bolt11.c

#include <bitcoin/address.h>
#include <bitcoin/base58.h>
#include <bitcoin/chainparams.h>
#include <bitcoin/script.h>
#include <ccan/array_size/array_size.h>
#include <ccan/cast/cast.h>
#include <ccan/endian/endian.h>
#include <ccan/tal/str/str.h>
#include <common/bech32.h>
#include <common/bech32_util.h>
#include <common/bolt11.h>
#include <common/utils.h>
#include <errno.h>
#include <hsmd/gen_hsm_wire.h>
#include <inttypes.h>
#include <lightningd/hsm_control.h>
#include <lightningd/jsonrpc.h>
#include <lightningd/lightningd.h>
#include <lightningd/log.h>
#include <stdarg.h>
#include <stdio.h>
#include <wire/wire.h>
#include <wire/wire_sync.h>

struct multiplier {
        const char letter;
        /* We can't represent p postfix to msat, so we multiply this by 10 */
        u64 m10;
};

/* BOLT #11:
 *
 * The following `multiplier` letters are defined:
 *
 * * `m` (milli): multiply by 0.001
 * * `u` (micro): multiply by 0.000001
 * * `n` (nano): multiply by 0.000000001
 * * `p` (pico): multiply by 0.000000000001
 */
static struct multiplier multipliers[] = {
        { 'm', 10 * MSAT_PER_BTC / 1000 },
        { 'u', 10 * MSAT_PER_BTC / 1000000 },
        { 'n', 10 * MSAT_PER_BTC / 1000000000 },
        { 'p', 10 * MSAT_PER_BTC / 1000000000000ULL }
};

/* If pad is false, we discard any bits which don't fit in the last byte.
 * Otherwise we add an extra byte */
static bool pull_bits(struct hash_u5 *hu5,
                      u5 **data, size_t *data_len, void *dst, size_t nbits,
                      bool pad)
{
        size_t n5 = nbits / 5;
        size_t len = 0;

        if (nbits % 5)
                n5++;

        if (*data_len < n5)
                return false;
        if (!bech32_convert_bits(dst, &len, 8, *data, n5, 5, pad))
                return false;
        if (hu5)
                hash_u5(hu5, *data, n5);
        *data += n5;
        *data_len -= n5;

        return true;
}

/* For pulling fields where we should have checked it will succeed already. */
#ifndef NDEBUG
#define pull_bits_certain(hu5, data, data_len, dst, nbits, pad)             \
        assert(pull_bits((hu5), (data), (data_len), (dst), (nbits), (pad)))
#else
#define pull_bits_certain pull_bits
#endif

/* Helper for pulling a variable-length big-endian int. */
static bool pull_uint(struct hash_u5 *hu5,
                      u5 **data, size_t *data_len,
                      u64 *val, size_t databits)
{
        be64 be_val;

        /* Too big. */
        if (databits > sizeof(be_val) * CHAR_BIT)
                return false;
        if (!pull_bits(hu5, data, data_len, &be_val, databits, true))
                return false;
        *val = be64_to_cpu(be_val) >> (sizeof(be_val) * CHAR_BIT - databits);
        return true;
}

static size_t num_u8(size_t num_u5)
{
        return (num_u5 * 5 + 4) / 8;
}

/* Frees bolt11, returns NULL. */
static struct bolt11 *decode_fail(struct bolt11 *b11, char **fail,
                                  const char *fmt, ...)
        PRINTF_FMT(3,4);

static struct bolt11 *decode_fail(struct bolt11 *b11, char **fail,
                                  const char *fmt, ...)
{
        va_list ap;

        va_start(ap, fmt);
        *fail = tal_vfmt(tal_parent(b11), fmt, ap);
        va_end(ap);
        return tal_free(b11);
}

/*
 * These handle specific fields in the payment request; returning the problem
 * if any, or NULL.
 */
static char *unknown_field(struct bolt11 *b11,
                           struct hash_u5 *hu5,
                           u5 **data, size_t *data_len,
                           u5 type, size_t length)
{
        struct bolt11_field *extra = tal(b11, struct bolt11_field);
        u8 u8data[num_u8(length)];

        extra->tag = type;
        extra->data = tal_dup_arr(extra, u5, *data, length, 0);
        list_add_tail(&b11->extra_fields, &extra->list);

        pull_bits_certain(hu5, data, data_len, u8data, length, false);
        (*data) += length;
        (*data_len) -= length;
        return NULL;
}

