Doc/library/base64.rst
!base64 --- Base16, Base32, Base64, Base85 Data Encodings.. module:: base64 :synopsis: RFC 4648: Base16, Base32, Base64 Data Encodings; Base85 and Ascii85
Source code: :source:Lib/base64.py
.. index:: pair: base64; encoding single: MIME; base64 encoding
This module provides functions for encoding binary data to printable
ASCII characters and decoding such encodings back to binary data.
This includes the :ref:encodings specified in <base64-rfc-4648>
:rfc:4648 (Base64, Base32 and Base16), the :ref:Base85 encoding <base64-base-85> specified in PDF 2.0 <https://pdfa.org/resource/iso-32000-2/>_, and non-standard variants
of Base85 used elsewhere.
There are two interfaces provided by this module. The modern interface
supports encoding :term:bytes-like objects <bytes-like object> to ASCII
:class:bytes, and decoding :term:bytes-like objects <bytes-like object> or
strings containing ASCII to :class:bytes. Both base-64 alphabets
defined in :rfc:4648 (normal, and URL- and filesystem-safe) are supported.
The :ref:legacy interface <base64-legacy> does not support decoding from strings, but it does
provide functions for encoding and decoding to and from :term:file objects <file object>. 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 :mod:email
package instead.
.. versionchanged:: 3.3 ASCII-only Unicode strings are now accepted by the decoding functions of the modern interface.
.. versionchanged:: 3.4
Any :term:bytes-like objects <bytes-like object> are now accepted by all
encoding and decoding functions in this module. Ascii85/Base85 support added.
.. _base64-rfc-4648:
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.
.. function:: b64encode(s, altchars=None, *, padded=True, wrapcol=0)
Encode the :term:bytes-like object s using Base64 and return the encoded
:class:bytes.
Optional altchars must be a :term: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.
If padded is true (default), pad the encoded data with the '=' character to a size multiple of 4. If padded is false, do not add the pad characters.
If wrapcol is non-zero, insert a newline (b'\n') character
after at most every wrapcol characters.
If wrapcol is zero (default), do not insert any newlines.
.. versionchanged:: 3.15 Added the padded and wrapcol parameters.
.. function:: b64decode(s, altchars=None, validate=False, *, padded=True, canonical=False) b64decode(s, altchars=None, validate=True, *, ignorechars, padded=True, canonical=False)
Decode the Base64 encoded :term:bytes-like object or ASCII string
s and return the decoded :class:bytes.
Optional altchars must be a :term:bytes-like object or ASCII string
of length 2 which specifies the alternative alphabet used instead of the
+ and / characters.
If padded is true, the last group of 4 base 64 alphabet characters must
be padded with the '=' character.
If padded is false, padding is neither required nor recognized:
the '=' character is not treated as padding but as a non-alphabet
character, which means it is silently discarded when validate is false,
or causes an :exc:~binascii.Error when validate is true unless
b'=' is included in ignorechars.
A :exc:binascii.Error exception is raised
if s is incorrectly padded.
If ignorechars is specified, it should be a :term:bytes-like object
containing characters to ignore from the input when validate is true.
If ignorechars contains the pad character '=', the pad characters
presented before the end of the encoded data and the excess pad characters
will be ignored.
The default value of validate is True if ignorechars is specified,
False otherwise.
If validate is false, characters that are neither
in the normal base-64 alphabet nor (if ignorechars is not specified)
the alternative alphabet are
discarded prior to the padding check, but the + and / characters
keep their meaning if they are not in altchars (they will be discarded
in future Python versions).
If validate is true, these non-alphabet characters in the input
result in a :exc:binascii.Error.
If canonical is true, non-zero padding bits are rejected.
See :func:binascii.a2b_base64 for details.
For more information about the strict base64 check, see :func:binascii.a2b_base64
.. versionchanged:: 3.15 Added the canonical, ignorechars, and padded parameters.
.. deprecated:: 3.15
Accepting the + and / characters with an alternative alphabet
is now deprecated.
.. function:: standard_b64encode(s)
Encode :term:bytes-like object s using the standard Base64 alphabet
and return the encoded :class:bytes.
.. function:: standard_b64decode(s)
Decode :term:bytes-like object or ASCII string s using the standard
Base64 alphabet and return the decoded :class:bytes.
.. function:: urlsafe_b64encode(s, *, padded=True)
Encode :term: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 :class:bytes. The result
can still contain = if padded is true (default).
.. versionchanged:: 3.15 Added the padded parameter.
