address: reject v2-v16 segwit addresses encoded with bech32

BIP-350 requires that segregated witness outputs of version 1 through 16
use the bech32m checksum, while only version 0 uses bech32. decodeSegWitAddress
only special-cased versions 0 and 1, so a witness program with version 2-16
encoded using the legacy bech32 checksum decoded successfully, in violation of
the spec and contrary to the BIP-350 reference decoder (which rejects any
non-zero witness version that is not bech32m).

Generalize the version 1 check to cover all versions >= 1, matching the
reference decode() function. Add the relevant BIP-350 INVALID_ADDRESS vectors
(v2 and v16 encoded with bech32) as a regression test; they decoded
successfully before this change and are now rejected.

Signed-off-by: Lrifton92 <Lrifton92@users.noreply.github.com>

address/address.go

@@ -275,16 +275,19 @@ func decodeSegWitAddress(address string) (byte, []byte, error) {
 			"version 0: %v", len(regrouped))
 	}
 
-	// For witness version 0, the bech32 encoding must be used.
+	// Per BIP-350, witness version 0 must use the bech32 encoding, while
+	// witness versions 1 through 16 must use the bech32m encoding. Previously
+	// only versions 0 and 1 were checked, which let a v2-v16 program encoded
+	// with the legacy bech32 checksum decode successfully in violation of the
+	// spec.
 	if version == 0 && bech32version != bech32.Version0 {
 		return 0, nil, fmt.Errorf("invalid checksum expected bech32 " +
 			"encoding for address with witness version 0")
 	}
 
-	// For witness version 1, the bech32m encoding must be used.
-	if version == 1 && bech32version != bech32.VersionM {
-		return 0, nil, fmt.Errorf("invalid checksum expected bech32m " +
-			"encoding for address with witness version 1")
+	if version >= 1 && bech32version != bech32.VersionM {
+		return 0, nil, fmt.Errorf("invalid checksum expected bech32m "+
+			"encoding for address with witness version %d", version)
 	}
 
 	return version, regrouped, nil

address/bip350_diff_test.go

@@ -0,0 +1,60 @@
+package address
+
+import (
+	"testing"
+)
+
+// TestSegWitAddressBIP350Vectors runs a subset of the official BIP-173/BIP-350
+// reference segwit address vectors against decodeSegWitAddress, which implements
+// the reference decode() function from those BIPs. It specifically guards the
+// rule that witness versions 1 through 16 MUST use the bech32m encoding (BIP-350
+// line "Addresses for segregated witness outputs version 1 through 16 use
+// Bech32m"), not just version 1.
+func TestSegWitAddressBIP350Vectors(t *testing.T) {
+	// Valid Bech32m vectors (BIP-350). These MUST decode without error. The HRP
+	// is validated at a higher layer; here we only validate segwit-encoding
+	// rules, so the HRP value is irrelevant.
+	valid := []string{
+		"bc1pw508d6qejxtdg4y5r3zarvary0c5xw7kw508d6qejxtdg4y5r3zarvary0c5xw7kt5nd6y",
+		"BC1SW50QGDZ25J",
+		"bc1zw508d6qejxtdg4y5r3zarvaryvaxxpcs",
+		"bc1p0xlxvlhemja6c4dqv22uapctqupfhlxm9h8z3k2e72q4k9hcz7vqzk5jj0",
+		// Valid Bech32 v0 vectors.
+		"BC1QW508D6QEJXTDG4Y5R3ZARVARY0C5XW7KV8F3T4",
+		"tb1qrp33g0q5c5txsp9arysrx4k6zdkfs4nce4xj0gdcccefvpysxf3q0sl5k7",
+	}
+	for _, addr := range valid {
+		if _, _, err := decodeSegWitAddress(addr); err != nil {
+			t.Errorf("BIP350 valid vector rejected: %q -> %v", addr, err)
+		}
+	}
+
+	// Invalid vectors from the BIP-350 INVALID_ADDRESS list whose defect lies in
+	// the segwit-encoding rules (not merely the HRP). Each MUST be rejected.
+	invalid := []struct {
+		name string
+		addr string
+	}{
+		// Witness version 1, Bech32 instead of Bech32m.
+		{"v1-bech32-not-bech32m", "bc1p0xlxvlhemja6c4dqv22uapctqupfhlxm9h8z3k2e72q4k9hcz7vqh2y7hd"},
+		// Witness version 2, Bech32 instead of Bech32m. This is the vector that
+		// previously decoded successfully because only v0/v1 were checked.
+		{"v2-bech32-not-bech32m", "tb1z0xlxvlhemja6c4dqv22uapctqupfhlxm9h8z3k2e72q4k9hcz7vqglt7rf"},
+		// Witness version 16, Bech32 instead of Bech32m.
+		{"v16-bech32-not-bech32m", "BC1S0XLXVLHEMJA6C4DQV22UAPCTQUPFHLXM9H8Z3K2E72Q4K9HCZ7VQ54WELL"},
+		// Witness version 0, Bech32m instead of Bech32.
+		{"v0-bech32m-not-bech32", "bc1qw508d6qejxtdg4y5r3zarvary0c5xw7kemeawh"},
+		// Invalid program length (1 byte).
+		{"program-length-1", "bc1pw5dgrnzv"},
+		// Invalid program length (41 bytes).
+		{"program-length-41", "bc1p0xlxvlhemja6c4dqv22uapctqupfhlxm9h8z3k2e72q4k9hcz7v8n0nx0muaewav253zgeav"},
+		// Invalid program length for witness version 0 (per BIP-141).
+		{"v0-program-length-16", "BC1QR508D6QEJXTDG4Y5R3ZARVARYV98GJ9P"},
+	}
+	for _, tc := range invalid {
+		if _, _, err := decodeSegWitAddress(tc.addr); err == nil {
+			t.Errorf("BIP350 invalid vector %s accepted: %q (expected error)",
+				tc.name, tc.addr)
+		}
+	}
+}