Cryptography?

Not sure if this is meant for 'Science & Math' but, it seemed the best place. :s

How would one crack a vernam cipher, without the (one-time)pad?
Assuming you know the pad is not 100% random, and certain numbers appear more than once in the key itself?

I've tried my (pathetic)best to find this on the net, and you are my only hope. :/

You probably can't crack it, but you haven't really provided enough info. What do you mean by "not 100% random"? Psuedo-random? And how is the pad derived from the key?

If the keylength is short, and enough is known about the encrypted data (either some expected marker or statistical properties), I suppose you could try to bruteforce it.

Yes, pseudo-random. Sorry, not the best english.

I'm not sure about anything with the pad.. I just know that the pad itself is not done proper, and it has the same numbers appear in it more then once.
ex.
723 198 126 123 192 297 126 123 723
^that is not the pad, or part of though, ofcourse.

I just have the encrypted message, and my only hint was "The pad is not truly random".

Just bruteforce can be used with that?

Is that what you ask?

Ah, I see. If this was given to you as a puzzle, the answer will be a simple trick.

To decrypt something which was encrypted with a Vernam cipher, you need to find a list of numbers, which, when xor'ed with the code you have, result in the data you're after. Normally, this list of numbers is completely random (a truly uncrackable 'one-time' pad) or pseudo-random (generated based on a key, which you could try to bruteforce).

However, in this case, since the key is absent but the challenge supposedly still solvable, you're probably looking for a simple list of numbers to xor with, e.g. 123 456 789 123 456 789 (something like that, but probably more complex).

You could of course post the entire challenge, and someone here might figure it out... but it will obviously be more fun to do it yourself.

Well, I don't want to post the challenge.. I want to actually work this myself, but.. I'm not 100% sure on how to go about.

Was looking for different methods of cracking a bad vernam cipher, but I find nothing except how to make one..
I am keep looking, but.. Any help on resources would be appreciated.

I also think, I figure out a word in the message, but not so sure, if I find a word, do I look for the numbers in other parts of the message, and change them to the other letter they match?
ex.
483 = T  112 = H  573 = E
and all 483 are T?

This is fun.. But confusing too, maybe something good come of it.

change them to the other letter they match?
No, that would be a substitution cipher.

You should be looking for larger patterns. A part where the numbers (erratically) increase, or something. Of course it could be anything, so I can't really help you much further.