internal/bech32: avoid panic on Unicode input

Reported by Joe Doyle of Trail of Bits.

internal/bech32/bech32.go

@@ -154,14 +154,21 @@ func Decode(s string) (hrp string, data []byte, err error) {
 			return "", nil, fmt.Errorf("invalid character human-readable part: s[%d]=%d", p, c)
 		}
 	}
-	s = strings.ToLower(s)
 	for p, c := range s[pos+1:] {
+		// Fold ASCII explicitly. Unicode case folding can turn a non-ASCII
+		// rune into a shorter valid charset member.
+		if c >= 'A' && c <= 'Z' {
+			c += 'a' - 'A'
+		}
 		d := strings.IndexRune(charset, c)
 		if d == -1 {
 			return "", nil, fmt.Errorf("invalid character data part: s[%d]=%v", p, c)
 		}
 		data = append(data, byte(d))
 	}
+	if len(data) < 6 {
+		return "", nil, fmt.Errorf("data part too short")
+	}
 	if !verifyChecksum(hrp, data) {
 		return "", nil, fmt.Errorf("invalid checksum")
 	}

internal/bech32/bech32_test.go

@@ -93,3 +93,23 @@ func TestBech32(t *testing.T) {
 		}
 	}
 }
+
+func TestDecodeShortDataPart(t *testing.T) {
+	kelvin := string(rune(0x212A))
+	for _, s := range []string{
+		"AA3100AC" + kelvin,
+		"BK1" + kelvin + "0JFM",
+		"AQM1KZCML",
+	} {
+		func() {
+			defer func() {
+				if r := recover(); r != nil {
+					t.Errorf("Decode(%+q) panicked: %v", s, r)
+				}
+			}()
+			if _, _, err := bech32.Decode(s); err == nil {
+				t.Errorf("Decode(%+q) = nil error, want error", s)
+			}
+		}()
+	}
+}