base64.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package base64 implements base64 encoding as specified by RFC 4648.
     6  package base64
     7  
     8  import (
     9  	"internal/byteorder"
    10  	"io"
    11  	"math"
    12  	"slices"
    13  	"strconv"
    14  )
    15  
    16  /*
    17   * Encodings
    18   */
    19  
    20  // An Encoding is a radix 64 encoding/decoding scheme, defined by a
    21  // 64-character alphabet. The most common encoding is the "base64"
    22  // encoding defined in RFC 4648 and used in MIME (RFC 2045) and PEM
    23  // (RFC 1421).  RFC 4648 also defines an alternate encoding, which is
    24  // the standard encoding with - and _ substituted for + and /.
    25  type Encoding struct {
    26  	encode    [64]byte   // mapping of symbol index to symbol byte value
    27  	decodeMap [256]uint8 // mapping of symbol byte value to symbol index
    28  	padChar   rune
    29  	strict    bool
    30  }
    31  
    32  const (
    33  	StdPadding rune = '=' // Standard padding character
    34  	NoPadding  rune = -1  // No padding
    35  )
    36  
    37  const (
    38  	decodeMapInitialize = "" +
    39  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    40  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    41  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    42  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    43  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    44  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    45  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    46  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    47  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    48  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    49  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    50  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    51  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    52  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    53  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    54  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff"
    55  	invalidIndex = '\xff'
    56  )
    57  
    58  // NewEncoding returns a new padded Encoding defined by the given alphabet,
    59  // which must be a 64-byte string that contains unique byte values and
    60  // does not contain the padding character or CR / LF ('\r', '\n').
    61  // The alphabet is treated as a sequence of byte values
    62  // without any special treatment for multi-byte UTF-8.
    63  // The resulting Encoding uses the default padding character ('='),
    64  // which may be changed or disabled via [Encoding.WithPadding].
    65  func NewEncoding(encoder string) *Encoding {
    66  	if len(encoder) != 64 {
    67  		panic("encoding alphabet is not 64-bytes long")
    68  	}
    69  
    70  	e := new(Encoding)
    71  	e.padChar = StdPadding
    72  	copy(e.encode[:], encoder)
    73  	copy(e.decodeMap[:], decodeMapInitialize)
    74  
    75  	for i := 0; i < len(encoder); i++ {
    76  		// Note: While we document that the alphabet cannot contain
    77  		// the padding character, we do not enforce it since we do not know
    78  		// if the caller intends to switch the padding from StdPadding later.
    79  		switch {
    80  		case encoder[i] == '\n' || encoder[i] == '\r':
    81  			panic("encoding alphabet contains newline character")
    82  		case e.decodeMap[encoder[i]] != invalidIndex:
    83  			panic("encoding alphabet includes duplicate symbols")
    84  		}
    85  		e.decodeMap[encoder[i]] = uint8(i)
    86  	}
    87  	return e
    88  }
    89  
    90  // WithPadding creates a new encoding identical to enc except
    91  // with a specified padding character, or [NoPadding] to disable padding.
    92  // The padding character must not be '\r' or '\n',
    93  // must not be contained in the encoding's alphabet,
    94  // must not be negative, and must be a rune equal or below '\xff'.
    95  // Padding characters above '\x7f' are encoded as their exact byte value
    96  // rather than using the UTF-8 representation of the codepoint.
    97  func (enc Encoding) WithPadding(padding rune) *Encoding {
    98  	switch {
    99  	case padding < NoPadding || padding == '\r' || padding == '\n' || padding > 0xff:
   100  		panic("invalid padding")
   101  	case padding != NoPadding && enc.decodeMap[byte(padding)] != invalidIndex:
   102  		panic("padding contained in alphabet")
   103  	}
   104  	enc.padChar = padding
   105  	return &enc
   106  }
   107  
   108  // Strict creates a new encoding identical to enc except with
   109  // strict decoding enabled. In this mode, the decoder requires that
   110  // trailing padding bits are zero, as described in RFC 4648 section 3.5.
   111  //
   112  // Note that the input is still malleable, as new line characters
   113  // (CR and LF) are still ignored.
   114  func (enc Encoding) Strict() *Encoding {
   115  	enc.strict = true
   116  	return &enc
   117  }
   118  
   119  // StdEncoding is the standard base64 encoding, as defined in RFC 4648.
   120  var StdEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")
   121  
   122  // URLEncoding is the alternate base64 encoding defined in RFC 4648.
