# Decrypt code without the key

• Sep 20th 2011, 11:36 PM
mark090480
Decrypt code without the key
Hi,

I have been given a code without the key to crack. I have been told that it is based on primes numbers and the code is made up of all letters. Any ideas where I should start? Is there some known maths formulas to try?

Thanks.
• Sep 20th 2011, 11:56 PM
chisigma
Re: Decrypt code without the key
Quote:

Originally Posted by mark090480
Hi,

I have been given a code without the key to crack. I have been told that it is based on primes numbers and the code is made up of all letters. Any ideas where I should start? Is there some known maths formulas to try?

Thanks.

Supposing thet You know the 'coding algorithm' but You don't know the key, then the code is 'perfectly sure' only if the following conditions are satisfied...

a) the key is 'perfectly random'...

b) the key has been used only for one message...

If a) or b) aren't satisfied, then Youn have 'some chance' to decrypt the message...

Kind regards

$\chi$ $\sigma$
• Sep 21st 2011, 12:07 AM
CaptainBlack
Re: Decrypt code without the key
Quote:

Originally Posted by chisigma
Supposing theta You know the 'coding algorithm' but You don't know the key, then the code is 'perfectly sure' only if the following conditions are satisfied...

a) the key is 'perfectly random'...

b) the key has been used only for one message...

If a) or b) aren't satisfied, then Youn have 'some chance' to decrypt the message...

Kind regards

$\chi$ $\sigma$

No, there are still possible attacks that are possible. For instance a short random key and a partially known text can be dealt with fairly easily (assuming known encryption algorithm) by exhausitve search.

The only provably secure cipher is a one time pad

CB
• Sep 21st 2011, 12:25 AM
mark090480
Re: Decrypt code without the key
Quote:

a) the key is 'perfectly random'...

b) the key has been used only for one message...
Yes as far as I know both a and b are true. I think it must be breakable or how could it be a question?

So the only way is to go through all algorithms? I understand that there should be some logic involved.
• Sep 21st 2011, 12:29 AM
CaptainBlack
Re: Decrypt code without the key
Quote:

Originally Posted by mark090480
Yes as far as I know both a and b are true. I think it must be breakable or how could it be a question?

So the only way is to go through all algorithms? I understand that there should be some logic involved.

It might help if you told us the provenance of this question, where does it come from what else do you know about the message etc...

Exact wording of the question and the cipher text would also help.

CB
• Sep 21st 2011, 12:37 AM
mark090480
Re: Decrypt code without the key
It is a question to learn about cryptography and computer languages. All I know is that it's based on primes, no more. The computer language does not matter, the algo is what counts.
• Sep 21st 2011, 12:57 AM
chisigma
Re: Decrypt code without the key
Quote:

Originally Posted by mark090480
Yes as far as I know both a and b are true. I think it must be breakable or how could it be a question?

So the only way is to go through all algorithms? I understand that there should be some logic involved.

In order to avoid misunderstandings as 'perfectly random key' one means that the key must have the same lenght of the message [if not the key is repetitive and not 'perfectly random'...] and it must have some basic statistical properties [about that may be a detailed discussion has to be made...]. If part of the message for some motivations is known, then if the conditions a) and b) are satisfied the remaining part of the message 'with high probably' remains unknown [also about this point a detailed discussion may be useful...]...

Kind regards

$\chi$ $\sigma$
• Sep 21st 2011, 01:04 AM
chisigma
Re: Decrypt code without the key
Quote:

Originally Posted by mark090480
...all I know is that it's based on primes...

Can You do more informations about that?... in the cipher algoritm is the so called RSA system, based on public and private keys, then the conditions a) and b) are far from be satisfied...

Kind regards

$\chi$ $\sigma$
• Sep 21st 2011, 01:06 AM
mark090480
Re: Decrypt code without the key
Quote:

one means that the key must have the same lenght of the message
I have no idea of the key's length. In the past we done some tasks regarding RSA etc.. but that was pretty easy as we knew the key. I am guessing this is not related too much to RSA. The tutor told us that this task is more about logical thinking and being creative. I think the answer when known will be easy :). The message given is all letters and random characters % & etc.. I am guessing that it's just using primes to somehow skip over the letters and land on the right letter. I have tried every combination of that and it doesn't work. Anyhow I will get creative and hopefully i think up something, like I said, I understand that this task should be easy :)
• Sep 21st 2011, 08:49 PM
CaptainBlack
Re: Decrypt code without the key
Quote:

Originally Posted by mark090480
Hi,

I have been given a code without the key to crack. I have been told that it is based on primes numbers and the code is made up of all letters. Any ideas where I should start? Is there some known maths formulas to try?

Thanks.

In the absence of any other information you start by looking at the cipher text, looking for peculiarities ..

CB
• Sep 6th 2012, 03:32 PM
humble
Re: Decrypt code without the key
"i have a cipher text to decryp. I have tried it a lot but i couldnot, i want ur help.More over i dnt knw the key and encryption technique used for it. if u plz help me. i would b vry thankful to u.