Problem: This vault uses ASCII encoding for the password. The source code for this vault is here: VaultDoor4.java
File: THE_FILE
Solution:
Find the bytes
106 , 85 , 53 , 116 , 95 , 52 , 95 , 98 ,
0x55, 0x6e, 0x43, 0x68, 0x5f, 0x30, 0x66, 0x5f,
0142, 0131, 0164, 063 , 0163, 0137, 0142, 071 ,
'e' , '9' , '2' , 'f' , '7' , '6' , 'a' , 'c' ,
Convert them into ascii. Its a mix of hex, octal, and decimal. https://www.rapidtables.com/convert/number/ascii-hex-bin-dec-converter.html
line 1 is decimal = jU5t_4_b
line 2 is hex = UnCh_0f_
line 3 is octal = bYt3s_b9
line 4 is itself = e92f76ac
Flag: picoCTF{jU5t_4_bUnCh_0f_bYt3s_b9e92f76ac}