diff --git a/common/bolt12.c b/common/bolt12.c
index 03d145094..be1eecd4d 100644
--- a/common/bolt12.c
+++ b/common/bolt12.c
@@ -437,3 +437,67 @@ bool bolt12_has_prefix(const char *str)
 	return bolt12_has_invoice_prefix(str) || bolt12_has_offer_prefix(str) ||
 	       bolt12_has_request_prefix(str);
 }
+
+/* Inclusive span of tlv range >= minfield and <= maxfield */
+size_t tlv_span(const u8 *tlvstream, size_t minfield, size_t maxfield,
+		size_t *startp)
+{
+	const u8 *cursor = tlvstream;
+	size_t tlvlen = tal_bytelen(tlvstream);
+	const u8 *start, *end;
+
+	start = end = NULL;
+	while (tlvlen) {
+		const u8 *before = cursor;
+		bigsize_t type = fromwire_bigsize(&cursor, &tlvlen);
+		bigsize_t len = fromwire_bigsize(&cursor, &tlvlen);
+		if (type >= minfield && start == NULL)
+			start = before;
+		if (type > maxfield)
+			break;
+		fromwire_pad(&cursor, &tlvlen, len);
+		end = cursor;
+	}
+	if (!start)
+		start = end;
+
+	if (startp)
+		*startp = start - tlvstream;
+	return end - start;
+}
+
+static void calc_offer(const u8 *tlvstream, struct sha256 *id)
+{
+	size_t start, len;
+
+	/* BOLT-offers #12:
+	 * A writer of an offer:
+	 *  - MUST NOT set any tlv fields greater or equal to 80, or tlv field 0.
+	 */
+	len = tlv_span(tlvstream, 1, 79, &start);
+	sha256(id, tlvstream + start, len);
+}
+
+void offer_offer_id(const struct tlv_offer *offer, struct sha256 *id)
+{
+	u8 *wire = tal_arr(tmpctx, u8, 0);
+
+	towire_tlv_offer(&wire, offer);
+	calc_offer(wire, id);
+}
+
+void invreq_offer_id(const struct tlv_invoice_request *invreq, struct sha256 *id)
+{
+	u8 *wire = tal_arr(tmpctx, u8, 0);
+
+	towire_tlv_invoice_request(&wire, invreq);
+	calc_offer(wire, id);
+}
+
+void invoice_offer_id(const struct tlv_invoice *invoice, struct sha256 *id)
+{
+	u8 *wire = tal_arr(tmpctx, u8, 0);
+
+	towire_tlv_invoice(&wire, invoice);
+	calc_offer(wire, id);
+}
diff --git a/common/bolt12.h b/common/bolt12.h
index 93801dc2d..a337a85e0 100644
--- a/common/bolt12.h
+++ b/common/bolt12.h
@@ -120,4 +120,21 @@ void offer_period_paywindow(const struct tlv_offer_recurrence *recurrence,
  */
 bool bolt12_has_prefix(const char *str);
 
+/**
+ * tlv_span: Find span of this inclusive range of tlv types
+ * @tlvstream: the tlv stream
+ * @minfield: lowest field to find
+ * @maxfield: highest field to find
+ * @start: (out) optional offset of start.
+ *
+ * Returns length, so 0 means nothing found.
