Prove that if only a single character is encrypted, then the shift cipher is perfectly secret.9/18/2023 ![]() ![]() Suppose each key is 1000 bytes.Įach day Alice sends Bob 1 picture showing her battle plans. Unfortunately they can't cross the battlefield to meet each other.īefore they got separated Alice and Bob, exchanged a 1000 page book full of keys, with 10 keys per page, for a total of 10,000 keys. Suppose Alice and Bob are at war with Eve and they need to exchange battle plans. In practice, this could be a big problem. When they run out of keys, they will need to meet again to exchange a new list. Every time they use a key they need to scratch it off their list and use the next key. In practice, to make the OTP useful Alice and Bob need to exchange a list of keys that they will agree to use in the future when they send messages to each other. ![]() So it tends to beg the question, that if Alice and Bob could meet to securely exchange a key the same size as the message, why wouldn't they just exchange the message ? the key must be randomly chosen, and the same size of the message The biggest problem with the one time pad (OTP) is the key: ![]()
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