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 import re
  39
  40 from binascii import b2a_base64, a2b_base64
  41 from email.utils import fix_eols
  42
  43 CRLF = '\r\n'
  44 NL = '\n'
  45 EMPTYSTRING = ''
  46
  47 # See also Charset.py
  48 MISC_LEN = 7
  49
  50
  51 \f
  52 # Helpers
  53 def base64_len(s):
  54     """Return the length of s when it is encoded with base64."""
  55     groups_of_3, leftover = divmod(len(s), 3)
  56     # 4 bytes out for each 3 bytes (or nonzero fraction thereof) in.
  57     # Thanks, Tim!
  58     n = groups_of_3 * 4
  59     if leftover:
  60         n += 4
  61     return n
  62
  63
  64 \f
  65 def header_encode(header, charset='iso-8859-1', keep_eols=False,
  66                   maxlinelen=76, eol=NL):
  67     """Encode a single header line with Base64 encoding in a given charset.
  68
  69     Defined in RFC 2045, this Base64 encoding is identical to normal Base64
  70     encoding, except that each line must be intelligently wrapped (respecting
  71     the Base64 encoding), and subsequent lines must start with a space.
  72
  73     charset names the character set to use to encode the header.  It defaults
  74     to iso-8859-1.
  75
  76     End-of-line characters (\\r, \\n, \\r\\n) will be automatically converted
  77     to the canonical email line separator \\r\\n unless the keep_eols
  78     parameter is True (the default is False).
  79
  80     Each line of the header will be terminated in the value of eol, which
  81     defaults to "\\n".  Set this to "\\r\\n" if you are using the result of
  82     this function directly in email.
  83
  84     The resulting string will be in the form:
  85
  86     "=?charset?b?WW/5ciBtYXp66XLrIHf8eiBhIGhhbXBzdGHuciBBIFlv+XIgbWF6euly?=\\n
  87       =?charset?b?6yB3/HogYSBoYW1wc3Rh7nIgQkMgWW/5ciBtYXp66XLrIHf8eiBhIGhh?="
  88
  89     with each line wrapped at, at most, maxlinelen characters (defaults to 76
  90     characters).
  91     """
  92     # Return empty headers unchanged
  93     if not header:
  94         return header
  95
  96     if not keep_eols:
  97         header = fix_eols(header)
  98
  99     # Base64 encode each line, in encoded chunks no greater than maxlinelen in
 100     # length, after the RFC chrome is added in.
 101     base64ed = []
 102     max_encoded = maxlinelen - len(charset) - MISC_LEN
 103     max_unencoded = max_encoded * 3 // 4
 104
 105     for i in range(0, len(header), max_unencoded):
 106         base64ed.append(b2a_base64(header[i:i+max_unencoded]))
 107
 108     # Now add the RFC chrome to each encoded chunk
 109     lines = []
 110     for line in base64ed:
 111         # Ignore the last character of each line if it is a newline
 112         if line.endswith(NL):
 113             line = line[:-1]
 114         # Add the chrome
 115         lines.append('=?%s?b?%s?=' % (charset, line))
 116     # Glue the lines together and return it.  BAW: should we be able to
 117     # specify the leading whitespace in the joiner?
 118     joiner = eol + ' '
 119     return joiner.join(lines)
 120
 121
 122 \f
 123 def encode(s, binary=True, maxlinelen=76, eol=NL):
 124     """Encode a string with base64.
 125
 126     Each line will be wrapped at, at most, maxlinelen characters (defaults to
 127     76 characters).
 128
 129     If binary is False, end-of-line characters will be converted to the
 130     canonical email end-of-line sequence \\r\\n.  Otherwise they will be left
 131     verbatim (this is the default).
 132
 133     Each line of encoded text will end with eol, which defaults to "\\n".  Set
 134     this to "\r\n" if you will be using the result of this function directly
 135     in an email.
 136     """
 137     if not s:
 138         return s
 139
 140     if not binary:
 141         s = fix_eols(s)
 142
 143     encvec = []
 144     max_unencoded = maxlinelen * 3 // 4
 145     for i in range(0, len(s), max_unencoded):
 146         # BAW: should encode() inherit b2a_base64()'s dubious behavior in
 147         # adding a newline to the encoded string?
 148         enc = b2a_base64(s[i:i + max_unencoded])
 149         if enc.endswith(NL) and eol <> NL:
 150             enc = enc[:-1] + eol
 151         encvec.append(enc)
 152     return EMPTYSTRING.join(encvec)
 153
 154
 155 # For convenience and backwards compatibility w/ standard base64 module
 156 body_encode = encode
 157 encodestring = encode
 158
 159
 160 \f
 161 def decode(s, convert_eols=None):
 162     """Decode a raw base64 string.
 163
 164     If convert_eols is set to a string value, all canonical email linefeeds,
 165     e.g. "\\r\\n", in the decoded text will be converted to the value of
 166     convert_eols.  os.linesep is a good choice for convert_eols if you are
 167     decoding a text attachment.
 168
 169     This function does not parse a full MIME header value encoded with
 170     base64 (like =?iso-8895-1?b?bmloISBuaWgh?=) -- please use the high
 171     level email.Header class for that functionality.
 172     """
 173     if not s:
 174         return s
 175
 176     dec = a2b_base64(s)
 177     if convert_eols:
 178         return dec.replace(CRLF, convert_eols)
 179     return dec
 180
 181
 182 # For convenience and backwards compatibility w/ standard base64 module
 183 body_decode = decode
 184 decodestring = decode