diff --git a/common/Makefile b/common/Makefile
index 5290afe19..de27d5c8f 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -11,6 +11,7 @@ COMMON_SRC :=					\
 	common/derive_basepoints.c		\
 	common/dev_disconnect.c			\
 	common/funding_tx.c			\
+	common/hash_u5.c			\
 	common/htlc_state.c			\
 	common/htlc_tx.c			\
 	common/htlc_wire.c			\
@@ -54,7 +55,7 @@ common/gen_htlc_state_names.h: common/htlc_state.h ccan/ccan/cdump/tools/cdump-e
 check-makefile: check-common-makefile
 
 check-common-makefile:
-	@if [ x"`LC_ALL=C ls common/*.h | grep -v ^common/gen_`" != x"`echo $(COMMON_HEADERS_NOGEN) | tr ' ' '\n' | LC_ALL=C sort`" ]; then echo COMMON_HEADERS_NOGEN incorrect; exit 1; fi
+	if [ x"`LC_ALL=C ls common/*.h | grep -v ^common/gen_`" != x"`echo $(COMMON_HEADERS_NOGEN) | tr ' ' '\n' | LC_ALL=C sort`" ]; then echo COMMON_HEADERS_NOGEN incorrect; exit 1; fi
 
 check-source-bolt: $(COMMON_SRC:%=bolt-check/%) $(COMMON_HEADERS:%=bolt-check/%)
 check-whitespace: $(COMMON_SRC:%=check-whitespace/%) $(COMMON_HEADERS:%=check-whitespace/%)
diff --git a/common/hash_u5.c b/common/hash_u5.c
new file mode 100644
index 000000000..0d3eb91a6
--- /dev/null
+++ b/common/hash_u5.c
@@ -0,0 +1,47 @@
+#include <ccan/endian/endian.h>
+#include <common/hash_u5.h>
+#include <string.h>
+
+void hash_u5_init(struct hash_u5 *hu5, const char *hrp)
+{
+        hu5->buf = 0;
+        hu5->num_bits = 0;
+        sha256_init(&hu5->hash);
+        sha256_update(&hu5->hash, hrp, strlen(hrp));
+}
+
+void hash_u5(struct hash_u5 *hu5, const u8 *u5, size_t len)
+{
+        size_t bits = len * 5;
+
+        while (bits) {
+                size_t n = 5;
+
+                if (bits < n)
+                        n = bits;
+
+                hu5->buf <<= n;
+                hu5->buf |= (*u5 >> (5-n));
+                bits -= n;
+                hu5->num_bits += n;
+
+                if (n == 5)
+                        u5++;
+
+                if (hu5->num_bits >= 32) {
+                        be32 be32 = cpu_to_be32(hu5->buf >> (hu5->num_bits-32));
+                        sha256_update(&hu5->hash, &be32, sizeof(be32));
+                        hu5->num_bits -= 32;
+                }
+        }
+}
+
+void hash_u5_done(struct hash_u5 *hu5, struct sha256 *res)
+{
+        if (hu5->num_bits) {
+                be32 be32 = cpu_to_be32(hu5->buf << (32 - hu5->num_bits));
+
+                sha256_update(&hu5->hash, &be32, (hu5->num_bits + 7) / 8);
+        }
+        sha256_done(&hu5->hash, res);
+}
diff --git a/common/hash_u5.h b/common/hash_u5.h
new file mode 100644
index 000000000..65ded9a6d
--- /dev/null
+++ b/common/hash_u5.h
@@ -0,0 +1,21 @@
+/* bech32 (thus bolt11) deal in 5-bit values */
+#ifndef LIGHTNING_COMMON_HASH_U5_H
+#define LIGHTNING_COMMON_HASH_U5_H
+#include "config.h"
+#include <ccan/crypto/sha256/sha256.h>
+#include <ccan/short_types/short_types.h>
+
+/* Type to annotate a 5 bit value. */
+typedef unsigned char u5;
+
+struct hash_u5 {
+        u64 buf;
+        unsigned int num_bits;
+        struct sha256_ctx hash;
+};
+
+void hash_u5_init(struct hash_u5 *hu5, const char *hrp);
+void hash_u5(struct hash_u5 *hu5, const u5 *u5, size_t len);
+void hash_u5_done(struct hash_u5 *hu5, struct sha256 *res);
+
+#endif /* LIGHTNING_COMMON_HASH_U5_H */