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Why is base64_encode() adding a slash “/” in the result?

Posted by: admin November 30, 2017 Leave a comment

Questions:

I am encoding the URL suffix of my application:

$url = 'subjects?_d=1';
echo base64_encode($url);

// Outputs
c3ViamVjdHM/X2Q9MQ==

Notice the slash before ‘X2’.

Why is this happening? I thought base64 only outputted A-Z, 0-9 and ‘=’ as padding? I have tried using an online base64 encoder to check, and it seems base64 always does this. I can’t tell if it’s the underscore “_” or the question mark “?” or the “=” perhaps?

Answers:

No. The Base64 alphabet includes A-Z, a-z, 0-9 and + and /.

You can replace them if you don’t care about portability towards other applications.

See: http://en.wikipedia.org/wiki/Base64#Variants_summary_table

You can use something like these to use your own symbols instead (replace - and _ by anything you want, as long as it is not in the base64 base alphabet, of course!).

The following example converts the normal base64 to base64url as specified in RFC 4648:

function base64url_encode($s) {
    return str_replace(array('+', '/'), array('-', '_'), base64_encode($s));
}

function base64url_decode($s) {
    return base64_decode(str_replace(array('-', '_'), array('+', '/'), $s));
}

Questions:
Answers:

Sorry, you thought wrong. A-Za-z0-9 only gets you 62 characters. Base64 uses two additional characters, in PHP’s case / and +.

Questions:
Answers:

There is nothing special in that.

The base 64 “alphabet” or “digits” are A-Z,a-z,0-9 plus two extra characters + (plus) and / (slash).

You can later encode / with %2f if you want.

Questions:
Answers:

A-Z is 26 characters.
0-9 is 10 characters.
= is one character. That gives a total of 37 characters, which is some way short of 64.

/ is one of the 64 characters. You can see a complete list on the wikipedia page.

Questions:
Answers:

In addition to all of the answers above, pointing out that / is part of the expected base64 alphabet, it should be noted that the particular reason you saw a / in your encoded string, is because when base64 encoding ASCII text, the only way to generate a / is to have a question mark in a position divisible by three.

Questions:
Answers:

For base64 the valid charset is:
ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/

the = is used as filler for the last bytes

M.

Questions:
Answers:

Not directly related, and enough people above have answered and explained solutions quite well.

However, going a bit outside of the scope of things. If you want readable base text, try looking into Base58. It’s worth considering if you want only alphanumeric characters.