A "One Time Pad" can be thought of a Vigenere cipher with...

  1. An infinitely long key
  2. A secure symmetric key
  3. Multiple ciphertext alphabets
  4. A columnar transposition

I had this question come up today in one of my mid-terms and to be honest it caught me off guard a little, I chose (2) but I'm having doubts and now thinking its (3). I'm pretty sure i'll get a quick response to this because its quite a basic question, this is from an initial test on intro to cryptography and I felt I did pretty well on the rest of the test but this is bugging me.

I know one time pad can have infinite period and can be truly unbreakable but that doesn't mean it has an infinitely long key! (does it). (4) is surely a null consideration which leads me to think its (2) or (3)!

  • 1
    $\begingroup$ What does "a secure symmetric key" mean? If (1) were worded "a random key as long as the plaintext", I would have chosen (1). $\endgroup$
    – mikeazo
    Commented Nov 11, 2015 at 18:58
  • $\begingroup$ I'd say definitely not 3 or 4. You could easily argue that 2 is correct if you say that you define "secure symmetric key" as "a random key as long as the plaintext". $\endgroup$
    – mikeazo
    Commented Nov 11, 2015 at 19:20
  • 3
    $\begingroup$ I think that (1) is clearly the the intended answer, but it's a badly formulated question. $\endgroup$ Commented Nov 11, 2015 at 19:33
  • 1
    $\begingroup$ @CodesInChaos, that's my guess too. I think that due to the wording you could argue (2). $\endgroup$
    – mikeazo
    Commented Nov 11, 2015 at 19:35
  • 1
    $\begingroup$ @pi-e to me "multiple ciphertext alphabets" means polyalphabetic cipher. Vigenere is a polyalphabetic cipher by itself, so (3) doesn't really add anything to the sentence. $\endgroup$
    – mikeazo
    Commented Nov 11, 2015 at 19:37

1 Answer 1


(1) is the correct answer. The Vengiere cipher phrase is a distractor. Basically all alphabets are in the cipher.

Note that the key does not need to be infinite, merely the same length as the plaintext and random. So in that regard, all of the answers are incorrect... But (1) is the "best" answer as it is the answer with the most entropy or randomness, which is the point of a one-time pad. You cannot draw any conclusions about entropy from the other answers.

  • $\begingroup$ There is no one-time pad implementation with an infinitely long key, as generating such a key would require an infinite computation and thus infinite energy. $\endgroup$ Commented Nov 12, 2015 at 21:54
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    $\begingroup$ I think I agree with you (1) is now the best answer either way i will consult with my professor regarding the ambiguity of the question (but these academics can often be staunch in their opinions). I think my fundamental understanding of a one time pad was my undoing in this question. $\endgroup$
    – pi-e
    Commented Nov 12, 2015 at 22:25
  • 1
    $\begingroup$ @SimonJohnson They didn't say it had an infinitely long key, just that it can be thought of as a cipher with an infinitely long key. $\endgroup$ Commented Nov 13, 2015 at 12:40

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