   123  // It is typically used in URLs and file names.
   124  var URLEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_")
   125  
   126  // RawStdEncoding is the standard raw, unpadded base64 encoding,
   127  // as defined in RFC 4648 section 3.2.
   128  // This is the same as [StdEncoding] but omits padding characters.
   129  var RawStdEncoding = StdEncoding.WithPadding(NoPadding)
   130  
   131  // RawURLEncoding is the unpadded alternate base64 encoding defined in RFC 4648.
   132  // It is typically used in URLs and file names.
   133  // This is the same as [URLEncoding] but omits padding characters.
   134  var RawURLEncoding = URLEncoding.WithPadding(NoPadding)
   135  
   136  /*
   137   * Encoder
   138   */
   139  
   140  // Encode encodes src using the encoding enc,
   141  // writing [Encoding.EncodedLen](len(src)) bytes to dst.
   142  //
   143  // The encoding pads the output to a multiple of 4 bytes,
   144  // so Encode is not appropriate for use on individual blocks
   145  // of a large data stream. Use [NewEncoder] instead.
   146  func (enc *Encoding) Encode(dst, src []byte) {
   147  	if len(src) == 0 {
   148  		return
   149  	}
   150  	// enc is a pointer receiver, so the use of enc.encode within the hot
   151  	// loop below means a nil check at every operation. Lift that nil check
   152  	// outside of the loop to speed up the encoder.
   153  	_ = enc.encode
   154  
   155  	for len(src) >= 3 {
   156  		// Convert 3x 8bit source bytes into 4 bytes
   157  		val := uint(src[0])<<16 | uint(src[1])<<8 | uint(src[2])
   158  
   159  		_ = dst[3] // Eliminate bounds checks below.
   160  		dst[0] = enc.encode[val>>18&0x3F]
   161  		dst[1] = enc.encode[val>>12&0x3F]
   162  		dst[2] = enc.encode[val>>6&0x3F]
   163  		dst[3] = enc.encode[val&0x3F]
   164  
   165  		src = src[3:]
   166  		dst = dst[4:]
   167  	}
   168  
   169  	// Add the remaining small block (if any).
   170  	switch len(src) {
   171  	case 0:
   172  		return
   173  	case 1:
   174  		val := uint(src[0]) << 16
   175  		dst[0] = enc.encode[val>>18&0x3F]
   176  		dst[1] = enc.encode[val>>12&0x3F]
   177  		if enc.padChar != NoPadding {
   178  			dst[2] = byte(enc.padChar)
   179  			dst[3] = byte(enc.padChar)
   180  		}
   181  	case 2:
   182  		val := uint(src[0])<<16 | uint(src[1])<<8
   183  		dst[0] = enc.encode[val>>18&0x3F]
   184  		dst[1] = enc.encode[val>>12&0x3F]
   185  		dst[2] = enc.encode[val>>6&0x3F]
   186  		if enc.padChar != NoPadding {
   187  			dst[3] = byte(enc.padChar)
   188  		}
   189  	}
   190  }
   191  
   192  // AppendEncode appends the base64 encoded src to dst
   193  // and returns the extended buffer.
   194  func (enc *Encoding) AppendEncode(dst, src []byte) []byte {
   195  	n := enc.EncodedLen(len(src))
   196  	dst = slices.Grow(dst, n)
   197  	enc.Encode(dst[len(dst):][:n], src)
   198  	return dst[:len(dst)+n]
   199  }
   200  
   201  // EncodeToString returns the base64 encoding of src.
   202  func (enc *Encoding) EncodeToString(src []byte) string {
   203  	buf := make([]byte, enc.EncodedLen(len(src)))
   204  	enc.Encode(buf, src)
   205  	return string(buf)
   206  }
   207  
   208  type encoder struct {
   209  	err  error
   210  	enc  *Encoding
   211  	w    io.Writer
   212  	buf  [3]byte    // buffered data waiting to be encoded
   213  	nbuf int        // number of bytes in buf
   214  	out  [1024]byte // output buffer
   215  }
   216  
   217  func (e *encoder) Write(p []byte) (n int, err error) {
   218  	if e.err != nil {
   219  		return 0, e.err
   220  	}
   221  
   222  	// Leading fringe.