/* BOLT #11:
 *
 * `p` (1): `data_length` 52.  256-bit SHA256 payment_hash.  Preimage of this
 * provides proof of payment
 */
static void decode_p(struct bolt11 *b11,
                     struct hash_u5 *hu5,
                     u5 **data, size_t *data_len,
                     size_t data_length, bool *have_p)
{
        /* BOLT #11:
         *
         * A payer... SHOULD use the first `p` field that it did NOT
         * skip as the payment hash.
         */
        if (*have_p) {
                unknown_field(b11, hu5, data, data_len, 'p', data_length);
                return;
        }

        /* BOLT #11:
         *
         * A reader... MUST skip over unknown fields, OR an `f` field
	 * with unknown `version`, OR `p`, `h`, or `n` fields that do
	 * NOT have `data_length`s of 52, 52, or 53, respectively.
	*/
        if (data_length != 52) {
                unknown_field(b11, hu5, data, data_len, 'p', data_length);
                return;
        }

        pull_bits_certain(hu5, data, data_len, &b11->payment_hash, 256, false);
        *have_p = true;
}

/* BOLT #11:
 *
 * `d` (13): `data_length` variable.  Short description of purpose of payment
 * (UTF-8), e.g. '1 cup of coffee' or 'ナンセンス 1杯'
 */
static void decode_d(struct bolt11 *b11,
                     struct hash_u5 *hu5,
                     u5 **data, size_t *data_len,
                     size_t data_length, bool *have_d)
{
        if (*have_d) {
                unknown_field(b11, hu5, data, data_len, 'd', data_length);
                return;
        }

        b11->description = tal_arrz(b11, char, num_u8(data_length) + 1);
        pull_bits_certain(hu5, data, data_len, (u8 *)b11->description,
                          data_length*5, false);
        *have_d = true;
}

/* BOLT #11:
 *
 * `h` (23): `data_length` 52.  256-bit description of purpose of payment
 * (SHA256).  This is used to commit to an associated description that is over
 * 639 bytes, but the transport mechanism for the description in that case is
 * transport specific and not defined here.
 */
static void decode_h(struct bolt11 *b11,
                     struct hash_u5 *hu5,
                     u5 **data, size_t *data_len,
                     size_t data_length, bool *have_h)
{
        if (*have_h) {
                unknown_field(b11, hu5, data, data_len, 'h', data_length);
                return;
        }

        /* BOLT #11:
	 *
	 * A reader... MUST skip over unknown fields, OR an `f` field
	 * with unknown `version`, OR `p`, `h`, or `n` fields that do
	 * NOT have `data_length`s of 52, 52, or 53, respectively. */
        if (data_length != 52) {
                unknown_field(b11, hu5, data, data_len, 'h', data_length);
                return;
        }

        b11->description_hash = tal(b11, struct sha256);
        pull_bits_certain(hu5, data, data_len, b11->description_hash, 256,
                          false);
        *have_h = true;
}

/* BOLT #11:
 *
 * `x` (6): `data_length` variable.  `expiry` time in seconds
 * (big-endian). Default is 3600 (1 hour) if not specified.
 */
#define DEFAULT_X 3600
static char *decode_x(struct bolt11 *b11,
                      struct hash_u5 *hu5,
                      u5 **data, size_t *data_len,
                      size_t data_length, const bool *have_x)
{
        if (*have_x)
                return unknown_field(b11, hu5, data, data_len, 'x',
                                     data_length);

        /* FIXME: Put upper limit in bolt 11 */
        if (!pull_uint(hu5, data, data_len, &b11->expiry, data_length * 5))
                return tal_fmt(b11, "x: length %zu chars is excessive",
                               *data_len);
        return NULL;
}

/* BOLT #11:
 *
 * `c` (24): `data_length` variable.  `min_final_cltv_expiry` to use for the
 * last HTLC in the route. Default is 9 if not specified.
 */
#define DEFAULT_C 9
static char *decode_c(struct bolt11 *b11,
                      struct hash_u5 *hu5,
                      u5 **data, size_t *data_len,
                      size_t data_length, const bool *have_c)
{
        u64 c;
        if (*have_c)
                return unknown_field(b11, hu5, data, data_len, 'c',
                                     data_length);

