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. This includes the encodings specified in RFC 4648 (Base64, Base32 and Base16), the Base85 encoding specified in PDF 2.0, and non-standard variants of Base85 used elsewhere.

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.

RFC 4648 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.

base64.b64encode(s, altchars=None, *, wrapcol=0)

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.

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.

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

Changed in version 3.15: Added the wrapcol parameter.

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

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 ignorechars is specified, it should be a 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 binascii.Error.

For more information about the strict base64 check, see binascii.a2b_base64()

Changed in version 3.15: Added the ignorechars parameter.

Deprecated since version 3.15: Accepting the + and / characters with an alternative alphabet is now deprecated.

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.

Deprecated since version 3.15: Accepting the + and / characters is now deprecated.

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.

Added 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.

Added 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.

Base85 Encodings

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 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 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 z85encode().

The functions present in this module differ in how they handle the following:

  • Whether to include and expect enclosing <~ and ~> markers

  • Whether to fold the input into multiple lines

  • The set of ASCII characters used for encoding

  • The encoding of zero-padding bytes applied to the input

Refer to the documentation of the individual functions for more information.

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 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 PDF streams must not use a leading <~, but they must be terminated with ~>.

Added 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 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 ValueError is raised.

ignorechars should be a 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.

Added 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.

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 be a multiple of 5 bytes, and thus the length of the data may not be preserved on decoding.

Added in version 3.4.

base64.b85decode(b)

Decode the base85-encoded bytes-like object or ASCII string b and return the decoded bytes.

Added in version 3.4.

base64.z85encode(s, pad=False)

Encode the bytes-like object s using Z85 (as used in ZeroMQ) and return the encoded 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 then be a multiple of 5 bytes, as required by the ZeroMQ standard.

Added in version 3.13.

Changed in version 3.15: The pad parameter was added.

base64.z85decode(s)

Decode the Z85-encoded bytes-like object or ASCII string s and return the decoded bytes.

Added in version 3.13.

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.

Added 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).

Added 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.

Binary-to-text encoding

This Wikipedia article describes the history of binary to text encoding techniques including those implemented by this module.

ISO 32000-2 Portable document format - Part 2: PDF 2.0

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.

RFC 1924 - A Compact Representation of IPv6 Addresses

Section 4.2 details the character set used in base85 encoding. The question of zero-padding is not mentioned, since IPV6 addresses by definition are a multiple of four bytes.

ZeroMQ RFC 32/Z85

The “Formal Specification” section provides the character set used in Z85.