The Numbers

Challenge Description

Seems like an interesting challenge. The downloaded file looks like this.

The numbers

PicoCTF Hint: The flag is in the format PICOCTF{}

Now, this is surprising. All the previous challenges have had the same flag format of picoCTF{...}.

I realised “P” is the 16th character of the alphabet, and “I” is the 9th. I concluded that the numbers most likely correspond the alphabets’ place in the alphabet. After all, the numbers are all within the range of 1 and 26.

Check

This also means that the flag will only contain uppercase letters.

Decryption script

Script to obtain the flag

enc_flag = "16 9 3 15 3 20 6 { 20 8 5 14 21 13 2 5 18 19 13 1 19 15 14 }"
 
alphabets = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
 
flag = ""
 
for i in enc_flag.split():
    if i.isnumeric():
        flag += alphabets[int(i)-1]
    else:
        flag += i
 
print(flag)

The above shows the python script I used to convert the numbers into the desired flag format.

Flag

PICOCTF{THENUMBERSMASON}