diff options
Diffstat (limited to 'Lib/email/_encoded_words.py')
| -rw-r--r-- | Lib/email/_encoded_words.py | 211 |
1 files changed, 211 insertions, 0 deletions
diff --git a/Lib/email/_encoded_words.py b/Lib/email/_encoded_words.py new file mode 100644 index 0000000..01fe42f --- /dev/null +++ b/Lib/email/_encoded_words.py @@ -0,0 +1,211 @@ +""" Routines for manipulating RFC2047 encoded words. + +This is currently a package-private API, but will be considered for promotion +to a public API if there is demand. + +""" + +# An ecoded word looks like this: +# +# =?charset[*lang]?cte?encoded_string?= +# +# for more information about charset see the charset module. Here it is one +# of the preferred MIME charset names (hopefully; you never know when parsing). +# cte (Content Transfer Encoding) is either 'q' or 'b' (ignoring case). In +# theory other letters could be used for other encodings, but in practice this +# (almost?) never happens. There could be a public API for adding entries +# to to the CTE tables, but YAGNI for now. 'q' is Quoted Printable, 'b' is +# Base64. The meaning of encoded_string should be obvious. 'lang' is optional +# as indicated by the brackets (they are not part of the syntax) but is almost +# never encountered in practice. +# +# The general interface for a CTE decoder is that it takes the encoded_string +# as its argument, and returns a tuple (cte_decoded_string, defects). The +# cte_decoded_string is the original binary that was encoded using the +# specified cte. 'defects' is a list of MessageDefect instances indicating any +# problems encountered during conversion. 'charset' and 'lang' are the +# corresponding strings extracted from the EW, case preserved. +# +# The general interface for a CTE encoder is that it takes a binary sequence +# as input and returns the cte_encoded_string, which is an ascii-only string. +# +# Each decoder must also supply a length function that takes the binary +# sequence as its argument and returns the length of the resulting encoded +# string. +# +# The main API functions for the module are decode, which calls the decoder +# referenced by the cte specifier, and encode, which adds the appropriate +# RFC 2047 "chrome" to the encoded string, and can optionally automatically +# select the shortest possible encoding. See their docstrings below for +# details. + +import re +import base64 +import binascii +import functools +from string import ascii_letters, digits +from email import errors + +# +# Quoted Printable +# + +# regex based decoder. +_q_byte_subber = functools.partial(re.compile(br'=([a-fA-F0-9]{2})').sub, + lambda m: bytes([int(m.group(1), 16)])) + +def decode_q(encoded): + encoded = encoded.replace(b'_', b' ') + return _q_byte_subber(encoded), [] + + +# dict mapping bytes to their encoded form +class QByteMap(dict): + + safe = b'-!*+/' + ascii_letters.encode('ascii') + digits.encode('ascii') + + def __missing__(self, key): + if key in self.safe: + self[key] = chr(key) + else: + self[key] = "={:02X}".format(key) + return self[key] + +_q_byte_map = QByteMap() + +# In headers spaces are mapped to '_'. +_q_byte_map[ord(' ')] = '_' + +def encode_q(bstring): + return ''.join(_q_byte_map[x] for x in bstring) + +def len_q(bstring): + return sum(len(_q_byte_map[x]) for x in bstring) + + +# +# Base64 +# + +def decode_b(encoded): + defects = [] + pad_err = len(encoded) % 4 + if pad_err: + defects.append(errors.InvalidBase64PaddingDefect()) + padded_encoded = encoded + b'==='[:4-pad_err] + else: + padded_encoded = encoded + try: + return base64.b64decode(padded_encoded, validate=True), defects + except binascii.Error: + # Since we had correct padding, this must an invalid char error. + defects = [errors.InvalidBase64CharactersDefect()] + # The non-alphabet characters are ignored as far as padding + # goes, but we don't know how many there are. So we'll just + # try various padding lengths until something works. + for i in 0, 1, 2, 3: + try: + return base64.b64decode(encoded+b'='*i, validate=False), defects + except binascii.Error: + if i==0: + defects.append(errors.InvalidBase64PaddingDefect()) + else: + # This should never happen. + raise AssertionError("unexpected binascii.Error") + +def encode_b(bstring): + return base64.b64encode(bstring).decode('ascii') + +def len_b(bstring): + groups_of_3, leftover = divmod(len(bstring), 3) + # 4 bytes out for each 3 bytes (or nonzero fraction thereof) in. + return groups_of_3 * 4 + (4 if leftover else 0) + + +_cte_decoders = { + 'q': decode_q, + 'b': decode_b, + } + +def decode(ew): + """Decode encoded word and return (string, charset, lang, defects) tuple. + + An RFC 2047/2243 encoded word has the form: + + =?charset*lang?cte?encoded_string?= + + where '*lang' may be omitted but the other parts may not be. + + This function expects exactly such a string (that is, it does not check the + syntax and may raise errors if the string is not well formed), and returns + the encoded_string decoded first from its Content Transfer Encoding and + then from the resulting bytes into unicode using the specified charset. If + the cte-decoded string does not successfully decode using the specified + character set, a defect is added to the defects list and the unknown octets + are replaced by the unicode 'unknown' character \uFDFF. + + The specified charset and language are returned. The default for language, + which is rarely if ever encountered, is the empty string. + + """ + _, charset, cte, cte_string, _ = ew.split('?') + charset, _, lang = charset.partition('*') + cte = cte.lower() + # Recover the original bytes and do CTE decoding. + bstring = cte_string.encode('ascii', 'surrogateescape') + bstring, defects = _cte_decoders[cte](bstring) + # Turn the CTE decoded bytes into unicode. + try: + string = bstring.decode(charset) + except UnicodeError: + defects.append(errors.UndecodableBytesDefect("Encoded word " + "contains bytes not decodable using {} charset".format(charset))) + string = bstring.decode(charset, 'surrogateescape') + except LookupError: + string = bstring.decode('ascii', 'surrogateescape') + if charset.lower() != 'unknown-8bit': + defects.append(errors.CharsetError("Unknown charset {} " + "in encoded word; decoded as unknown bytes".format(charset))) + return string, charset, lang, defects + + +_cte_encoders = { + 'q': encode_q, + 'b': encode_b, + } + +_cte_encode_length = { + 'q': len_q, + 'b': len_b, + } + +def encode(string, charset='utf-8', encoding=None, lang=''): + """Encode string using the CTE encoding that produces the shorter result. + + Produces an RFC 2047/2243 encoded word of the form: + + =?charset*lang?cte?encoded_string?= + + where '*lang' is omitted unless the 'lang' parameter is given a value. + Optional argument charset (defaults to utf-8) specifies the charset to use + to encode the string to binary before CTE encoding it. Optional argument + 'encoding' is the cte specifier for the encoding that should be used ('q' + or 'b'); if it is None (the default) the encoding which produces the + shortest encoded sequence is used, except that 'q' is preferred if it is up + to five characters longer. Optional argument 'lang' (default '') gives the + RFC 2243 language string to specify in the encoded word. + + """ + if charset == 'unknown-8bit': + bstring = string.encode('ascii', 'surrogateescape') + else: + bstring = string.encode(charset) + if encoding is None: + qlen = _cte_encode_length['q'](bstring) + blen = _cte_encode_length['b'](bstring) + # Bias toward q. 5 is arbitrary. + encoding = 'q' if qlen - blen < 5 else 'b' + encoded = _cte_encoders[encoding](bstring) + if lang: + lang = '*' + lang + return "=?{}{}?{}?{}?=".format(charset, lang, encoding, encoded) |