Why does a base64 encoded string have an = sign at the end
Q Does a base64 string always end with =?
A: No. (the word USB is base64 encoded into dXNi)
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.
You will not have a '=' sign if your string has a multiple of 3 characters number, because Base64 encoding takes each three bytes (a character=1 byte) and represents them as four printable characters in the ASCII standard.
Example:
(a) If you want to encode
ABCDEFG <=> [ABC] [DEF] [G
Base64 will deal with the first block (producing 4 characters) and the second (as they are complete). But for the third it will add a double == in the output in order to complete the 4 needed characters. Thus, the result will be QUJD REVG Rw== (without spaces).
(b) If you want to encode ABCDEFGH <=> [ABC] [DEF] [GH
similarly, it will add just a single = in the end of the output to get 4 characters.
The result will be QUJD REVG R0g= (without spaces).
It serves as padding.
A more complete answer is that a base64 encoded string doesn't always end with a =, it will only end with one or two = if they are required to pad the string out to the proper length.
From Wikipedia:
The final '==' sequence indicates that the last group contained only one byte, and '=' indicates that it contained two bytes.
Thus, this is some sort of padding.
Its defined in RFC 2045 as a special padding character if fewer than 24 bits are available at the end of the encoded data.
- No.
- To pad the Base64-encoded string to a multiple of 4 characters in length, so that it can be decoded correctly.