In the challenge they gave us an encrypted data and a encryption algorithm.

prefix="Hello. Your flag is DarkCTF{"
suffix="}."
main_string="c an u br ea k th is we ir d en or yp ti on".split()

clear_text = prefix + flag + suffix
enc_text = ""
for letter in clear_text:
		 c1 = ord(letter) / 16
		 c2 = ord(letter) % 16
		 enc_text += main_string[c1]
		 enc_text += main_string[c2]

print enc_text

The python code doesn't work because there is no defined flag variable.
But we can understand the algorithm.

Firstly there is a list defined as main_string.
At for loop, every byte is setting to latter orderly.
Then bytes's unicode number divided into 16 and quotient set as c1, remainder set as c2.
In the list select an item by c1 and c2 values, then add it to enc_text variable.
When all bytes were converted then the encrypted data will appear.

Here is the encrypted data:
eawethkthorthorthonutiuckirthoniskisuucthththorthanthisucthirisbruceaeathanisutheneabrkeaeathisenbrctheneacisirkonbristhwebranbrkkonbrisbranthypbrbrkonkirbrciskkoneatibrbrbrbrtheakonbrisbrckoneauisubrbreacthenkoneaypbrbrisyputi

When we look at that we can understand how made it.
Lets code the reverse encryption script.

main_string="c an u br ea k th is we ir d en or yp ti on".split()
data = "**ENCRYPTED_DATA**"

chck = ""
raw = ""
at = 0

for letter in data:
	chck += letter
	if chck in main_string:
		if at%2 == 0:
			c1 = main_string.index(chck)*16
			at+=1
		else:
			c2 = main_string.index(chck)
			raw += chr(c1+c2)
			at+=1
		chck=""
print(raw)

Again for loop set data's bytes to latter variable.
Bytes were added to check list then if the check string is in the main_string list, chck can continue.
If not the loop appends another byte to chck string.
When reading the data script, we need to seperate to the c1 and c2 variables.
c1 variable multiplied by 16, c2 added to the c1.
The integer gives a unicode number, when it's converted to the ascii type we get the decrypted letter.
When all latters are added to raw list, decrypted data appears.

And the final message is: Hello. Your flag is DarkCTF{0k@y_7h15_71m3_Y0u_N33d_70_Br3@k_M3}.