lightningd/cryptomsg.c b/lightningd/cryptomsg.c
index 75835393f8fb..a9a1cf77f698 100644
--- a/lightningd/cryptomsg.c
+++ b/lightningd/cryptomsg.c
@@ -259,6 +259,13 @@ struct io_plan *peer_write_message(struct io_conn *conn,
 							cs->sk.u.u8);
 	assert(ret == 0);
 	assert(clen == sizeof(l) + 16);
+#ifdef SUPERVERBOSE
+	status_trace("# encrypt l: cleartext=0x%s, AD=NULL, sn=0x%s, sk=0x%s => 0x%s",
+		     tal_hexstr(trc, &l, sizeof(l)),
+		     tal_hexstr(trc, npub, sizeof(npub)),
+		     tal_hexstr(trc, &cs->sk, sizeof(cs->sk)),
+		     tal_hexstr(trc, cs->out, clen));
+#endif
 
 	/* BOLT #8:
 	 *
@@ -277,6 +284,13 @@ struct io_plan *peer_write_message(struct io_conn *conn,
 							cs->sk.u.u8);
 	assert(ret == 0);
 	assert(clen == mlen + 16);
+#ifdef SUPERVERBOSE
+	status_trace("# encrypt m: cleartext=0x%s, AD=NULL, sn=0x%s, sk=0x%s => 0x%s",
+		     tal_hexstr(trc, msg, mlen),
+		     tal_hexstr(trc, npub, sizeof(npub)),
+		     tal_hexstr(trc, &cs->sk, sizeof(cs->sk)),
+		     tal_hexstr(trc, cs->out + 18, clen));
+#endif
 
 	maybe_rotate_key(&cs->sn, &cs->sk, &cs->s_ck);
 