        /* FIXME: Put upper limit in bolt 11 */
        if (!pull_uint(hu5, data, data_len, &c, data_length * 5))
                return tal_fmt(b11, "c: length %zu chars is excessive",
                               *data_len);
        b11->min_final_cltv_expiry = c;
        /* Can overflow, since c is 64 bits but value must be < 32 bits */
        if (b11->min_final_cltv_expiry != c)
                return tal_fmt(b11, "c: %"PRIu64" is too large", c);

        return NULL;
}

static char *decode_n(struct bolt11 *b11,
                      struct hash_u5 *hu5,
                      u5 **data, size_t *data_len,
                      size_t data_length, bool *have_n)
{
        if (*have_n)
                return unknown_field(b11, hu5, data, data_len, 'n',
                                     data_length);

        /* BOLT #11:
	 *
	 * A reader... MUST skip over unknown fields, OR an `f` field
	 * with unknown `version`, OR `p`, `h`, or `n` fields that do
	 * NOT have `data_length`s of 52, 52, or 53, respectively. */
        if (data_length != 53)
                return unknown_field(b11, hu5, data, data_len, 'n',
                                     data_length);

        pull_bits_certain(hu5, data, data_len, &b11->receiver_id.k,
			  data_length * 5, false);
        if (!node_id_valid(&b11->receiver_id))
                return tal_fmt(b11, "n: invalid pubkey %s",
			       type_to_string(tmpctx, struct node_id,
					      &b11->receiver_id));

        *have_n = true;
        return NULL;
}

/* BOLT #11:
 *
 * `f` (9): `data_length` variable, depending on version. Fallback
 * on-chain address: for Bitcoin, this starts with a 5-bit `version`
 * and contains a witness program or P2PKH or P2SH address.
 */
static char *decode_f(struct bolt11 *b11,
                      struct hash_u5 *hu5,
                      u5 **data, size_t *data_len,
                      size_t data_length)
{
        u64 version;
	u8 *fallback;

        if (!pull_uint(hu5, data, data_len, &version, 5))
                return tal_fmt(b11, "f: data_length %zu short", data_length);
        data_length--;

        /* BOLT #11:
         *
	 * for Bitcoin payments... MUST set an `f` field to a valid
	 * witness version and program, OR to `17` followed by a
	 * public key hash, OR to `18` followed by a script hash.
	*/
        if (version == 17) {
                /* Pay to pubkey hash (P2PKH) */
                struct bitcoin_address pkhash;
                if (num_u8(data_length) != sizeof(pkhash))
                        return tal_fmt(b11, "f: pkhash length %zu",
                                       data_length);

                pull_bits_certain(hu5, data, data_len, &pkhash, data_length*5,
                                  false);
                fallback = scriptpubkey_p2pkh(b11, &pkhash);
        } else if (version == 18) {
                /* Pay to pubkey script hash (P2SH) */
                struct ripemd160 shash;
                if (num_u8(data_length) != sizeof(shash))
                        return tal_fmt(b11, "f: p2sh length %zu",
                                       data_length);

                pull_bits_certain(hu5, data, data_len, &shash, data_length*5,
                                  false);
                fallback = scriptpubkey_p2sh_hash(b11, &shash);
        } else if (version < 17) {
                u8 *f = tal_arr(b11, u8, data_length * 5 / 8);
                if (version == 0) {
                        if (tal_count(f) != 20 && tal_count(f) != 32)
                                return tal_fmt(b11,
                                               "f: witness v0 bad length %zu",
                                               data_length);
                }
                pull_bits_certain(hu5, data, data_len, f, data_length * 5,
                                  false);
                fallback = scriptpubkey_witness_raw(b11, version,
						    f, tal_count(f));
                tal_free(f);
        } else
                return unknown_field(b11, hu5, data, data_len, 'f',
                                     data_length);

	if (b11->fallbacks == NULL)
		b11->fallbacks = tal_arr(b11, const u8 *, 1);
	else
		tal_resize(&b11->fallbacks, tal_count(b11->fallbacks) + 1);

	b11->fallbacks[tal_count(b11->fallbacks)-1]
		= tal_steal(b11->fallbacks, fallback);
        return NULL;
}

