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  	"encoding/binary"
    10  	"io"
    11  	"slices"
    12  	"strconv"
    13  )
    14  
    15  /*
    16   * Encodings
    17   */
    18  
    19  // An Encoding is a radix 64 encoding/decoding scheme, defined by a
    20  // 64-character alphabet. The most common encoding is the "base64"
    21  // encoding defined in RFC 4648 and used in MIME (RFC 2045) and PEM
    22  // (RFC 1421).  RFC 4648 also defines an alternate encoding, which is
    23  // the standard encoding with - and _ substituted for + and /.
    24  type Encoding struct {
    25  	encode    [64]byte   // mapping of symbol index to symbol byte value
    26  	decodeMap [256]uint8 // mapping of symbol byte value to symbol index
    27  	padChar   rune
    28  	strict    bool
    29  }
    30  
    31  const (
    32  	StdPadding rune = '=' // Standard padding character
    33  	NoPadding  rune = -1  // No padding
    34  )
    35  
    36  const (
    37  	decodeMapInitialize = "" +
    38  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    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  	invalidIndex = '\xff'
    55  )
    56  
    57  // NewEncoding returns a new padded Encoding defined by the given alphabet,
    58  // which must be a 64-byte string that contains unique byte values and
    59  // does not contain the padding character or CR / LF ('\r', '\n').
    60  // The alphabet is treated as a sequence of byte values
    61  // without any special treatment for multi-byte UTF-8.
    62  // The resulting Encoding uses the default padding character ('='),
    63  // which may be changed or disabled via [Encoding.WithPadding].
    64  func NewEncoding(encoder string) *Encoding {
    65  	if len(encoder) != 64 {
    66  		panic("encoding alphabet is not 64-bytes long")
    67  	}
    68  
    69  	e := new(Encoding)
    70  	e.padChar = StdPadding
    71  	copy(e.encode[:], encoder)
    72  	copy(e.decodeMap[:], decodeMapInitialize)
    73  
    74  	for i := 0; i < len(encoder); i++ {
    75  		// Note: While we document that the alphabet cannot contain
    76  		// the padding character, we do not enforce it since we do not know
    77  		// if the caller intends to switch the padding from StdPadding later.
    78  		switch {
    79  		case encoder[i] == '\n' || encoder[i] == '\r':
    80  			panic("encoding alphabet contains newline character")
    81  		case e.decodeMap[encoder[i]] != invalidIndex:
    82  			panic("encoding alphabet includes duplicate symbols")
    83  		}
    84  		e.decodeMap[encoder[i]] = uint8(i)
    85  	}
    86  	return e
    87  }
    88  
    89  // WithPadding creates a new encoding identical to enc except
    90  // with a specified padding character, or [NoPadding] to disable padding.
    91  // The padding character must not be '\r' or '\n',
    92  // must not be contained in the encoding's alphabet,
    93  // must not be negative, and must be a rune equal or below '\xff'.
    94  // Padding characters above '\x7f' are encoded as their exact byte value
    95  // rather than using the UTF-8 representation of the codepoint.
    96  func (enc Encoding) WithPadding(padding rune) *Encoding {
    97  	switch {
    98  	case padding < NoPadding || padding == '\r' || padding == '\n' || padding > 0xff:
    99  		panic("invalid padding")
   100  	case padding != NoPadding && enc.decodeMap[byte(padding)] != invalidIndex:
   101  		panic("padding contained in alphabet")
   102  	}
   103  	enc.padChar = padding
   104  	return &enc
   105  }
   106  
   107  // Strict creates a new encoding identical to enc except with
   108  // strict decoding enabled. In this mode, the decoder requires that
   109  // trailing padding bits are zero, as described in RFC 4648 section 3.5.
   110  //
   111  // Note that the input is still malleable, as new line characters
   112  // (CR and LF) are still ignored.
   113  func (enc Encoding) Strict() *Encoding {
   114  	enc.strict = true
   115  	return &enc
   116  }
   117  
   118  // StdEncoding is the standard base64 encoding, as defined in RFC 4648.
   119  var StdEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/")
   120  
   121  // URLEncoding is the alternate base64 encoding defined in RFC 4648.
