Is it ok to remove the equal signs from a base64 string?

python base64 md5
ensnare picture ensnare · Jan 26, 2012 · Viewed 23.7k times · Source

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.

Answer

Brian picture Brian · Jan 26, 2012

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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