+*/
+size_t tlv_span(const u8 *tlvstream, size_t minfield, size_t maxfield,
+		size_t *start);
+
+/* Get offer_id referred to by various structures. */
+void offer_offer_id(const struct tlv_offer *offer, struct sha256 *id);
+void invreq_offer_id(const struct tlv_invoice_request *invreq, struct sha256 *id);
+void invoice_offer_id(const struct tlv_invoice *invoice, struct sha256 *id);
+
 #endif /* LIGHTNING_COMMON_BOLT12_H */
diff --git a/common/test/run-bolt12_decode.c b/common/test/run-bolt12_decode.c
index 1138561f4..9043b876e 100644
--- a/common/test/run-bolt12_decode.c
+++ b/common/test/run-bolt12_decode.c
@@ -49,12 +49,18 @@ int features_unsupported(const struct feature_set *our_features UNNEEDED,
 /* Generated stub for fromwire */
 const u8 *fromwire(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, void *copy UNNEEDED, size_t n UNNEEDED)
 { fprintf(stderr, "fromwire called!\n"); abort(); }
+/* Generated stub for fromwire_bigsize */
+bigsize_t fromwire_bigsize(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_bigsize called!\n"); abort(); }
 /* Generated stub for fromwire_bool */
 bool fromwire_bool(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 { fprintf(stderr, "fromwire_bool called!\n"); abort(); }
 /* Generated stub for fromwire_fail */
 void *fromwire_fail(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 { fprintf(stderr, "fromwire_fail called!\n"); abort(); }
+/* Generated stub for fromwire_pad */
+void fromwire_pad(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, size_t num UNNEEDED)
+{ fprintf(stderr, "fromwire_pad called!\n"); abort(); }
 /* Generated stub for fromwire_secp256k1_ecdsa_signature */
 void fromwire_secp256k1_ecdsa_signature(const u8 **cursor UNNEEDED, size_t *max UNNEEDED,
 					secp256k1_ecdsa_signature *signature UNNEEDED)
diff --git a/common/test/run-bolt12_period.c b/common/test/run-bolt12_period.c
index 3d6ab9a63..9c29e154a 100644
--- a/common/test/run-bolt12_period.c
+++ b/common/test/run-bolt12_period.c
@@ -52,12 +52,18 @@ bool from_bech32_charset(const tal_t *ctx UNNEEDED,
 /* Generated stub for fromwire */
 const u8 *fromwire(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, void *copy UNNEEDED, size_t n UNNEEDED)
 { fprintf(stderr, "fromwire called!\n"); abort(); }
+/* Generated stub for fromwire_bigsize */
+bigsize_t fromwire_bigsize(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_bigsize called!\n"); abort(); }
 /* Generated stub for fromwire_bool */
 bool fromwire_bool(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 { fprintf(stderr, "fromwire_bool called!\n"); abort(); }
 /* Generated stub for fromwire_fail */
 void *fromwire_fail(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 { fprintf(stderr, "fromwire_fail called!\n"); abort(); }
+/* Generated stub for fromwire_pad */
+void fromwire_pad(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, size_t num UNNEEDED)
+{ fprintf(stderr, "fromwire_pad called!\n"); abort(); }
 /* Generated stub for fromwire_secp256k1_ecdsa_signature */
 void fromwire_secp256k1_ecdsa_signature(const u8 **cursor UNNEEDED, size_t *max UNNEEDED,
 					secp256k1_ecdsa_signature *signature UNNEEDED)
diff --git a/common/test/run-tlv_span.c b/common/test/run-tlv_span.c
new file mode 100644
index 000000000..0fc9d03c8
--- /dev/null
+++ b/common/test/run-tlv_span.c
@@ -0,0 +1,133 @@
+#include "config.h"
+#include "../bolt12.c"
+#include "../bigsize.c"
+#include "../../wire/fromwire.c"
+#include <common/setup.h>
+#include <stdio.h>
+
+/* AUTOGENERATED MOCKS START */
+/* Generated stub for amount_asset_is_main */
