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  	"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  	for len(src) >= 3 {
   155  		// Convert 3x 8bit source bytes into 4 bytes
   156  		val := uint(src[0])<<16 | uint(src[1])<<8 | uint(src[2])
   157  
   158  		_ = dst[3] // Eliminate bounds checks below.
   159  		dst[0] = enc.encode[val>>18&0x3F]
   160  		dst[1] = enc.encode[val>>12&0x3F]
   161  		dst[2] = enc.encode[val>>6&0x3F]
   162  		dst[3] = enc.encode[val&0x3F]
   163  
   164  		src = src[3:]
   165  		dst = dst[4:]
   166  	}
   167  
   168  	// Add the remaining small block (if any).
   169  	switch len(src) {
   170  	case 0:
   171  		return
   172  	case 1:
   173  		val := uint(src[0]) << 16
   174  		dst[0] = enc.encode[val>>18&0x3F]
   175  		dst[1] = enc.encode[val>>12&0x3F]
   176  		if enc.padChar != NoPadding {
   177  			dst[2] = byte(enc.padChar)
   178  			dst[3] = byte(enc.padChar)
   179  		}
   180  	case 2:
   181  		val := uint(src[0])<<16 | uint(src[1])<<8
   182  		dst[0] = enc.encode[val>>18&0x3F]
   183  		dst[1] = enc.encode[val>>12&0x3F]
   184  		dst[2] = enc.encode[val>>6&0x3F]
   185  		if enc.padChar != NoPadding {
   186  			dst[3] = byte(enc.padChar)
   187  		}
   188  	}
   189  }
   190  
   191  // AppendEncode appends the base64 encoded src to dst
   192  // and returns the extended buffer.
   193  func (enc *Encoding) AppendEncode(dst, src []byte) []byte {
   194  	n := enc.EncodedLen(len(src))
   195  	dst = slices.Grow(dst, n)
   196  	enc.Encode(dst[len(dst):][:n], src)
   197  	return dst[:len(dst)+n]
   198  }
   199  
   200  // EncodeToString returns the base64 encoding of src.
   201  func (enc *Encoding) EncodeToString(src []byte) string {
   202  	buf := make([]byte, enc.EncodedLen(len(src)))
   203  	enc.Encode(buf, src)
   204  	return string(buf)
   205  }
   206  
   207  type encoder struct {
   208  	err  error
   209  	enc  *Encoding
   210  	w    io.Writer
   211  	buf  [3]byte    // buffered data waiting to be encoded
   212  	nbuf int        // number of bytes in buf
   213  	out  [1024]byte // output buffer
   214  }
   215  
   216  func (e *encoder) Write(p []byte) (n int, err error) {
   217  	if e.err != nil {
   218  		return 0, e.err
   219  	}
   220  
   221  	// Leading fringe.
   222  	if e.nbuf > 0 {
   223  		var i int
   224  		for i = 0; i < len(p) && e.nbuf < 3; i++ {
   225  			e.buf[e.nbuf] = p[i]
   226  			e.nbuf++
   227  		}
   228  		n += i
   229  		p = p[i:]
   230  		if e.nbuf < 3 {
   231  			return
   232  		}
   233  		e.enc.Encode(e.out[:], e.buf[:])
   234  		if _, e.err = e.w.Write(e.out[:4]); e.err != nil {
   235  			return n, e.err
   236  		}
   237  		e.nbuf = 0
   238  	}
   239  
   240  	// Large interior chunks.
   241  	for len(p) >= 3 {
   242  		nn := len(e.out) / 4 * 3
   243  		if nn > len(p) {
   244  			nn = len(p)
   245  			nn -= nn % 3
   246  		}
   247  		e.enc.Encode(e.out[:], p[:nn])
   248  		if _, e.err = e.w.Write(e.out[0 : nn/3*4]); e.err != nil {
   249  			return n, e.err
   250  		}
   251  		n += nn
   252  		p = p[nn:]
   253  	}
   254  
   255  	// Trailing fringe.
   256  	copy(e.buf[:], p)
   257  	e.nbuf = len(p)
   258  	n += len(p)
   259  	return
   260  }
   261  
   262  // Close flushes any pending output from the encoder.
   263  // It is an error to call Write after calling Close.
   264  func (e *encoder) Close() error {
   265  	// If there's anything left in the buffer, flush it out
   266  	if e.err == nil && e.nbuf > 0 {
   267  		e.enc.Encode(e.out[:], e.buf[:e.nbuf])
   268  		_, e.err = e.w.Write(e.out[:e.enc.EncodedLen(e.nbuf)])
   269  		e.nbuf = 0
   270  	}
   271  	return e.err
   272  }
   273  
   274  // NewEncoder returns a new base64 stream encoder. Data written to
   275  // the returned writer will be encoded using enc and then written to w.
