server.py

from Crypto.Cipher import AES
from Crypto.Util.strxor import strxor
from binascii import hexlify, unhexlify

def sign(key, message):
    try:
        ECB = AES.new(key, AES.MODE_ECB)
        messageblocks = [message[i:i + 16] for i in range(0, len(message), 16)]
        tag = ECB.encrypt(messageblocks[0])
        for i in range(1,len(messageblocks)):
            tag = ECB.encrypt(strxor(messageblocks[i], tag))
        return hexlify(tag)
    except:
        print("\nYou can't sign that way! No padding done here boy!")
        exit()

if __name__ == '__main__':

    flag = "\nWhat? Nooooooooo!!! xiomara{1_b0w_d0wn_70_y0u!}"
    key = b'YELLOW SUBMARINE'
    print("\nYou wanna challenge me? You trying to break my signing scheme? LOLLLLLL ><")
    print("Anyways, try hard for that boy!")
    print("Press 0 to get your message signed and 1 to submit a forgery...Pffff! Seriously?")
    while(True):
        try:
            inp = raw_input("\n")
            if(inp=="0"):
                hex_msg = raw_input("\nGimme your hex encoded message\n")
                msg = unhexlify(hex_msg)
                hex_tag = sign(key, msg)
                print("\nThere you go! Here's my hex encoded tag!")
                print(hex_tag)
            else:
                print("\nOh! So, you are up for it?")
                print("\nAlright! Gimme just two different hex encoded messages that could sign to the same tag!")
                msg1 = unhexlify(raw_input("\nMessage #1: \n"))
                msg2 = unhexlify(raw_input("\nMessage #2: \n"))
                if(msg1 == msg2):
                    print("\nI am not fool boy! Get back and do the job like a grown up!")
                    exit()
                if(msg1 != msg2 and sign(key, msg1)==sign(key, msg2)):
                    print(flag)
                    exit()
                else:
                    print("\nOops! They don't match! Told ya! Hard work my son...Better luck next time!")
                    exit()
        except:
            exit()