+bool amount_asset_is_main(struct amount_asset *asset UNNEEDED)
+{ fprintf(stderr, "amount_asset_is_main called!\n"); abort(); }
+/* Generated stub for amount_asset_to_sat */
+struct amount_sat amount_asset_to_sat(struct amount_asset *asset UNNEEDED)
+{ fprintf(stderr, "amount_asset_to_sat called!\n"); abort(); }
+/* Generated stub for amount_sat */
+struct amount_sat amount_sat(u64 satoshis UNNEEDED)
+{ fprintf(stderr, "amount_sat called!\n"); abort(); }
+/* Generated stub for amount_sat_add */
+ bool amount_sat_add(struct amount_sat *val UNNEEDED,
+				       struct amount_sat a UNNEEDED,
+				       struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_add called!\n"); abort(); }
+/* Generated stub for amount_sat_eq */
+bool amount_sat_eq(struct amount_sat a UNNEEDED, struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_eq called!\n"); abort(); }
+/* Generated stub for amount_sat_greater_eq */
+bool amount_sat_greater_eq(struct amount_sat a UNNEEDED, struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_greater_eq called!\n"); abort(); }
+/* Generated stub for amount_sat_sub */
+ bool amount_sat_sub(struct amount_sat *val UNNEEDED,
+				       struct amount_sat a UNNEEDED,
+				       struct amount_sat b UNNEEDED)
+{ fprintf(stderr, "amount_sat_sub called!\n"); abort(); }
+/* Generated stub for amount_sat_to_asset */
+struct amount_asset amount_sat_to_asset(struct amount_sat *sat UNNEEDED, const u8 *asset UNNEEDED)
+{ fprintf(stderr, "amount_sat_to_asset called!\n"); abort(); }
+/* Generated stub for amount_tx_fee */
+struct amount_sat amount_tx_fee(u32 fee_per_kw UNNEEDED, size_t weight UNNEEDED)
+{ fprintf(stderr, "amount_tx_fee called!\n"); abort(); }
+/* Generated stub for features_unsupported */
+int features_unsupported(const struct feature_set *our_features UNNEEDED,
+			 const u8 *their_features UNNEEDED,
+			 enum feature_place p UNNEEDED)
+{ fprintf(stderr, "features_unsupported called!\n"); abort(); }
+/* Generated stub for from_bech32_charset */
+bool from_bech32_charset(const tal_t *ctx UNNEEDED,
+			 const char *bech32 UNNEEDED, size_t bech32_len UNNEEDED,
+			 char **hrp UNNEEDED, u8 **data UNNEEDED)
+{ fprintf(stderr, "from_bech32_charset called!\n"); abort(); }
+/* Generated stub for fromwire_tlv_invoice */
+struct tlv_invoice *fromwire_tlv_invoice(const tal_t *ctx UNNEEDED,
+						  const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_tlv_invoice called!\n"); abort(); }
+/* Generated stub for fromwire_tlv_invoice_request */
+struct tlv_invoice_request *fromwire_tlv_invoice_request(const tal_t *ctx UNNEEDED,
+						  const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_tlv_invoice_request called!\n"); abort(); }
+/* Generated stub for fromwire_tlv_offer */
+struct tlv_offer *fromwire_tlv_offer(const tal_t *ctx UNNEEDED,
+						  const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
+{ fprintf(stderr, "fromwire_tlv_offer called!\n"); abort(); }
+/* Generated stub for merkle_tlv */
+void merkle_tlv(const struct tlv_field *fields UNNEEDED, struct sha256 *merkle UNNEEDED)
+{ fprintf(stderr, "merkle_tlv called!\n"); abort(); }
+/* Generated stub for sighash_from_merkle */
+void sighash_from_merkle(const char *messagename UNNEEDED,
+			 const char *fieldname UNNEEDED,
+			 const struct sha256 *merkle UNNEEDED,
+			 struct sha256 *sighash UNNEEDED)
+{ fprintf(stderr, "sighash_from_merkle called!\n"); abort(); }
+/* Generated stub for to_bech32_charset */
+char *to_bech32_charset(const tal_t *ctx UNNEEDED,
+			const char *hrp UNNEEDED, const u8 *data UNNEEDED)
