common: add check that pico-valued invoices are round numbers.

Otherwise you can ask for a sub-millisatoshi amount, which is dumb and
violates the spec.

See-also: https://github.com/lightningnetwork/lightning-rfc/pull/736
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: We now reject invoices which ask for sub-millisatoshi amounts

common/bolt11.c

@@ -649,6 +649,17 @@ struct bolt11 *bolt11_decode(const tal_t *ctx, const char *str,
 		 * amount required for payment.
 		*/
                 b11->msat = tal(b11, struct amount_msat);
+		/* BOLT-50143e388e16a449a92ed574fc16eb35b51426b9 #11:
+		 *
+		 * - if multiplier is `p` and the last decimal of `amount` is
+		 *   not 0:
+		 *    - MUST fail the payment.
+		 */
+		if (amount * m10 % 10 != 0)
+			return decode_fail(b11, fail,
+					   "Invalid sub-millisatoshi amount"
+					   " '%sp'", amountstr);
+
                 b11->msat->millisatoshis = amount * m10 / 10; /* Raw: raw amount multiplier calculation */
         }
 

common/test/run-bolt11.c

@@ -562,6 +562,13 @@ int main(void)
 	assert(!bolt11_decode(tmpctx, "lnbc2500x1pvjluezpp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqypqdq5xysxxatsyp3k7enxv4jsxqzpujr6jxr9gq9pv6g46y7d20jfkegkg4gljz2ea2a3m9lmvvr95tq2s0kvu70u3axgelz3kyvtp2ywwt0y8hkx2869zq5dll9nelr83zzqqpgl2zg", NULL, &fail));
 	assert(streq(fail, "Invalid amount postfix 'x'"));
 
+	/* BOLT- #11:
+	 * > ### Invalid sub-millisatoshi precision.
+	 * > lnbc2500000001p1pvjluezpp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqypqdq5xysxxatsyp3k7enxv4jsxqzpu7hqtk93pkf7sw55rdv4k9z2vj050rxdr6za9ekfs3nlt5lr89jqpdmxsmlj9urqumg0h9wzpqecw7th56tdms40p2ny9q4ddvjsedzcplva53s
+	 */
+	assert(!bolt11_decode(tmpctx, "lnbc2500000001p1pvjluezpp5qqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqqqsyqcyq5rqwzqfqypqdq5xysxxatsyp3k7enxv4jsxqzpu7hqtk93pkf7sw55rdv4k9z2vj050rxdr6za9ekfs3nlt5lr89jqpdmxsmlj9urqumg0h9wzpqecw7th56tdms40p2ny9q4ddvjsedzcplva53s", NULL, &fail));
+	assert(streq(fail, "Invalid sub-millisatoshi amount '2500000001p'"));
+
 	/* FIXME: Test the others! */
 	wally_cleanup(0);
 	tal_free(tmpctx);