base64.go

Documentation: encoding/base64

     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  	"strconv"
    12  )
    13  
    14  /*
    15   * Encodings
    16   */
    17  
    18  // An Encoding is a radix 64 encoding/decoding scheme, defined by a
    19  // 64-character alphabet. The most common encoding is the "base64"
    20  // encoding defined in RFC 4648 and used in MIME (RFC 2045) and PEM
    21  // (RFC 1421).  RFC 4648 also defines an alternate encoding, which is
    22  // the standard encoding with - and _ substituted for + and /.
    23  type Encoding struct {
    24  	encode    [64]byte
    25  	decodeMap [256]byte
    26  	padChar   rune
    27  	strict    bool
    28  }
    29  
    30  const (
    31  	StdPadding          rune = '=' // Standard padding character
    32  	NoPadding           rune = -1  // No padding
    33  	decodeMapInitialize      = "" +
    34  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    35  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    36  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    37  		"\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff\xff" +
    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  )
    51  
    52  const encodeStd = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
    53  const encodeURL = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
    54  
    55  // NewEncoding returns a new padded Encoding defined by the given alphabet,
    56  // which must be a 64-byte string that does not contain the padding character
    57  // or CR / LF ('\r', '\n').
    58  // The resulting Encoding uses the default padding character ('='),
    59  // which may be changed or disabled via WithPadding.
    60  func NewEncoding(encoder string) *Encoding {
    61  	if len(encoder) != 64 {
    62  		panic("encoding alphabet is not 64-bytes long")
    63  	}
    64  	for i := 0; i < len(encoder); i++ {
    65  		if encoder[i] == '\n' || encoder[i] == '\r' {
    66  			panic("encoding alphabet contains newline character")
    67  		}
    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  		e.decodeMap[encoder[i]] = byte(i)
    77  	}
    78  	return e
    79  }
    80  
    81  // WithPadding creates a new encoding identical to enc except
    82  // with a specified padding character, or NoPadding to disable padding.
    83  // The padding character must not be '\r' or '\n', must not
    84  // be contained in the encoding's alphabet and must be a rune equal or
    85  // below '\xff'.
    86  func (enc Encoding) WithPadding(padding rune) *Encoding {
    87  	if padding == '\r' || padding == '\n' || padding > 0xff {
    88  		panic("invalid padding")
    89  	}
    90  
    91  	for i := 0; i < len(enc.encode); i++ {
    92  		if rune(enc.encode[i]) == padding {
    93  			panic("padding contained in alphabet")
    94  		}
    95  	}
    96  
    97  	enc.padChar = padding
    98  	return &enc
    99  }
   100  
   101  // Strict creates a new encoding identical to enc except with
   102  // strict decoding enabled. In this mode, the decoder requires that
   103  // trailing padding bits are zero, as described in RFC 4648 section 3.5.
   104  //
   105  // Note that the input is still malleable, as new line characters
   106  // (CR and LF) are still ignored.
   107  func (enc Encoding) Strict() *Encoding {
   108  	enc.strict = true
   109  	return &enc
   110  }
   111  
   112  // StdEncoding is the standard base64 encoding, as defined in
   113  // RFC 4648.
   114  var StdEncoding = NewEncoding(encodeStd)
   115  
   116  // URLEncoding is the alternate base64 encoding defined in RFC 4648.
   117  // It is typically used in URLs and file names.
   118  var URLEncoding = NewEncoding(encodeURL)
   119  
   120  // RawStdEncoding is the standard raw, unpadded base64 encoding,
   121  // as defined in RFC 4648 section 3.2.
   122  // This is the same as StdEncoding but omits padding characters.
   123  var RawStdEncoding = StdEncoding.WithPadding(NoPadding)
   124  
   125  // RawURLEncoding is the unpadded alternate base64 encoding defined in RFC 4648.
   126  // It is typically used in URLs and file names.
   127  // This is the same as URLEncoding but omits padding characters.
   128  var RawURLEncoding = URLEncoding.WithPadding(NoPadding)
   129  
   130  /*
   131   * Encoder
   132   */
   133  
   134  // Encode encodes src using the encoding enc, writing
   135  // EncodedLen(len(src)) bytes to dst.
   136  //
   137  // The encoding pads the output to a multiple of 4 bytes,
   138  // so Encode is not appropriate for use on individual blocks
   139  // of a large data stream. Use NewEncoder() instead.