   223  	if e.nbuf > 0 {
   224  		var i int
   225  		for i = 0; i < len(p) && e.nbuf < 3; i++ {
   226  			e.buf[e.nbuf] = p[i]
   227  			e.nbuf++
   228  		}
   229  		n += i
   230  		p = p[i:]
   231  		if e.nbuf < 3 {
   232  			return
   233  		}
   234  		e.enc.Encode(e.out[:], e.buf[:])
   235  		if _, e.err = e.w.Write(e.out[:4]); e.err != nil {
   236  			return n, e.err
   237  		}
   238  		e.nbuf = 0
   239  	}
   240  
   241  	// Large interior chunks.
   242  	for len(p) >= 3 {
   243  		nn := len(e.out) / 4 * 3
   244  		if nn > len(p) {
   245  			nn = len(p)
   246  			nn -= nn % 3
   247  		}
   248  		e.enc.Encode(e.out[:], p[:nn])
   249  		if _, e.err = e.w.Write(e.out[0 : nn/3*4]); e.err != nil {
   250  			return n, e.err
   251  		}
   252  		n += nn
   253  		p = p[nn:]
   254  	}
   255  
   256  	// Trailing fringe.
   257  	copy(e.buf[:], p)
   258  	e.nbuf = len(p)
   259  	n += len(p)
   260  	return
   261  }
   262  
   263  // Close flushes any pending output from the encoder.
   264  // It is an error to call Write after calling Close.
   265  func (e *encoder) Close() error {
   266  	// If there's anything left in the buffer, flush it out
   267  	if e.err == nil && e.nbuf > 0 {
   268  		e.enc.Encode(e.out[:], e.buf[:e.nbuf])
   269  		_, e.err = e.w.Write(e.out[:e.enc.EncodedLen(e.nbuf)])
   270  		e.nbuf = 0
   271  	}
   272  	return e.err
   273  }
   274  
   275  // NewEncoder returns a new base64 stream encoder. Data written to
   276  // the returned writer will be encoded using enc and then written to w.
   277  // Base64 encodings operate in 4-byte blocks; when finished
   278  // writing, the caller must Close the returned encoder to flush any
   279  // partially written blocks.
   280  func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser {
   281  	return &encoder{enc: enc, w: w}
   282  }
   283  
   284  // EncodedLen returns the length in bytes of the base64 encoding
   285  // of an input buffer of length n.
   286  // It panics if the encoded length overflows int,
   287  // which can happen only if n > [math.MaxInt]/4*3.
   288  func (enc *Encoding) EncodedLen(n int) int {
   289  	if enc.padChar == NoPadding {
   290  		if n > math.MaxInt/4*3+2 {
   291  			panic("encoded length overflows int")
   292  		}
   293  		return n/3*4 + (n%3*8+5)/6 // minimum # chars at 6 bits per char
   294  	}
   295  	if n > math.MaxInt/4*3 {
   296  		panic("encoded length overflows int")
   297  	}
   298  	return (n + 2) / 3 * 4 // minimum # 4-char quanta, 3 bytes each
   299  }
   300  
   301  /*
   302   * Decoder
   303   */
   304  
   305  type CorruptInputError int64
   306  
   307  func (e CorruptInputError) Error() string {
   308  	return "illegal base64 data at input byte " + strconv.FormatInt(int64(e), 10)
   309  }
   310  
   311  // decodeQuantum decodes up to 4 base64 bytes. The received parameters are
   312  // the destination buffer dst, the source buffer src and an index in the
   313  // source buffer si.
   314  // It returns the number of bytes read from src, the number of bytes written
   315  // to dst, and an error, if any.
   316  func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err error) {
   317  	// Decode quantum using the base64 alphabet
   318  	var dbuf [4]byte
   319  	dlen := 4
   320  
   321  	// Lift the nil check outside of the loop.
   322  	_ = enc.decodeMap
   323  
   324  	for j := 0; j < len(dbuf); j++ {
   325  		if len(src) == si {
   326  			switch {
   327  			case j == 0:
   328  				return si, 0, nil
   329  			case j == 1, enc.padChar != NoPadding:
   330  				return si, 0, CorruptInputError(si - j)
   331  			}
   332  			dlen = j
   333  			break
   334  		}
   335  		in := src[si]
   336  		si++
   337  
   338  		out := enc.decodeMap[in]
   339  		if out != 0xff {
   340  			dbuf[j] = out
   341  			continue
   342  		}
   343  
   344  		if in == '\n' || in == '\r' {
   345  			j--
   346  			continue
   347  		}
   348  
   349  		if rune(in) != enc.padChar {
   350  			return si, 0, CorruptInputError(si - 1)
   351  		}
   352  
   353  		// We've reached the end and there's padding
   354  		switch j {
   355  		case 0, 1:
   356  			// incorrect padding
   357  			return si, 0, CorruptInputError(si - 1)
   358  		case 2:
   359  			// "==" is expected, the first "=" is already consumed.
