From fdbdcf1560f690f95905b7b8a38344e7b3aa65d5 Mon Sep 17 00:00:00 2001
From: Daniel McNally <mcnallydp@gmail.com>
Date: Mon, 19 Feb 2018 10:18:16 -0500
Subject: [PATCH] zpay32: handle segwit prefixes > 2 chars

This change fixes a bug when an invoice is decoded for a network
whose bech32 segwit prefix is longer than 2 characters. The length
of the Bech32HRPSegwit network parameter is used to determine
where in the human-readable portion of the invoice the amount
begins, rather than assuming it begins after the first four
characters.

Decode() now throws an error when the encoded invoice does
not match the active network.

Changes the minimum hrp length check to >= 3 instead of >= 4.

Also removes a redundant "if ...; err != nil check" that was raising
a warning in invoice.go.
---
 zpay32/invoice.go | 37 +++++++++++++------------------------
 1 file changed, 13 insertions(+), 24 deletions(-)

diff --git a/zpay32/invoice.go b/zpay32/invoice.go
index 0856d3ea..4dbbfeec 100644
--- a/zpay32/invoice.go
+++ b/zpay32/invoice.go
@@ -265,9 +265,9 @@ func NewInvoice(net *chaincfg.Params, paymentHash [32]byte,
 	return invoice, nil
 }
 
-// Decode parses the provided encoded invoice, and returns a decoded Invoice in
-// case it is valid by BOLT-0011.
-func Decode(invoice string) (*Invoice, error) {
+// Decode parses the provided encoded invoice and returns a decoded Invoice if
+// it is valid by BOLT-0011 and matches the provided active network.
+func Decode(invoice string, net *chaincfg.Params) (*Invoice, error) {
 	decodedInvoice := Invoice{}
 
 	// Decode the invoice using the modified bech32 decoder.
@@ -276,9 +276,9 @@ func Decode(invoice string) (*Invoice, error) {
 		return nil, err
 	}
 
-	// We expect the human-readable part to at least have ln + two chars
+	// We expect the human-readable part to at least have ln + one char
 	// encoding the network.
-	if len(hrp) < 4 {
+	if len(hrp) < 3 {
 		return nil, fmt.Errorf("hrp too short")
 	}
 
@@ -288,24 +288,17 @@ func Decode(invoice string) (*Invoice, error) {
 	}
 
 	// The next characters should be a valid prefix for a segwit BIP173
-	// address. This will also determine which network this invoice is
-	// meant for.
-	var net *chaincfg.Params
-	if strings.HasPrefix(hrp[2:], chaincfg.MainNetParams.Bech32HRPSegwit) {
-		net = &chaincfg.MainNetParams
-	} else if strings.HasPrefix(hrp[2:], chaincfg.TestNet3Params.Bech32HRPSegwit) {
-		net = &chaincfg.TestNet3Params
-	} else if strings.HasPrefix(hrp[2:], chaincfg.SimNetParams.Bech32HRPSegwit) {
-		net = &chaincfg.SimNetParams
-	} else {
+	// address that match the active network.
+	if !strings.HasPrefix(hrp[2:], net.Bech32HRPSegwit) {
 		return nil, fmt.Errorf("unknown network")
 	}
 	decodedInvoice.Net = net
 
-	// Optionally, if there's anything left of the HRP, it encodes the
-	// payment amount.
-	if len(hrp) > 4 {
-		amount, err := decodeAmount(hrp[4:])
+	// Optionally, if there's anything left of the HRP after ln + the segwit
+	// prefix, we try to decode this as the payment amount.
+	var netPrefixLength = len(net.Bech32HRPSegwit) + 2
+	if len(hrp) > netPrefixLength {
+		amount, err := decodeAmount(hrp[netPrefixLength:])
 		if err != nil {
 			return nil, err
 		}
@@ -573,11 +566,7 @@ func parseData(invoice *Invoice, data []byte, net *chaincfg.Params) error {
 
 	// The rest are tagged parts.
 	tagData := data[7:]
-	if err := parseTaggedFields(invoice, tagData, net); err != nil {
-		return err
-	}
-
-	return nil
+	return parseTaggedFields(invoice, tagData, net)
 }
 
 // parseTimestamp converts a 35-bit timestamp (encoded in base32) to uint64.