"base64" --- Base16, Base32, Base64, Base85 Data Encodings
**********************************************************

**Source code:** Lib/base64.py

======================================================================

This module provides functions for encoding binary data to printable
ASCII characters and decoding such encodings back to binary data. It
provides encoding and decoding functions for the encodings specified
in **RFC 4648**, which defines the Base16, Base32, and Base64
algorithms, and for the de-facto standard Ascii85 and Base85
encodings.

The **RFC 4648** encodings are suitable for encoding binary data so
that it can be safely sent by email, used as parts of URLs, or
included as part of an HTTP POST request.  The encoding algorithm is
not the same as the **uuencode** program.

There are two interfaces provided by this module.  The modern
interface supports encoding *bytes-like objects* to ASCII "bytes", and
decoding *bytes-like objects* or strings containing ASCII to "bytes".
Both base-64 alphabets defined in **RFC 4648** (normal, and URL- and
filesystem-safe) are supported.

The legacy interface does not support decoding from strings, but it
does provide functions for encoding and decoding to and from *file
objects*.  It only supports the Base64 standard alphabet, and it adds
newlines every 76 characters as per **RFC 2045**.  Note that if you
are looking for **RFC 2045** support you probably want to be looking
at the "email" package instead.

Changed in version 3.3: ASCII-only Unicode strings are now accepted by
the decoding functions of the modern interface.

Changed in version 3.4: Any *bytes-like objects* are now accepted by
all encoding and decoding functions in this module.  Ascii85/Base85
support added.

The modern interface provides:

base64.b64encode(s, altchars=None)

   Encode the *bytes-like object* *s* using Base64 and return the
   encoded "bytes".

   Optional *altchars* must be a *bytes-like object* of length 2 which
   specifies an alternative alphabet for the "+" and "/" characters.
   This allows an application to e.g. generate URL or filesystem safe
   Base64 strings.  The default is "None", for which the standard
   Base64 alphabet is used.

   May assert or raise a a "ValueError" if the length of *altchars* is
   not 2.  Raises a "TypeError" if *altchars* is not a *bytes-like
   object*.

base64.b64decode(s, altchars=None, validate=False)

   Decode the Base64 encoded *bytes-like object* or ASCII string *s*
   and return the decoded "bytes".

   Optional *altchars* must be a *bytes-like object* or ASCII string
   of length 2 which specifies the alternative alphabet used instead
   of the "+" and "/" characters.

   A "binascii.Error" exception is raised if *s* is incorrectly
   padded.

   If *validate* is "False" (the default), characters that are neither
   in the normal base-64 alphabet nor the alternative alphabet are
   discarded prior to the padding check.  If *validate* is "True",
   these non-alphabet characters in the input result in a
   "binascii.Error".

   May assert or raise a "ValueError" if the length of *altchars* is
   not 2.

base64.standard_b64encode(s)

   Encode *bytes-like object* *s* using the standard Base64 alphabet
   and return the encoded "bytes".

base64.standard_b64decode(s)

   Decode *bytes-like object* or ASCII string *s* using the standard
   Base64 alphabet and return the decoded "bytes".

base64.urlsafe_b64encode(s)

   Encode *bytes-like object* *s* using the URL- and filesystem-safe
   alphabet, which substitutes "-" instead of "+" and "_" instead of
   "/" in the standard Base64 alphabet, and return the encoded
   "bytes".  The result can still contain "=".

base64.urlsafe_b64decode(s)

   Decode *bytes-like object* or ASCII string *s* using the URL- and
   filesystem-safe alphabet, which substitutes "-" instead of "+" and
   "_" instead of "/" in the standard Base64 alphabet, and return the
   decoded "bytes".

base64.b32encode(s)

   Encode the *bytes-like object* *s* using Base32 and return the
   encoded "bytes".

base64.b32decode(s, casefold=False, map01=None)

   Decode the Base32 encoded *bytes-like object* or ASCII string *s*
   and return the decoded "bytes".

   Optional *casefold* is a flag specifying whether a lowercase
   alphabet is acceptable as input.  For security purposes, the
   default is "False".

   **RFC 4648** allows for optional mapping of the digit 0 (zero) to
   the letter O (oh), and for optional mapping of the digit 1 (one) to
   either the letter I (eye) or letter L (el).  The optional argument
   *map01* when not "None", specifies which letter the digit 1 should
   be mapped to (when *map01* is not "None", the digit 0 is always
   mapped to the letter O).  For security purposes the default is
   "None", so that 0 and 1 are not allowed in the input.

   A "binascii.Error" is raised if *s* is incorrectly padded or if
   there are non-alphabet characters present in the input.

base64.b32hexencode(s)

   Similar to "b32encode()" but uses the Extended Hex Alphabet, as
   defined in **RFC 4648**.

   New in version 3.10.

base64.b32hexdecode(s, casefold=False)

   Similar to "b32decode()" but uses the Extended Hex Alphabet, as
   defined in **RFC 4648**.

