Is My Homemade Encryption Method Secure Enough for Real-World Use?

[LizardKing23]

I made up this encryption method on an airplane, and so far no one in my dorm has gotten it. Since I live in a college dorm, it may or may not be a good testing area, so I figured I'd try it out here.

PRMDC JKIHF
LQHEH UHMRZ
JKIQG WIYHA

 

[shmoe]

Having no idea what kind of method you used to encrypt that, it's impossible to crack. We could make a guess at the method, like a substitution cipher, and possibly decode it to be something meaningful but we'd have no way of knowing if you didn't just use a one-time pad and our "meaningful" message wasn't just a random chance.

 

[LizardKing23]

thats true, i know i didnt supply nearly enough information
i just wanted to see if anyone here is hardcore into cryptography and would try to crack it
would it help if i said that it's a plato quote?

 

[shmoe]

Anyone into cryptography should know it was futile and wouldn't bother.

Even knowing it's a plato quote, we still don't have enough. As it is, I could come up with an encryption method that would decode what you've got into any plato quote you like. eg. Here's two encoded messages using a scheme I've just come up with that are both plato quotes:

ASED

and

GHUT

I'll even tell you one means "Courage is a kind of salvation" and the other "Courage is knowing what not to fear" but you have absolutely no way of knowing which is which.

 

[tomfitzyuk]

When does a code become decipherable?

Surely, given a finate number of data points (i.e. cipertext) there is an infinite number of solutions to it? What information needs to be known for it to be breakable?

 

[shmoe]

Knowing what was used to encode it can go a long way, and having some idea of what it says won't hurt. Newspapers often have "Cryptograms" that are made from a simple substitution cipher. It will have many characters so it's unlikely that the wrong substitutions will make a coherent message and they'll sometimes make the encoded message the answer to a riddle so the message is meaningful. These are meant to be broken of course, but are examples of how not to use a substitution cipher if you want security.

Just knowing the method can do the trick as well. Any RSA encryption can be broken given enough time, though done properly it's an extremely long time (ideally long after the data is no longer sensitive).

Using the same code over and over can do you in. Say a baseball coach has 5 different signals to mean "steal second". Over the course of one game, he can use these without trouble. If he used the same set of signals to mean "steal second" for an entire season, people will catch on (if they're paying attention).

It really depends on the method used, and also how it's used as to just how secure your encryption is, as well as the resources and ability of your attackers.

 

[jim mcnamara]

Normally, good 2-way crypto algorithms are given lots of public exposure, testing, verification. What you did depends on method only, very likely.

Have a quick read about why crypto is harder than it seems -
http://www.schneier.com/essay-037.html