S.H.E.L.L CTF 2021: Algoric-Shift | szymanski.ninja

Algoric-Shift

Category: Cryptography

chal

50 points

ciphered text : HESL{LRAT5PN51010T_CNPH1R}3

Try decrypting:

file: script.py

script.py

text = 'flag{...}'

key = [3,1,2]

li0 = []
li1 = []
li2 = []
for i in range(0,len(text)):
    if i % 3 == 0:
        li0.append(text[i])
    elif (i - 1) % 3 == 0:
        li1.append(text[i])
    elif (i - 2) % 3 == 0:
        li2.append(text[i])
li = []
for i in range(len(li1)): 
    li.append(li1[i]) 
    li.append(li2[i])
    li.append(li0[i])

# print(li)
print("The ciphered text is :")
ciphered_txt = (''.join(li))
print(ciphered_txt)

Solution

I’ve changed given Python script as below to reverse flag encryption.

text = 'HESL{LRAT5PN51010T_CNPH1R}3'

key = [3,1,2]

li0 = []
li1 = []
li2 = []
for i in range(0,len(text)):
    if i % 3 == 0:
        li2.append(text[i])
    elif (i - 1) % 3 == 0:
        li0.append(text[i])
    elif (i - 2) % 3 == 0:
        li1.append(text[i])
li = []
for i in range(len(li1)): 
    li.append(li1[i]) 
    li.append(li2[i])
    li.append(li0[i])

print("The ciphered text is :")
ciphered_txt = (''.join(li))
print(ciphered_txt)

Output:

SHELL{TRAN5P051T10N_C1PH3R}

Flag

SHELL{TRAN5P051T10N_C1PH3R}