Lib/email/base64mime.py

   1 # Copyright (C) 2002-2006 Python Software Foundation
   2 # Author: Ben Gertzfield
   3 # Contact: email-sig@python.org
   4
   5 """Base64 content transfer encoding per RFCs 2045-2047.
   6
   7 This module handles the content transfer encoding method defined in RFC 2045
   8 to encode arbitrary 8-bit data using the three 8-bit bytes in four 7-bit
   9 characters encoding known as Base64.
  10
  11 It is used in the MIME standards for email to attach images, audio, and text
  12 using some 8-bit character sets to messages.
  13
  14 This module provides an interface to encode and decode both headers and bodies
  15 with Base64 encoding.
  16
  17 RFC 2045 defines a method for including character set information in an
  18 `encoded-word' in a header.  This method is commonly used for 8-bit real names
  19 in To:, From:, Cc:, etc. fields, as well as Subject: lines.
  20
  21 This module does not do the line wrapping or end-of-line character conversion
  22 necessary for proper internationalized headers; it only does dumb encoding and
  23 decoding.  To deal with the various line wrapping issues, use the email.header
  24 module.
  25 """
  26
  27 __all__ = [
  28     'base64_len',
  29     'body_decode',
  30     'body_encode',
  31     'decode',
  32     'decodestring',
  33     'encode',
  34     'encodestring',
  35     'header_encode',
  36     ]
  37
  38
  39 from binascii import b2a_base64, a2b_base64
  40 from email.utils import fix_eols
  41
  42 CRLF = '\r\n'
  43 NL = '\n'
  44 EMPTYSTRING = ''
  45
  46 # See also Charset.py
  47 MISC_LEN = 7
  48
  49
  50 \f
  51 # Helpers
  52 def base64_len(s):
  53     """Return the length of s when it is encoded with base64."""
  54     groups_of_3, leftover = divmod(len(s), 3)
  55     # 4 bytes out for each 3 bytes (or nonzero fraction thereof) in.
  56     # Thanks, Tim!
  57     n = groups_of_3 * 4
  58     if leftover:
  59         n += 4
  60     return n
  61
  62
  63 \f
  64 def header_encode(header, charset='iso-8859-1', keep_eols=False,
  65                   maxlinelen=76, eol=NL):
  66     """Encode a single header line with Base64 encoding in a given charset.
  67
  68     Defined in RFC 2045, this Base64 encoding is identical to normal Base64
  69     encoding, except that each line must be intelligently wrapped (respecting
  70     the Base64 encoding), and subsequent lines must start with a space.
  71
  72     charset names the character set to use to encode the header.  It defaults
  73     to iso-8859-1.
  74
  75     End-of-line characters (\\r, \\n, \\r\\n) will be automatically converted
  76     to the canonical email line separator \\r\\n unless the keep_eols
  77     parameter is True (the default is False).
  78
  79     Each line of the header will be terminated in the value of eol, which
  80     defaults to "\\n".  Set this to "\\r\\n" if you are using the result of
  81     this function directly in email.
  82
  83     The resulting string will be in the form:
  84
  85     "=?charset?b?WW/5ciBtYXp66XLrIHf8eiBhIGhhbXBzdGHuciBBIFlv+XIgbWF6euly?=\\n
  86       =?charset?b?6yB3/HogYSBoYW1wc3Rh7nIgQkMgWW/5ciBtYXp66XLrIHf8eiBhIGhh?="
  87
  88     with each line wrapped at, at most, maxlinelen characters (defaults to 76
  89     characters).
  90     """
  91     # Return empty headers unchanged
  92     if not header:
  93         return header
  94
  95     if not keep_eols:
  96         header = fix_eols(header)
  97
  98     # Base64 encode each line, in encoded chunks no greater than maxlinelen in
  99     # length, after the RFC chrome is added in.
 100     base64ed = []
 101     max_encoded = maxlinelen - len(charset) - MISC_LEN
 102     max_unencoded = max_encoded * 3 // 4
 103
 104     for i in range(0, len(header), max_unencoded):
 105         base64ed.append(b2a_base64(header[i:i+max_unencoded]))
 106
 107     # Now add the RFC chrome to each encoded chunk
 108     lines = []
 109     for line in base64ed:
 110         # Ignore the last character of each line if it is a newline
 111         if line.endswith(NL):
 112             line = line[:-1]
 113         # Add the chrome
 114         lines.append('=?%s?b?%s?=' % (charset, line))
 115     # Glue the lines together and return it.  BAW: should we be able to
 116     # specify the leading whitespace in the joiner?
 117     joiner = eol + ' '
 118     return joiner.join(lines)
 119
 120
 121 \f
 122 def encode(s, binary=True, maxlinelen=76, eol=NL):
 123     """Encode a string with base64.
 124
 125     Each line will be wrapped at, at most, maxlinelen characters (defaults to
 126     76 characters).
 127
 128     If binary is False, end-of-line characters will be converted to the
 129     canonical email end-of-line sequence \\r\\n.  Otherwise they will be left
 130     verbatim (this is the default).
 131
 132     Each line of encoded text will end with eol, which defaults to "\\n".  Set
 133     this to "\r\n" if you will be using the result of this function directly
 134     in an email.
 135     """
 136     if not s:
 137         return s
 138
 139     if not binary:
 140         s = fix_eols(s)
 141
 142     encvec = []
 143     max_unencoded = maxlinelen * 3 // 4
 144     for i in range(0, len(s), max_unencoded):
 145         # BAW: should encode() inherit b2a_base64()'s dubious behavior in
 146         # adding a newline to the encoded string?
 147         enc = b2a_base64(s[i:i + max_unencoded])
 148         if enc.endswith(NL) and eol != NL:
 149             enc = enc[:-1] + eol
 150         encvec.append(enc)
 151     return EMPTYSTRING.join(encvec)
 152
 153
 154 # For convenience and backwards compatibility w/ standard base64 module
 155 body_encode = encode
 156 encodestring = encode
 157
 158
 159 \f
 160 def decode(s, convert_eols=None):
 161     """Decode a raw base64 string.
 162
 163     If convert_eols is set to a string value, all canonical email linefeeds,
 164     e.g. "\\r\\n", in the decoded text will be converted to the value of
 165     convert_eols.  os.linesep is a good choice for convert_eols if you are
 166     decoding a text attachment.
 167
 168     This function does not parse a full MIME header value encoded with
 169     base64 (like =?iso-8895-1?b?bmloISBuaWgh?=) -- please use the high
 170     level email.header class for that functionality.
 171     """
 172     if not s:
 173         return s
 174
 175     dec = a2b_base64(s)
 176     if convert_eols:
 177         return dec.replace(CRLF, convert_eols)
 178     return dec
 179
 180
 181 # For convenience and backwards compatibility w/ standard base64 module
 182 body_decode = decode
 183 decodestring = decode