.. function:: urlsafe_b64decode(s, *, padded=False)
Decode :term: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
:class:bytes.
.. versionchanged:: 3.15 Added the padded parameter. Padding of input is no longer required by default.
.. deprecated:: 3.15
Accepting the + and / characters is now deprecated.
.. function:: b32encode(s, *, padded=True, wrapcol=0)
Encode the :term:bytes-like object s using Base32 and return the
encoded :class:bytes.
If padded is true (default), pad the encoded data with the '=' character to a size multiple of 8. If padded is false, do not add the pad characters.
If wrapcol is non-zero, insert a newline (b'\n') character
after at most every wrapcol characters.
If wrapcol is zero (default), do not add any newlines.
.. versionchanged:: 3.15 Added the padded and wrapcol parameters.
.. function:: b32decode(s, casefold=False, map01=None, *, padded=True, ignorechars=b'', canonical=False)
Decode the Base32 encoded :term:bytes-like object or ASCII string s and
return the decoded :class: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.
If padded is true, the last group of 8 base 32 alphabet characters must
be padded with the '=' character.
If padded is false, padding is neither required nor recognized:
the '=' character is not treated as padding but as a non-alphabet
character, which means it raises an :exc:~binascii.Error unless
b'=' is included in ignorechars.
ignorechars should be a :term:bytes-like object containing characters
to ignore from the input.
If canonical is true, non-zero padding bits are rejected.
See :func:binascii.a2b_base32 for details.
A :exc:binascii.Error is raised if s is
incorrectly padded or if there are non-alphabet characters present in the
input.
.. versionchanged:: 3.15 Added the canonical, ignorechars, and padded parameters.
.. function:: b32hexencode(s, *, padded=True, wrapcol=0)
Similar to :func:b32encode but uses the Extended Hex Alphabet, as defined in
:rfc:4648.
.. versionadded:: 3.10
.. versionchanged:: 3.15 Added the padded and wrapcol parameters.
.. function:: b32hexdecode(s, casefold=False, *, padded=True, ignorechars=b'', canonical=False)
Similar to :func: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.
.. versionadded:: 3.10
.. versionchanged:: 3.15 Added the canonical, ignorechars, and padded parameters.
.. function:: b16encode(s, *, wrapcol=0)
Encode the :term:bytes-like object s using Base16 and return the
encoded :class:bytes.
If wrapcol is non-zero, insert a newline (b'\n') character
after at most every wrapcol characters.
If wrapcol is zero (default), do not add any newlines.
.. versionchanged:: 3.15 Added the wrapcol parameter.
.. function:: b16decode(s, casefold=False, *, ignorechars=b'')
Decode the Base16 encoded :term:bytes-like object or ASCII string s and
return the decoded :class:bytes.
Optional casefold is a flag specifying whether a
lowercase alphabet is acceptable as input. For security purposes, the default
is False.
ignorechars should be a :term:bytes-like object containing characters
to ignore from the input.
A :exc:binascii.Error is raised if s is
incorrectly padded or if there are non-alphabet characters present in the
input.
.. versionchanged:: 3.15 Added the ignorechars parameter.
.. _base64-base-85:
Base85 encoding is a family of algorithms which represent four bytes
using five ASCII characters. Originally implemented in the Unix
btoa(1) utility, a version of it was later adopted by Adobe in the
PostScript language and is standardized in PDF 2.0 (ISO 32000-2).
This version, in both its btoa and PDF variants, is implemented by
:func:a85encode.
A separate version, using a different output character set, was
defined as an April Fool's joke in :rfc:1924 but is now used by Git
and other software. This version is implemented by :func:b85encode.
Finally, a third version, using yet another output character set
designed for safe inclusion in programming language strings, is
defined by ZeroMQ and implemented here by :func:z85encode.
The functions present in this module differ in how they handle the following:
<~ and ~> markers.Refer to the documentation of the individual functions for more information.
.. function:: a85encode(b, *, foldspaces=False, wrapcol=0, pad=False, adobe=False)
Encode the :term:bytes-like object b using Ascii85 and return the
encoded :class: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 encoding used in PDF.
If wrapcol is non-zero, insert a newline (b'\n') character
after at most every wrapcol characters.
If wrapcol is zero (default), do not insert any newlines.
pad controls whether zero-padding applied to the end of the input
is fully retained in the output encoding, as done by btoa,
producing an exact multiple of 5 bytes of output. This is not part
of the standard encoding used in PDF, as it does not preserve the
length of the data.
adobe controls whether the encoded byte sequence is framed with
<~ and ~>, as in a PostScript base-85 string literal. Note
that while ASCII85Decode streams in PDF documents must be
terminated with ~>, they must not use a leading <~.