static bool fromwire_route_info(const u8 **cursor, size_t *max,
                                struct route_info *route_info)
{
        fromwire_node_id(cursor, max, &route_info->pubkey);
        fromwire_short_channel_id(cursor, max, &route_info->short_channel_id);
        route_info->fee_base_msat = fromwire_u32(cursor, max);
        route_info->fee_proportional_millionths = fromwire_u32(cursor, max);
        route_info->cltv_expiry_delta = fromwire_u16(cursor, max);
        return *cursor != NULL;
}

static void towire_route_info(u8 **pptr, const struct route_info *route_info)
{
        towire_node_id(pptr, &route_info->pubkey);
        towire_short_channel_id(pptr, &route_info->short_channel_id);
        towire_u32(pptr, route_info->fee_base_msat);
        towire_u32(pptr, route_info->fee_proportional_millionths);
        towire_u16(pptr, route_info->cltv_expiry_delta);
}

/* BOLT #11:
 *
 * `r` (3): `data_length` variable.  One or more entries containing
 * extra routing information for a private route; there may be more
 * than one `r` field
 *
 *   * `pubkey` (264 bits)
 *   * `short_channel_id` (64 bits)
 *   * `fee_base_msat` (32 bits, big-endian)
 *   * `fee_proportional_millionths` (32 bits, big-endian)
 *   * `cltv_expiry_delta` (16 bits, big-endian)
 */
static char *decode_r(struct bolt11 *b11,
                      struct hash_u5 *hu5,
                      u5 **data, size_t *data_len,
                      size_t data_length)
{
        size_t rlen = data_length * 5 / 8;
        u8 *r8 = tal_arr(tmpctx, u8, rlen);
        size_t n = 0;
        struct route_info *r = tal_arr(tmpctx, struct route_info, n);
        const u8 *cursor = r8;

        /* Route hops don't split in 5 bit boundaries, so convert whole thing */
        pull_bits_certain(hu5, data, data_len, r8, data_length * 5, false);

        do {
		struct route_info ri;
                if (!fromwire_route_info(&cursor, &rlen, &ri)) {
                        return tal_fmt(b11, "r: hop %zu truncated", n);
                }
		tal_arr_expand(&r, ri);
        } while (rlen);

        /* Append route */
	tal_arr_expand(&b11->routes, tal_steal(b11, r));
        return NULL;
}

struct bolt11 *new_bolt11(const tal_t *ctx,
			  const struct amount_msat *msat TAKES)
{
        struct bolt11 *b11 = tal(ctx, struct bolt11);

        list_head_init(&b11->extra_fields);
        b11->description = NULL;
        b11->description_hash = NULL;
        b11->fallbacks = NULL;
        b11->routes = NULL;
        b11->msat = NULL;
        b11->expiry = DEFAULT_X;
        b11->min_final_cltv_expiry = DEFAULT_C;

        if (msat)
                b11->msat = tal_dup(b11, struct amount_msat, msat);
        return b11;
}

/* Decodes and checks signature; returns NULL on error. */
struct bolt11 *bolt11_decode(const tal_t *ctx, const char *str,
                             const char *description, char **fail)
{
        char *hrp, *amountstr, *prefix;
        u5 *data;
        size_t data_len;
        struct bolt11 *b11 = new_bolt11(ctx, NULL);
        u8 sig_and_recid[65];
        secp256k1_ecdsa_recoverable_signature sig;
        struct hash_u5 hu5;
        struct sha256 hash;
        bool have_p = false, have_n = false, have_d = false, have_h = false,
                have_x = false, have_c = false;

        b11->routes = tal_arr(b11, struct route_info *, 0);

	/* BOLT #11:
	 *
	 * If a URI scheme is desired, the current recommendation is to either
	 * use 'lightning:' as a prefix before the BOLT-11 encoding
	 */
	if (strstarts(str, "lightning:") || strstarts(str, "LIGHTNING:"))
		str += strlen("lightning:");

        if (strlen(str) < 8)
                return decode_fail(b11, fail, "Bad bech32 string");

        hrp = tal_arr(tmpctx, char, strlen(str) - 6);
        data = tal_arr(tmpctx, u5, strlen(str) - 8);

        if (!bech32_decode(hrp, data, &data_len, str, (size_t)-1))
                return decode_fail(b11, fail, "Bad bech32 string");

        /* For signature checking at the end. */
        hash_u5_init(&hu5, hrp);

