This is a transposition cipher challenge. We first download the corrupted message by using wget.
Scrambled message
After running cat message.txt, we can see a scrambled message.
Scrambled message: heTfl g as iicpCTo{7F4NRP051N5_16_35P3X51N3_V6E5926A}4.
PicoCTF Hint: Split the message up into blocks of 3 and see how the first block is scrambled
Understanding the problem
By observing the scrambled message, we can infer that the first 2 words of the message should be “The flag”.
Let’s zoom into the first 3 blocks (of 3), which will make up the first 2 words.
First block: heT
Second block: fl
Third block: g a
Spaces in between characters
We must not neglect the spaces in between characters. They should also be treated as normal characters as they were also scrambled during the encryption process.
From the above 3 blocks. we can conclude that the third character of each block should be the first. This will be followed by the first and second character of the block.
Decryption
Decryption script for the Flag
Since we have established that each block should be in this format instead:
<third-chr><first-chr><second-chr>
I used a Python script to decrypt the scrambled message to obtain the flag.
enc_flag = "heTfl g as iicpCTo{7F4NRP051N5_16_35P3X51N3_V6E5926A}4"flag = ""blocks_of_3 = [enc_flag[i:i+3] for i in range(0, len(enc_flag), 3)]for block in blocks_of_3: flag += block[2] + block[0] + block[1]print(flag)
Running the script reveals the flag in plaintext.
Decrypted message: The flag is picoCTF{7R4N5P051N6_15_3XP3N51V3_56E6924A}