   360  			// skip over newlines
   361  			for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
   362  				si++
   363  			}
   364  			if si == len(src) {
   365  				// not enough padding
   366  				return si, 0, CorruptInputError(len(src))
   367  			}
   368  			if rune(src[si]) != enc.padChar {
   369  				// incorrect padding
   370  				return si, 0, CorruptInputError(si - 1)
   371  			}
   372  
   373  			si++
   374  		}
   375  
   376  		// skip over newlines
   377  		for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
   378  			si++
   379  		}
   380  		if si < len(src) {
   381  			// trailing garbage
   382  			err = CorruptInputError(si)
   383  		}
   384  		dlen = j
   385  		break
   386  	}
   387  
   388  	// Convert 4x 6bit source bytes into 3 bytes
   389  	val := uint(dbuf[0])<<18 | uint(dbuf[1])<<12 | uint(dbuf[2])<<6 | uint(dbuf[3])
   390  	dbuf[2], dbuf[1], dbuf[0] = byte(val>>0), byte(val>>8), byte(val>>16)
   391  	switch dlen {
   392  	case 4:
   393  		dst[2] = dbuf[2]
   394  		dbuf[2] = 0
   395  		fallthrough
   396  	case 3:
   397  		dst[1] = dbuf[1]
   398  		if enc.strict && dbuf[2] != 0 {
   399  			return si, 0, CorruptInputError(si - 1)
   400  		}
   401  		dbuf[1] = 0
   402  		fallthrough
   403  	case 2:
   404  		dst[0] = dbuf[0]
   405  		if enc.strict && (dbuf[1] != 0 || dbuf[2] != 0) {
   406  			return si, 0, CorruptInputError(si - 2)
   407  		}
   408  	}
   409  
   410  	return si, dlen - 1, err
   411  }
   412  
   413  // AppendDecode appends the base64 decoded src to dst
   414  // and returns the extended buffer.
   415  // If the input is malformed, it returns the partially decoded src and an error.
   416  // New line characters (\r and \n) are ignored.
   417  func (enc *Encoding) AppendDecode(dst, src []byte) ([]byte, error) {
   418  	// Compute the output size without padding to avoid over allocating.
   419  	n := len(src)
   420  	for n > 0 && rune(src[n-1]) == enc.padChar {
   421  		n--
   422  	}
   423  	n = decodedLen(n, NoPadding)
   424  
   425  	dst = slices.Grow(dst, n)
   426  	n, err := enc.Decode(dst[len(dst):][:n], src)
   427  	return dst[:len(dst)+n], err
   428  }
   429  
   430  // DecodeString returns the bytes represented by the base64 string s.
   431  // If the input is malformed, it returns the partially decoded data and
   432  // [CorruptInputError]. New line characters (\r and \n) are ignored.
   433  func (enc *Encoding) DecodeString(s string) ([]byte, error) {
   434  	dbuf := make([]byte, enc.DecodedLen(len(s)))
   435  	n, err := enc.Decode(dbuf, []byte(s))
   436  	return dbuf[:n], err
   437  }
   438  
   439  type decoder struct {
   440  	err     error
   441  	readErr error // error from r.Read
   442  	enc     *Encoding
   443  	r       io.Reader
   444  	end     bool       // saw a padded group: no more input may follow
   445  	total   int64      // input consumed so far, excluding filtered newlines
   446  	buf     [1024]byte // leftover input
   447  	nbuf    int
   448  	out     []byte // leftover decoded output
   449  	outbuf  [1024 / 4 * 3]byte
   450  }
   451  
   452  // rebaseError updates the offset of a CorruptInputError returned by decoding
   453  // d.buf so that it refers to a position in the whole input stream.
   454  func (d *decoder) rebaseError(err error) error {
   455  	if e, ok := err.(CorruptInputError); ok {
   456  		return CorruptInputError(int64(e) + d.total)
   457  	}
   458  	return err
   459  }
   460  
   461  func (d *decoder) Read(p []byte) (n int, err error) {
   462  	// Use leftover decoded output from last read.
