General Question

[Tubbieh]

I have a encrypted version of a file and the unencrypted version of the same file, is there any way to figure out the encryption method, and come up with decryption program or what ever?

[Goatboy]

Look up Known Plaintext Cryptanalysis.

[msbachman]

Curious, how do you know you have identical files without knowing the encryption method? You have to decrypt it to know, correct?

[Goatboy]

Depending on the method used to encrypt, you may be able to get by with only part of the file. For example, a ZIP file may contain many encrypted documents, and you might have just one of them. Or let's say you have the decrypted version of a document, and you know that many others are encrypted the same way. You could use this knowledge to work out the key.

[Tubbieh]

After a bit of researching I found out that i can use [adaptive] chosen plaintext. How can I go about discovering the cipher with this method?

edit:

Just to clarify I have a bunch of files[1.txt,2.txt,3.txt,4.txt,...100.txt] in encrypted and unencrypted form. I can put in any numbers or text I want, im trying to figure out the cipher. [That didn't clarify at all]...

-- Thu Dec 09, 2010 10:57 pm --

Curious, how do you know you have identical files without knowing the encryption method? You have to decrypt it to know, correct?

I have a program that encrypts my own files [so I have the original and the decrypted], and I want the cipher so I can decrypt other peoples files too .

[msbachman]

I have a program that encrypts my own files [so I have the original and the decrypted], and I want the cipher so I can decrypt other peoples files too

If you post the program, I am sure that people would find it a fun challenge.

[Tubbieh]

Byond, Its a game engine. A simplified version of C++. The only thing is getting the encrypted and decrypted files is extremely hard to explain. I can post the Files if you guys want or whatever.