This problem is worth 400 points in the "Crypto and Stego" category. This was one of the least solved challenges in the whole cmpetition with only 2% of teams being able to solve it. I was the second to solve this challenge.
Not that easy, only fives!!
TFTTTTFFTFTTTTFFTTTFFTTFTTFFFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFTFTFTFTTFFTFFFFTTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFTFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFTFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFTFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFFTFTFFFFTFTTFFFFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFTFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFTFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFTFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFTFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFTFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFTFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFTFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFTFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFFTFTFFFFTFTTFFFFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFTFFFTFFFTFTFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFTFFFTFFFTFTFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFTFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFFFFTTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFTFFFTFFFTFFFTFTFTTFFTFTTTFTTTFTFFFFFTFFFTFFFTFTFTTFFTF
Taking a look at the ciphertext provided in my_data.dat, we recognize that there are only two characters, T and F. So, let's convert to binary and see if we get any notable results. As a reference, here is the conversion chart for the standard printable characters to their binary, octal, hex, and decimal representations.
We notice that each character in binary representation must have a 0 in the first bit position. So, let's convert T to 0 and F to 1.
010000110100001100011001001111010001000101111101110111010100110100010001011111011101110101001101111000010111110111011101010011011110000101111101110111010100110100010001011111011101110101001101111000010101110111011101010011010001000101111101110111010100110111100001011111011101110101001101111000010111110111010101010011011110000101011101110111010100110100010001010111011101010101001101000100010101110111011101010011010001000101111101110111010100110111100001011111011101010101001101000100010111110111011101010011010001000101111101110101010100110111100001011111011101110101001101101011110100111100010001010111011101110101001101000100010101110111011101010011011110000101011101110111010100110100010001010111011101110101001101000100010101110111010101010011011110000101111101110111010100110100010001010111011101110101001101111000010101110111011101010011011110000101111101110101010100110111100001011111011101110101001101111000010101110111011101010011010001000101111101110101010100110100010001011111011101010101001101111000010111110111010101010011011110000101111101110111010100110100010001010111011101010101001101000100010101110111011101010011010001000101111101110101010100110100010001011111011101110101001101101011110100111111100001011111011101110101001101000100010111110111011101010011011110000101111101110111010100110111100001011111011101110101001101111000010101110111010101010011010001000101111101110111010100110111100001010111011101010101001101000100010111110111011101010011010001000101011101110111010100110100010001010111011101110101001101000100010111110111011101010011010001000101011101110101010100110100010001010111011101110101001101111000010101110111011101010011010001000101011101110111010100110100010001011111011101110101001101000100010111110111011101010011010001000101011101110111010100110100010001011111011101110101001101
When we try to convert this binary into text, we realize that some bytes form unprintable characters. So, let's reverse the original ciphertext. This is what we get.
FTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFTFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFTFTFFFTFFFTFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFTFTFFFTFFFTFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFFFFTTFTFFFFTFTFFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFTFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFTFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFTFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFTFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFTFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFTFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFTFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFTFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFFFFTTFTFFFFTFTFFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFTFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFTFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFTFTFFFTFFFTFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTTFFFFTFFTTFTFTFTFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFTFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTTFFFFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFTTFTFTFFFTFFFTFFFFFTFTTTFTTTFTFFFFTTFTTFFTTTFFTTTTFTFFTTTTFT
This time, the first character is an F so let's convert F to 0 and T to 1.
010011010100010001000001011101110100110101000100010001010111011101001101010001000100000101110111010011010100010001000001011101110100110101000100010001010111011101001101010001000100010101111000010011010100010001000101011101110100110101010100010001010111011101001101010001000100000101110111010011010100010001000101011101110100110101000100010001010111011101001101010001000100000101110111010011010101010001000101011110000100110101000100010000010111011101001101010101000100010101111000010011010100010001000001011110000100110101000100010000010111100001001101010001000100000101110111010011010100010001000001011110000000110100001010010011010100010001000001011101110100110101010100010000010111011101001101010001000100010101110111010011010101010001000101011101110100110101000100010000010111100001001101010101000100000101111000010011010101010001000001011101110100110101010100010000010111011101001101010001000100010101111000010011010100010001000001011110000100110101010100010000010111100001001101010001000100010101111000010011010100010001000101011101110100110101000100010000010111100001001101010101000100010101110111010011010100010001000101011101110100110101000100010001010111100001001101010001000100010101110111010011010100010001000101011101110000110100001010010011010100010001000001011110000100110101010100010000010111011101001101010001000100000101110111010011010101010001000001011110000100110101000100010000010111011101001101010001000100010101110111010011010101010001000101011101110100110101000100010001010111100001001101010101000100000101111000010011010100010001000001011110000100110101000100010000010111011101001101010001000100010101111000010011010100010001000001011101110100110101000100010000010111100001001101010001000100000101111000010011010100010001000001011101110100110101000100010000010111011101000011011001110011110100111101
We convert this binary to its corresponding ASCII vaues and all the characters are valid.
MDAwMDEwMDAwMDAwMDEwMDExMDEwMTEwMDAwMDEwMDEwMDAwMTExMDAwMTExMDAxMDAxMDAwMDAxMDAwMTAwMDEwMTEwMDAxMTAxMTAwMTAwMDExMDAxMTAxMDExMDEwMDAxMTEwMDEwMDExMDEwMDEwMDAxMTAwMDAwMTAxMDAwMDEwMTEwMDExMTAxMDAxMDAwMDExMDAwMDAxMDAxMDAwMDAwCg==
This resulting string is encoded by some base, most probably base 64 or base 32 by the two equal signs at the end. Let's try base64.
000010000000010011010110000010010000111000111001001000001000100010110001101100100011001101011010001110010011010010001100000101000010110011101001000011000001001000000
When we decode it from base64, we get this string that looks like it's in binary. However, there are unprintable characters when we attempt to convert this ciphertext to its ASCII equivalent. It doesn't work if we reverse the string and try again. It's good to note that the string starts and ends with at least 4 0's and thus is not binary.
Another cipher in which there are 2 characters is baconian cipher. Here is the conversion chart for reference.
When we decode our ciphertext, we get BACONCIPHERISNOTGOODTOENCRYPTDATA, which is our flag.
JISCTF{BACONCIPHERISNOTGOODTOENCRYPTDATA}