diff --git a/bitcoin/short_channel_id.c b/bitcoin/short_channel_id.c
index 98fe4748b714..3cfab21ae9bd 100644
--- a/bitcoin/short_channel_id.c
+++ b/bitcoin/short_channel_id.c
@@ -2,6 +2,7 @@
 #include <bitcoin/short_channel_id.h>
 #include <ccan/tal/str/str.h>
 #include <common/randbytes.h>
+#include <common/utils.h>
 #include <stdio.h>
 #include <wire/wire.h>
 
@@ -40,17 +41,27 @@ bool mk_short_channel_id(struct short_channel_id *scid,
 bool short_channel_id_from_str(const char *str, size_t strlen,
 			       struct short_channel_id *dst)
 {
-	u32 blocknum, txnum;
-	u16 outnum;
-	int matches;
+	const char *x1, *x2;
+	u64 blocknum, txnum, outnum;
 
-	char buf[strlen + 1];
-	memcpy(buf, str, strlen);
-	buf[strlen] = 0;
+	x1 = memchr(str, 'x', strlen);
+	if (!x1)
+		return false;
+
+	x2 = memchr(x1 + 1, 'x', strlen - (x1 + 1 - str));
+	if (!x2)
+		return false;
+
+	if (!str_to_u64(str, x1 - str, &blocknum))
+		return false;
+
+	if (!str_to_u64(x1 + 1, x2 - (x1 + 1), &txnum))
+		return false;
+
+	if (!str_to_u64(x2 + 1, (str + strlen) - (x2 + 1), &outnum))
+		return false;
 
-	matches = sscanf(buf, "%ux%ux%hu", &blocknum, &txnum, &outnum);
-	return matches == 3
-		&& mk_short_channel_id(dst, blocknum, txnum, outnum);
+	return mk_short_channel_id(dst, blocknum, txnum, outnum);
 }
 