   140  func (enc *Encoding) Encode(dst, src []byte) {
   141  	if len(src) == 0 {
   142  		return
   143  	}
   144  	// enc is a pointer receiver, so the use of enc.encode within the hot
   145  	// loop below means a nil check at every operation. Lift that nil check
   146  	// outside of the loop to speed up the encoder.
   147  	_ = enc.encode
   148  
   149  	di, si := 0, 0
   150  	n := (len(src) / 3) * 3
   151  	for si < n {
   152  		// Convert 3x 8bit source bytes into 4 bytes
   153  		val := uint(src[si+0])<<16 | uint(src[si+1])<<8 | uint(src[si+2])
   154  
   155  		dst[di+0] = enc.encode[val>>18&0x3F]
   156  		dst[di+1] = enc.encode[val>>12&0x3F]
   157  		dst[di+2] = enc.encode[val>>6&0x3F]
   158  		dst[di+3] = enc.encode[val&0x3F]
   159  
   160  		si += 3
   161  		di += 4
   162  	}
   163  
   164  	remain := len(src) - si
   165  	if remain == 0 {
   166  		return
   167  	}
   168  	// Add the remaining small block
   169  	val := uint(src[si+0]) << 16
   170  	if remain == 2 {
   171  		val |= uint(src[si+1]) << 8
   172  	}
   173  
   174  	dst[di+0] = enc.encode[val>>18&0x3F]
   175  	dst[di+1] = enc.encode[val>>12&0x3F]
   176  
   177  	switch remain {
   178  	case 2:
   179  		dst[di+2] = enc.encode[val>>6&0x3F]
   180  		if enc.padChar != NoPadding {
   181  			dst[di+3] = byte(enc.padChar)
   182  		}
   183  	case 1:
   184  		if enc.padChar != NoPadding {
   185  			dst[di+2] = byte(enc.padChar)
   186  			dst[di+3] = byte(enc.padChar)
   187  		}
   188  	}
   189  }
   190  
   191  // EncodeToString returns the base64 encoding of src.
   192  func (enc *Encoding) EncodeToString(src []byte) string {
   193  	buf := make([]byte, enc.EncodedLen(len(src)))
   194  	enc.Encode(buf, src)
   195  	return string(buf)
   196  }
   197  
   198  type encoder struct {
   199  	err  error
   200  	enc  *Encoding
   201  	w    io.Writer
   202  	buf  [3]byte    // buffered data waiting to be encoded
   203  	nbuf int        // number of bytes in buf
   204  	out  [1024]byte // output buffer
   205  }
   206  
   207  func (e *encoder) Write(p []byte) (n int, err error) {
   208  	if e.err != nil {
   209  		return 0, e.err
   210  	}
   211  
   212  	// Leading fringe.
   213  	if e.nbuf > 0 {
   214  		var i int
   215  		for i = 0; i < len(p) && e.nbuf < 3; i++ {
   216  			e.buf[e.nbuf] = p[i]
   217  			e.nbuf++
   218  		}
   219  		n += i
   220  		p = p[i:]
   221  		if e.nbuf < 3 {
   222  			return
   223  		}
   224  		e.enc.Encode(e.out[:], e.buf[:])
   225  		if _, e.err = e.w.Write(e.out[:4]); e.err != nil {
   226  			return n, e.err
   227  		}
   228  		e.nbuf = 0
   229  	}
   230  
   231  	// Large interior chunks.
   232  	for len(p) >= 3 {
   233  		nn := len(e.out) / 4 * 3
   234  		if nn > len(p) {
   235  			nn = len(p)
   236  			nn -= nn % 3
   237  		}
   238  		e.enc.Encode(e.out[:], p[:nn])
   239  		if _, e.err = e.w.Write(e.out[0 : nn/3*4]); e.err != nil {
   240  			return n, e.err
   241  		}
   242  		n += nn
   243  		p = p[nn:]
   244  	}
   245  
   246  	// Trailing fringe.
   247  	copy(e.buf[:], p)
   248  	e.nbuf = len(p)
   249  	n += len(p)
   250  	return
   251  }
   252  
   253  // Close flushes any pending output from the encoder.
   254  // It is an error to call Write after calling Close.
   255  func (e *encoder) Close() error {
   256  	// If there's anything left in the buffer, flush it out
   257  	if e.err == nil && e.nbuf > 0 {
   258  		e.enc.Encode(e.out[:], e.buf[:e.nbuf])
   259  		_, e.err = e.w.Write(e.out[:e.enc.EncodedLen(e.nbuf)])
   260  		e.nbuf = 0
   261  	}
   262  	return e.err
   263  }
   264  
   265  // NewEncoder returns a new base64 stream encoder. Data written to
   266  // the returned writer will be encoded using enc and then written to w.
