I am attending cryptography course on Coursera. I am stuck at one question which I cannot answer right even after few tries.
Which of the following are true about Vigenere cipher? (Check all that apply)
- A Vigenere cipher with key of length 100 can be broken (in a reasonable amount of time) using exhaustive search of the key space.
- The Vigenere cipher can always be broken, regardles of the length of the key and regardless of th elength of plaintext being encyrpted
- The Vigenere cipher is computationally infeasible to break if the key has length 100, even if 1000s of characters of plaintext are encrypted.
- The Vigenere cipher is perfectly secret if the length of the key is equal to the length of the messages in the message space.
I am not sure about answer nr. 1 if it can be broken in reasonable amount of time. Non-Computationally for sure not.
I think the second answer is also wrong as by definition if the key and message are the same length it is perfectly secret.(well the key has to be randomly chosen...)
Third answer is also wrong since the key is repeating.
The fourth one is true as I mentioned at second one...
I am not sure what possibilities I have tried but I cannot pass this one :)