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ciphers
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base64.py
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ciphers
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base64.py
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B64_CHARSET
=
"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
def
base64_encode
(
data
:
bytes
)
->
bytes
:
"""Encodes data according to RFC4648.
The data is first transformed to binary and appended with binary digits so that its
length becomes a multiple of 6, then each 6 binary digits will match a character in
the B64_CHARSET string. The number of appended binary digits would later determine
how many "=" signs should be added, the padding.
For every 2 binary digits added, a "=" sign is added in the output.
We can add any binary digits to make it a multiple of 6, for instance, consider the
following example:
"AA" -> 0010100100101001 -> 001010 010010 1001
As can be seen above, 2 more binary digits should be added, so there's 4
possibilities here: 00, 01, 10 or 11.
That being said, Base64 encoding can be used in Steganography to hide data in these
appended digits.
>>> from base64 import b64encode
>>> a = b"This pull request is part of Hacktoberfest20!"
>>> b = b"https://tools.ietf.org/html/rfc4648"
>>> c = b"A"
>>> base64_encode(a) == b64encode(a)
True
>>> base64_encode(b) == b64encode(b)
True
>>> base64_encode(c) == b64encode(c)
True
>>> base64_encode("abc")
Traceback (most recent call last):
...
TypeError: a bytes-like object is required, not 'str'
"""
# Make sure the supplied data is a bytes-like object
if
not
isinstance
(
data
,
bytes
):
raise
TypeError
(
f"a bytes-like object is required, not '
{
data
.
__class__
.
__name__
}
'"
)
binary_stream
=
""
.
join
(
bin
(
byte
)[
2
:].
zfill
(
8
)
for
byte
in
data
)
padding_needed
=
len
(
binary_stream
)
%
6
!=
0
if
padding_needed
:
# The padding that will be added later
padding
=
b"="
*
((
6
-
len
(
binary_stream
)
%
6
)
//
2
)
# Append binary_stream with arbitrary binary digits (0's by default) to make its
# length a multiple of 6.
binary_stream
+=
"0"
*
(
6
-
len
(
binary_stream
)
%
6
)
else
:
padding
=
b""
# Encode every 6 binary digits to their corresponding Base64 character
return
(
""
.
join
(
B64_CHARSET
[
int
(
binary_stream
[
index
:
index
+
6
],
2
)]
for
index
in
range
(
0
,
len
(
binary_stream
),
6
)
).
encode
()
+
padding
)
def
base64_decode
(
encoded_data
:
str
)
->
bytes
:
"""Decodes data according to RFC4648.
This does the reverse operation of base64_encode.
We first transform the encoded data back to a binary stream, take off the
previously appended binary digits according to the padding, at this point we
would have a binary stream whose length is multiple of 8, the last step is
to convert every 8 bits to a byte.
>>> from base64 import b64decode
>>> a = "VGhpcyBwdWxsIHJlcXVlc3QgaXMgcGFydCBvZiBIYWNrdG9iZXJmZXN0MjAh"
>>> b = "aHR0cHM6Ly90b29scy5pZXRmLm9yZy9odG1sL3JmYzQ2NDg="
>>> c = "QQ=="
>>> base64_decode(a) == b64decode(a)
True
>>> base64_decode(b) == b64decode(b)
True
>>> base64_decode(c) == b64decode(c)
True
>>> base64_decode("abc")
Traceback (most recent call last):
...
AssertionError: Incorrect padding
"""
# Make sure encoded_data is either a string or a bytes-like object
if
not
isinstance
(
encoded_data
,
bytes
)
and
not
isinstance
(
encoded_data
,
str
):
raise
TypeError
(
"argument should be a bytes-like object or ASCII string, not "
f"'
{
encoded_data
.
__class__
.
__name__
}
'"
)
# In case encoded_data is a bytes-like object, make sure it contains only
# ASCII characters so we convert it to a string object
if
isinstance
(
encoded_data
,
bytes
):
try
:
encoded_data
=
encoded_data
.
decode
(
"utf-8"
)
except
UnicodeDecodeError
:
raise
ValueError
(
"base64 encoded data should only contain ASCII characters"
)
padding
=
encoded_data
.
count
(
"="
)
# Check if the encoded string contains non base64 characters
if
padding
:
assert
all
(
char
in
B64_CHARSET
for
char
in
encoded_data
[:
-
padding
]
),
"Invalid base64 character(s) found."
else
:
assert
all
(
char
in
B64_CHARSET
for
char
in
encoded_data
),
"Invalid base64 character(s) found."
# Check the padding
assert
len
(
encoded_data
)
%
4
==
0
and
padding
<
3
,
"Incorrect padding"
if
padding
:
# Remove padding if there is one
encoded_data
=
encoded_data
[:
-
padding
]
binary_stream
=
""
.
join
(
bin
(
B64_CHARSET
.
index
(
char
))[
2
:].
zfill
(
6
)
for
char
in
encoded_data
)[:
-
padding
*
2
]
else
:
binary_stream
=
""
.
join
(
bin
(
B64_CHARSET
.
index
(
char
))[
2
:].
zfill
(
6
)
for
char
in
encoded_data
)
data
=
[
int
(
binary_stream
[
index
:
index
+
8
],
2
)
for
index
in
range
(
0
,
len
(
binary_stream
),
8
)
]
return
bytes
(
data
)
if
__name__
==
"__main__"
:
import
doctest
doctest
.
testmod
()
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