   267  // Base64 encodings operate in 4-byte blocks; when finished
   268  // writing, the caller must Close the returned encoder to flush any
   269  // partially written blocks.
   270  func NewEncoder(enc *Encoding, w io.Writer) io.WriteCloser {
   271  	return &encoder{enc: enc, w: w}
   272  }
   273  
   274  // EncodedLen returns the length in bytes of the base64 encoding
   275  // of an input buffer of length n.
   276  func (enc *Encoding) EncodedLen(n int) int {
   277  	if enc.padChar == NoPadding {
   278  		return (n*8 + 5) / 6 // minimum # chars at 6 bits per char
   279  	}
   280  	return (n + 2) / 3 * 4 // minimum # 4-char quanta, 3 bytes each
   281  }
   282  
   283  /*
   284   * Decoder
   285   */
   286  
   287  type CorruptInputError int64
   288  
   289  func (e CorruptInputError) Error() string {
   290  	return "illegal base64 data at input byte " + strconv.FormatInt(int64(e), 10)
   291  }
   292  
   293  // decodeQuantum decodes up to 4 base64 bytes. The received parameters are
   294  // the destination buffer dst, the source buffer src and an index in the
   295  // source buffer si.
   296  // It returns the number of bytes read from src, the number of bytes written
   297  // to dst, and an error, if any.
   298  func (enc *Encoding) decodeQuantum(dst, src []byte, si int) (nsi, n int, err error) {
   299  	// Decode quantum using the base64 alphabet
   300  	var dbuf [4]byte
   301  	dlen := 4
   302  
   303  	// Lift the nil check outside of the loop.
   304  	_ = enc.decodeMap
   305  
   306  	for j := 0; j < len(dbuf); j++ {
   307  		if len(src) == si {
   308  			switch {
   309  			case j == 0:
   310  				return si, 0, nil
   311  			case j == 1, enc.padChar != NoPadding:
   312  				return si, 0, CorruptInputError(si - j)
   313  			}
   314  			dlen = j
   315  			break
   316  		}
   317  		in := src[si]
   318  		si++
   319  
   320  		out := enc.decodeMap[in]
   321  		if out != 0xff {
   322  			dbuf[j] = out
   323  			continue
   324  		}
   325  
   326  		if in == '\n' || in == '\r' {
   327  			j--
   328  			continue
   329  		}
   330  
   331  		if rune(in) != enc.padChar {
   332  			return si, 0, CorruptInputError(si - 1)
   333  		}
   334  
   335  		// We've reached the end and there's padding
   336  		switch j {
   337  		case 0, 1:
   338  			// incorrect padding
   339  			return si, 0, CorruptInputError(si - 1)
   340  		case 2:
   341  			// "==" is expected, the first "=" is already consumed.
   342  			// skip over newlines
   343  			for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
   344  				si++
   345  			}
   346  			if si == len(src) {
   347  				// not enough padding
   348  				return si, 0, CorruptInputError(len(src))
   349  			}
   350  			if rune(src[si]) != enc.padChar {
   351  				// incorrect padding
   352  				return si, 0, CorruptInputError(si - 1)
   353  			}
   354  
   355  			si++
   356  		}
   357  
   358  		// skip over newlines
   359  		for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
   360  			si++
   361  		}
   362  		if si < len(src) {
   363  			// trailing garbage
   364  			err = CorruptInputError(si)
   365  		}
   366  		dlen = j
   367  		break
   368  	}
   369  
   370  	// Convert 4x 6bit source bytes into 3 bytes
   371  	val := uint(dbuf[0])<<18 | uint(dbuf[1])<<12 | uint(dbuf[2])<<6 | uint(dbuf[3])
   372  	dbuf[2], dbuf[1], dbuf[0] = byte(val>>0), byte(val>>8), byte(val>>16)
   373  	switch dlen {
   374  	case 4:
   375  		dst[2] = dbuf[2]
   376  		dbuf[2] = 0
   377  		fallthrough
   378  	case 3:
   379  		dst[1] = dbuf[1]
   380  		if enc.strict && dbuf[2] != 0 {
   381  			return si, 0, CorruptInputError(si - 1)
   382  		}
   383  		dbuf[1] = 0
   384  		fallthrough
   385  	case 2:
   386  		dst[0] = dbuf[0]
   387  		if enc.strict && (dbuf[1] != 0 || dbuf[2] != 0) {
   388  			return si, 0, CorruptInputError(si - 2)
   389  		}
   390  	}
   391  
   392  	return si, dlen - 1, err
   393  }
   394  
   395  // DecodeString returns the bytes represented by the base64 string s.
