https://www.rfc-editor.org/rfc/rfc4568.html#section-6.1 says:
When base64 decoding the key and salt, padding characters (i.e.,
one or two "=" at the end of the base64-encoded data) are discarded
(see [RFC3548] for details).
https://www.rfc-editor.org/rfc/rfc3548#section-2.2 says:
In some circumstances, the use of padding ("=") in base encoded data
is not required nor used. In the general case, when assumptions on
size of transported data cannot be made, padding is required to yield
correct decoded data.
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