   463  	if len(d.out) > 0 {
   464  		n = copy(p, d.out)
   465  		d.out = d.out[n:]
   466  		return n, nil
   467  	}
   468  
   469  	if d.err != nil {
   470  		return 0, d.err
   471  	}
   472  
   473  	// This code assumes that d.r strips supported whitespace ('\r' and '\n').
   474  
   475  	// Refill buffer.
   476  	for d.nbuf < 4 && d.readErr == nil {
   477  		nn := len(p) / 3 * 4
   478  		if nn < 4 {
   479  			nn = 4
   480  		}
   481  		if nn > len(d.buf) {
   482  			nn = len(d.buf)
   483  		}
   484  		nn, d.readErr = d.r.Read(d.buf[d.nbuf:nn])
   485  		d.nbuf += nn
   486  	}
   487  
   488  	// A padded group must end the stream: decoding the same input as a whole
   489  	// reports any input after it as garbage.
   490  	if d.end && d.nbuf > 0 {
   491  		d.err = CorruptInputError(d.total)
   492  		return 0, d.err
   493  	}
   494  
   495  	if d.nbuf < 4 {
   496  		if d.enc.padChar == NoPadding && d.nbuf > 0 {
   497  			// Decode final fragment, without padding.
   498  			var nw int
   499  			nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:d.nbuf])
   500  			d.err = d.rebaseError(d.err)
   501  			d.nbuf = 0
   502  			d.out = d.outbuf[:nw]
   503  			n = copy(p, d.out)
   504  			d.out = d.out[n:]
   505  			if n > 0 || len(p) == 0 && len(d.out) > 0 {
   506  				return n, nil
   507  			}
   508  			if d.err != nil {
   509  				return 0, d.err
   510  			}
   511  		}
   512  		d.err = d.readErr
   513  		if d.err == io.EOF && d.nbuf > 0 {
   514  			d.err = io.ErrUnexpectedEOF
   515  		}
   516  		return 0, d.err
   517  	}
   518  
   519  	// Decode chunk into p, or d.out and then p if p is too small.
   520  	nr := d.nbuf / 4 * 4
   521  	nw := d.nbuf / 4 * 3
   522  	if nw > len(p) {
   523  		nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:nr])
   524  		d.out = d.outbuf[:nw]
   525  		n = copy(p, d.out)
   526  		d.out = d.out[n:]
   527  	} else {
   528  		n, d.err = d.enc.Decode(p, d.buf[:nr])
   529  	}
   530  	if d.err != nil {
   531  		d.err = d.rebaseError(d.err)
   532  	} else if d.enc.padChar != NoPadding && d.buf[nr-1] == byte(d.enc.padChar) {
   533  		// The decoded chunk ended with a padded group.
   534  		d.end = true
   535  	}
   536  	d.total += int64(nr)
   537  	d.nbuf -= nr
   538  	copy(d.buf[:d.nbuf], d.buf[nr:])
   539  	return n, d.err
   540  }
   541  
   542  // Decode decodes src using the encoding enc. It writes at most
   543  // [Encoding.DecodedLen](len(src)) bytes to dst and returns the number of bytes
   544  // written. The caller must ensure that dst is large enough to hold all
   545  // the decoded data. If src contains invalid base64 data, it will return the
   546  // number of bytes successfully written and [CorruptInputError].
   547  // New line characters (\r and \n) are ignored.
   548  func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
   549  	if len(src) == 0 {
   550  		return 0, nil
   551  	}
   552  
   553  	// Lift the nil check outside of the loop. enc.decodeMap is directly
   554  	// used later in this function, to let the compiler know that the
   555  	// receiver can't be nil.
