encoding/base32: don't panic when decoding "AAAA==".

Edit encoding/base64's internals and tests to match encoding/base32.

Properly handling line breaks in padding is left for another CL.

R=dsymonds
CC=golang-dev
https://golang.org/cl/7693044

src/pkg/encoding/base32/base32.go

@@ -236,7 +236,6 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 		var dbuf [8]byte
 		dlen := 8
 
-		// do the top bytes contain any data?
 		for j := 0; j < 8; {
 			if len(src) == 0 {
 				return n, false, CorruptInputError(len(osrc) - len(src) - j)
@@ -248,15 +247,26 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 				continue
 			}
 			if in == '=' && j >= 2 && len(src) < 8 {
-				// We've reached the end and there's
-				// padding, the rest should be padded
-				for k := 0; k < 8-j-1; k++ {
+				// We've reached the end and there's padding
+				if len(src)+j < 8-1 {
+					// not enough padding
+					return n, false, CorruptInputError(len(osrc))
+				}
+				for k := 0; k < 8-1-j; k++ {
 					if len(src) > k && src[k] != '=' {
+						// incorrect padding
 						return n, false, CorruptInputError(len(osrc) - len(src) + k - 1)
 					}
 				}
-				dlen = j
-				end = true
+				dlen, end = j, true
+				// 7, 5 and 2 are not valid padding lengths, and so 1, 3 and 6 are not
+				// valid dlen values. See RFC 4648 Section 6 "Base 32 Encoding" listing
+				// the five valid padding lengths, and Section 9 "Illustrations and
+				// Examples" for an illustration for how the the 1st, 3rd and 6th base32
+				// src bytes do not yield enough information to decode a dst byte.
+				if dlen == 1 || dlen == 3 || dlen == 6 {
+					return n, false, CorruptInputError(len(osrc) - len(src) - 1)
+				}
 				break
 			}
 			dbuf[j] = enc.decodeMap[in]
@@ -269,16 +279,16 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 		// Pack 8x 5-bit source blocks into 5 byte destination
 		// quantum
 		switch dlen {
-		case 7, 8:
+		case 8:
 			dst[4] = dbuf[6]<<5 | dbuf[7]
 			fallthrough
-		case 6, 5:
+		case 7:
 			dst[3] = dbuf[4]<<7 | dbuf[5]<<2 | dbuf[6]>>3
 			fallthrough
-		case 4:
+		case 5:
 			dst[2] = dbuf[3]<<4 | dbuf[4]>>1
 			fallthrough
-		case 3:
+		case 4:
 			dst[1] = dbuf[1]<<6 | dbuf[2]<<1 | dbuf[3]>>4
 			fallthrough
 		case 2:
@@ -288,11 +298,11 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 		switch dlen {
 		case 2:
 			n += 1
-		case 3, 4:
+		case 4:
 			n += 2
 		case 5:
 			n += 3
-		case 6, 7:
+		case 7:
 			n += 4
 		case 8:
 			n += 5

