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269 lines
9.4 KiB
ReStructuredText
269 lines
9.4 KiB
ReStructuredText
:mod:`base64` --- Base16, Base32, Base64, Base85 Data Encodings
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===============================================================
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.. module:: base64
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:synopsis: RFC 3548: Base16, Base32, Base64 Data Encodings;
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Base85 and Ascii85
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.. index::
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pair: base64; encoding
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single: MIME; base64 encoding
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This module provides functions for encoding binary data to printable
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ASCII characters and decoding such encodings back to binary data.
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It provides encoding and decoding functions for the encodings specified in
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:rfc:`3548`, which defines the Base16, Base32, and Base64 algorithms,
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and for the de-facto standard Ascii85 and Base85 encodings.
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The :rfc:`3548` encodings are suitable for encoding binary data so that it can
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safely sent by email, used as parts of URLs, or included as part of an HTTP
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POST request. The encoding algorithm is not the same as the
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:program:`uuencode` program.
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There are two :rfc:`3548` interfaces provided by this module. The modern
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interface supports encoding and decoding ASCII byte string objects using all
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three :rfc:`3548` defined alphabets (normal, URL-safe, and filesystem-safe).
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Additionally, the decoding functions of the modern interface also accept
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Unicode strings containing only ASCII characters. The legacy interface provides
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for encoding and decoding to and from file-like objects as well as byte
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strings, but only using the Base64 standard alphabet.
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.. versionchanged:: 3.3
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ASCII-only Unicode strings are now accepted by the decoding functions of
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the modern interface.
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.. versionchanged:: 3.4
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Any :term:`bytes-like object`\ s are now accepted by all
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encoding and decoding functions in this module. Ascii85/Base85 support added.
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The modern interface provides:
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.. function:: b64encode(s, altchars=None)
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Encode a byte string using Base64.
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*s* is the string to encode. Optional *altchars* must be a string of at least
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length 2 (additional characters are ignored) which specifies an alternative
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alphabet for the ``+`` and ``/`` characters. This allows an application to e.g.
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generate URL or filesystem safe Base64 strings. The default is ``None``, for
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which the standard Base64 alphabet is used.
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The encoded byte string is returned.
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.. function:: b64decode(s, altchars=None, validate=False)
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Decode a Base64 encoded byte string.
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*s* is the byte string to decode. Optional *altchars* must be a string of
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at least length 2 (additional characters are ignored) which specifies the
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alternative alphabet used instead of the ``+`` and ``/`` characters.
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The decoded string is returned. A :exc:`binascii.Error` exception is raised
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if *s* is incorrectly padded.
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If *validate* is ``False`` (the default), non-base64-alphabet characters are
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discarded prior to the padding check. If *validate* is ``True``,
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non-base64-alphabet characters in the input result in a
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:exc:`binascii.Error`.
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.. function:: standard_b64encode(s)
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Encode byte string *s* using the standard Base64 alphabet.
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.. function:: standard_b64decode(s)
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Decode byte string *s* using the standard Base64 alphabet.
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.. function:: urlsafe_b64encode(s)
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Encode byte string *s* using a URL-safe alphabet, which substitutes ``-`` instead of
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``+`` and ``_`` instead of ``/`` in the standard Base64 alphabet. The result
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can still contain ``=``.
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.. function:: urlsafe_b64decode(s)
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Decode byte string *s* using a URL-safe alphabet, which substitutes ``-`` instead of
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``+`` and ``_`` instead of ``/`` in the standard Base64 alphabet.
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.. function:: b32encode(s)
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Encode a byte string using Base32. *s* is the string to encode. The encoded string
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is returned.
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.. function:: b32decode(s, casefold=False, map01=None)
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Decode a Base32 encoded byte string.
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*s* is the byte string to decode. Optional *casefold* is a flag specifying
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whether a lowercase alphabet is acceptable as input. For security purposes,
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the default is ``False``.
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:rfc:`3548` allows for optional mapping of the digit 0 (zero) to the letter O
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(oh), and for optional mapping of the digit 1 (one) to either the letter I (eye)
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or letter L (el). The optional argument *map01* when not ``None``, specifies
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which letter the digit 1 should be mapped to (when *map01* is not ``None``, the
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digit 0 is always mapped to the letter O). For security purposes the default is
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``None``, so that 0 and 1 are not allowed in the input.
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The decoded byte string is returned. A :exc:`binascii.Error` is raised if *s* is
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incorrectly padded or if there are non-alphabet characters present in the
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string.
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.. function:: b16encode(s)
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Encode a byte string using Base16.