.. versionadded:: 3.4
.. function:: a85decode(b, *, foldspaces=False, adobe=False, ignorechars=b' \t\n\r\v', canonical=False)
Decode the Ascii85 encoded :term:bytes-like object or ASCII string b and
return the decoded :class: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 used in PDF and PostScript.
adobe controls whether the <~ and ~> markers are
present. While the leading <~ is not required, the input must
end with ~>, or a :exc:ValueError is raised.
ignorechars should be a :term:bytes-like object containing characters
to ignore from the input.
This should only contain whitespace characters, and by
default contains all whitespace characters in ASCII.
If canonical is true, non-canonical encodings are rejected.
See :func:binascii.a2b_ascii85 for details.
.. versionadded:: 3.4
.. versionchanged:: 3.15 Added the canonical parameter. Single-character final groups are now always rejected as encoding violations.
.. function:: b85encode(b, pad=False, *, wrapcol=0)
Encode the :term:bytes-like object b using base85 (as used in e.g.
git-style binary diffs) and return the encoded :class:bytes.
The input is padded with b'\0' so its length is a multiple of 4
bytes before encoding. If pad is true, all the resulting
characters are retained in the output, which will always be a
multiple of 5 bytes, and thus the length of the data may not be
preserved on decoding.
If wrapcol is non-zero, insert a newline (b'\n') character
after at most every wrapcol characters.
If wrapcol is zero (default), do not add any newlines.
.. versionadded:: 3.4
.. versionchanged:: 3.15 Added the wrapcol parameter.
.. function:: b85decode(b, *, ignorechars=b'', canonical=False)
Decode the base85-encoded :term:bytes-like object or ASCII string b and
return the decoded :class:bytes.
ignorechars should be a :term:bytes-like object containing characters
to ignore from the input.
If canonical is true, non-canonical encodings are rejected.
See :func:binascii.a2b_base85 for details.
.. versionadded:: 3.4
.. versionchanged:: 3.15 Added the canonical and ignorechars parameters. Single-character final groups are now always rejected as encoding violations.
.. function:: z85encode(s, pad=False, *, wrapcol=0)
Encode the :term:bytes-like object s using Z85 (as used in ZeroMQ)
and return the encoded :class:bytes.
The input is padded with b'\0' so its length is a multiple of 4
bytes before encoding. If pad is true, all the resulting
characters are retained in the output, which will always be a
multiple of 5 bytes, as required by the ZeroMQ standard.
If wrapcol is non-zero, insert a newline (b'\n') character
after at most every wrapcol characters.
If wrapcol is zero (default), do not add any newlines.
.. versionadded:: 3.13
.. versionchanged:: 3.15 The pad parameter was added.
.. versionchanged:: 3.15 Added the wrapcol parameter.
.. function:: z85decode(s, *, ignorechars=b'', canonical=False)
Decode the Z85-encoded :term:bytes-like object or ASCII string s and
return the decoded :class:bytes.
ignorechars should be a :term:bytes-like object containing characters
to ignore from the input.
If canonical is true, non-canonical encodings are rejected.
See :func:binascii.a2b_base85 for details.
.. versionadded:: 3.13
.. versionchanged:: 3.15 Added the canonical and ignorechars parameters. Single-character final groups are now always rejected as encoding violations.
.. _base64-legacy:
.. function:: 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 :term:file objects <file object>. input will be read until input.readline() returns an
empty bytes object.
.. function:: decodebytes(s)
Decode the :term:bytes-like object s, which must contain one or more
lines of base64 encoded data, and return the decoded :class:bytes.
.. versionadded:: 3.1
.. function:: 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 :term:file objects <file object>. input will be read until input.read() returns
an empty bytes object. :func: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).
.. function:: encodebytes(s)
Encode the :term:bytes-like object s, which can contain arbitrary binary
data, and return :class: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).
.. versionadded:: 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'
.. _base64-security:
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.
.. seealso::
Module :mod: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.
ISO 32000-2 Portable document format - Part 2: PDF 2.0 <https://pdfa.org/resource/iso-32000-2/>_
Section 7.4.3, "ASCII85Decode Filter," provides the definition
of the Ascii85 encoding used in PDF and PostScript, including
the output character set and the details of data length preservation
using zero-padding and partial output groups.
ZeroMQ RFC 32/Z85 <https://rfc.zeromq.org/spec/32/>_
The "Formal Specification" section provides the character set used in Z85.