   This version does not allow the digit 0 (zero) to the letter O (oh)
   and digit 1 (one) to either the letter I (eye) or letter L (el)
   mappings, all these characters are included in the Extended Hex
   Alphabet and are not interchangeable.

   New in version 3.10.

base64.b16encode(s)

   Encode the *bytes-like object* *s* using Base16 and return the
   encoded "bytes".

base64.b16decode(s, casefold=False)

   Decode the Base16 encoded *bytes-like object* or ASCII string *s*
   and return the decoded "bytes".

   Optional *casefold* is a flag specifying whether a lowercase
   alphabet is acceptable as input.  For security purposes, the
   default is "False".

   A "binascii.Error" is raised if *s* is incorrectly padded or if
   there are non-alphabet characters present in the input.

base64.a85encode(b, *, foldspaces=False, wrapcol=0, pad=False, adobe=False)

   Encode the *bytes-like object* *b* using Ascii85 and return the
   encoded "bytes".

   *foldspaces* is an optional flag that uses the special short
   sequence 'y' instead of 4 consecutive spaces (ASCII 0x20) as
   supported by 'btoa'. This feature is not supported by the
   "standard" Ascii85 encoding.

   *wrapcol* controls whether the output should have newline ("b'\n'")
   characters added to it. If this is non-zero, each output line will
   be at most this many characters long.

   *pad* controls whether the input is padded to a multiple of 4
   before encoding. Note that the "btoa" implementation always pads.

   *adobe* controls whether the encoded byte sequence is framed with
   "<~" and "~>", which is used by the Adobe implementation.

   New in version 3.4.

base64.a85decode(b, *, foldspaces=False, adobe=False, ignorechars=b' \t\n\r\x0b')

   Decode the Ascii85 encoded *bytes-like object* or ASCII string *b*
   and return the decoded "bytes".

   *foldspaces* is a flag that specifies whether the 'y' short
   sequence should be accepted as shorthand for 4 consecutive spaces
   (ASCII 0x20). This feature is not supported by the "standard"
   Ascii85 encoding.

   *adobe* controls whether the input sequence is in Adobe Ascii85
   format (i.e. is framed with <~ and ~>).

   *ignorechars* should be a *bytes-like object* or ASCII string
   containing characters to ignore from the input. This should only
   contain whitespace characters, and by default contains all
   whitespace characters in ASCII.

   New in version 3.4.

base64.b85encode(b, pad=False)

   Encode the *bytes-like object* *b* using base85 (as used in e.g.
   git-style binary diffs) and return the encoded "bytes".

   If *pad* is true, the input is padded with "b'\0'" so its length is
   a multiple of 4 bytes before encoding.

   New in version 3.4.

base64.b85decode(b)

   Decode the base85-encoded *bytes-like object* or ASCII string *b*
   and return the decoded "bytes".  Padding is implicitly removed, if
   necessary.

   New in version 3.4.

The legacy interface:

base64.decode(input, output)

   Decode the contents of the binary *input* file and write the
   resulting binary data to the *output* file. *input* and *output*
   must be *file objects*. *input* will be read until
   "input.readline()" returns an empty bytes object.

base64.decodebytes(s)

   Decode the *bytes-like object* *s*, which must contain one or more
   lines of base64 encoded data, and return the decoded "bytes".

   New in version 3.1.

base64.encode(input, output)

   Encode the contents of the binary *input* file and write the
   resulting base64 encoded data to the *output* file. *input* and
   *output* must be *file objects*. *input* will be read until
   "input.read()" returns an empty bytes object. "encode()" inserts a
   newline character ("b'\n'") after every 76 bytes of the output, as
   well as ensuring that the output always ends with a newline, as per
   **RFC 2045** (MIME).

base64.encodebytes(s)

   Encode the *bytes-like object* *s*, which can contain arbitrary
   binary data, and return "bytes" containing the base64-encoded data,
   with newlines ("b'\n'") inserted after every 76 bytes of output,
   and ensuring that there is a trailing newline, as per **RFC 2045**
   (MIME).

   New in version 3.1.

An example usage of the module:

>>> import base64
>>> encoded = base64.b64encode(b'data to be encoded')
>>> encoded
b'ZGF0YSB0byBiZSBlbmNvZGVk'
>>> data = base64.b64decode(encoded)
>>> data
b'data to be encoded'


Security Considerations
=======================

A new security considerations section was added to **RFC 4648**
(section 12); it's recommended to review the security section for any
code deployed to production.

See also:

  Module "binascii"
     Support module containing ASCII-to-binary and binary-to-ASCII
     conversions.

  **RFC 1521** - MIME (Multipurpose Internet Mail Extensions) Part
  One: Mechanisms for Specifying and Describing the Format of Internet
  Message Bodies
     Section 5.2, "Base64 Content-Transfer-Encoding," provides the
     definition of the base64 encoding.
