diff --git a/common/sphinx.c b/common/sphinx.c
index 972d84279..e43c01ba9 100644
--- a/common/sphinx.c
+++ b/common/sphinx.c
@@ -785,3 +785,27 @@ u8 *unwrap_onionreply(const tal_t *ctx,
 		return tal_free(final);
 	return final;
 }
+
+u8 *sphinx_decompress(const tal_t *ctx, const u8 *compressed,
+		      struct secret *shared_secret)
+{
+	size_t compressedlen = tal_bytelen(compressed);
+	size_t prefill_size = TOTAL_PACKET_SIZE - compressedlen;
+	u8 *dst;
+	int p = 0;
+
+	assert(prefill_size >= 0);
+	assert(compressedlen >= VERSION_SIZE + PUBKEY_SIZE + HMAC_SIZE);
+	dst = tal_arrz(ctx, u8, TOTAL_PACKET_SIZE);
+	write_buffer(
+	    dst, compressed,
+	    VERSION_SIZE + PUBKEY_SIZE + ROUTING_INFO_SIZE - prefill_size, &p);
+
+	/* We can just XOR here since we initialized the array with zeros. */
+	sphinx_prefill_stream_xor(dst + p, prefill_size, shared_secret);
+	p += prefill_size;
+
+	write_buffer(dst, compressed + compressedlen - HMAC_SIZE, HMAC_SIZE,
+		     &p);
+	return dst;
+}
diff --git a/common/sphinx.h b/common/sphinx.h
index 28fd588e1..fe5b946a9 100644
--- a/common/sphinx.h
+++ b/common/sphinx.h
@@ -246,6 +246,13 @@ u8 *serialize_compressed_onion(const tal_t *ctx,
 bool sphinx_path_set_rendezvous(struct sphinx_path *sp,
 				const struct node_id *rendezvous_id);
 
+/**
+ * Given a compressed onion expand it by re-generating the prefiller and
+ * inserting it in the appropriate place.
+ */
+u8 *sphinx_decompress(const tal_t *ctx, const u8 *compressed,
+		      struct secret *shared_secret);
+
 #if DEVELOPER
 /* Override to force us to reject valid onion packets */
 extern bool dev_fail_process_onionpacket;
diff --git a/devtools/onion.c b/devtools/onion.c
index f58756394..b20e33a51 100644
--- a/devtools/onion.c
+++ b/devtools/onion.c
@@ -290,6 +290,30 @@ static void runtest(const char *filename)
 	tal_free(ctx);
 }
 
+static void decompress(char *hexprivkey, char *hexonion)
+{
+	struct privkey rendezvous_key;
+	size_t onionlen = hex_data_size(strlen(hexonion));
+	u8 *compressed, *decompressed;
+	struct pubkey ephkey;
+	struct secret shared_secret;
+
+	if (!hex_decode(hexprivkey, strlen(hexprivkey), &rendezvous_key, sizeof(rendezvous_key)))
+		errx(1, "Invalid private key hex '%s'", hexprivkey);
+
+	compressed = tal_arr(NULL, u8, onionlen);
+	if (!hex_decode(hexonion, strlen(hexonion), compressed, onionlen))
+		errx(1, "Invalid onion hex '%s'", hexonion);
+
+	if (onionlen < HMAC_SIZE + 1 + PUBKEY_SIZE)
+		errx(1, "Onion is too short to contain the version, ephemeral key and HMAC");
+
+	pubkey_from_der(compressed + 1, PUBKEY_SIZE, &ephkey);
+
+	decompressed = sphinx_decompress(NULL, compressed, &shared_secret);
+	printf("Decompressed Onion: %s\n", tal_hex(NULL, decompressed));
+}
+
 /* Tal wrappers for opt. */
 static void *opt_allocfn(size_t size)
 {
@@ -353,6 +377,15 @@ int main(int argc, char **argv)
 			do_generate(argc, argv, assocdata, NULL);
 		else
 			do_generate(argc, argv, assocdata, &rendezvous_id);
+	} else if (streq(method, "decompress")) {
+		if (argc != 4) {
+			errx(2,
+			     "'%s decompress' requires a private key and a "
+			     "compressed onion",
+			     argv[0]);
+		}
+
+		decompress(argv[2], argv[3]);
 	} else if (streq(method, "decode")) {
 		do_decode(argc, argv, assocdata);
 	} else {