        /* BOLT #11:
         *
	 * The human-readable part of a Lightning invoice consists of two sections:
	 * 1. `prefix`: `ln` + BIP-0173 currency prefix (e.g. `lnbc` for Bitcoin mainnet,
	 *    `lntb` for Bitcoin testnet, and `lnbcrt` for Bitcoin regtest)
	 * 1. `amount`: optional number in that currency, followed by an optional
	 *    `multiplier` letter. The unit encoded here is the 'social' convention of a payment unit -- in the case of Bitcoin the unit is 'bitcoin' NOT satoshis.
	*/
        prefix = tal_strndup(tmpctx, hrp, strcspn(hrp, "0123456789"));

        /* BOLT #11:
         *
	 * A reader...if it does NOT understand the `prefix`... MUST fail the payment.
	 */
        if (!strstarts(prefix, "ln"))
                return decode_fail(b11, fail,
                                   "Prefix '%s' does not start with ln", prefix);

        b11->chain = chainparams_by_bip173(prefix + 2);
        if (!b11->chain)
                return decode_fail(b11, fail, "Unknown chain %s", prefix + 2);

        /* BOLT #11:
         *
         *   - if the `amount` is empty:
         * */
        amountstr = tal_strdup(tmpctx, hrp + strlen(prefix));
        if (streq(amountstr, "")) {
                /* BOLT #11:
                 *
	         * - SHOULD indicate to the payer that amount is unspecified.
                 */
                b11->msat = NULL;
        } else {
                u64 m10 = 10;
                u64 amount;
                char *end;

                /* Gather and trim multiplier */
                end = amountstr + strlen(amountstr)-1;
                for (size_t i = 0; i < ARRAY_SIZE(multipliers); i++) {
                        if (*end == multipliers[i].letter) {
                                m10 = multipliers[i].m10;
                                *end = '\0';
                                break;
                        }
                }

                /* BOLT #11:
                 *
                 * if `amount` contains a non-digit OR is followed by
		 * anything except a `multiplier` (see table above)... MUST fail the
		 * payment.
                 **/
                amount = strtoull(amountstr, &end, 10);
                if (amount == ULLONG_MAX && errno == ERANGE)
                        return decode_fail(b11, fail,
                                           "Invalid amount '%s'", amountstr);
                if (!*amountstr || *end)
                        return decode_fail(b11, fail,
                                           "Invalid amount postfix '%s'", end);

                /* BOLT #11:
                 *
                 * if the `multiplier` is present...  MUST multiply
		 * `amount` by the `multiplier` value to derive the
		 * amount required for payment.
		*/
                b11->msat = tal(b11, struct amount_msat);
                b11->msat->millisatoshis = amount * m10 / 10; /* Raw: raw amount multiplier calculation */
        }

        /* BOLT #11:
         *
	 * The data part of a Lightning invoice consists of multiple sections:
	 *
	 * 1. `timestamp`: seconds-since-1970 (35 bits, big-endian)
	 * 1. zero or more tagged parts
	 * 1. `signature`: Bitcoin-style signature of above (520 bits)
	 */
        if (!pull_uint(&hu5, &data, &data_len, &b11->timestamp, 35))
                return decode_fail(b11, fail, "Can't get 35-bit timestamp");

        while (data_len > 520 / 5) {
                const char *problem = NULL;
                u64 type, data_length;

                /* BOLT #11:
                 *
                 * Each Tagged Field is of the form:
                 *
                 * 1. `type` (5 bits)
                 * 1. `data_length` (10 bits, big-endian)
                 * 1. `data` (`data_length` x 5 bits)
                 */
                if (!pull_uint(&hu5, &data, &data_len, &type, 5)
                    || !pull_uint(&hu5, &data, &data_len, &data_length, 10))
                        return decode_fail(b11, fail,
                                           "Can't get tag and length");

