bech32: Ensure HRP is lowercase when encoding.

BIP173 specifically calls out that encoders must always output an all
lowercase bech32 string and that the lowercase form is used when
determining a character's value for calculating the checksum.

Currently, the implementation does not respect either of those
requirements.

This modifies the Encode function to convert the provided HRP to
lowercase to ensure the requirements are satisfied and adds tests
accordingly.

bech32/bech32.go

@@ -150,12 +150,13 @@ func bech32VerifyChecksum(hrp string, data []byte) bool {
 }
 
 // DecodeNoLimit decodes a bech32 encoded string, returning the human-readable
-// part and the data part excluding the checksum. This function does NOT
+// part and the data part excluding the checksum.  This function does NOT
 // validate against the BIP-173 maximum length allowed for bech32 strings and
 // is meant for use in custom applications (such as lightning network payment
 // requests), NOT on-chain addresses.
 //
-// Note that the returned data is 5-bit (base32) encoded.
+// Note that the returned data is 5-bit (base32) encoded and the human-readable
+// part will be lowercase.
 func DecodeNoLimit(bech string) (string, []byte, error) {
 	// The minimum allowed size of a bech32 string is 8 characters, since it
 	// needs a non-empty HRP, a separator, and a 6 character checksum.
@@ -234,7 +235,8 @@ func DecodeNoLimit(bech string) (string, []byte, error) {
 // Decode decodes a bech32 encoded string, returning the human-readable part and
 // the data part excluding the checksum.
 //
-// Note that the returned data is 5-bit (base32) encoded.
+// Note that the returned data is 5-bit (base32) encoded and the human-readable
+// part will be lowercase.
 func Decode(bech string) (string, []byte, error) {
 	// The maximum allowed length for a bech32 string is 90.
 	if len(bech) > 90 {
@@ -244,13 +246,14 @@ func Decode(bech string) (string, []byte, error) {
 	return DecodeNoLimit(bech)
 }
 
-// Encode encodes a byte slice into a bech32 string with the
-// human-readable part hrb. Note that the bytes must each encode 5 bits
-// (base32).
+// Encode encodes a byte slice into a bech32 string with the given
+// human-readable part (HRP).  The HRP will be converted to lowercase if needed
+// since mixed cased encodings are not permitted and lowercase is used for
+// checksum purposes.  Note that the bytes must each encode 5 bits (base32).
 func Encode(hrp string, data []byte) (string, error) {
-
-	// The resulting bech32 string is the concatenation of the hrp, the
-	// separator 1, data and the 6-byte checksum.
+	// The resulting bech32 string is the concatenation of the lowercase hrp,
+	// the separator 1, data and the 6-byte checksum.
+	hrp = strings.ToLower(hrp)
 	var bldr strings.Builder
 	bldr.Grow(len(hrp) + 1 + len(data) + 6)
 	bldr.WriteString(hrp)

bech32/bech32_test.go

@@ -86,6 +86,95 @@ func TestBech32(t *testing.T) {
 	}
 }
 
+// TestMixedCaseEncode ensures mixed case HRPs are converted to lowercase as
+// expected when encoding and that decoding the produced encoding when converted
+// to all uppercase produces the lowercase HRP and original data.
+func TestMixedCaseEncode(t *testing.T) {
+	tests := []struct {
+		name    string
+		hrp     string
+		data    string
+		encoded string
+	}{{
+		name:    "all uppercase HRP with no data",
+		hrp:     "A",
+		data:    "",
+		encoded: "a12uel5l",
+	}, {
+		name:    "all uppercase HRP with data",
+		hrp:     "UPPERCASE",
+		data:    "787878",
+		encoded: "uppercase10pu8sss7kmp",
+	}, {
+		name:    "mixed case HRP even offsets uppercase",
+		hrp:     "AbCdEf",
+		data:    "00443214c74254b635cf84653a56d7c675be77df",
+		encoded: "abcdef1qpzry9x8gf2tvdw0s3jn54khce6mua7lmqqqxw",
+	}, {
+		name:    "mixed case HRP odd offsets uppercase ",
+		hrp:     "aBcDeF",
+		data:    "00443214c74254b635cf84653a56d7c675be77df",
+		encoded: "abcdef1qpzry9x8gf2tvdw0s3jn54khce6mua7lmqqqxw",
+	}, {
+		name:    "all lowercase HRP",
+		hrp:     "abcdef",
+		data:    "00443214c74254b635cf84653a56d7c675be77df",
+		encoded: "abcdef1qpzry9x8gf2tvdw0s3jn54khce6mua7lmqqqxw",
+	}}
+
+	for _, test := range tests {
+		// Convert the text hex to bytes, convert those bytes from base256 to
+		// base32, then ensure the encoded result with the HRP provided in the
+		// test data is as expected.
+		data, err := hex.DecodeString(test.data)
+		if err != nil {
+			t.Errorf("%q: invalid hex %q: %v", test.name, test.data, err)
+			continue
+		}
+		convertedData, err := ConvertBits(data, 8, 5, true)
+		if err != nil {
+			t.Errorf("%q: unexpected convert bits error: %v", test.name,
+				err)
+			continue
+		}
+		gotEncoded, err := Encode(test.hrp, convertedData)
+		if err != nil {
+			t.Errorf("%q: unexpected encode error: %v", test.name, err)
+			continue
+		}
+		if gotEncoded != test.encoded {
+			t.Errorf("%q: mismatched encoding -- got %q, want %q", test.name,
+				gotEncoded, test.encoded)
+			continue
+		}
+
+		// Ensure the decoding the expected lowercase encoding converted to all
+		// uppercase produces the lowercase HRP and original data.
+		gotHRP, gotData, err := Decode(strings.ToUpper(test.encoded))
+		if err != nil {
+			t.Errorf("%q: unexpected decode error: %v", test.name, err)
+			continue
+		}
+		wantHRP := strings.ToLower(test.hrp)
+		if gotHRP != wantHRP {
+			t.Errorf("%q: mismatched decoded HRP -- got %q, want %q", test.name,
+				gotHRP, wantHRP)
+			continue
+		}
+		convertedGotData, err := ConvertBits(gotData, 5, 8, false)
+		if err != nil {
+			t.Errorf("%q: unexpected convert bits error: %v", test.name,
+				err)
+			continue
+		}
+		if !bytes.Equal(convertedGotData, data) {
+			t.Errorf("%q: mismatched data -- got %x, want %x", test.name,
+				convertedGotData, data)
+			continue
+		}
+	}
+}
+
 // TestCanDecodeUnlimtedBech32 tests whether decoding a large bech32 string works
 // when using the DecodeNoLimit version
 func TestCanDecodeUnlimtedBech32(t *testing.T) {
@@ -118,7 +207,6 @@ func TestCanDecodeUnlimtedBech32(t *testing.T) {
 // cycle of a bech32 string. It also  reports the allocation count, which we
 // expect to be 2 for a fully optimized cycle.
 func BenchmarkEncodeDecodeCycle(b *testing.B) {
-
 	// Use a fixed, 49-byte raw data for testing.
 	inputData, err := hex.DecodeString("cbe6365ddbcda9a9915422c3f091c13f8c7b2f263b8d34067bd12c274408473fa764871c9dd51b1bb34873b3473b633ed1")
 	if err != nil {

bech32/example_test.go

@@ -45,5 +45,5 @@ func ExampleEncode() {
 	fmt.Println("Encoded Data:", encoded)
 
 	// Output:
-	// Encoded Data: customHrp!11111q123jhxapqv3shgcgumastr
+	// Encoded Data: customhrp!11111q123jhxapqv3shgcgkxpuhe
 }