   122  // It is typically used in URLs and file names.
   123  var URLEncoding = NewEncoding("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_")
   124  
   125  // RawStdEncoding is the standard raw, unpadded base64 encoding,
   126  // as defined in RFC 4648 section 3.2.
   127  // This is the same as [StdEncoding] but omits padding characters.
   128  var RawStdEncoding = StdEncoding.WithPadding(NoPadding)
   129  
   130  // RawURLEncoding is the unpadded alternate base64 encoding defined in RFC 4648.
   131  // It is typically used in URLs and file names.
   132  // This is the same as [URLEncoding] but omits padding characters.
   133  var RawURLEncoding = URLEncoding.WithPadding(NoPadding)
   134  
   135  /*
   136   * Encoder
   137   */
   138  
   139  // Encode encodes src using the encoding enc,
   140  // writing [Encoding.EncodedLen](len(src)) bytes to dst.
   141  //
   142  // The encoding pads the output to a multiple of 4 bytes,
   143  // so Encode is not appropriate for use on individual blocks
   144  // of a large data stream. Use [NewEncoder] instead.
   145  func (enc *Encoding) Encode(dst, src []byte) {
   146  	if len(src) == 0 {
   147  		return
   148  	}
   149  	// enc is a pointer receiver, so the use of enc.encode within the hot
   150  	// loop below means a nil check at every operation. Lift that nil check
   151  	// outside of the loop to speed up the encoder.
   152  	_ = enc.encode
   153  
   154  	di, si := 0, 0
   155  	n := (len(src) / 3) * 3
   156  	for si < n {
   157  		// Convert 3x 8bit source bytes into 4 bytes
   158  		val := uint(src[si+0])<<16 | uint(src[si+1])<<8 | uint(src[si+2])
   159  
   160  		dst[di+0] = enc.encode[val>>18&0x3F]
   161  		dst[di+1] = enc.encode[val>>12&0x3F]
   162  		dst[di+2] = enc.encode[val>>6&0x3F]
   163  		dst[di+3] = enc.encode[val&0x3F]
   164  
   165  		si += 3
   166  		di += 4
   167  	}
   168  
   169  	remain := len(src) - si
   170  	if remain == 0 {
   171  		return
   172  	}
   173  	// Add the remaining small block
   174  	val := uint(src[si+0]) << 16
   175  	if remain == 2 {
   176  		val |= uint(src[si+1]) << 8
   177  	}
   178  
   179  	dst[di+0] = enc.encode[val>>18&0x3F]
   180  	dst[di+1] = enc.encode[val>>12&0x3F]
   181  
   182  	switch remain {
   183  	case 2:
   184  		dst[di+2] = enc.encode[val>>6&0x3F]
   185  		if enc.padChar != NoPadding {
   186  			dst[di+3] = byte(enc.padChar)
   187  		}
   188  	case 1:
   189  		if enc.padChar != NoPadding {
   190  			dst[di+2] = byte(enc.padChar)
   191  			dst[di+3] = byte(enc.padChar)
   192  		}
   193  	}
   194  }
   195  
   196  // AppendEncode appends the base64 encoded src to dst
   197  // and returns the extended buffer.
   198  func (enc *Encoding) AppendEncode(dst, src []byte) []byte {
   199  	n := enc.EncodedLen(len(src))
   200  	dst = slices.Grow(dst, n)
   201  	enc.Encode(dst[len(dst):][:n], src)
   202  	return dst[:len(dst)+n]
   203  }
   204  
   205  // EncodeToString returns the base64 encoding of src.
   206  func (enc *Encoding) EncodeToString(src []byte) string {
   207  	buf := make([]byte, enc.EncodedLen(len(src)))
   208  	enc.Encode(buf, src)
   209  	return string(buf)
   210  }
   211  
   212  type encoder struct {
   213  	err  error
   214  	enc  *Encoding
   215  	w    io.Writer
   216  	buf  [3]byte    // buffered data waiting to be encoded
   217  	nbuf int        // number of bytes in buf
   218  	out  [1024]byte // output buffer
   219  }
   220  
   221  func (e *encoder) Write(p []byte) (n int, err error) {
   222  	if e.err != nil {
   223  		return 0, e.err
   224  	}
   225  
   226  	// Leading fringe.
