Base64URL

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Base64URL is a modification of the main Base64 standard, the purpose of which is the ability to use the encoding result as filename or URL address. The Base64URL is described in RFC 4648 § 5, where it is mentioned that the standard Base64 alphabet contains invalid characters for URLs and filenames.

The first problem is that the main standard uses “+” as the 62rd character and “=” as padding character. Both characters have a special meaning in the URI address: “+” is interpreted as space, while “=” is used to send data via query string as “key=value” pair. As you understand, using these symbols may lead to various errors.

The second problem — the main standard uses “/” as the 63rd character, which both for URL addresses and for file system locations, represents the directory separator. Therefore in this case errors are guaranteed.

To avoid the errors above, it was proposed to use a “safe alphabet” for URL addresses and filenames. Thus, the Base64URL was born. It uses the same algorithm as the main standard, but differs in the following:

Replaces “+” by “-” (minus)
Replaces “/” by “_” (underline)
Does not require a padding character
Forbids line separators

For example, the main standard will encode <<???>> to PDw/Pz8+Pg== while Base64URL will convert it to PDw_Pz8-Pg. As you can see, only special characters have been changed, while the letters and digits have remained intact.

If you want to see it in action, check the following tools:

This page would be incomplete without Base64URL characters table. So, meet it:

Uppercase Letters
Index	Character
0	A
1	B
2	C
3	D
4	E
5	F
6	G
7	H
8	I
9	J
10	K
11	L
12	M
13	N
14	O
15	P
16	Q
17	R
18	S
19	T
20	U
21	V
22	W
23	X
24	Y
25	Z

Lowercase Letters
Index	Character
26	a
27	b
28	c
29	d
30	e
31	f
32	g
33	h
34	i
35	j
36	k
37	l
38	m
39	n
40	o
41	p
42	q
43	r
44	s
45	t
46	u
47	v
48	w
49	x
50	y
51	z

Digits
Index	Character
52	0
53	1
54	2
55	3
56	4
57	5
58	6
59	7
60	8
61	9

Symbols
Index	Character
62	-
63	_

Given all of the above, a Base64URL value can be defined using the following regular expression:

^[A-Za-z0-9_-]+$

I would also like to provide some additional links:

Base64URL in PHP

Comments (21)

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Faizul, 18 june 2020 at 18:27 #

I want to decode some string that I received from app sniffing.

I will be pleased if you help me to decode such following code:

abG9nb191cmw=, maHR0cDovLzUxLjE1LjIwOS45MDo4ODAwL2RhdGEvaW1nL2ZpbGVzLzU2NjU5X2FzcG9yLmpwZw==

Please let me know how to decode the above-mentioned characters.

Thanks

reply
- Zaidan, 22 july 2020 at 13:03 #↑
  
  Found one kinda look like that while reversing android apk, thought it was Base64 but turns out it was AES256 encryption.
  So that might be AES256
  
  reply
DavidSpector, 9 june 2021 at 13:19 #

This is a great standard, and one that is overdue. But those responsible for JavaScript and PHP (at least), need to add it to their languages so programmers can learn about this and use it directly.

reply
- cs32, 16 july 2022 at 22:03 #↑
  
  @DavidSpector, it would be convenient to have built-in base64url methods in PHP / JS, but, frankly, implementing them is very trivial:
  
  PHP:
  php function base64url($str) { return str_replace(['+','/','='], ['-','_',''], base64_encode($str)); } // decode base64url encoded data function unbase64url($str) { return base64_decode(str_replace(['-','_'], ['+','/'], $str)); }
  
  JS:
  js function base64url(str) { return btoa(str).replace(/=+$/,'').replace(/\+/g,'-').replace(/\//g,'_'); } function unbase64url(str) { return atob(str.replace(/-/g,'+').replace(/_/g,'/')); }
  
  reply
  - Slay, 3 august 2022 at 06:31 #↑
    
    @cs32,
    it's was wrong:
    // decode base64url encoded data function unbase64url($str) { return base64_decode(str_replace(['-','_'], ['+','/'], $str)); }
    
    so is correct:
    function unbase64url($str) { return base64_decode(str_replace(['-','_'], ['+','/'], base64_decode($str)); }
    
    reply
Dominik, 25 november 2023 at 18:19 #

Thanks for the article, clear and understandable. My only question is what exactly do you mean by "Does not require a padding character"? So it means that in some cases it is allowed to use it? When is it allowed and when is it not? It'd be great to extend that section.

