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c3f55be7dd
When attempting to base64-decode a payload of invalid length (1 mod 4), properly recognize and handle it. The given data will be returned as-is, i.e. not decoded, along with a new defect, InvalidBase64LengthDefect.
233 lines
8.3 KiB
Python
233 lines
8.3 KiB
Python
""" Routines for manipulating RFC2047 encoded words.
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This is currently a package-private API, but will be considered for promotion
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to a public API if there is demand.
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"""
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# An ecoded word looks like this:
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#
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# =?charset[*lang]?cte?encoded_string?=
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#
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# for more information about charset see the charset module. Here it is one
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# of the preferred MIME charset names (hopefully; you never know when parsing).
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# cte (Content Transfer Encoding) is either 'q' or 'b' (ignoring case). In
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# theory other letters could be used for other encodings, but in practice this
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# (almost?) never happens. There could be a public API for adding entries
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# to the CTE tables, but YAGNI for now. 'q' is Quoted Printable, 'b' is
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# Base64. The meaning of encoded_string should be obvious. 'lang' is optional
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# as indicated by the brackets (they are not part of the syntax) but is almost
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# never encountered in practice.
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#
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# The general interface for a CTE decoder is that it takes the encoded_string
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# as its argument, and returns a tuple (cte_decoded_string, defects). The
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# cte_decoded_string is the original binary that was encoded using the
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# specified cte. 'defects' is a list of MessageDefect instances indicating any
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# problems encountered during conversion. 'charset' and 'lang' are the
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# corresponding strings extracted from the EW, case preserved.
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#
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# The general interface for a CTE encoder is that it takes a binary sequence
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# as input and returns the cte_encoded_string, which is an ascii-only string.
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#
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# Each decoder must also supply a length function that takes the binary
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# sequence as its argument and returns the length of the resulting encoded
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# string.
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#
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# The main API functions for the module are decode, which calls the decoder
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# referenced by the cte specifier, and encode, which adds the appropriate
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# RFC 2047 "chrome" to the encoded string, and can optionally automatically
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# select the shortest possible encoding. See their docstrings below for
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# details.
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import re
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import base64
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import binascii
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import functools
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from string import ascii_letters, digits
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from email import errors
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__all__ = ['decode_q',
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'encode_q',
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'decode_b',
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'encode_b',
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'len_q',
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'len_b',
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'decode',
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'encode',
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]
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#
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# Quoted Printable
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#
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# regex based decoder.
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_q_byte_subber = functools.partial(re.compile(br'=([a-fA-F0-9]{2})').sub,
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lambda m: bytes.fromhex(m.group(1).decode()))
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def decode_q(encoded):
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encoded = encoded.replace(b'_', b' ')
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return _q_byte_subber(encoded), []
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# dict mapping bytes to their encoded form
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class _QByteMap(dict):
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safe = b'-!*+/' + ascii_letters.encode('ascii') + digits.encode('ascii')
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def __missing__(self, key):
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if key in self.safe:
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self[key] = chr(key)
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else:
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self[key] = "={:02X}".format(key)
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return self[key]
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_q_byte_map = _QByteMap()
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# In headers spaces are mapped to '_'.
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_q_byte_map[ord(' ')] = '_'
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def encode_q(bstring):
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return ''.join(_q_byte_map[x] for x in bstring)
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def len_q(bstring):
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return sum(len(_q_byte_map[x]) for x in bstring)
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#
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# Base64
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#
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def decode_b(encoded):
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# First try encoding with validate=True, fixing the padding if needed.
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# This will succeed only if encoded includes no invalid characters.
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pad_err = len(encoded) % 4
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missing_padding = b'==='[:4-pad_err] if pad_err else b''
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try:
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return (
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base64.b64decode(encoded + missing_padding, validate=True),
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[errors.InvalidBase64PaddingDefect()] if pad_err else [],
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)
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except binascii.Error:
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# Since we had correct padding, this is likely an invalid char error.
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#
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# The non-alphabet characters are ignored as far as padding
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# goes, but we don't know how many there are. So try without adding
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# padding to see if it works.
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try:
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return (
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base64.b64decode(encoded, validate=False),
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[errors.InvalidBase64CharactersDefect()],
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)
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except binascii.Error:
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# Add as much padding as could possibly be necessary (extra padding
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# is ignored).