                /* Can't exceed total data remaining. */
                if (data_length > data_len)
                        return decode_fail(b11, fail, "%c: truncated",
                                           bech32_charset[type]);

                switch (bech32_charset[type]) {
                case 'p':
                        decode_p(b11, &hu5, &data, &data_len, data_length,
                                 &have_p);
                        break;

                case 'd':
                        decode_d(b11, &hu5, &data, &data_len, data_length,
                                 &have_d);
                        break;

                case 'h':
                        decode_h(b11, &hu5, &data, &data_len, data_length,
                                 &have_h);
                        break;

                case 'n':
                        problem = decode_n(b11, &hu5, &data,
                                           &data_len, data_length,
                                           &have_n);
                        break;

                case 'x':
                        problem = decode_x(b11, &hu5, &data,
                                           &data_len, data_length,
                                           &have_x);
                        break;

                case 'c':
                        problem = decode_c(b11, &hu5, &data,
                                           &data_len, data_length,
                                           &have_c);
                        break;

                case 'f':
                        problem = decode_f(b11, &hu5, &data,
                                           &data_len, data_length);
                        break;
                case 'r':
                        problem = decode_r(b11, &hu5, &data, &data_len,
                                           data_length);
                        break;
                default:
                        unknown_field(b11, &hu5, &data, &data_len,
                                      bech32_charset[type], data_length);
                }
                if (problem)
                        return decode_fail(b11, fail, "%s", problem);
        }

        if (!have_p)
                return decode_fail(b11, fail, "No valid 'p' field found");

        if (have_h) {
                struct sha256 sha;

                /* BOLT #11:
		 *
                 * A reader... MUST check that the SHA2 256-bit hash
		 * in the `h` field exactly matches the hashed
		 * description.
                 */
                if (!description)
                        return decode_fail(b11, fail,
                                           "h: no description to check");
                sha256(&sha, description, strlen(description));
                if (!sha256_eq(b11->description_hash, &sha))
                        return decode_fail(b11, fail,
                                           "h: does not match description");
        }

        hash_u5_done(&hu5, &hash);

        /* BOLT #11:
         *
         * A writer...MUST set `signature` to a valid 512-bit
	 * secp256k1 signature of the SHA2 256-bit hash of the
	 * human-readable part, represented as UTF-8 bytes,
	 * concatenated with the data part (excluding the signature)
	 * with 0 bits appended to pad the data to the next byte
	 * boundary, with a trailing byte containing the recovery ID
	 * (0, 1, 2, or 3).
	 */
        if (!pull_bits(NULL, &data, &data_len, sig_and_recid, 520, false))
                return decode_fail(b11, fail, "signature truncated");

        assert(data_len == 0);

        if (!secp256k1_ecdsa_recoverable_signature_parse_compact
            (secp256k1_ctx, &sig, sig_and_recid, sig_and_recid[64]))
                return decode_fail(b11, fail, "signature invalid");

        secp256k1_ecdsa_recoverable_signature_convert(secp256k1_ctx,
                                                      &b11->sig, &sig);

        /* BOLT #11:
	 *
         * A reader...  MUST check that the `signature` is valid (see
	 * the `n` tagged field specified below). ... A reader...
	 * MUST use the `n` field to validate the signature instead of
	 * performing signature recovery.
	 */
        if (!have_n) {
		struct pubkey k;
                if (!secp256k1_ecdsa_recover(secp256k1_ctx,
                                             &k.pubkey,
                                             &sig,
                                             (const u8 *)&hash))
                        return decode_fail(b11, fail,
                                           "signature recovery failed");
		node_id_from_pubkey(&b11->receiver_id, &k);
        } else {
		struct pubkey k;
		/* n parsing checked this! */
		if (!pubkey_from_node_id(&k, &b11->receiver_id))
			abort();
                if (!secp256k1_ecdsa_verify(secp256k1_ctx, &b11->sig,
                                            (const u8 *)&hash,
                                            &k.pubkey))
                        return decode_fail(b11, fail, "invalid signature");
        }

        return b11;
}

/* Helper for pushing a variable-length big-endian int. */
static void push_varlen_uint(u5 **data, u64 val, size_t nbits)
{
        be64 be_val = cpu_to_be64(val << (64 - nbits));
        bech32_push_bits(data, &be_val, nbits);
}

/* BOLT #11:
 *
 * Each Tagged Field is of the form:
 *
 * 1. `type` (5 bits)
 * 1. `data_length` (10 bits, big-endian)
 * 1. `data` (`data_length` x 5 bits)
 */
static void push_field(u5 **data, char type, const void *src, size_t nbits)
{
        assert(bech32_charset_rev[(unsigned char)type] >= 0);
        push_varlen_uint(data, bech32_charset_rev[(unsigned char)type], 5);
        push_varlen_uint(data, (nbits + 4) / 5, 10);
        bech32_push_bits(data, src, nbits);
}