reply
Newton, 5 january 2024 at 05:03 #

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when preparing her own post-graduate research in addition to, most importantly,
regarding providing all the ideas in a single blog post.
If we had been aware of your web page a year ago, we will have been kept from
the needless measures we were implementing. Thanks to you.
toys for adults

reply
nqhXncMU, 6 january 2024 at 12:38 #

-5 OR 998=(SELECT 998 FROM PG_SLEEP(15))--

reply
nqhXncMU, 6 january 2024 at 12:54 #

555'||DBMS_PIPE.RECEIVE_MESSAGE(CHR(98)||CHR(98)||CHR(98),15)||'

reply
gBqsPxAZ, 6 january 2024 at 14:58 #

AgLz6K5O'; waitfor delay '0:0:15' --

reply
gBqsPxAZ, 6 january 2024 at 15:02 #

(select(0)from(select(sleep(15)))v)/*'+(select(0)from(select(sleep(15)))v)+'"+(select(0)from(select(sleep(15)))v)+"*/

reply
gBqsPxAZ, 6 january 2024 at 15:03 #

555'||DBMS_PIPE.RECEIVE_MESSAGE(CHR(98)||CHR(98)||CHR(98),15)||'

reply
nqhXncMU, 6 january 2024 at 23:05 #

lvij0vd8'; waitfor delay '0:0:15' --

reply
nqhXncMU, 6 january 2024 at 23:25 #

v0WuCun8' OR 718=(SELECT 718 FROM PG_SLEEP(15))--

reply
nqhXncMU, 6 january 2024 at 23:34 #

0'XOR(if(now()=sysdate(),sleep(15),0))XOR'Z

reply
ncMUFCMU, 7 january 2024 at 00:36 #

BJVuPLLk') OR 319=(SELECT 319 FROM PG_SLEEP(15))--

reply
ncMUFCMU, 7 january 2024 at 00:46 #

555*DBMS_PIPE.RECEIVE_MESSAGE(CHR(99)||CHR(99)||CHR(99),15)

reply
ncMUFCMU, 7 january 2024 at 00:52 #

if(now()=sysdate(),sleep(15),0)

reply
ncMUFCMU, 7 january 2024 at 00:56 #

0"XOR(if(now()=sysdate(),sleep(15),0))XOR"Z

reply
Tjpatel, 7 february 2024 at 19:00 #

I know some of the are base64 but when use that the other results are unknown
so I want to decrypt them eyJ0eXBlIjoiU1NIIiwibmFtZSI6IlZQTjTEsEsgMSIsInNzaFR1bm5lbENvbmZpZyI6. 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

reply
Pawel, 21 march 2024 at 11:20 #

Hi,

Can anyone tell me what does +$ means in replace method:
js
function base64url(str) {
return btoa(str).replace(/=+$/,'').replace(/\+/g,'-').replace(/\//g,'_');
}
?

reply

Base64.Guru

A virtual teacher who reveals to you the great secrets of Base64

Base64URL

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Index	Character
0	A
1	B
2	C
3	D
4	E
5	F
6	G
7	H
8	I
9	J
10	K
11	L
12	M
13	N
14	O
15	P
16	Q
17	R
18	S
19	T
20	U
21	V
22	W
23	X
24	Y
25	Z

Index	Character
26	a
27	b
28	c
29	d
30	e
31	f
32	g
33	h
34	i
35	j
36	k
37	l
38	m
39	n
40	o
41	p
42	q
43	r
44	s
45	t
46	u
47	v
48	w
49	x
50	y
51	z

Index	Character
0	A
1	B
2	C
3	D
4	E
5	F
6	G
7	H
8	I
9	J
10	K
11	L
12	M
13	N
14	O
15	P
16	Q
17	R
18	S
19	T
20	U
21	V
22	W
23	X
24	Y
25	Z

Index	Character
26	a
27	b
28	c
29	d
30	e
31	f
32	g
33	h
34	i
35	j
36	k
37	l
38	m
39	n
40	o
41	p
42	q
43	r
44	s
45	t
46	u
47	v
48	w
49	x
50	y
51	z

Index	Character
0	A
1	B
2	C
3	D
4	E
5	F
6	G
7	H
8	I
9	J
10	K
11	L
12	M
13	N
14	O
15	P
16	Q
17	R
18	S
19	T
20	U
21	V
22	W
23	X
24	Y
25	Z

Index	Character
26	a
27	b
28	c
29	d
30	e
31	f
32	g
33	h
34	i
35	j
36	k
37	l
38	m
39	n
40	o
41	p
42	q
43	r
44	s
45	t
46	u
47	v
48	w
49	x
50	y
51	z