Having just finished reading The Code Book by Simon Singh, which I found extremely illuminating, nevertheless I remain deeply puzzled about one aspect in the book regarding the one-time pad cipher. To cut to the chase, I don't understand how a would-be hacker would be able to break a given OTP cipher (thereby deriving its plaintext) even if he or she already knew the key as well as its cybertext. Unlike the key of a Vigenere cipher, for instance, a key for a given OTP cipher is entirely random, therefore resistant to frequency analysis. The only workaround I can think of is that the OTP is itself a kind of Vigenere cipher? This, however, only takes me so far, sorry to relate. So to summarise: how can a hacker obtain an OTP's plaintext from its key? Many thanks!

• The valid recipient also has the ciphertext and the key - how does he decrypt the ciphertext to recover the plaintext? Nov 22, 2022 at 18:48
• It's ciphertext - i.e. the output of a cipher, not cybertext :) Nov 22, 2022 at 21:12

I don't understand how a would-be hacker would be able to break a given OTP cipher (thereby deriving its plaintext) even if he or she already knew the key as well as its cybertext.

That’s like asking, “I don’t understand how to make a noise if all I have is a klaxon horn”. Um - blow the horn! 😄

With an OTP, the “key” is a stream of random data of the same length as the plaintext message you want to encrypt.

• You encrypt by XOR-ing each byte of the plaintext (P), with the corresponding byte of the random stream key (K). The result of that process is the ciphertext (C)(not “cybertext”).

• You decrypt by XOR-ing each byte of the ciphertext C, with the corresponding byte of the exact same random stream key K. The result of that process is the original plaintext P.

So if you have C and K, that is all you need to get P.