/* BOLT #11:
 *
 * SHOULD use the minimum `data_length` possible for `x` and `c` fields.
 */
static void push_varlen_field(u5 **data, char type, u64 val)
{
        assert(bech32_charset_rev[(unsigned char)type] >= 0);
        push_varlen_uint(data, bech32_charset_rev[(unsigned char)type], 5);

        for (size_t nbits = 5; nbits < 65; nbits += 5) {
                if ((val >> nbits) == 0) {
                        push_varlen_uint(data, nbits / 5, 10);
                        push_varlen_uint(data, val,  nbits);
                        return;
                }
        }
        /* Can't be encoded in <= 60 bits. */
        abort();
}

/* BOLT #11:
 *
 * `f` (9): `data_length` variable, depending on version. Fallback
 * on-chain address: for Bitcoin, this starts with a 5-bit `version`
 * and contains a witness program or P2PKH or P2SH address.
 */
static void push_fallback_addr(u5 **data, u5 version, const void *addr, u16 addr_len)
{
        push_varlen_uint(data, bech32_charset_rev[(unsigned char)'f'], 5);
        push_varlen_uint(data, ((5 + addr_len * CHAR_BIT) + 4) / 5, 10);
        push_varlen_uint(data, version, 5);
        bech32_push_bits(data, addr, addr_len * CHAR_BIT);
}

static void encode_p(u5 **data, const struct sha256 *hash)
{
        push_field(data, 'p', hash, 256);
}

static void encode_d(u5 **data, const char *description)
{
        push_field(data, 'd', description, strlen(description) * CHAR_BIT);
}

static void encode_h(u5 **data, const struct sha256 *hash)
{
        push_field(data, 'h', hash, 256);
}

static void encode_n(u5 **data, const struct node_id *id)
{
        assert(node_id_valid(id));
        push_field(data, 'n', id->k, sizeof(id->k) * CHAR_BIT);
}

static void encode_x(u5 **data, u64 expiry)
{
        push_varlen_field(data, 'x', expiry);
}

static void encode_c(u5 **data, u16 min_final_cltv_expiry)
{
        push_varlen_field(data, 'c', min_final_cltv_expiry);
}

static void encode_f(u5 **data, const u8 *fallback)
{
        struct bitcoin_address pkh;
        struct ripemd160 sh;
        struct sha256 wsh;

        /* BOLT #11:
         *
	 * for Bitcoin payments... MUST set an `f` field to a valid
	 * witness version and program, OR to `17` followed by a
	 * public key hash, OR to `18` followed by a script hash.
         */
        if (is_p2pkh(fallback, &pkh)) {
                push_fallback_addr(data, 17, &pkh, sizeof(pkh));
        } else if (is_p2sh(fallback, &sh)) {
                push_fallback_addr(data, 18, &sh, sizeof(sh));
        } else if (is_p2wpkh(fallback, &pkh)) {
                push_fallback_addr(data, 0, &pkh, sizeof(pkh));
        } else if (is_p2wsh(fallback, &wsh)) {
                push_fallback_addr(data, 0, &wsh, sizeof(wsh));
        } else if (tal_count(fallback)
                   && fallback[0] >= 0x50
                   && fallback[0] < (0x50+16)) {
                /* Other (future) witness versions: turn OP_N into N */
                push_fallback_addr(data, fallback[0] - 0x50, fallback + 1,
                                   tal_count(fallback) - 1);
        } else {
                /* Copy raw. */
                push_field(data, 'f',
                           fallback, tal_count(fallback) * CHAR_BIT);
        }
}

static void encode_r(u5 **data, const struct route_info *r)
{
        u8 *rinfo = tal_arr(NULL, u8, 0);

        for (size_t i = 0; i < tal_count(r); i++)
                towire_route_info(&rinfo, &r[i]);

        push_field(data, 'r', rinfo, tal_count(rinfo) * CHAR_BIT);
        tal_free(rinfo);
}

static void encode_extra(u5 **data, const struct bolt11_field *extra)
{
        size_t len;

        push_varlen_uint(data, extra->tag, 5);
        push_varlen_uint(data, tal_count(extra->data), 10);