diff --git a/lightningd/test/Makefile b/lightningd/test/Makefile
new file mode 100644
index 000000000000..84076a26d836
--- /dev/null
+++ b/lightningd/test/Makefile
@@ -0,0 +1,16 @@
+check: lightningd/tests
+
+# Note that these actually #include everything they need, except ccan/ and bitcoin/.
+# That allows for unit testing of statics, and special effects.
+LIGHTNINGD_TEST_SRC := $(wildcard lightningd/test/run-*.c)
+LIGHTNINGD_TEST_OBJS := $(LIGHTNINGD_TEST_SRC:.c=.o)
+LIGHTNINGD_TEST_PROGRAMS := $(LIGHTNINGD_TEST_OBJS:.o=)
+
+update-mocks: $(LIGHTNINGD_TEST_SRC:%=update-mocks/%)
+
+$(LIGHTNINGD_TEST_PROGRAMS): $(CCAN_OBJS) $(BITCOIN_OBJS) $(WIRE_OBJS) $(LIBBASE58_OBJS) libsecp256k1.a utils.o $(LIGHTNINGD_HANDSHAKE_GEN_SRC:.c=.o)
+
+$(LIGHTNINGD_TEST_OBJS): $(LIGHTNINGD_HEADERS) $(BITCOIN_HEADERS) $(CORE_HEADERS) $(GEN_HEADERS) $(WIRE_HEADERS) $(CCAN_HEADERS) $(LIBBASE58_HEADERS)
+
+lightningd/tests: $(LIGHTNINGD_TEST_PROGRAMS:%=unittest/%)
+
diff --git a/lightningd/test/run-cryptomsg.c b/lightningd/test/run-cryptomsg.c
new file mode 100644
index 000000000000..2d6489bb9dad
--- /dev/null
+++ b/lightningd/test/run-cryptomsg.c
@@ -0,0 +1,112 @@
+#include <assert.h>
+#include <ccan/str/hex/hex.h>
+#include <ccan/tal/str/str.h>
+#include <status.h>
+#include <stdio.h>
+#include <wire/wire_io.h>
+
+#undef io_read
+#undef io_write
+
+static char *read_buf;
+static size_t read_buf_len;
+
+static void do_read(void *buf, size_t len)
+{
+	assert(len <= read_buf_len);
+	memcpy(buf, read_buf, len);
+	read_buf += len;
+	read_buf_len -= len;
+}
+
+#define io_read(conn, p, len, next, arg)			\
+	(do_read((p), (len)), (next)((conn), (arg)), NULL)
+
+static char *write_buf;
+
+static void do_write(const void *buf, size_t len)
+{
+	size_t oldlen = tal_count(write_buf);
+	tal_resize(&write_buf, oldlen + len);
+	memcpy(write_buf + oldlen, buf, len);
+}
+
+#define io_write(conn, p, len, next, arg) \
+	(do_write((p), (len)), (next)((conn), (arg)), NULL)
+
+#define status_trace(fmt, ...) \
+	printf(fmt "\n", __VA_ARGS__)
+
+#include "../cryptomsg.c"
+
+const void *trc;
+
+static struct io_plan *check_msg_write(struct io_conn *conn, struct peer *peer)
+{
+	assert(tal_count(write_buf) == 2 + 16 + 5 + 16);
+	return NULL;
+}
+
+static struct io_plan *check_msg_read(struct io_conn *conn, struct peer *peer,
+				      u8 *msg)
+{
+	assert(tal_count(msg) == 5);
+	assert(memcmp(msg, "hello", 5) == 0);
+	return NULL;
+}
+
+static struct sha256 sha256_from_hex(const char *hex)
+{
+	struct sha256 sha256;
+	hex += 2;
+	if (!hex_decode(hex, strlen(hex), &sha256, sizeof(sha256)))
+		abort();
+	return sha256;
+}
+
+int main(void)
+{
+	tal_t *tmpctx = tal_tmpctx(NULL);
+	struct crypto_state *cs_out, *cs_in;
+	struct sha256 sk, rk, ck;
+	const void *msg = tal_dup_arr(tmpctx, char, "hello", 5, 0);
+	size_t i;
+
+	trc = tal_tmpctx(tmpctx);
+
+	/* BOLT #8:
+	 *
+	 * name: transport-initiator successful handshake
+	 *...
+	 * # ck,temp_k3=0x919219dbb2920afa8db80f9a51787a840bcf111ed8d588caf9ab4be716e42b01,0x981a46c820fb7a241bc8184ba4bb1f01bcdfafb00dde80098cb8c38db9141520
+	 * # encryptWithAD(0x981a46c820fb7a241bc8184ba4bb1f01bcdfafb00dde80098cb8c38db9141520, 0x000000000000000000000000, 0x5dcb5ea9b4ccc755e0e3456af3990641276e1d5dc9afd82f974d90a47c918660, <empty>)
+	 * # t=0x8dc68b1c466263b47fdf31e560e139ba
+	 * output: 0x00b9e3a702e93e3a9948c2ed6e5fd7590a6e1c3a0344cfc9d5b57357049aa22355361aa02e55a8fc28fef5bd6d71ad0c38228dc68b1c466263b47fdf31e560e139ba
+	 * # HKDF(0x919219dbb2920afa8db80f9a51787a840bcf111ed8d588caf9ab4be716e42b01,zero)
+	 * output: sk,rk=0x969ab31b4d288cedf6218839b27a3e2140827047f2c0f01bf5c04435d43511a9,0xbb9020b8965f4df047e07f955f3c4b88418984aadc5cdb35096b9ea8fa5c3442
+	 */
+	ck = sha256_from_hex("0x919219dbb2920afa8db80f9a51787a840bcf111ed8d588caf9ab4be716e42b01");
+	sk = sha256_from_hex("0x969ab31b4d288cedf6218839b27a3e2140827047f2c0f01bf5c04435d43511a9");
+	rk = sha256_from_hex("0xbb9020b8965f4df047e07f955f3c4b88418984aadc5cdb35096b9ea8fa5c3442");
+
+	cs_out = crypto_state(tmpctx, &sk, &rk, &ck, &ck, 0, 0);
+	cs_in = crypto_state(tmpctx, &rk, &sk, &ck, &ck, 0, 0);
+
+	for (i = 0; i < 1002; i++) {
+		write_buf = tal_arr(tmpctx, char, 0);
+
+		peer_write_message(NULL, cs_out, msg, check_msg_write);
+		if ((i % 500) < 2)
+			status_trace("output %zu: 0x%s", i,
+				     tal_hex(tmpctx, write_buf));
+
+		read_buf = write_buf;
+		read_buf_len = tal_count(read_buf);
+		write_buf = tal_arr(tmpctx, char, 0);
+
+		peer_read_message(NULL, cs_in, check_msg_read);
+		assert(read_buf_len == 0);
+	}
+	tal_free(tmpctx);
+	return 0;
+}