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142
ciphers/base58.py
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142
ciphers/base58.py
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B64_CHARSET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
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def base64_encode(data: bytes) -> bytes:
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"""Encodes data according to RFC4648.
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The data is first transformed to binary and appended with binary digits so that its
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length becomes a multiple of 6, then each 6 binary digits will match a character in
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the B64_CHARSET string. The number of appended binary digits would later determine
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how many "=" signs should be added, the padding.
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For every 2 binary digits added, a "=" sign is added in the output.
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We can add any binary digits to make it a multiple of 6, for instance, consider the
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following example:
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"AA" -> 0010100100101001 -> 001010 010010 1001
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As can be seen above, 2 more binary digits should be added, so there's 4
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possibilities here: 00, 01, 10 or 11.
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That being said, Base64 encoding can be used in Steganography to hide data in these
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appended digits.
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>>> from base64 import b64encode
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>>> a = b"This pull request is part of Hacktoberfest20!"
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>>> b = b"https://tools.ietf.org/html/rfc4648"
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>>> c = b"A"
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>>> base64_encode(a) == b64encode(a)
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True
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>>> base64_encode(b) == b64encode(b)
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True
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>>> base64_encode(c) == b64encode(c)
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True
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>>> base64_encode("abc")
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Traceback (most recent call last):
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...
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TypeError: a bytes-like object is required, not 'str'
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"""
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# Make sure the supplied data is a bytes-like object
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if not isinstance(data, bytes):
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msg = f"a bytes-like object is required, not '{data.__class__.__name__}'"
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raise TypeError(msg)
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binary_stream = "".join(bin(byte)[2:].zfill(8) for byte in data)
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padding_needed = len(binary_stream) % 6 != 0
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if padding_needed:
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# The padding that will be added later
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padding = b"=" * ((6 - len(binary_stream) % 6) // 2)
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# Append binary_stream with arbitrary binary digits (0's by default) to make its
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# length a multiple of 6.
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binary_stream += "0" * (6 - len(binary_stream) % 6)
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else:
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padding = b""
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# Encode every 6 binary digits to their corresponding Base64 character
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return (
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"".join(
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B64_CHARSET[int(binary_stream[index : index + 6], 2)]
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for index in range(0, len(binary_stream), 6)
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).encode()
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+ padding
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)
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def base64_decode(encoded_data: str) -> bytes:
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"""Decodes data according to RFC4648.
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This does the reverse operation of base64_encode.
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We first transform the encoded data back to a binary stream, take off the
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previously appended binary digits according to the padding, at this point we
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would have a binary stream whose length is multiple of 8, the last step is
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to convert every 8 bits to a byte.
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>>> from base64 import b64decode
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>>> a = "VGhpcyBwdWxsIHJlcXVlc3QgaXMgcGFydCBvZiBIYWNrdG9iZXJmZXN0MjAh"
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>>> b = "aHR0cHM6Ly90b29scy5pZXRmLm9yZy9odG1sL3JmYzQ2NDg="
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>>> c = "QQ=="
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>>> base64_decode(a) == b64decode(a)
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True
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>>> base64_decode(b) == b64decode(b)
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True
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>>> base64_decode(c) == b64decode(c)
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True
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>>> base64_decode("abc")
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Traceback (most recent call last):
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...
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AssertionError: Incorrect padding
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"""
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# Make sure encoded_data is either a string or a bytes-like object
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if not isinstance(encoded_data, bytes) and not isinstance(encoded_data, str):
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msg = (
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"argument should be a bytes-like object or ASCII string, "
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f"not '{encoded_data.__class__.__name__}'"
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)
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raise TypeError(msg)
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# In case encoded_data is a bytes-like object, make sure it contains only
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# ASCII characters so we convert it to a string object
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if isinstance(encoded_data, bytes):
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try:
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encoded_data = encoded_data.decode("utf-8")
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except UnicodeDecodeError:
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raise ValueError("base64 encoded data should only contain ASCII characters")
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padding = encoded_data.count("=")
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# Check if the encoded string contains non base64 characters
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if padding:
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assert all(
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char in B64_CHARSET for char in encoded_data[:-padding]
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), "Invalid base64 character(s) found."
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else:
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assert all(
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char in B64_CHARSET for char in encoded_data
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), "Invalid base64 character(s) found."
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# Check the padding
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assert len(encoded_data) % 4 == 0 and padding < 3, "Incorrect padding"
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if padding:
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# Remove padding if there is one
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encoded_data = encoded_data[:-padding]
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binary_stream = "".join(
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bin(B64_CHARSET.index(char))[2:].zfill(6) for char in encoded_data
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)[: -padding * 2]
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else:
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binary_stream = "".join(
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bin(B64_CHARSET.index(char))[2:].zfill(6) for char in encoded_data
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)
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data = [
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int(binary_stream[index : index + 8], 2)
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for index in range(0, len(binary_stream), 8)
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]
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return bytes(data)
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if __name__ == "__main__":
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import doctest
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doctest.testmod()
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