encoding/base32: support custom and disabled padding when decoding

CL 38634 added support for custom (and disabled) padding characters
when encoding, but didn't update the decoding paths. This adds
decoding support.

Fixes #20854

Change-Id: I9fb1a0aaebb27f1204c9f726a780d5784eb71024
Reviewed-on: https://go-review.googlesource.com/47341
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/base32/base32.go

@@ -279,19 +279,28 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 		dlen := 8
 
 		for j := 0; j < 8; {
-			if len(src) == 0 {
+
+			// We have reached the end and are missing padding
+			if len(src) == 0 && enc.padChar != NoPadding {
 				return n, false, CorruptInputError(olen - len(src) - j)
 			}
+
+			// We have reached the end and are not expecing any padding
+			if len(src) == 0 && enc.padChar == NoPadding {
+				dlen, end = j, true
+				break
+			}
+
 			in := src[0]
 			src = src[1:]
-			if in == '=' && j >= 2 && len(src) < 8 {
+			if in == byte(enc.padChar) && j >= 2 && len(src) < 8 {
 				// We've reached the end and there's padding
 				if len(src)+j < 8-1 {
 					// not enough padding
 					return n, false, CorruptInputError(olen)
 				}
 				for k := 0; k < 8-1-j; k++ {
-					if len(src) > k && src[k] != '=' {
+					if len(src) > k && src[k] != byte(enc.padChar) {
 						// incorrect padding
 						return n, false, CorruptInputError(olen - len(src) + k - 1)
 					}
@@ -484,4 +493,11 @@ func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
 
 // DecodedLen returns the maximum length in bytes of the decoded data
 // corresponding to n bytes of base32-encoded data.
-func (enc *Encoding) DecodedLen(n int) int { return n / 8 * 5 }
+func (enc *Encoding) DecodedLen(n int) int {
+	if enc.padChar == NoPadding {
+		// +6 represents the missing padding
+		return (n + 6) / 8 * 5
+	}
+
+	return n / 8 * 5
+}

src/encoding/base32/base32_test.go

@@ -490,3 +490,91 @@ func TestWithoutPadding(t *testing.T) {
 		}
 	}
 }
+
+func TestDecodeWithPadding(t *testing.T) {
+	encodings := []*Encoding{
+		StdEncoding,
+		StdEncoding.WithPadding('-'),
+		StdEncoding.WithPadding(NoPadding),
+	}
+
+	for i, enc := range encodings {
+		for _, pair := range pairs {
+
+			input := pair.decoded
+			encoded := enc.EncodeToString([]byte(input))
+
+			decoded, err := enc.DecodeString(encoded)
+			if err != nil {
+				t.Errorf("DecodeString Error for encoding %d (%q): %v", i, input, err)
+			}
+
+			if input != string(decoded) {
+				t.Errorf("Unexpected result for encoding %d: got %q; want %q", i, decoded, input)
+			}
+		}
+	}
+}
+
+func TestDecodeWithWrongPadding(t *testing.T) {
+	encoded := StdEncoding.EncodeToString([]byte("foobar"))
+
+	_, err := StdEncoding.WithPadding('-').DecodeString(encoded)
+	if err == nil {
+		t.Error("expected error")
+	}
+
+	_, err = StdEncoding.WithPadding(NoPadding).DecodeString(encoded)
+	if err == nil {
+		t.Error("expected error")
+	}
+}
+
+func TestEncodedDecodedLen(t *testing.T) {
+	type test struct {
+		in      int
+		wantEnc int
+		wantDec int
+	}
+	data := bytes.Repeat([]byte("x"), 100)
+	for _, test := range []struct {
+		name  string
+		enc   *Encoding
+		cases []test
+	}{
+		{"StdEncoding", StdEncoding, []test{
+			{0, 0, 0},
+			{1, 8, 5},
+			{5, 8, 5},
+			{6, 16, 10},
+			{10, 16, 10},
+		}},
+		{"NoPadding", StdEncoding.WithPadding(NoPadding), []test{
+			{0, 0, 0},
+			{1, 2, 5},
+			{2, 4, 5},
+			{5, 8, 5},
+			{6, 10, 10},
+			{7, 12, 10},
+			{10, 16, 10},
+			{11, 18, 15},
+		}},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			for _, tc := range test.cases {
+				encLen := test.enc.EncodedLen(tc.in)
+				decLen := test.enc.DecodedLen(encLen)
+				enc := test.enc.EncodeToString(data[:tc.in])
+				if len(enc) != encLen {
+					t.Fatalf("EncodedLen(%d) = %d but encoded to %q (%d)", tc.in, encLen, enc, len(enc))
+				}
+				if encLen != tc.wantEnc {
+					t.Fatalf("EncodedLen(%d) = %d; want %d", tc.in, encLen, tc.wantEnc)
+				}
+				if decLen != tc.wantDec {
+					t.Fatalf("DecodedLen(%d) = %d; want %d", encLen, decLen, tc.wantDec)
+				}
+			}
+		})
+	}
+}