   276  // Base64 encodings operate in 4-byte blocks; when finished
   277  // writing, the caller must Close the returned encoder to flush any
   278  // partially written blocks.
   279  func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser {
   280  	return &encoder{enc: enc, w: w}
   281  }
   282  
   283  // EncodedLen returns the length in bytes of the base64 encoding
   284  // of an input buffer of length n.
   285  func (enc *Encoding) EncodedLen(n int) int {
   286  	if enc.padChar == NoPadding {
   287  		return n/3*4 + (n%3*8+5)/6 // minimum # chars at 6 bits per char
   288  	}
   289  	return (n + 2) / 3 * 4 // minimum # 4-char quanta, 3 bytes each
   290  }
   291  
   292  /*
   293   * Decoder
   294   */
   295  
   296  type CorruptInputError int64
   297  
   298  func (e CorruptInputError) Error() string {
   299  	return "illegal base64 data at input byte " + strconv.FormatInt(int64(e), 10)
   300  }
   301  
   302  // decodeQuantum decodes up to 4 base64 bytes. The received parameters are
   303  // the destination buffer dst, the source buffer src and an index in the
   304  // source buffer si.
   305  // It returns the number of bytes read from src, the number of bytes written
   306  // to dst, and an error, if any.
   307  func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err error) {
   308  	// Decode quantum using the base64 alphabet
   309  	var dbuf [4]byte
   310  	dlen := 4
   311  
   312  	// Lift the nil check outside of the loop.
   313  	_ = enc.decodeMap
   314  
   315  	for j := 0; j < len(dbuf); j++ {
   316  		if len(src) == si {
   317  			switch {
   318  			case j == 0:
   319  				return si, 0, nil
   320  			case j == 1, enc.padChar != NoPadding:
   321  				return si, 0, CorruptInputError(si - j)
   322  			}
   323  			dlen = j
   324  			break
   325  		}
   326  		in := src[si]
   327  		si++
   328  
   329  		out := enc.decodeMap[in]
   330  		if out != 0xff {
   331  			dbuf[j] = out
   332  			continue
   333  		}
   334  
   335  		if in == '\n' || in == '\r' {
   336  			j--
   337  			continue
   338  		}
   339  
   340  		if rune(in) != enc.padChar {
   341  			return si, 0, CorruptInputError(si - 1)
   342  		}
   343  
   344  		// We've reached the end and there's padding
   345  		switch j {
   346  		case 0, 1:
   347  			// incorrect padding
   348  			return si, 0, CorruptInputError(si - 1)
   349  		case 2:
   350  			// "==" is expected, the first "=" is already consumed.
   351  			// skip over newlines
   352  			for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
   353  				si++
   354  			}
   355  			if si == len(src) {
   356  				// not enough padding
   357  				return si, 0, CorruptInputError(len(src))
   358  			}
   359  			if rune(src[si]) != enc.padChar {
   360  				// incorrect padding
   361  				return si, 0, CorruptInputError(si - 1)
   362  			}
   363  
   364  			si++
   365  		}
   366  
   367  		// skip over newlines
   368  		for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
   369  			si++
   370  		}
   371  		if si < len(src) {
   372  			// trailing garbage
   373  			err = CorruptInputError(si)
   374  		}
   375  		dlen = j
   376  		break
   377  	}
   378  
   379  	// Convert 4x 6bit source bytes into 3 bytes
   380  	val := uint(dbuf[0])<<18 | uint(dbuf[1])<<12 | uint(dbuf[2])<<6 | uint(dbuf[3])
   381  	dbuf[2], dbuf[1], dbuf[0] = byte(val>>0), byte(val>>8), byte(val>>16)
   382  	switch dlen {
   383  	case 4:
   384  		dst[2] = dbuf[2]
   385  		dbuf[2] = 0
   386  		fallthrough
   387  	case 3:
   388  		dst[1] = dbuf[1]
   389  		if enc.strict && dbuf[2] != 0 {
   390  			return si, 0, CorruptInputError(si - 1)
   391  		}
   392  		dbuf[1] = 0
   393  		fallthrough
   394  	case 2:
   395  		dst[0] = dbuf[0]
   396  		if enc.strict && (dbuf[1] != 0 || dbuf[2] != 0) {
   397  			return si, 0, CorruptInputError(si - 2)
   398  		}
   399  	}
   400  
   401  	return si, dlen - 1, err
   402  }
   403  
   404  // AppendDecode appends the base64 decoded src to dst
   405  // and returns the extended buffer.
   406  // If the input is malformed, it returns the partially decoded src and an error.
   407  // New line characters (\r and \n) are ignored.
