common: add a descriptor checksum computation module

This is stolen from William's clightning-dumpkeys (https://github.com/jb55/clightning-dumpkeys),
itself adapted from 42b66a6b81/src/script/descriptor.cpp (L25)

Co-authored-by: William Casarin <jb55@jb55.com>
Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

common/Makefile

@@ -21,6 +21,7 @@ COMMON_SRC_NOGEN :=				\
 	common/daemon_conn.c			\
 	common/decode_array.c			\
 	common/derive_basepoints.c		\
+	common/descriptor_checksum.c		\
 	common/dev_disconnect.c			\
 	common/dijkstra.c			\
 	common/ecdh_hsmd.c			\

common/descriptor_checksum.c

@@ -0,0 +1,82 @@
+#include <ccan/short_types/short_types.h>
+#include <common/descriptor_checksum.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+
+static const char CHECKSUM_CHARSET[] = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";
+
+static const char INPUT_CHARSET[] =
+	"0123456789()[],'/*abcdefgh@:$%{}"
+	"IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~"
+	"ijklmnopqrstuvwxyzABCDEFGH`#\"\\ ";
+
+static inline int charset_find(char ch) {
+	for (size_t i = 0; i < sizeof(INPUT_CHARSET); i++) {
+		if (INPUT_CHARSET[i] == ch)
+			return i;
+	}
+	return -1;
+}
+
+static u64 polymod(u64 c, int val)
+{
+	u8 c0 = c >> 35;
+	c = ((c & 0x7ffffffff) << 5) ^ val;
+	if (c0 & 1) c ^= 0xf5dee51989;
+	if (c0 & 2) c ^= 0xa9fdca3312;
+	if (c0 & 4) c ^= 0x1bab10e32d;
+	if (c0 & 8) c ^= 0x3706b1677a;
+	if (c0 & 16) c ^= 0x644d626ffd;
+	return c;
+}
+
+
+bool descriptor_checksum(const char *descriptor, int desc_size,
+			 struct descriptor_checksum *checksum)
+{
+	checksum->csum[0] = 0;
+
+	int j;
+	u64 c = 1;
+	int cls = 0;
+	int clscount = 0;
+
+	for (int i = 0; i < desc_size; i++) {
+		char ch = descriptor[i];
+		int pos = charset_find(ch);
+		if (pos == -1) {
+			checksum->csum[0] = 0;
+			return false;
+		}
+		/* Emit a symbol for the position inside the group, for every
+		 * character. */
+		c = polymod(c, pos & 31);
+
+		/* Accumulate the group numbers */
+		cls = cls * 3 + (pos >> 5);
+
+		if (++clscount == 3) {
+			c = polymod(c, cls);
+			cls = 0;
+			clscount = 0;
+		}
+	}
+
+	if (clscount > 0)
+		c = polymod(c, cls);
+
+	/* Shift further to determine the checksum. */
+	for (j = 0; j < DESCRIPTOR_CHECKSUM_LENGTH; ++j)
+		c = polymod(c, 0);
+
+	/* Prevent appending zeroes from not affecting the checksum. */
+	c ^= 1;
+
+	for (j = 0; j < DESCRIPTOR_CHECKSUM_LENGTH; ++j)
+		checksum->csum[j] = CHECKSUM_CHARSET[(c >> (5 * (7 - j))) & 31];
+
+	checksum->csum[DESCRIPTOR_CHECKSUM_LENGTH] = 0;
+
+	return true;
+}

common/descriptor_checksum.h

@@ -0,0 +1,16 @@
+#ifndef LIGHTNING_COMMON_DESCRIPTOR_CHECKSUM_H
+#define LIGHTNING_COMMON_DESCRIPTOR_CHECKSUM_H
+#include "config.h"
+#include <stdbool.h>
+
+/* https://github.com/bitcoin/bitcoin/blob/master/doc/descriptors.md#reference */
+#define DESCRIPTOR_CHECKSUM_LENGTH 8
+
+struct descriptor_checksum {
+	char csum[DESCRIPTOR_CHECKSUM_LENGTH + 1];
+};
+
+bool descriptor_checksum(const char *descriptor, int desc_size,
+			 struct descriptor_checksum *checksum);
+
+#endif /* LIGHTNING_COMMON_DESCRIPTOR_CHECKSUM_H */