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try:
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return (
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base64.b64decode(encoded + b'==', validate=False),
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[errors.InvalidBase64CharactersDefect(),
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errors.InvalidBase64PaddingDefect()],
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)
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except binascii.Error:
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# This only happens when the encoded string's length is 1 more
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# than a multiple of 4, which is invalid.
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#
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# bpo-27397: Just return the encoded string since there's no
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# way to decode.
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return encoded, [errors.InvalidBase64LengthDefect()]
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def encode_b(bstring):
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return base64.b64encode(bstring).decode('ascii')
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def len_b(bstring):
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groups_of_3, leftover = divmod(len(bstring), 3)
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# 4 bytes out for each 3 bytes (or nonzero fraction thereof) in.
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return groups_of_3 * 4 + (4 if leftover else 0)
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_cte_decoders = {
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'q': decode_q,
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'b': decode_b,
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}
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def decode(ew):
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"""Decode encoded word and return (string, charset, lang, defects) tuple.
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An RFC 2047/2243 encoded word has the form:
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=?charset*lang?cte?encoded_string?=
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where '*lang' may be omitted but the other parts may not be.
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This function expects exactly such a string (that is, it does not check the
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syntax and may raise errors if the string is not well formed), and returns
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the encoded_string decoded first from its Content Transfer Encoding and
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then from the resulting bytes into unicode using the specified charset. If
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the cte-decoded string does not successfully decode using the specified
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character set, a defect is added to the defects list and the unknown octets
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are replaced by the unicode 'unknown' character \\uFDFF.
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The specified charset and language are returned. The default for language,
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which is rarely if ever encountered, is the empty string.
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"""
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_, charset, cte, cte_string, _ = ew.split('?')
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charset, _, lang = charset.partition('*')
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cte = cte.lower()
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# Recover the original bytes and do CTE decoding.
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bstring = cte_string.encode('ascii', 'surrogateescape')
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bstring, defects = _cte_decoders[cte](bstring)
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# Turn the CTE decoded bytes into unicode.
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try:
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string = bstring.decode(charset)
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except UnicodeError:
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defects.append(errors.UndecodableBytesDefect("Encoded word "
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"contains bytes not decodable using {} charset".format(charset)))
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string = bstring.decode(charset, 'surrogateescape')
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except LookupError:
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string = bstring.decode('ascii', 'surrogateescape')
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if charset.lower() != 'unknown-8bit':
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defects.append(errors.CharsetError("Unknown charset {} "
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"in encoded word; decoded as unknown bytes".format(charset)))
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return string, charset, lang, defects
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_cte_encoders = {
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'q': encode_q,
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'b': encode_b,
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}
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_cte_encode_length = {
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'q': len_q,
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'b': len_b,
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}
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def encode(string, charset='utf-8', encoding=None, lang=''):
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"""Encode string using the CTE encoding that produces the shorter result.
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Produces an RFC 2047/2243 encoded word of the form:
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=?charset*lang?cte?encoded_string?=
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where '*lang' is omitted unless the 'lang' parameter is given a value.
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Optional argument charset (defaults to utf-8) specifies the charset to use
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to encode the string to binary before CTE encoding it. Optional argument
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'encoding' is the cte specifier for the encoding that should be used ('q'
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or 'b'); if it is None (the default) the encoding which produces the
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shortest encoded sequence is used, except that 'q' is preferred if it is up
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to five characters longer. Optional argument 'lang' (default '') gives the
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RFC 2243 language string to specify in the encoded word.
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"""
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if charset == 'unknown-8bit':
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bstring = string.encode('ascii', 'surrogateescape')
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else:
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bstring = string.encode(charset)
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if encoding is None:
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qlen = _cte_encode_length['q'](bstring)
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blen = _cte_encode_length['b'](bstring)
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# Bias toward q. 5 is arbitrary.
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encoding = 'q' if qlen - blen < 5 else 'b'
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encoded = _cte_encoders[encoding](bstring)
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if lang:
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lang = '*' + lang
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return "=?{}{}?{}?{}?=".format(charset, lang, encoding, encoded)
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