   227  	if e.nbuf > 0 {
   228  		var i int
   229  		for i = 0; i < len(p) && e.nbuf < 3; i++ {
   230  			e.buf[e.nbuf] = p[i]
   231  			e.nbuf++
   232  		}
   233  		n += i
   234  		p = p[i:]
   235  		if e.nbuf < 3 {
   236  			return
   237  		}
   238  		e.enc.Encode(e.out[:], e.buf[:])
   239  		if _, e.err = e.w.Write(e.out[:4]); e.err != nil {
   240  			return n, e.err
   241  		}
   242  		e.nbuf = 0
   243  	}
   244  
   245  	// Large interior chunks.
   246  	for len(p) >= 3 {
   247  		nn := len(e.out) / 4 * 3
   248  		if nn > len(p) {
   249  			nn = len(p)
   250  			nn -= nn % 3
   251  		}
   252  		e.enc.Encode(e.out[:], p[:nn])
   253  		if _, e.err = e.w.Write(e.out[0 : nn/3*4]); e.err != nil {
   254  			return n, e.err
   255  		}
   256  		n += nn
   257  		p = p[nn:]
   258  	}
   259  
   260  	// Trailing fringe.
   261  	copy(e.buf[:], p)
   262  	e.nbuf = len(p)
   263  	n += len(p)
   264  	return
   265  }
   266  
   267  // Close flushes any pending output from the encoder.
   268  // It is an error to call Write after calling Close.
   269  func (e *encoder) Close() error {
   270  	// If there's anything left in the buffer, flush it out
   271  	if e.err == nil && e.nbuf > 0 {
   272  		e.enc.Encode(e.out[:], e.buf[:e.nbuf])
   273  		_, e.err = e.w.Write(e.out[:e.enc.EncodedLen(e.nbuf)])
   274  		e.nbuf = 0
   275  	}
   276  	return e.err
   277  }
   278  
   279  // NewEncoder returns a new base64 stream encoder. Data written to
   280  // the returned writer will be encoded using enc and then written to w.
   281  // Base64 encodings operate in 4-byte blocks; when finished
   282  // writing, the caller must Close the returned encoder to flush any
   283  // partially written blocks.
   284  func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser {
   285  	return &encoder{enc: enc, w: w}
   286  }
   287  
   288  // EncodedLen returns the length in bytes of the base64 encoding
   289  // of an input buffer of length n.
   290  func (enc *Encoding) EncodedLen(n int) int {
   291  	if enc.padChar == NoPadding {
   292  		return n/3*4 + (n%3*8+5)/6 // minimum # chars at 6 bits per char
   293  	}
   294  	return (n + 2) / 3 * 4 // minimum # 4-char quanta, 3 bytes each
   295  }
   296  
   297  /*
   298   * Decoder
   299   */
   300  
   301  type CorruptInputError int64
   302  
   303  func (e CorruptInputError) Error() string {
   304  	return "illegal base64 data at input byte " + strconv.FormatInt(int64(e), 10)
   305  }
   306  
   307  // decodeQuantum decodes up to 4 base64 bytes. The received parameters are
   308  // the destination buffer dst, the source buffer src and an index in the
   309  // source buffer si.
   310  // It returns the number of bytes read from src, the number of bytes written
   311  // to dst, and an error, if any.
   312  func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err error) {
   313  	// Decode quantum using the base64 alphabet
   314  	var dbuf [4]byte
   315  	dlen := 4
   316  
   317  	// Lift the nil check outside of the loop.
