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298 lines
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ReStructuredText
298 lines
10 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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**Source code:** :source:`Lib/base64.py`
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.. index::
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pair: base64; encoding
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single: MIME; base64 encoding
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--------------
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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 interfaces provided by this module. The modern interface
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supports encoding :term:`bytes-like objects <bytes-like object>` to ASCII
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:class:`bytes`, and decoding :term:`bytes-like objects <bytes-like object>` or
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strings containing ASCII to :class:`bytes`. Both base-64 alphabets
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defined in :rfc:`3548` (normal, and URL- and filesystem-safe) are supported.
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The legacy interface does not support decoding from strings, but it does
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provide functions for encoding and decoding to and from :term:`file objects
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<file object>`. It only supports the Base64 standard alphabet, and it adds
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newlines every 76 characters as per :rfc:`2045`. Note that if you are looking
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for :rfc:`2045` support you probably want to be looking at the :mod:`email`
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package instead.
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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 objects <bytes-like object>` 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 the :term:`bytes-like object` *s* using Base64 and return the encoded
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:class:`bytes`.
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Optional *altchars* must be a :term:`bytes-like object` 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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.. function:: b64decode(s, altchars=None, validate=False)
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Decode the Base64 encoded :term:`bytes-like object` or ASCII string
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*s* and return the decoded :class:`bytes`.
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Optional *altchars* must be a :term:`bytes-like object` or ASCII 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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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), characters that are neither
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in the normal base-64 alphabet nor the alternative alphabet are
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discarded prior to the padding check. If *validate* is ``True``,
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these non-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 :term:`bytes-like object` *s* using the standard Base64 alphabet
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and return the encoded :class:`bytes`.
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.. function:: standard_b64decode(s)
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Decode :term:`bytes-like object` or ASCII string *s* using the standard
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Base64 alphabet and return the decoded :class:`bytes`.
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.. function:: urlsafe_b64encode(s)
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Encode :term:`bytes-like object` *s* using the
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URL- and filesystem-safe alphabet, which
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substitutes ``-`` instead of ``+`` and ``_`` instead of ``/`` in the
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standard Base64 alphabet, and return the encoded :class:`bytes`. The result
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can still contain ``=``.
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.. function:: urlsafe_b64decode(s)
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Decode :term:`bytes-like object` or ASCII string *s*
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using the URL- and filesystem-safe
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alphabet, which substitutes ``-`` instead of ``+`` and ``_`` instead of
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``/`` in the standard Base64 alphabet, and return the decoded
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:class:`bytes`.
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.. function:: b32encode(s)
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Encode the :term:`bytes-like object` *s* using Base32 and return the
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encoded :class:`bytes`.
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.. function:: b32decode(s, casefold=False, map01=None)
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Decode the Base32 encoded :term:`bytes-like object` or ASCII string *s* and
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return the decoded :class:`bytes`.
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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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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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input.
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.. function:: b16encode(s)
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Encode the :term:`bytes-like object` *s* using Base16 and return the
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encoded :class:`bytes`.
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.. function:: b16decode(s, casefold=False)
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Decode the Base16 encoded :term:`bytes-like object` or ASCII string *s* and
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return the decoded :class:`bytes`.
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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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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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input.
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.. function:: a85encode(b, *, foldspaces=False, wrapcol=0, pad=False, adobe=False)
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Encode the :term:`bytes-like object` *b* using Ascii85 and return the
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encoded :class:`bytes`.
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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 (``b'\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 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(b, *, foldspaces=False, adobe=False, ignorechars=b' \\t\\n\\r\\v')
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Decode the Ascii85 encoded :term:`bytes-like object` or ASCII string *b* and
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return the decoded :class:`bytes`.
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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 :term:`bytes-like object` or ASCII string
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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(b, pad=False)
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Encode the :term:`bytes-like object` *b* using base85 (as used in e.g.
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git-style binary diffs) and return the encoded :class:`bytes`.
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If *pad* is true, the input is padded with ``b'\0'`` so its length is a
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multiple of 4 bytes before encoding.
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.. versionadded:: 3.4
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.. function:: b85decode(b)
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Decode the base85-encoded :term:`bytes-like object` or ASCII string *b* and
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return the decoded :class:`bytes`. 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.readline()`` returns an
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empty bytes object.
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.. function:: decodebytes(s)
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Decode the :term:`bytes-like object` *s*, which must contain one or more
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lines of base64 encoded data, and return the decoded :class:`bytes`.
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.. versionadded:: 3.1
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.. function:: decodestring(s)
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Deprecated alias of :func:`decodebytes`.
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.. deprecated:: 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` inserts a newline character (``b'\n'``)
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after every 76 bytes of the output, as well as ensuring that the output
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always ends with a newline, as per :rfc:`2045` (MIME).
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.. function:: encodebytes(s)
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Encode the :term:`bytes-like object` *s*, which can contain arbitrary binary
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data, and return :class:`bytes` containing the base64-encoded data, with newlines
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(``b'\n'``) inserted after every 76 bytes of output, and ensuring that
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there is a trailing newline, as per :rfc:`2045` (MIME).
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.. versionadded:: 3.1
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.. function:: encodestring(s)
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Deprecated alias of :func:`encodebytes`.
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.. deprecated:: 3.1
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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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