text: #! /usr/bin/env python
  
  """RFC 3548: Base16, Base32, Base64 Data Encodings"""
  
  # Modified 04-Oct-1995 by Jack Jansen to use binascii module
  # Modified 30-Dec-2003 by Barry Warsaw to add full RFC 3548 support
  
  import re
  import struct
  import binascii
  
  
  __all__ = [
      # Legacy interface exports traditional RFC 1521 Base64 encodings
      'encode', 'decode', 'encodestring', 'decodestring',
      # Generalized interface for other encodings
      'b64encode', 'b64decode', 'b32encode', 'b32decode',
      'b16encode', 'b16decode',
      # Standard Base64 encoding
      'standard_b64encode', 'standard_b64decode',
      # Some common Base64 alternatives.  As referenced by RFC 3458, see thread
      # starting at:
      #
      # http://zgp.org/pipermail/p2p-hackers/2001-September/000316.html
      'urlsafe_b64encode', 'urlsafe_b64decode',
      ]
  
  _translation = [chr(_x) for _x in range(256)]
  EMPTYSTRING = ''
  
  
  def _translate(s, altchars):
      translation = _translation[:]
      for k, v in altchars.items():
          translation[ord(k)] = v
      return s.translate(''.join(translation))
  
  
  
  # Base64 encoding/decoding uses binascii
  
  def b64encode(s, altchars=None):
      """Encode a string using Base64.
  
      s is the string to encode.  Optional altchars must be a string of at least
      length 2 (additional characters are ignored) which specifies an
      alternative alphabet for the '+' and '/' characters.  This allows an
      application to e.g. generate url or filesystem safe Base64 strings.
  
      The encoded string is returned.
      """
      # Strip off the trailing newline
      encoded = binascii.b2a_base64(s)[:-1]
      if altchars is not None:
          return _translate(encoded, {'+': altchars[0], '/': altchars[1]})
      return encoded
  
  
  def b64decode(s, altchars=None):
      """Decode a Base64 encoded string.
  
      s is the string to decode.  Optional altchars must be a string of at least
      length 2 (additional characters are ignored) which specifies the
      alternative alphabet used instead of the '+' and '/' characters.
  
      The decoded string is returned.  A TypeError is raised if s were
      incorrectly padded or if there are non-alphabet characters present in the
      string.
      """
      if altchars is not None:
          s = _translate(s, {altchars[0]: '+', altchars[1]: '/'})
      try:
          return binascii.a2b_base64(s)
      except binascii.Error, msg:
          # Transform this exception for consistency
          raise TypeError(msg)
  
  
  def standard_b64encode(s):
      """Encode a string using the standard Base64 alphabet.
  
      s is the string to encode.  The encoded string is returned.
      """
      return b64encode(s)
  
  def standard_b64decode(s):
      """Decode a string encoded with the standard Base64 alphabet.
  
      s is the string to decode.  The decoded string is returned.  A TypeError
      is raised if the string is incorrectly padded or if there are non-alphabet
      characters present in the string.
      """
      return b64decode(s)
  
  def urlsafe_b64encode(s):
      """Encode a string using a url-safe Base64 alphabet.
  
      s is the string to encode.  The encoded string is returned.  The alphabet
      uses '-' instead of '+' and '_' instead of '/'.
      """
      return b64encode(s, '-_')
  
  def urlsafe_b64decode(s):
      """Decode a string encoded with the standard Base64 alphabet.
  
      s is the string to decode.  The decoded string is returned.  A TypeError
      is raised if the string is incorrectly padded or if there are non-alphabet
      characters present in the string.
  