   318  	_ = enc.decodeMap
   319  
   320  	for j := 0; j < len(dbuf); j++ {
   321  		if len(src) == si {
   322  			switch {
   323  			case j == 0:
   324  				return si, 0, nil
   325  			case j == 1, enc.padChar != NoPadding:
   326  				return si, 0, CorruptInputError(si - j)
   327  			}
   328  			dlen = j
   329  			break
   330  		}
   331  		in := src[si]
   332  		si++
   333  
   334  		out := enc.decodeMap[in]
   335  		if out != 0xff {
   336  			dbuf[j] = out
   337  			continue
   338  		}
   339  
   340  		if in == '\n' || in == '\r' {
   341  			j--
   342  			continue
   343  		}
   344  
   345  		if rune(in) != enc.padChar {
   346  			return si, 0, CorruptInputError(si - 1)
   347  		}
   348  
   349  		// We've reached the end and there's padding
   350  		switch j {
   351  		case 0, 1:
   352  			// incorrect padding
   353  			return si, 0, CorruptInputError(si - 1)
   354  		case 2:
   355  			// "==" is expected, the first "=" is already consumed.
   356  			// skip over newlines
   357  			for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
   358  				si++
   359  			}
   360  			if si == len(src) {
   361  				// not enough padding
   362  				return si, 0, CorruptInputError(len(src))
   363  			}
   364  			if rune(src[si]) != enc.padChar {
   365  				// incorrect padding
   366  				return si, 0, CorruptInputError(si - 1)
   367  			}
   368  
   369  			si++
   370  		}
   371  
   372  		// skip over newlines
   373  		for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
   374  			si++
   375  		}
   376  		if si < len(src) {
   377  			// trailing garbage
   378  			err = CorruptInputError(si)
   379  		}
   380  		dlen = j
   381  		break
   382  	}
   383  
   384  	// Convert 4x 6bit source bytes into 3 bytes
   385  	val := uint(dbuf[0])<<18 | uint(dbuf[1])<<12 | uint(dbuf[2])<<6 | uint(dbuf[3])
   386  	dbuf[2], dbuf[1], dbuf[0] = byte(val>>0), byte(val>>8), byte(val>>16)
   387  	switch dlen {
   388  	case 4:
   389  		dst[2] = dbuf[2]
   390  		dbuf[2] = 0
   391  		fallthrough
   392  	case 3:
   393  		dst[1] = dbuf[1]
   394  		if enc.strict && dbuf[2] != 0 {
   395  			return si, 0, CorruptInputError(si - 1)
   396  		}
   397  		dbuf[1] = 0
   398  		fallthrough
   399  	case 2:
   400  		dst[0] = dbuf[0]
   401  		if enc.strict && (dbuf[1] != 0 || dbuf[2] != 0) {
   402  			return si, 0, CorruptInputError(si - 2)
   403  		}
   404  	}
   405  
   406  	return si, dlen - 1, err
   407  }
   408  
   409  // AppendDecode appends the base64 decoded src to dst
   410  // and returns the extended buffer.
   411  // If the input is malformed, it returns the partially decoded src and an error.
   412  // New line characters (\r and \n) are ignored.
   413  func (enc *Encoding) AppendDecode(dst, src []byte) ([]byte, error) {
   414  	// Compute the output size without padding to avoid over allocating.
   415  	n := len(src)
   416  	for n > 0 && rune(src[n-1]) == enc.padChar {
   417  		n--
   418  	}
   419  	n = decodedLen(n, NoPadding)
   420  
   421  	dst = slices.Grow(dst, n)
   422  	n, err := enc.Decode(dst[len(dst):][:n], src)
   423  	return dst[:len(dst)+n], err
   424  }
   425  
   426  // DecodeString returns the bytes represented by the base64 string s.
   427  // If the input is malformed, it returns the partially decoded data and
   428  // [CorruptInputError]. New line characters (\r and \n) are ignored.
   429  func (enc *Encoding) DecodeString(s string) ([]byte, error) {
   430  	dbuf := make([]byte, enc.DecodedLen(len(s)))
   431  	n, err := enc.Decode(dbuf, []byte(s))
   432  	return dbuf[:n], err
   433  }
   434  
   435  type decoder struct {
   436  	err     error
   437  	readErr error // error from r.Read
   438  	enc     *Encoding
   439  	r       io.Reader
   440  	buf     [1024]byte // leftover input
   441  	nbuf    int
   442  	out     []byte // leftover decoded output
   443  	outbuf  [1024 / 4 * 3]byte
   444  }
   445  
   446  func (d *decoder) Read(p []byte) (n int, err error) {
   447  	// Use leftover decoded output from last read.
