Unknown encryption

Okay, so I get a dump of a text that has been encrypted with a home-made algorithm. Which I wanna try and break.

But where should I start?

If anyone has some experience with something like this, I would like to hear what your first step would be. And maybe second and third steps also :)

Thanks in advance.

Frequencies, starting with standard letter frequency. Though having absolutely no knowledge about the encryption whatsoever leaves you pretty crippled.

Start by entering simple strings and see what the result is. Then gradually increase the length of the string to try and get a feel for what the algorithm is doing. It's kind of a brute force method but not a blind one. Your mileage may vary. I'm assuming you can do these steps. After you get used to seeing normal strings in their hashed formats, it should begin to come natural for you to reverse the steps starting with a hash.

If this is good advice, then I'm glad I could contribute. Otherwise, see what someone more skilled in this area can suggest.

suid wrote: Start by entering simple strings and see what the result is. Or well, yeah, this; if you have access to deciding and encrypting your own plaintexts. Based on the initial post though, I assume that isn't the case.

If it's a polyalphabetic cipher you may want to try using Friedman's Index of Coincidence method. That'll probably only work if you've got a pretty good sized set of crypt-text.

Thanks for the comments guys.

Yea, I have never tried decrypting from this level and it’s not a polyalphabetic cipher. I think I might have to ask my friend for a bit more information.

Think it might be impossible, since I don’t really know what I’m looking for, what language ect..

Therma wrote: it’s not a polyalphabetic cipher

Out of curiosity, how do you know that? Did your friend give you more information about this cipher that you didn't tell us?