src/pkg/encoding/base32/base32_test.go

@@ -137,27 +137,48 @@ func TestDecoderBuffering(t *testing.T) {
 }
 
 func TestDecodeCorrupt(t *testing.T) {
-	type corrupt struct {
-		e string
-		p int
-	}
-	examples := []corrupt{
+	testCases := []struct {
+		input  string
+		offset int // -1 means no corruption.
+	}{
+		{"", -1},
 		{"!!!!", 0},
 		{"x===", 0},
 		{"AA=A====", 2},
 		{"AAA=AAAA", 3},
 		{"MMMMMMMMM", 8},
 		{"MMMMMM", 0},
-	}
-
-	for _, e := range examples {
-		dbuf := make([]byte, StdEncoding.DecodedLen(len(e.e)))
-		_, err := StdEncoding.Decode(dbuf, []byte(e.e))
+		{"A=", 1},
+		{"AA=", 3},
+		{"AA==", 4},
+		{"AA===", 5},
+		{"AAAA=", 5},
+		{"AAAA==", 6},
+		{"AAAAA=", 6},
+		{"AAAAA==", 7},
+		{"A=======", 1},
+		{"AA======", -1},
+		{"AAA=====", 3},
+		{"AAAA====", -1},
+		{"AAAAA===", -1},
+		{"AAAAAA==", 6},
+		{"AAAAAAA=", -1},
+		{"AAAAAAAA", -1},
+	}
+	for _, tc := range testCases {
+		dbuf := make([]byte, StdEncoding.DecodedLen(len(tc.input)))
+		_, err := StdEncoding.Decode(dbuf, []byte(tc.input))
+		if tc.offset == -1 {
+			if err != nil {
+				t.Error("Decoder wrongly detected coruption in", tc.input)
+			}
+			continue
+		}
 		switch err := err.(type) {
 		case CorruptInputError:
-			testEqual(t, "Corruption in %q at offset %v, want %v", e.e, int(err), e.p)
+			testEqual(t, "Corruption in %q at offset %v, want %v", tc.input, int(err), tc.offset)
 		default:
-			t.Error("Decoder failed to detect corruption in", e)
+			t.Error("Decoder failed to detect corruption in", tc)
 		}
 	}
 }

src/pkg/encoding/base64/base64.go

@@ -227,9 +227,8 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 				continue
 			}
 			if in == '=' && j >= 2 && len(src) < 4 {
-				// We've reached the end and there's
-				// padding
-				if len(src) == 0 && j == 2 {
+				// We've reached the end and there's padding
+				if len(src)+j < 4-1 {
 					// not enough padding
 					return n, false, CorruptInputError(len(osrc))
 				}
@@ -237,8 +236,7 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 					// incorrect padding
 					return n, false, CorruptInputError(len(osrc) - len(src) - 1)
 				}
-				dlen = j
-				end = true
+				dlen, end = j, true
 				break
 			}
 			dbuf[j] = enc.decodeMap[in]

src/pkg/encoding/base64/base64_test.go

@@ -142,11 +142,11 @@ func TestDecoderBuffering(t *testing.T) {
 }
 
 func TestDecodeCorrupt(t *testing.T) {
-	type corrupt struct {
-		e string
-		p int
-	}
-	examples := []corrupt{
+	testCases := []struct {
+		input  string
+		offset int // -1 means no corruption.
+	}{
+		{"", -1},
 		{"!!!!", 0},
 		{"x===", 1},
 		{"AA=A", 2},
@@ -154,18 +154,27 @@ func TestDecodeCorrupt(t *testing.T) {
 		{"AAAAA", 4},
 		{"AAAAAA", 4},
 		{"A=", 1},
+		{"A==", 1},
 		{"AA=", 3},
+		{"AA==", -1},
+		{"AAA=", -1},
+		{"AAAA", -1},
 		{"AAAAAA=", 7},
 	}
-
-	for _, e := range examples {
-		dbuf := make([]byte, StdEncoding.DecodedLen(len(e.e)))
-		_, err := StdEncoding.Decode(dbuf, []byte(e.e))
+	for _, tc := range testCases {
+		dbuf := make([]byte, StdEncoding.DecodedLen(len(tc.input)))
+		_, err := StdEncoding.Decode(dbuf, []byte(tc.input))
+		if tc.offset == -1 {
+			if err != nil {
+				t.Error("Decoder wrongly detected coruption in", tc.input)
+			}
+			continue
+		}
 		switch err := err.(type) {
 		case CorruptInputError:
-			testEqual(t, "Corruption in %q at offset %v, want %v", e.e, int(err), e.p)
+			testEqual(t, "Corruption in %q at offset %v, want %v", tc.input, int(err), tc.offset)
 		default:
-			t.Error("Decoder failed to detect corruption in", e)
+			t.Error("Decoder failed to detect corruption in", tc)
 		}
 	}
 }