/usr/lib/swipl/library/base64.pl
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 base64_encoded(+Plain, -Encoded, +Options) is det
base64_encoded(-Plain, +Encoded, +Options) is det
General the base64 encoding and decoding. This predicate subsumes base64/2 and base64url/2, providing control over padding, the characters used for encoding and the output type. Options:
charset(+Charset)
Define the encoding character set to use. The (default) classic uses the classical rfc2045 characters. The value url uses URL and file name friendly characters. See base64url/2. The value openbsd uses the OpenBSD password-file alphabet.
padding(+Boolean)
If true (default), the output is padded with = characters.
as(+Type)
Defines the type of the output. One of string (default) or atom.
encoding(+Encoding)
Encoding to use for translation between (Unicode) text and bytes (Base64 is an encoding for bytes). Default is utf8.
Arguments:
Plain- is an atom or string containing the unencoded (plain) text.
Encoded- is an atom or string containing the base64 encoded version of Plain.