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*s* is the string to encode. The encoded byte string is returned.
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.. function:: b16decode(s, casefold=False)
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Decode a Base16 encoded byte string.
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*s* is the string to decode. Optional *casefold* is a flag specifying whether a
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lowercase alphabet is acceptable as input. For security purposes, the default
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is ``False``.
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The decoded byte string is returned. A :exc:`TypeError` is raised if *s* were
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incorrectly padded or if there are non-alphabet characters present in the
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string.
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.. function:: a85encode(s, *, foldspaces=False, wrapcol=0, pad=False, adobe=False)
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Encode a byte string using Ascii85.
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*s* is the string to encode. The encoded byte string is returned.
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*foldspaces* is an optional flag that uses the special short sequence 'y'
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instead of 4 consecutive spaces (ASCII 0x20) as supported by 'btoa'. This
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feature is not supported by the "standard" Ascii85 encoding.
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*wrapcol* controls whether the output should have newline ('\n')
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characters added to it. If this is non-zero, each output line will be
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at most this many characters long.
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*pad* controls whether the input string is padded to a multiple of 4
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before encoding. Note that the ``btoa`` implementation always pads.
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*adobe* controls whether the encoded byte sequence is framed with ``<~``
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and ``~>``, which is used by the Adobe implementation.
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.. versionadded:: 3.4
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.. function:: a85decode(s, *, foldspaces=False, adobe=False, ignorechars=b' \t\n\r\v')
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Decode an Ascii85 encoded byte string.
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*s* is the byte string to decode.
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*foldspaces* is a flag that specifies whether the 'y' short sequence
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should be accepted as shorthand for 4 consecutive spaces (ASCII 0x20).
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This feature is not supported by the "standard" Ascii85 encoding.
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*adobe* controls whether the input sequence is in Adobe Ascii85 format
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(i.e. is framed with <~ and ~>).
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*ignorechars* should be a byte string containing characters to ignore
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from the input. This should only contain whitespace characters, and by
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default contains all whitespace characters in ASCII.
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.. versionadded:: 3.4
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.. function:: b85encode(s, pad=False)
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Encode a byte string using base85, as used in e.g. git-style binary
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diffs.
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If *pad* is true, the input is padded with "\\0" so its length is a
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multiple of 4 characters before encoding.
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.. versionadded:: 3.4
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.. function:: b85decode(b)
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Decode base85-encoded byte string. Padding is implicitly removed, if
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necessary.
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.. versionadded:: 3.4
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.. note::
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Both Base85 and Ascii85 have an expansion factor of 5 to 4 (5 Base85 or
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Ascii85 characters can encode 4 binary bytes), while the better-known
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Base64 has an expansion factor of 6 to 4. They are therefore more
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efficient when space expensive. They differ by details such as the
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character map used for encoding.
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The legacy interface:
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.. function:: decode(input, output)
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Decode the contents of the binary *input* file and write the resulting binary
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data to the *output* file. *input* and *output* must be :term:`file objects
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<file object>`. *input* will be read until ``input.read()`` returns an empty
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bytes object.
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.. function:: decodebytes(s)
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decodestring(s)
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Decode the byte string *s*, which must contain one or more lines of base64
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encoded data, and return a byte string containing the resulting binary data.
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``decodestring`` is a deprecated alias.
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.. versionadded:: 3.1
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.. function:: encode(input, output)
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Encode the contents of the binary *input* file and write the resulting base64
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encoded data to the *output* file. *input* and *output* must be :term:`file
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objects <file object>`. *input* will be read until ``input.read()`` returns
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an empty bytes object. :func:`encode` returns the encoded data plus a trailing
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newline character (``b'\n'``).
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.. function:: encodebytes(s)
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encodestring(s)
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Encode the byte string *s*, which can contain arbitrary binary data, and
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return a byte string containing one or more lines of base64-encoded data.
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:func:`encodebytes` returns a string containing one or more lines of
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base64-encoded data always including an extra trailing newline (``b'\n'``).
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``encodestring`` is a deprecated alias.
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An example usage of the module:
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>>> import base64
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>>> encoded = base64.b64encode(b'data to be encoded')
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>>> encoded
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b'ZGF0YSB0byBiZSBlbmNvZGVk'
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>>> data = base64.b64decode(encoded)
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>>> data
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b'data to be encoded'
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.. seealso::
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Module :mod:`binascii`
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Support module containing ASCII-to-binary and binary-to-ASCII conversions.
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:rfc:`1521` - MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies
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Section 5.2, "Base64 Content-Transfer-Encoding," provides the definition of the
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base64 encoding.
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