+{ fprintf(stderr, "to_bech32_charset called!\n"); abort(); }
+/* Generated stub for towire */
+void towire(u8 **pptr UNNEEDED, const void *data UNNEEDED, size_t len UNNEEDED)
+{ fprintf(stderr, "towire called!\n"); abort(); }
+/* Generated stub for towire_bool */
+void towire_bool(u8 **pptr UNNEEDED, bool v UNNEEDED)
+{ fprintf(stderr, "towire_bool called!\n"); abort(); }
+/* Generated stub for towire_secp256k1_ecdsa_signature */
+void towire_secp256k1_ecdsa_signature(u8 **pptr UNNEEDED,
+			      const secp256k1_ecdsa_signature *signature UNNEEDED)
+{ fprintf(stderr, "towire_secp256k1_ecdsa_signature called!\n"); abort(); }
+/* Generated stub for towire_sha256 */
+void towire_sha256(u8 **pptr UNNEEDED, const struct sha256 *sha256 UNNEEDED)
+{ fprintf(stderr, "towire_sha256 called!\n"); abort(); }
+/* Generated stub for towire_tlv_invoice */
+void towire_tlv_invoice(u8 **pptr UNNEEDED, const struct tlv_invoice *record UNNEEDED)
+{ fprintf(stderr, "towire_tlv_invoice called!\n"); abort(); }
+/* Generated stub for towire_tlv_invoice_request */
+void towire_tlv_invoice_request(u8 **pptr UNNEEDED, const struct tlv_invoice_request *record UNNEEDED)
+{ fprintf(stderr, "towire_tlv_invoice_request called!\n"); abort(); }
+/* Generated stub for towire_tlv_offer */
+void towire_tlv_offer(u8 **pptr UNNEEDED, const struct tlv_offer *record UNNEEDED)
+{ fprintf(stderr, "towire_tlv_offer called!\n"); abort(); }
+/* Generated stub for towire_u32 */
+void towire_u32(u8 **pptr UNNEEDED, u32 v UNNEEDED)
+{ fprintf(stderr, "towire_u32 called!\n"); abort(); }
+/* Generated stub for towire_u64 */
+void towire_u64(u8 **pptr UNNEEDED, u64 v UNNEEDED)
+{ fprintf(stderr, "towire_u64 called!\n"); abort(); }
+/* Generated stub for towire_u8 */
+void towire_u8(u8 **pptr UNNEEDED, u8 v UNNEEDED)
+{ fprintf(stderr, "towire_u8 called!\n"); abort(); }
+/* Generated stub for towire_u8_array */
+void towire_u8_array(u8 **pptr UNNEEDED, const u8 *arr UNNEEDED, size_t num UNNEEDED)
+{ fprintf(stderr, "towire_u8_array called!\n"); abort(); }
+/* AUTOGENERATED MOCKS END */
+
+int main(int argc, char *argv[])
+{
+	u8 *wire;
+	size_t len, start;
+
+	common_setup(argv[0]);
+
+	wire = tal_hexdata(tmpctx, "0010b8538094dbd70d8a0f0439d8e64f766f022006226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f0801020a0b73696d706c6520746573741621035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d502006226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f582103aa144bcfb083a73e1f51416003083e63c6c68951c04733ab4d042d0cc2237f46f0408d43842db95b6377612674bb21e45ef4e1643289db27e32edb5c15422664b8ca7b052759729cafd7260634a6de053a292ca1859914a4b21e4b8e20e20d2f911b",
+			   strlen("0010b8538094dbd70d8a0f0439d8e64f766f022006226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f0801020a0b73696d706c6520746573741621035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d502006226e46111a0b59caaf126043eb5bbf28c34f3a5e332a1fc7b2b73cf188910f582103aa144bcfb083a73e1f51416003083e63c6c68951c04733ab4d042d0cc2237f46f0408d43842db95b6377612674bb21e45ef4e1643289db27e32edb5c15422664b8ca7b052759729cafd7260634a6de053a292ca1859914a4b21e4b8e20e20d2f911b"));
+	len = tlv_span(wire, 0, UINT64_MAX, &start);
+	assert(start == 0);
+	assert(len == tal_bytelen(wire));
+
+	len = tlv_span(wire, 1, UINT64_MAX, &start);
+	assert(start == strlen("0010b8538094dbd70d8a0f0439d8e64f766f") / 2);
+	assert(len == tal_bytelen(wire) - start);
+
+	len = tlv_span(wire, 0, 1, &start);
+	assert(start == 0);
+	assert(len == strlen("0010b8538094dbd70d8a0f0439d8e64f766f") / 2);
+	common_shutdown();
+	return 0;
+}