(full writeup at the link)

write a script to bruteforce:

li = dict(zip("ABCDEFGHIJKLMNOPQRSTUVWXYZ",range(26))) # letter -> integer
il = dict(zip(range(26),"ABCDEFGHIJKLMNOPQRSTUVWXYZ")) # integer -> letter

cipher = "CGULKVIPFRGDOOCSJTRRVMORCQDZG"

for i in range(1, 26):
    sub = 0 # counter for what to subtract by
    m = "" # message
    for l in cipher:
        nc = (li[l] + (i - sub)) % 26 # get integer representation of new letter
        m += il[nc] # add letter to message
        sub -= 1 # reduce the subtraction counter by 1
    print(m)

Only reasonable answer in response is "GLASSESAREUSEFULDONOTLOSETHEM" flag is: hsctf{GLASSESAREUSEFULDONOTLOSETHEM}

Original writeup (https://nullpxl.com/post/hsctf6-jsoninfo-keithbot-alostcause/).