 char *fmt_short_channel_id(const tal_t *ctx, struct short_channel_id scid)
diff --git a/bitcoin/test/run-short_channel_id.c b/bitcoin/test/run-short_channel_id.c
new file mode 100644
index 000000000000..af947f40db7e
--- /dev/null
+++ b/bitcoin/test/run-short_channel_id.c
@@ -0,0 +1,109 @@
+#include "config.h"
+#include "../short_channel_id.c"
+#include <assert.h>
+#include <common/setup.h>
+#include <stdio.h>
+
+/* AUTOGENERATED MOCKS START */
+/* Generated stub for fromwire_u64 */
+u64 fromwire_u64(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_u64 called!\n"); abort(); }
+/* Generated stub for randbytes_ */
+void randbytes_(void *bytes UNNEEDED, size_t num_bytes UNNEEDED, u64 *offset UNNEEDED)
+{ fprintf(stderr, "randbytes_ called!\n"); abort(); }
+/* Generated stub for towire_u64 */
+void towire_u64(u8 **pptr UNNEEDED, u64 v UNNEEDED)
+{ fprintf(stderr, "towire_u64 called!\n"); abort(); }
+/* AUTOGENERATED MOCKS END */
+
+int main(int argc, char *argv[])
+{
+	struct short_channel_id scid, exp;
+	struct short_channel_id_dir scidd;
+
+	common_setup(argv[0]);
+
+	/* Valid canonical short_channel_id strings */
+	assert(short_channel_id_from_str("539268x845x1", strlen("539268x845x1"), &scid));
+	assert(mk_short_channel_id(&exp, 539268, 845, 1));
+	assert(scid.u64 == exp.u64);
+	assert(short_channel_id_blocknum(scid) == 539268);
+	assert(short_channel_id_txnum(scid) == 845);
+	assert(short_channel_id_outnum(scid) == 1);
+
+	/* Boundary valid values */
+	assert(short_channel_id_from_str("0x0x0", strlen("0x0x0"), &scid));
+	assert(mk_short_channel_id(&exp, 0, 0, 0));
+	assert(scid.u64 == exp.u64);
+
+	assert(short_channel_id_from_str("16777215x16777215x65535",
+					 strlen("16777215x16777215x65535"), &scid));
+	assert(mk_short_channel_id(&exp, 16777215, 16777215, 65535));
+	assert(scid.u64 == exp.u64);
+
+	/* Formatting */
+	char *str = fmt_short_channel_id(tmpctx, scid);
+	assert(streq(str, "16777215x16777215x65535"));
+
+	/* Valid short_channel_id_dir strings */
+	assert(mk_short_channel_id(&exp, 539268, 845, 1));
+	assert(short_channel_id_dir_from_str("539268x845x1/0", strlen("539268x845x1/0"), &scidd));
+	assert(scidd.scid.u64 == exp.u64);
+	assert(scidd.dir == 0);
+
+	assert(short_channel_id_dir_from_str("539268x845x1/1", strlen("539268x845x1/1"), &scidd));
+	assert(scidd.dir == 1);
+
+	/* Invalid trailing garbage */
+	assert(!short_channel_id_from_str("1x2x3garbage", strlen("1x2x3garbage"), &scid));
+	assert(!short_channel_id_from_str("1x2x3/0", strlen("1x2x3/0"), &scid));
+	assert(!short_channel_id_from_str("1x2x3x4", strlen("1x2x3x4"), &scid));
+	assert(!short_channel_id_from_str("1x2x3 4", strlen("1x2x3 4"), &scid));
+
+	/* Invalid signs and negative components */
+	assert(!short_channel_id_from_str("1x2x-1", strlen("1x2x-1"), &scid));
+	assert(!short_channel_id_from_str("-1x2x3", strlen("-1x2x3"), &scid));
+	assert(!short_channel_id_from_str("1x-2x3", strlen("1x-2x3"), &scid));
+	assert(!short_channel_id_from_str("+1x2x3", strlen("+1x2x3"), &scid));
+	assert(!short_channel_id_from_str("1x+2x3", strlen("1x+2x3"), &scid));
+	assert(!short_channel_id_from_str("1x2x+3", strlen("1x2x+3"), &scid));
+
+	/* Invalid whitespace */
+	assert(!short_channel_id_from_str(" 1x2x3", strlen(" 1x2x3"), &scid));
+	assert(!short_channel_id_from_str("1x2x3 ", strlen("1x2x3 "), &scid));
+	assert(!short_channel_id_from_str("1x 2x3", strlen("1x 2x3"), &scid));
+	assert(!short_channel_id_from_str("1x2x 3", strlen("1x2x 3"), &scid));
+
+	/* Incomplete or empty */
+	assert(!short_channel_id_from_str("", 0, &scid));
+	assert(!short_channel_id_from_str("1", 1, &scid));
+	assert(!short_channel_id_from_str("1x", 2, &scid));
+	assert(!short_channel_id_from_str("1x2", 3, &scid));
+	assert(!short_channel_id_from_str("1x2x", 4, &scid));
+	assert(!short_channel_id_from_str("x2x3", 4, &scid));
+	assert(!short_channel_id_from_str("1xx3", 4, &scid));
+
+	/* Out of range components (including > 2^16, > 2^32, > 2^64) */
+	assert(!short_channel_id_from_str("16777216x1x1", strlen("16777216x1x1"), &scid));
+	assert(!short_channel_id_from_str("1x16777216x1", strlen("1x16777216x1"), &scid));
+	assert(!short_channel_id_from_str("1x1x65536", strlen("1x1x65536"), &scid));
+	assert(!short_channel_id_from_str("1x1x65537", strlen("1x1x65537"), &scid));
+	assert(!short_channel_id_from_str("4294967296x1x1", strlen("4294967296x1x1"), &scid));
+	assert(!short_channel_id_from_str("4294967297x1x1", strlen("4294967297x1x1"), &scid));
+	assert(!short_channel_id_from_str("1x4294967296x1", strlen("1x4294967296x1"), &scid));
+	assert(!short_channel_id_from_str("1x4294967297x1", strlen("1x4294967297x1"), &scid));
+	assert(!short_channel_id_from_str("1x1x4294967296", strlen("1x1x4294967296"), &scid));
+	assert(!short_channel_id_from_str("18446744073709551616x1x1", strlen("18446744073709551616x1x1"), &scid));
+	assert(!short_channel_id_from_str("1x18446744073709551616x1", strlen("1x18446744073709551616x1"), &scid));
+	assert(!short_channel_id_from_str("1x1x18446744073709551616", strlen("1x1x18446744073709551616"), &scid));
+	assert(!short_channel_id_from_str("99999999999999999999x1x1", strlen("99999999999999999999x1x1"), &scid));
+
+	/* Invalid short_channel_id_dir */
+	assert(!short_channel_id_dir_from_str("1x2x3garbage/0", strlen("1x2x3garbage/0"), &scidd));
+	assert(!short_channel_id_dir_from_str("1x2x3/2", strlen("1x2x3/2"), &scidd));
+	assert(!short_channel_id_dir_from_str("1x2x3/", strlen("1x2x3/"), &scidd));
+	assert(!short_channel_id_dir_from_str("1x2x3/00", strlen("1x2x3/00"), &scidd));
+
+	common_shutdown();
+	return 0;
+}