Would it be more secure if we used multiple keys in a One-Time-Pad encryption?

  • 2
    $\begingroup$ Welcome to crypto.se - Here is some advice to help improve your question: It is not exactly clear what you are asking. Please consider providing a demonstration of what you have in mind, and be specific regarding what your questions about it are. My current guess at your question leads me to believe it is likely a duplicate of this question, but we can't be sure until you provide more detail. $\endgroup$ – Ella Rose Jan 17 '19 at 1:34
  • $\begingroup$ @EllaRose I think I've understood. The OP might be asking $c_i = p_i \oplus k1_i \oplus k2_i \cdots \oplus km_i$. If each of the key streams $k1,\ldots,km$ are, in short, randomly generated then yes. $\endgroup$ – kelalaka Jan 17 '19 at 6:55

An OTP already provides provable confidentiality in the case that the key is as large as the plaintext, and that the key itself is completely random in an information theoretic sense. Provable security in this case means that even a computationally-unbounded attacker would be absolutely incapable of decrypting the ciphertext if they are not given the key. You can't get more secure than that. Adding another key to an already unbreakable cipher does not make it "more" unbreakable.

Do note that an OTP does not protect the integrity of the plaintext in any way, regardless of how many keys are used on top of each other. An attacker can flip any bit in the ciphertext and know that the corresponding bit in the plaintext will also be flipped. To avoid this, a message authentication code, or MAC, is required. A MAC needs a key of its own, so any extra key material can be used for integrity.

| improve this answer | |
  • 2
    $\begingroup$ When employing a OTP, you shouldn't use pad material for the MAC key. That's dangerous. You really need to keep it separate, ephemeral and just in the mind. Plus then of course the obligatory KDF. You seem to be conf-lating/using integrity with authenticity. $\endgroup$ – Paul Uszak Jan 19 '19 at 17:43
  • 1
    $\begingroup$ @PaulUszak OP is asking about two pads, so he can use the second for integrity and the first for confidentiality. I am fully aware that a single pad the size of the plaintext cannot be used for integrity as well as confidentiality without sacrificing the information theoretic security of the system. $\endgroup$ – forest Jan 21 '19 at 2:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.