In this lab we'll do some exercises related to OTP and stream ciphers.

Charlie manages to capture a last communication which turns out to be the most important, so it is crucial he decrypts it. However, this time Alice used the Vigenere cipher, with a key that Charlie knows has 7 characters.

The ciphertext is in the file attached. Try the method of multiplying probabilities as I explained in class and see if you can decrypt the ciphertext.

These are the known frequencies of the plaintext:

{'A': 0.07048643054277828,
'C': 0.01577161913523459,
'B': 0.012074517019319227,
'E': 0.13185372585096597,
'D': 0.043393514259429625,
'G': 0.01952621895124195,
'F': 0.023867295308187673,
'I': 0.06153403863845446,
'H': 0.08655128794848206,
'K': 0.007566697332106716,
'J': 0.0017594296228150873,
'M': 0.029657313707451703,
'L': 0.04609015639374425,
'O': 0.07679967801287949,
'N': 0.060217341306347746,
'Q': 0.0006382244710211592,
'P': 0.014357175712971482,
'S': 0.05892939282428703,
'R': 0.05765294388224471,
'U': 0.02749540018399264,
'T': 0.09984475620975161,
'W': 0.01892824287028519,
'V': 0.011148804047838086,
'Y': 0.023045078196872126,
'X': 0.0005289788408463661,
'Z': 0.00028173873045078196}

In class we explained that the one time pad is malleable (i.e. we can easily change the encrypted plaintext by simply modifying the ciphertext). Let’s see a concrete example. Suppose you are told that the one time pad encryption of the message “attack at dawn” is 09e1c5f70a65ac51626bc3d25f17 (the plaintext letters are encoded as 8-bit ASCII and the given ciphertext is written in hex). What would be the one time pad encryption of the message “attack at dusk” under the same OTP key?