   396  func (enc *Encoding) DecodeString(s string) ([]byte, error) {
   397  	dbuf := make([]byte, enc.DecodedLen(len(s)))
   398  	n, err := enc.Decode(dbuf, []byte(s))
   399  	return dbuf[:n], err
   400  }
   401  
   402  type decoder struct {
   403  	err     error
   404  	readErr error // error from r.Read
   405  	enc     *Encoding
   406  	r       io.Reader
   407  	buf     [1024]byte // leftover input
   408  	nbuf    int
   409  	out     []byte // leftover decoded output
   410  	outbuf  [1024 / 4 * 3]byte
   411  }
   412  
   413  func (d *decoder) Read(p []byte) (n int, err error) {
   414  	// Use leftover decoded output from last read.
   415  	if len(d.out) > 0 {
   416  		n = copy(p, d.out)
   417  		d.out = d.out[n:]
   418  		return n, nil
   419  	}
   420  
   421  	if d.err != nil {
   422  		return 0, d.err
   423  	}
   424  
   425  	// This code assumes that d.r strips supported whitespace ('\r' and '\n').
   426  
   427  	// Refill buffer.
   428  	for d.nbuf < 4 && d.readErr == nil {
   429  		nn := len(p) / 3 * 4
   430  		if nn < 4 {
   431  			nn = 4
   432  		}
   433  		if nn > len(d.buf) {
   434  			nn = len(d.buf)
   435  		}
   436  		nn, d.readErr = d.r.Read(d.buf[d.nbuf:nn])
   437  		d.nbuf += nn
   438  	}
   439  
   440  	if d.nbuf < 4 {
   441  		if d.enc.padChar == NoPadding && d.nbuf > 0 {
   442  			// Decode final fragment, without padding.
   443  			var nw int
   444  			nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:d.nbuf])
   445  			d.nbuf = 0
   446  			d.out = d.outbuf[:nw]
   447  			n = copy(p, d.out)
   448  			d.out = d.out[n:]
   449  			if n > 0 || len(p) == 0 && len(d.out) > 0 {
   450  				return n, nil
   451  			}
   452  			if d.err != nil {
   453  				return 0, d.err
   454  			}
   455  		}
   456  		d.err = d.readErr
   457  		if d.err == io.EOF && d.nbuf > 0 {
   458  			d.err = io.ErrUnexpectedEOF
   459  		}
   460  		return 0, d.err
   461  	}
   462  
   463  	// Decode chunk into p, or d.out and then p if p is too small.
   464  	nr := d.nbuf / 4 * 4
   465  	nw := d.nbuf / 4 * 3
   466  	if nw > len(p) {
   467  		nw, d.err = d.enc.Decode(d.outbuf[:], d.buf[:nr])
   468  		d.out = d.outbuf[:nw]
   469  		n = copy(p, d.out)
   470  		d.out = d.out[n:]
   471  	} else {
   472  		n, d.err = d.enc.Decode(p, d.buf[:nr])
   473  	}
   474  	d.nbuf -= nr
   475  	copy(d.buf[:d.nbuf], d.buf[nr:])
   476  	return n, d.err
   477  }
   478  
   479  // Decode decodes src using the encoding enc. It writes at most
   480  // DecodedLen(len(src)) bytes to dst and returns the number of bytes
   481  // written. If src contains invalid base64 data, it will return the
   482  // number of bytes successfully written and CorruptInputError.
   483  // New line characters (\r and \n) are ignored.
   484  func (enc *Encoding) Decode(dst, src []byte) (n int, err error) {
   485  	if len(src) == 0 {
   486  		return 0, nil
   487  	}
   488  
   489  	// Lift the nil check outside of the loop. enc.decodeMap is directly
   490  	// used later in this function, to let the compiler know that the
   491  	// receiver can't be nil.