      The alphabet uses '-' instead of '+' and '_' instead of '/'.
      """
      return b64decode(s, '-_')
  
  
  
  # Base32 encoding/decoding must be done in Python
  _b32alphabet = {
      0: 'A',  9: 'J', 18: 'S', 27: '3',
      1: 'B', 10: 'K', 19: 'T', 28: '4',
      2: 'C', 11: 'L', 20: 'U', 29: '5',
      3: 'D', 12: 'M', 21: 'V', 30: '6',
      4: 'E', 13: 'N', 22: 'W', 31: '7',
      5: 'F', 14: 'O', 23: 'X',
      6: 'G', 15: 'P', 24: 'Y',
      7: 'H', 16: 'Q', 25: 'Z',
      8: 'I', 17: 'R', 26: '2',
      }
  
  _b32tab = _b32alphabet.items()
  _b32tab.sort()
  _b32tab = [v for k, v in _b32tab]
  _b32rev = dict([(v, long(k)) for k, v in _b32alphabet.items()])
  
  
  def b32encode(s):
      """Encode a string using Base32.
  
      s is the string to encode.  The encoded string is returned.
      """
      parts = []
      quanta, leftover = divmod(len(s), 5)
      # Pad the last quantum with zero bits if necessary
      if leftover:
          s += ('\0' * (5 - leftover))
          quanta += 1
      for i in range(quanta):
          # c1 and c2 are 16 bits wide, c3 is 8 bits wide.  The intent of this
          # code is to process the 40 bits in units of 5 bits.  So we take the 1
          # leftover bit of c1 and tack it onto c2.  Then we take the 2 leftover
          # bits of c2 and tack them onto c3.  The shifts and masks are intended
          # to give us values of exactly 5 bits in width.
          c1, c2, c3 = struct.unpack('!HHB', s[i*5:(i+1)*5])
          c2 += (c1 & 1) << 16 # 17 bits wide
          c3 += (c2 & 3) << 8  # 10 bits wide
          parts.extend([_b32tab[c1 >> 11],         # bits 1 - 5
                        _b32tab[(c1 >> 6) & 0x1f], # bits 6 - 10
                        _b32tab[(c1 >> 1) & 0x1f], # bits 11 - 15
                        _b32tab[c2 >> 12],         # bits 16 - 20 (1 - 5)
                        _b32tab[(c2 >> 7) & 0x1f], # bits 21 - 25 (6 - 10)
                        _b32tab[(c2 >> 2) & 0x1f], # bits 26 - 30 (11 - 15)
                        _b32tab[c3 >> 5],          # bits 31 - 35 (1 - 5)
                        _b32tab[c3 & 0x1f],        # bits 36 - 40 (1 - 5)
                        ])
      encoded = EMPTYSTRING.join(parts)
      # Adjust for any leftover partial quanta
      if leftover == 1:
          return encoded[:-6] + '======'
      elif leftover == 2:
          return encoded[:-4] + '===='
      elif leftover == 3:
          return encoded[:-3] + '==='
      elif leftover == 4:
          return encoded[:-1] + '='
      return encoded
  
  
  def b32decode(s, casefold=False, map01=None):
      """Decode a Base32 encoded string.
  
      s is the string to decode.  Optional casefold is a flag specifying whether
      a lowercase alphabet is acceptable as input.  For security purposes, the
      default is False.
  
      RFC 3548 allows for optional mapping of the digit 0 (zero) to the letter O
      (oh), and for optional mapping of the digit 1 (one) to either the letter I
      (eye) or letter L (el).  The optional argument map01 when not None,
      specifies which letter the digit 1 should be mapped to (when map01 is not
      None, the digit 0 is always mapped to the letter O).  For security
      purposes the default is None, so that 0 and 1 are not allowed in the
      input.
  
