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I have a string that I'm encoding into base64 to conserve space. Is it a big deal if I remove the equal sign at the end? Would this significantly decrease entropy? What can I do to ensure the length of the resulting string is fixed?
>>> base64.b64encode(combined.digest(), altchars="AB") 'PeFC3irNFx8fuzwjAzAfEAup9cz6xujsf2gAIH2GdUM=' Thanks.
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Q Does a base64 string always end with = ? Q Why does an = get appended at the end? A: As a short answer: The last character ( = sign) is added only as a complement (padding) in the final process of encoding a message with a special number of characters.
The equals sign "=" represents a padding, usually seen at the end of a Base64 encoded sequence. The size in bytes is divisible by three (bits divisible by 24): All bits are encoded normally.
When decoding Base64 text, four characters are typically converted back to three bytes. The only exceptions are when padding characters exist. A single = indicates that the four characters will decode to only two bytes, while == indicates that the four characters will decode to only a single byte. For example: Encoded.
The base 64 digits in ascending order from zero are the uppercase characters 'A' to 'Z', lowercase characters 'a' to 'z', numerals '0' to '9', and the symbols '+' and '/'. % is not allowed in base64 encoding.
Every 3 bytes you need to encode as Base64 are converted to 4 ASCII characters and the '=' character is used to pad the result so that there are always a multiple of 4 encoded characters. If you have an exact multiple of 3 bytes then you will get no equal sign. One spare byte means you get two '=' characters at the end. Two spare bytes means you get one '=' character at the end. depending on how you decode the string it may or may not see this as a valid string. With the example string you have, it doesn't decode, but some simple strings I've tried do decode.
You can read this page for a better understanding of base64 strings and encoding/decoding.
http://www.nczonline.net/blog/2009/12/08/computer-science-in-javascript-base64-encoding/
There are free online encoder/decoders that you can use to check your output string
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