   448  	if len(d.out) > 0 {
   449  		n = copy(p, d.out)
   450  		d.out = d.out[n:]
   451  		return n, nil
   452  	}
   453  
   454  	if d.err != nil {
   455  		return 0, d.err
   456  	}
   457  
   458  	// This code assumes that d.r strips supported whitespace ('\r' and '\n').
   459  
   460  	// Refill buffer.
   461  	for d.nbuf < 4 && d.readErr == nil {
   462  		nn := len(p) / 3 * 4
   463  		if nn < 4 {
   464  			nn = 4
   465  		}
   466  		if nn > len(d.buf) {
   467  			nn = len(d.buf)
   468  		}
   469  		nn, d.readErr = d.r.Read(d.buf[d.nbuf:nn])
   470  		d.nbuf += nn
   471  	}
   472  
   473  	if d.nbuf < 4 {
   474  		if d.enc.padChar == NoPadding && d.nbuf > 0 {
   475  			// Decode final fragment, without padding.
   476  			var nw int
   477  			nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:d.nbuf])
   478  			d.nbuf = 0
   479  			d.out = d.outbuf[:nw]
   480  			n = copy(p, d.out)
   481  			d.out = d.out[n:]
   482  			if n > 0 || len(p) == 0 && len(d.out) > 0 {
   483  				return n, nil
   484  			}
   485  			if d.err != nil {
   486  				return 0, d.err
   487  			}
   488  		}
   489  		d.err = d.readErr
   490  		if d.err == io.EOF && d.nbuf > 0 {
   491  			d.err = io.ErrUnexpectedEOF
   492  		}
   493  		return 0, d.err
   494  	}
   495  
   496  	// Decode chunk into p, or d.out and then p if p is too small.
   497  	nr := d.nbuf / 4 * 4
   498  	nw := d.nbuf / 4 * 3
   499  	if nw > len(p) {
   500  		nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:nr])
   501  		d.out = d.outbuf[:nw]
   502  		n = copy(p, d.out)
   503  		d.out = d.out[n:]
   504  	} else {
   505  		n, d.err = d.enc.Decode(p, d.buf[:nr])
   506  	}
   507  	d.nbuf -= nr
   508  	copy(d.buf[:d.nbuf], d.buf[nr:])
   509  	return n, d.err
   510  }
   511  
   512  // Decode decodes src using the encoding enc. It writes at most
   513  // [Encoding.DecodedLen](len(src)) bytes to dst and returns the number of bytes
   514  // written. The caller must ensure that dst is large enough to hold all
   515  // the decoded data. If src contains invalid base64 data, it will return the
   516  // number of bytes successfully written and [CorruptInputError].
   517  // New line characters (\r and \n) are ignored.
   518  func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
   519  	if len(src) == 0 {
   520  		return 0, nil
   521  	}
   522  
   523  	// Lift the nil check outside of the loop. enc.decodeMap is directly
   524  	// used later in this function, to let the compiler know that the
   525  	// receiver can't be nil.