      The decoded string is returned.  A TypeError is raised if s were
      incorrectly padded or if there are non-alphabet characters present in the
      string.
      """
      quanta, leftover = divmod(len(s), 8)
      if leftover:
          raise TypeError('Incorrect padding')
      # Handle section 2.4 zero and one mapping.  The flag map01 will be either
      # False, or the character to map the digit 1 (one) to.  It should be
      # either L (el) or I (eye).
      if map01:
          s = _translate(s, {'0': 'O', '1': map01})
      if casefold:
          s = s.upper()
      # Strip off pad characters from the right.  We need to count the pad
      # characters because this will tell us how many null bytes to remove from
      # the end of the decoded string.
      padchars = 0
      mo = re.search('(?P<pad>[=]*)$', s)
      if mo:
          padchars = len(mo.group('pad'))
          if padchars > 0:
              s = s[:-padchars]
      # Now decode the full quanta
      parts = []
      acc = 0
      shift = 35
      for c in s:
          val = _b32rev.get(c)
          if val is None:
              raise TypeError('Non-base32 digit found')
          acc += _b32rev[c] << shift
          shift -= 5
          if shift < 0:
              parts.append(binascii.unhexlify('%010x' % acc))
              acc = 0
              shift = 35
      # Process the last, partial quanta
      last = binascii.unhexlify('%010x' % acc)
      if padchars == 0:
          last = ''                       # No characters
      elif padchars == 1:
          last = last[:-1]
      elif padchars == 3:
          last = last[:-2]
      elif padchars == 4:
          last = last[:-3]
      elif padchars == 6:
          last = last[:-4]
      else:
          raise TypeError('Incorrect padding')
      parts.append(last)
      return EMPTYSTRING.join(parts)
  
  
  
  # RFC 3548, Base 16 Alphabet specifies uppercase, but hexlify() returns
  # lowercase.  The RFC also recommends against accepting input case
  # insensitively.
  def b16encode(s):
      """Encode a string using Base16.
  
      s is the string to encode.  The encoded string is returned.
      """
      return binascii.hexlify(s).upper()
  
  
  def b16decode(s, casefold=False):
      """Decode a Base16 encoded string.
  
      s is the string to decode.  Optional casefold is a flag specifying whether
      a lowercase alphabet is acceptable as input.  For security purposes, the
      default is False.
  
      The decoded string is returned.  A TypeError is raised if s were
      incorrectly padded or if there are non-alphabet characters present in the
      string.
      """
      if casefold:
          s = s.upper()
      if re.search('[^0-9A-F]', s):
          raise TypeError('Non-base16 digit found')
      return binascii.unhexlify(s)
  
  
  
  # Legacy interface.  This code could be cleaned up since I don't believe
  # binascii has any line length limitations.  It just doesn't seem worth it
  # though.
  
  MAXLINESIZE = 76 # Excluding the CRLF
  MAXBINSIZE = (MAXLINESIZE//4)*3
  
  def encode(input, output):
      """Encode a file."""
      while True:
          s = input.read(MAXBINSIZE)
          if not s:
              break
          while len(s) < MAXBINSIZE:
              ns = input.read(MAXBINSIZE-len(s))
              if not ns:
                  break
              s += ns
          line = binascii.b2a_base64(s)
          output.write(line)
  
  
  def decode(input, output):
      """Decode a file."""
      while True:
          line = input.readline()
          if not line:
              break
          s = binascii.a2b_base64(line)
          output.write(s)
  
  
  def encodestring(s):
      """Encode a string into multiple lines of base-64 data."""
      pieces = []
      for i in range(0, len(s), MAXBINSIZE):
          chunk = s[i : i + MAXBINSIZE]
          pieces.append(binascii.b2a_base64(chunk))
      return "".join(pieces)
  
  
  def decodestring(s):
      """Decode a string."""
      return binascii.a2b_base64(s)
  
  
  
  # Useable as a script...
  def test():
      """Small test program"""
      import sys, getopt
      try:
          opts, args = getopt.getopt(sys.argv[1:], 'deut')
      except getopt.error, msg:
          sys.stdout = sys.stderr
          print msg
          print """usage: %s [-d|-e|-u|-t] [file|-]
          -d, -u: decode
          -e: encode (default)
          -t: encode and decode string 'Aladdin:open sesame'"""%sys.argv[0]
          sys.exit(2)
      func = encode
      for o, a in opts:
          if o == '-e': func = encode
          if o == '-d': func = decode
          if o == '-u': func = decode
          if o == '-t': test1(); return
      if args and args[0] != '-':
          with open(args[0], 'rb') as f:
              func(f, sys.stdout)
      else:
          func(sys.stdin, sys.stdout)
  
  
  def test1():
      s0 = "Aladdin:open sesame"
      s1 = encodestring(s0)
      s2 = decodestring(s1)
      print s0, repr(s1), s2
  
  
  if __name__ == '__main__':
      test()
  