   408  func (enc *Encoding) AppendDecode(dst, src []byte) ([]byte, error) {
   409  	// Compute the output size without padding to avoid over allocating.
   410  	n := len(src)
   411  	for n > 0 && rune(src[n-1]) == enc.padChar {
   412  		n--
   413  	}
   414  	n = decodedLen(n, NoPadding)
   415  
   416  	dst = slices.Grow(dst, n)
   417  	n, err := enc.Decode(dst[len(dst):][:n], src)
   418  	return dst[:len(dst)+n], err
   419  }
   420  
   421  // DecodeString returns the bytes represented by the base64 string s.
   422  // If the input is malformed, it returns the partially decoded data and
   423  // [CorruptInputError]. New line characters (\r and \n) are ignored.
   424  func (enc *Encoding) DecodeString(s string) ([]byte, error) {
   425  	dbuf := make([]byte, enc.DecodedLen(len(s)))
   426  	n, err := enc.Decode(dbuf, []byte(s))
   427  	return dbuf[:n], err
   428  }
   429  
   430  type decoder struct {
   431  	err     error
   432  	readErr error // error from r.Read
   433  	enc     *Encoding
   434  	r       io.Reader
   435  	buf     [1024]byte // leftover input
   436  	nbuf    int
   437  	out     []byte // leftover decoded output
   438  	outbuf  [1024 / 4 * 3]byte
   439  }
   440  
   441  func (d *decoder) Read(p []byte) (n int, err error) {
   442  	// Use leftover decoded output from last read.
   443  	if len(d.out) > 0 {
   444  		n = copy(p, d.out)
   445  		d.out = d.out[n:]
   446  		return n, nil
   447  	}
   448  
   449  	if d.err != nil {
   450  		return 0, d.err
   451  	}
   452  
   453  	// This code assumes that d.r strips supported whitespace ('\r' and '\n').
   454  
   455  	// Refill buffer.
   456  	for d.nbuf < 4 && d.readErr == nil {
   457  		nn := len(p) / 3 * 4
   458  		if nn < 4 {
   459  			nn = 4
   460  		}
   461  		if nn > len(d.buf) {
   462  			nn = len(d.buf)
   463  		}
   464  		nn, d.readErr = d.r.Read(d.buf[d.nbuf:nn])
   465  		d.nbuf += nn
   466  	}
   467  
   468  	if d.nbuf < 4 {
   469  		if d.enc.padChar == NoPadding && d.nbuf > 0 {
   470  			// Decode final fragment, without padding.
   471  			var nw int
   472  			nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:d.nbuf])
   473  			d.nbuf = 0
   474  			d.out = d.outbuf[:nw]
   475  			n = copy(p, d.out)
   476  			d.out = d.out[n:]
   477  			if n > 0 || len(p) == 0 && len(d.out) > 0 {
   478  				return n, nil
   479  			}
   480  			if d.err != nil {
   481  				return 0, d.err
   482  			}
   483  		}
   484  		d.err = d.readErr
   485  		if d.err == io.EOF && d.nbuf > 0 {
   486  			d.err = io.ErrUnexpectedEOF
   487  		}
   488  		return 0, d.err
   489  	}
   490  
   491  	// Decode chunk into p, or d.out and then p if p is too small.
   492  	nr := d.nbuf / 4 * 4
   493  	nw := d.nbuf / 4 * 3
   494  	if nw > len(p) {
   495  		nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:nr])
   496  		d.out = d.outbuf[:nw]
   497  		n = copy(p, d.out)
   498  		d.out = d.out[n:]
   499  	} else {
   500  		n, d.err = d.enc.Decode(p, d.buf[:nr])
   501  	}
   502  	d.nbuf -= nr
   503  	copy(d.buf[:d.nbuf], d.buf[nr:])
   504  	return n, d.err
   505  }
   506  
   507  // Decode decodes src using the encoding enc. It writes at most
   508  // [Encoding.DecodedLen](len(src)) bytes to dst and returns the number of bytes
   509  // written. The caller must ensure that dst is large enough to hold all
   510  // the decoded data. If src contains invalid base64 data, it will return the
   511  // number of bytes successfully written and [CorruptInputError].
   512  // New line characters (\r and \n) are ignored.
   513  func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
   514  	if len(src) == 0 {
   515  		return 0, nil
   516  	}
   517  
   518  	// Lift the nil check outside of the loop. enc.decodeMap is directly
   519  	// used later in this function, to let the compiler know that the
   520  	// receiver can't be nil.