   556  	_ = enc.decodeMap
   557  
   558  	si := 0
   559  	for strconv.IntSize >= 64 && len(src)-si >= 8 && len(dst)-n >= 8 {
   560  		src2 := src[si : si+8]
   561  		if dn, ok := assemble64(
   562  			enc.decodeMap[src2[0]],
   563  			enc.decodeMap[src2[1]],
   564  			enc.decodeMap[src2[2]],
   565  			enc.decodeMap[src2[3]],
   566  			enc.decodeMap[src2[4]],
   567  			enc.decodeMap[src2[5]],
   568  			enc.decodeMap[src2[6]],
   569  			enc.decodeMap[src2[7]],
   570  		); ok {
   571  			byteorder.BEPutUint64(dst[n:], dn)
   572  			n += 6
   573  			si += 8
   574  		} else {
   575  			var ninc int
   576  			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   577  			n += ninc
   578  			if err != nil {
   579  				return n, err
   580  			}
   581  		}
   582  	}
   583  
   584  	for len(src)-si >= 4 && len(dst)-n >= 4 {
   585  		src2 := src[si : si+4]
   586  		if dn, ok := assemble32(
   587  			enc.decodeMap[src2[0]],
   588  			enc.decodeMap[src2[1]],
   589  			enc.decodeMap[src2[2]],
   590  			enc.decodeMap[src2[3]],
   591  		); ok {
   592  			byteorder.BEPutUint32(dst[n:], dn)
   593  			n += 3
   594  			si += 4
   595  		} else {
   596  			var ninc int
   597  			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   598  			n += ninc
   599  			if err != nil {
   600  				return n, err
   601  			}
   602  		}
   603  	}
   604  
   605  	for si < len(src) {
   606  		var ninc int
   607  		si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   608  		n += ninc
   609  		if err != nil {
   610  			return n, err
   611  		}
   612  	}
   613  	return n, err
   614  }
   615  
   616  // assemble32 assembles 4 base64 digits into 3 bytes.
   617  // Each digit comes from the decode map, and will be 0xff
   618  // if it came from an invalid character.
   619  func assemble32(n1, n2, n3, n4 byte) (dn uint32, ok bool) {
   620  	// Check that all the digits are valid. If any of them was 0xff, their
   621  	// bitwise OR will be 0xff.
   622  	if n1|n2|n3|n4 == 0xff {
   623  		return 0, false
   624  	}
   625  	return uint32(n1)<<26 |
   626  			uint32(n2)<<20 |
   627  			uint32(n3)<<14 |
   628  			uint32(n4)<<8,
   629  		true
   630  }
   631  
   632  // assemble64 assembles 8 base64 digits into 6 bytes.
   633  // Each digit comes from the decode map, and will be 0xff
   634  // if it came from an invalid character.
   635  func assemble64(n1, n2, n3, n4, n5, n6, n7, n8 byte) (dn uint64, ok bool) {
   636  	// Check that all the digits are valid. If any of them was 0xff, their
   637  	// bitwise OR will be 0xff.
   638  	if n1|n2|n3|n4|n5|n6|n7|n8 == 0xff {
   639  		return 0, false
   640  	}
   641  	return uint64(n1)<<58 |
   642  			uint64(n2)<<52 |
   643  			uint64(n3)<<46 |
   644  			uint64(n4)<<40 |
   645  			uint64(n5)<<34 |
   646  			uint64(n6)<<28 |
   647  			uint64(n7)<<22 |
   648  			uint64(n8)<<16,
   649  		true
   650  }
   651  
   652  type newlineFilteringReader struct {
   653  	wrapped io.Reader
   654  }
   655  
   656  func (r *newlineFilteringReader) Read(p []byte) (int, error) {
   657  	n, err := r.wrapped.Read(p)
   658  	for n > 0 {
   659  		offset := 0
   660  		for i, b := range p[:n] {
   661  			if b != '\r' && b != '\n' {
   662  				if i != offset {
   663  					p[offset] = b
   664  				}
   665  				offset++
   666  			}
   667  		}
   668  		if offset > 0 {
   669  			return offset, err
   670  		}
   671  		// Previous buffer entirely whitespace, read again
   672  		n, err = r.wrapped.Read(p)
   673  	}
   674  	return n, err
   675  }
   676  
   677  // NewDecoder constructs a new base64 stream decoder.
   678  func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
   679  	return &decoder{enc: enc, r: &newlineFilteringReader{r}}
   680  }
   681  
   682  // DecodedLen returns the maximum length in bytes of the decoded data
   683  // corresponding to n bytes of base64-encoded data.
   684  func (enc *Encoding) DecodedLen(n int) int {
   685  	return decodedLen(n, enc.padChar)
   686  }
   687  
   688  func decodedLen(n int, padChar rune) int {
   689  	if padChar == NoPadding {
   690  		// Unpadded data may end with partial block of 2-3 characters.
   691  		return n/4*3 + n%4*6/8
   692  	}
   693  	// Padded base64 should always be a multiple of 4 characters in length.
   694  	return n / 4 * 3
   695  }
   696
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