simple

Crypto/Simple

Solved by 張力

CTF{$!mi14r_7o_th3_h0@0m3w@rk5?} This problem use AES OFB mode to encrypt flag and more then one message. As the diagram shown above, if one use same key, IV and OFB mode to encrypt more than one message, It will be a many-time pad vulnerability.

Every corresponding block in each message is xor-ed with same text and we know that each message is consisted of string.letters + string.digits

With these two knowledge, We can decrypt the flag.

unfortunately, in this particular problem we can many combination of flag that is possible answer. Therefore I list out all the possible answer and choose the right one. (Obviously, you should know which homework is similar to this one)