   521  	_ = enc.decodeMap
   522  
   523  	si := 0
   524  	for strconv.IntSize >= 64 && len(src)-si >= 8 && len(dst)-n >= 8 {
   525  		src2 := src[si : si+8]
   526  		if dn, ok := assemble64(
   527  			enc.decodeMap[src2[0]],
   528  			enc.decodeMap[src2[1]],
   529  			enc.decodeMap[src2[2]],
   530  			enc.decodeMap[src2[3]],
   531  			enc.decodeMap[src2[4]],
   532  			enc.decodeMap[src2[5]],
   533  			enc.decodeMap[src2[6]],
   534  			enc.decodeMap[src2[7]],
   535  		); ok {
   536  			byteorder.BEPutUint64(dst[n:], dn)
   537  			n += 6
   538  			si += 8
   539  		} else {
   540  			var ninc int
   541  			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   542  			n += ninc
   543  			if err != nil {
   544  				return n, err
   545  			}
   546  		}
   547  	}
   548  
   549  	for len(src)-si >= 4 && len(dst)-n >= 4 {
   550  		src2 := src[si : si+4]
   551  		if dn, ok := assemble32(
   552  			enc.decodeMap[src2[0]],
   553  			enc.decodeMap[src2[1]],
   554  			enc.decodeMap[src2[2]],
   555  			enc.decodeMap[src2[3]],
   556  		); ok {
   557  			byteorder.BEPutUint32(dst[n:], dn)
   558  			n += 3
   559  			si += 4
   560  		} else {
   561  			var ninc int
   562  			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   563  			n += ninc
   564  			if err != nil {
   565  				return n, err
   566  			}
   567  		}
   568  	}
   569  
   570  	for si < len(src) {
   571  		var ninc int
   572  		si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   573  		n += ninc
   574  		if err != nil {
   575  			return n, err
   576  		}
   577  	}
   578  	return n, err
   579  }
   580  
   581  // assemble32 assembles 4 base64 digits into 3 bytes.
   582  // Each digit comes from the decode map, and will be 0xff
   583  // if it came from an invalid character.
   584  func assemble32(n1, n2, n3, n4 byte) (dn uint32, ok bool) {
   585  	// Check that all the digits are valid. If any of them was 0xff, their
   586  	// bitwise OR will be 0xff.
   587  	if n1|n2|n3|n4 == 0xff {
   588  		return 0, false
   589  	}
   590  	return uint32(n1)<<26 |
   591  			uint32(n2)<<20 |
   592  			uint32(n3)<<14 |
   593  			uint32(n4)<<8,
   594  		true
   595  }
   596  
   597  // assemble64 assembles 8 base64 digits into 6 bytes.
   598  // Each digit comes from the decode map, and will be 0xff
   599  // if it came from an invalid character.
   600  func assemble64(n1, n2, n3, n4, n5, n6, n7, n8 byte) (dn uint64, ok bool) {
   601  	// Check that all the digits are valid. If any of them was 0xff, their
   602  	// bitwise OR will be 0xff.
   603  	if n1|n2|n3|n4|n5|n6|n7|n8 == 0xff {
   604  		return 0, false
   605  	}
   606  	return uint64(n1)<<58 |
   607  			uint64(n2)<<52 |
   608  			uint64(n3)<<46 |
   609  			uint64(n4)<<40 |
   610  			uint64(n5)<<34 |
   611  			uint64(n6)<<28 |
   612  			uint64(n7)<<22 |
   613  			uint64(n8)<<16,
   614  		true
   615  }
   616  
   617  type newlineFilteringReader struct {
   618  	wrapped io.Reader
   619  }
   620  
   621  func (r *newlineFilteringReader) Read(p []byte) (int, error) {
   622  	n, err := r.wrapped.Read(p)
   623  	for n > 0 {
   624  		offset := 0
   625  		for i, b := range p[:n] {
   626  			if b != '\r' && b != '\n' {
   627  				if i != offset {
   628  					p[offset] = b
   629  				}
   630  				offset++
   631  			}
   632  		}
   633  		if offset > 0 {
   634  			return offset, err
   635  		}
   636  		// Previous buffer entirely whitespace, read again
   637  		n, err = r.wrapped.Read(p)
   638  	}
   639  	return n, err
   640  }
   641  
   642  // NewDecoder constructs a new base64 stream decoder.
   643  func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
   644  	return &decoder{enc: enc, r: &newlineFilteringReader{r}}
   645  }
   646  
   647  // DecodedLen returns the maximum length in bytes of the decoded data
   648  // corresponding to n bytes of base64-encoded data.
   649  func (enc *Encoding) DecodedLen(n int) int {
   650  	return decodedLen(n, enc.padChar)
   651  }
   652  
   653  func decodedLen(n int, padChar rune) int {
   654  	if padChar == NoPadding {
   655  		// Unpadded data may end with partial block of 2-3 characters.
   656  		return n/4*3 + n%4*6/8
   657  	}
   658  	// Padded base64 should always be a multiple of 4 characters in length.
   659  	return n / 4 * 3
   660  }
   661
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