# A "one time pad" can be thought of a Vigenere cipher with

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)!

• What does "a secure symmetric key" mean? If (1) were worded "a random key as long as the plaintext", I would have chosen (1). Nov 11, 2015 at 18:58
• 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". Nov 11, 2015 at 19:20
• I think that (1) is clearly the the intended answer, but it's a badly formulated question. Nov 11, 2015 at 19:33
• @CodesInChaos, that's my guess too. I think that due to the wording you could argue (2). Nov 11, 2015 at 19:35
• @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. Nov 11, 2015 at 19:37