   492  	_ = enc.decodeMap
   493  
   494  	si := 0
   495  	for strconv.IntSize >= 64 && len(src)-si >= 8 && len(dst)-n >= 8 {
   496  		src2 := src[si : si+8]
   497  		if dn, ok := assemble64(
   498  			enc.decodeMap[src2[0]],
   499  			enc.decodeMap[src2[1]],
   500  			enc.decodeMap[src2[2]],
   501  			enc.decodeMap[src2[3]],
   502  			enc.decodeMap[src2[4]],
   503  			enc.decodeMap[src2[5]],
   504  			enc.decodeMap[src2[6]],
   505  			enc.decodeMap[src2[7]],
   506  		); ok {
   507  			binary.BigEndian.PutUint64(dst[n:], dn)
   508  			n += 6
   509  			si += 8
   510  		} else {
   511  			var ninc int
   512  			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   513  			n += ninc
   514  			if err != nil {
   515  				return n, err
   516  			}
   517  		}
   518  	}
   519  
   520  	for len(src)-si >= 4 && len(dst)-n >= 4 {
   521  		src2 := src[si : si+4]
   522  		if dn, ok := assemble32(
   523  			enc.decodeMap[src2[0]],
   524  			enc.decodeMap[src2[1]],
   525  			enc.decodeMap[src2[2]],
   526  			enc.decodeMap[src2[3]],
   527  		); ok {
   528  			binary.BigEndian.PutUint32(dst[n:], dn)
   529  			n += 3
   530  			si += 4
   531  		} else {
   532  			var ninc int
   533  			si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   534  			n += ninc
   535  			if err != nil {
   536  				return n, err
   537  			}
   538  		}
   539  	}
   540  
   541  	for si < len(src) {
   542  		var ninc int
   543  		si, ninc, err = enc.decodeQuantum(dst[n:], src, si)
   544  		n += ninc
   545  		if err != nil {
   546  			return n, err
   547  		}
   548  	}
   549  	return n, err
   550  }
   551  
   552  // assemble32 assembles 4 base64 digits into 3 bytes.
   553  // Each digit comes from the decode map, and will be 0xff
   554  // if it came from an invalid character.
   555  func assemble32(n1, n2, n3, n4 byte) (dn uint32, ok bool) {
   556  	// Check that all the digits are valid. If any of them was 0xff, their
   557  	// bitwise OR will be 0xff.
   558  	if n1|n2|n3|n4 == 0xff {
   559  		return 0, false
   560  	}
   561  	return uint32(n1)<<26 |
   562  			uint32(n2)<<20 |
   563  			uint32(n3)<<14 |
   564  			uint32(n4)<<8,
   565  		true
   566  }
   567  
   568  // assemble64 assembles 8 base64 digits into 6 bytes.
   569  // Each digit comes from the decode map, and will be 0xff
   570  // if it came from an invalid character.
   571  func assemble64(n1, n2, n3, n4, n5, n6, n7, n8 byte) (dn uint64, ok bool) {
   572  	// Check that all the digits are valid. If any of them was 0xff, their
   573  	// bitwise OR will be 0xff.
   574  	if n1|n2|n3|n4|n5|n6|n7|n8 == 0xff {
   575  		return 0, false
   576  	}
   577  	return uint64(n1)<<58 |
   578  			uint64(n2)<<52 |
   579  			uint64(n3)<<46 |
   580  			uint64(n4)<<40 |
   581  			uint64(n5)<<34 |
   582  			uint64(n6)<<28 |
   583  			uint64(n7)<<22 |
   584  			uint64(n8)<<16,
   585  		true
   586  }
   587  
   588  type newlineFilteringReader struct {
   589  	wrapped io.Reader
   590  }
   591  
   592  func (r *newlineFilteringReader) Read(p []byte) (int, error) {
   593  	n, err := r.wrapped.Read(p)
   594  	for n > 0 {
   595  		offset := 0
   596  		for i, b := range p[:n] {
   597  			if b != '\r' && b != '\n' {
   598  				if i != offset {
   599  					p[offset] = b
   600  				}
   601  				offset++
   602  			}
   603  		}
   604  		if offset > 0 {
   605  			return offset, err
   606  		}
   607  		// Previous buffer entirely whitespace, read again
   608  		n, err = r.wrapped.Read(p)
   609  	}
   610  	return n, err
   611  }
   612  
   613  // NewDecoder constructs a new base64 stream decoder.
   614  func NewDecoder(enc *Encoding, r io.Reader) io.Reader {
   615  	return &decoder{enc: enc, r: &newlineFilteringReader{r}}
   616  }
   617  
   618  // DecodedLen returns the maximum length in bytes of the decoded data
   619  // corresponding to n bytes of base64-encoded data.
   620  func (enc *Encoding) DecodedLen(n int) int {
   621  	if enc.padChar == NoPadding {
   622  		// Unpadded data may end with partial block of 2-3 characters.
   623  		return n * 6 / 8
   624  	}
   625  	// Padded base64 should always be a multiple of 4 characters in length.
   626  	return n / 4 * 3
   627  }
   628
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