Here we get an Jwt token, and immidiattly it gets revoked, by entering the blacklist.

However, we want to somehow find access token which is valid, and also not in the blacklist.

Base64

Base64 is an encoding method, in which there are 64 chars

ABCDEFGHIJKLMNOPQRSTUVWXYZ      ← 26
abcdefghijklmnopqrstuvwxyz      ← 26
0123456789                      ← 10
+ /                             ← 2
------------------------------
Total: 64 characters

Okay, It takes every 3 bytes, and decode it to 4 chars. For example, given this string: Man: | Character | ASCII | Binary | | ——— | —– | ———- | | M | 77 | 01001101 | | a | 97 | 01100001 | | n | 110 | 01101110 |

Which together is: 01001101 01100001 01101110

Then we split to 4 chunks: 010011 010110 000101 101110

Binary	Decimal
010011	19
010110	22
000101	5
101110	46

and mapping to base64 alphabet: | Decimal | Character | | ——- | ——— | | 19 | T | | 22 | W | | 5 | F | | 46 | u |

So the result will be TWFu.

However, what happens if we get less then 3 bytes?

We padd with zeros, and then, we add = to each fake byte. (if we had 1 byte, we add 2 =, and if 2 bytes, we add 1 =)

Input	Bytes	Padding Needed	Final Base64
"H"	1	2 =	SA==
"Hi"	2	1 =	SGk=
"Man"	3	0	TWFu

Okay, so in our case, we might be able to pad = at the end, and when decoding the access token we still get the same content! But this specific access token isn’t in the blacklist.

Flag: Do_n0t_r3v0ke_3nc0d3dTokenz_Mam3ne-Us3_th3_JTI_f1eld