CRYptograPI

Problem

Description

A Ngawi Spy was arrested in Lempuyangan Railway Station while eating a bread. He was arrested becuse he wanted to deliver an encrypted secret message to another spy. It was known that the message is encrypted by bitwise operating the message with decimal digits of Pi. Can you decrypt the secret message?

Message.txt

75 5b 5f 12 4d 12 51 50 47 15 5b 58 42 5e 5b 46 12 18 60 5e 5b 45 14 5a 40 18 47 5a 52 19 53 5c 53 5f 02 14 77 50 59 55 7f 6d 70 6d 7f 48 44 06 53 04 7c 73 5c 69 0d 68 5e 0d 5a 0d 74 57 6d 67 03 45 54 05 5a 61 6f 0f 77 5f 02 70 04 50 44

Hints

• How many bitwise operation do you know?

• Pi has unending decimal digits...

Solution

Quite straightforward, we search up for the decimal digits up to the same length as the ciphertext. Then we XOR each digits with the corresponding hex values. Initially I was a bit confused since I XORed each of the ciphertext with int(pi[i]) but it doesn't work.

Solve.py

cipher = '755b5f124d12515047155b58425e5b461218605e5b45145a4018475a5219535c535f0214775059557f6d706d7f48440653047c735c690d685e0d5a0d74576d67034554055a616f0f775f0270045044'
pi = '1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679821480865132823066470938446095505822317253594081284811174502840'

byte_cipher = bytes.fromhex(cipher)
decoded = []

for i in range(len(byte_cipher)):
    decoded.append(byte_cipher[i] ^ ord(pi[i]))

print(''.join(chr(i) for i in decoded))

Flag

FindITCTF{s3b4IKnY4_j4n9An_T3rl4lU_9Eg4B4h}