        /* extra->data is already u5s, so do this raw. */
        len = tal_count(*data);
        tal_resize(data, len + tal_count(extra->data));
        memcpy(*data + len, extra->data, tal_count(extra->data));
}

/* Encodes, even if it's nonsense. */
char *bolt11_encode_(const tal_t *ctx,
                     const struct bolt11 *b11, bool n_field,
		     bool (*sign)(const u5 *u5bytes,
				  const u8 *hrpu8,
				  secp256k1_ecdsa_recoverable_signature *rsig,
                                  void *arg),
		     void *arg)
{
        u5 *data = tal_arr(tmpctx, u5, 0);
        char *hrp, *output;
        u64 amount;
        struct bolt11_field *extra;
        secp256k1_ecdsa_recoverable_signature rsig;
        u8 sig_and_recid[65];
        u8 *hrpu8;
        int recid;

        /* BOLT #11:
         *
	 * A writer:
	 * - MUST encode `prefix` using the currency required for successful payment.
	 * - if a specific minimum `amount` is required for successful payment:
	 *   - MUST include that `amount`.
	 * - MUST encode `amount` as a positive decimal integer with no leading 0s.
	 * - SHOULD use the shortest representation possible, by using the largest multiplier or omitting the multiplier.
	 */
        if (b11->msat) {
                char postfix;
		u64 msat = b11->msat->millisatoshis; /* Raw: best-multiplier calc */
                if (msat % MSAT_PER_BTC == 0) {
                        postfix = '\0';
                        amount = msat / MSAT_PER_BTC;
                } else {
                        size_t i;
                        for (i = 0; i < ARRAY_SIZE(multipliers)-1; i++) {
                                if (!(msat * 10 % multipliers[i].m10))
                                        break;
                        }
                        postfix = multipliers[i].letter;
                        amount = msat * 10 / multipliers[i].m10;
                }
                hrp = tal_fmt(tmpctx, "ln%s%"PRIu64"%c",
                              b11->chain->bip173_name, amount, postfix);
        } else
                hrp = tal_fmt(tmpctx, "ln%s", b11->chain->bip173_name);

        /* BOLT #11:
         *
	 * 1. `timestamp`: seconds-since-1970 (35 bits, big-endian)
	 * 1. zero or more tagged parts
	 * 1. `signature`: Bitcoin-style signature of above (520 bits)
         */
        push_varlen_uint(&data, b11->timestamp, 35);

        /* BOLT #11:
         *
	 * if a writer offers more than one of any field type,
	 * it... MUST specify the most-preferred field first, followed
	 * by less-preferred fields, in order.
	 */
        /* Thus we do built-in fields, then extras last. */
        encode_p(&data, &b11->payment_hash);

        if (b11->description)
                encode_d(&data, b11->description);

        if (b11->description_hash)
                encode_h(&data, b11->description_hash);

        if (n_field)
                encode_n(&data, &b11->receiver_id);

        if (b11->expiry != DEFAULT_X)
                encode_x(&data, b11->expiry);

        if (b11->min_final_cltv_expiry != DEFAULT_C)
                encode_c(&data, b11->min_final_cltv_expiry);

        for (size_t i = 0; i < tal_count(b11->fallbacks); i++)
                encode_f(&data, b11->fallbacks[i]);

        for (size_t i = 0; i < tal_count(b11->routes); i++)
                encode_r(&data, b11->routes[i]);

        list_for_each(&b11->extra_fields, extra, list)
                encode_extra(&data, extra);

        /* FIXME: towire_ should check this? */
        if (tal_count(data) > 65535)
                return NULL;

        /* Need exact length here */
        hrpu8 = tal_dup_arr(tmpctx, u8, (const u8 *)hrp, strlen(hrp), 0);
        if (!sign(data, hrpu8, &rsig, arg))
                return NULL;

        secp256k1_ecdsa_recoverable_signature_serialize_compact(
                secp256k1_ctx,
                sig_and_recid,
                &recid,
                &rsig);
        sig_and_recid[64] = recid;

        bech32_push_bits(&data, sig_and_recid, sizeof(sig_and_recid) * CHAR_BIT);

        output = tal_arr(ctx, char, strlen(hrp) + tal_count(data) + 8);
        if (!bech32_encode(output, hrp, data, tal_count(data), (size_t)-1))
                output = tal_free(output);

        return output;
}