   526  	_ = enc.decodeMap
   527  
   528  	si := 0
   529  	for strconv.IntSize >= 64 && len(src)-si >= 8 && len(dst)-n >= 8 {
   530  		src2 := src[si : si+8]
   531  		if dn, ok := assemble64(
   532  			enc.decodeMap[src2[0]],
   533  			enc.decodeMap[src2[1]],
   534  			enc.decodeMap[src2[2]],
   535  			enc.decodeMap[src2[3]],
   536  			enc.decodeMap[src2[4]],
   537  			enc.decodeMap[src2[5]],
   538  			enc.decodeMap[src2[6]],
   539  			enc.decodeMap[src2[7]],
   540  		); ok {
   541  			binary.BigEndian.PutUint64(dst[n:], dn)
   542  			n += 6
   543  			si += 8
   544  		} else {
   545  			var ninc int
   546  			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   547  			n += ninc
   548  			if err != nil {
   549  				return n, err
   550  			}
   551  		}
   552  	}
   553  
   554  	for len(src)-si >= 4 && len(dst)-n >= 4 {
   555  		src2 := src[si : si+4]
   556  		if dn, ok := assemble32(
   557  			enc.decodeMap[src2[0]],
   558  			enc.decodeMap[src2[1]],
   559  			enc.decodeMap[src2[2]],
   560  			enc.decodeMap[src2[3]],
   561  		); ok {
   562  			binary.BigEndian.PutUint32(dst[n:], dn)
   563  			n += 3
   564  			si += 4
   565  		} else {
   566  			var ninc int
   567  			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   568  			n += ninc
   569  			if err != nil {
   570  				return n, err
   571  			}
   572  		}
   573  	}
   574  
   575  	for si < len(src) {
   576  		var ninc int
   577  		si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   578  		n += ninc
   579  		if err != nil {
   580  			return n, err
   581  		}
   582  	}
   583  	return n, err
   584  }
   585  
   586  // assemble32 assembles 4 base64 digits into 3 bytes.
   587  // Each digit comes from the decode map, and will be 0xff
   588  // if it came from an invalid character.
   589  func assemble32(n1, n2, n3, n4 byte) (dn uint32, ok bool) {
   590  	// Check that all the digits are valid. If any of them was 0xff, their
   591  	// bitwise OR will be 0xff.
   592  	if n1|n2|n3|n4 == 0xff {
   593  		return 0, false
   594  	}
   595  	return uint32(n1)<<26 |
   596  			uint32(n2)<<20 |
   597  			uint32(n3)<<14 |
   598  			uint32(n4)<<8,
   599  		true
   600  }
   601  
   602  // assemble64 assembles 8 base64 digits into 6 bytes.
   603  // Each digit comes from the decode map, and will be 0xff
   604  // if it came from an invalid character.
   605  func assemble64(n1, n2, n3, n4, n5, n6, n7, n8 byte) (dn uint64, ok bool) {
   606  	// Check that all the digits are valid. If any of them was 0xff, their
   607  	// bitwise OR will be 0xff.
   608  	if n1|n2|n3|n4|n5|n6|n7|n8 == 0xff {
   609  		return 0, false
   610  	}
   611  	return uint64(n1)<<58 |
   612  			uint64(n2)<<52 |
   613  			uint64(n3)<<46 |
   614  			uint64(n4)<<40 |
   615  			uint64(n5)<<34 |
   616  			uint64(n6)<<28 |
   617  			uint64(n7)<<22 |
   618  			uint64(n8)<<16,
   619  		true
   620  }
   621  
   622  type newlineFilteringReader struct {
   623  	wrapped io.Reader
   624  }
   625  
   626  func (r *newlineFilteringReader) Read(p []byte) (int, error) {
   627  	n, err := r.wrapped.Read(p)
   628  	for n > 0 {
   629  		offset := 0
   630  		for i, b := range p[:n] {
   631  			if b != '\r' && b != '\n' {
   632  				if i != offset {
   633  					p[offset] = b
   634  				}
   635  				offset++
   636  			}
   637  		}
   638  		if offset > 0 {
   639  			return offset, err
   640  		}
   641  		// Previous buffer entirely whitespace, read again
   642  		n, err = r.wrapped.Read(p)
   643  	}
   644  	return n, err
   645  }
   646  
   647  // NewDecoder constructs a new base64 stream decoder.
   648  func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
   649  	return &decoder{enc: enc, r: &newlineFilteringReader{r}}
   650  }
   651  
   652  // DecodedLen returns the maximum length in bytes of the decoded data
   653  // corresponding to n bytes of base64-encoded data.
   654  func (enc *Encoding) DecodedLen(n int) int {
   655  	return decodedLen(n, enc.padChar)
   656  }
   657  
   658  func decodedLen(n int, padChar rune) int {
   659  	if padChar == NoPadding {
   660  		// Unpadded data may end with partial block of 2-3 characters.
   661  		return n/4*3 + n%4*6/8
   662  	}
   663  	// Padded base64 should always be a multiple of 4 characters in length.
   664  	return n / 4 * 3
   665  }
   666
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