Programming

Hi everybody, I know this is a linux forum but I've got a question about encryption. If you want to encrypt (and decrypt) more than one byte at a time do you have to choose sufficiently large numbers when generating the keys for the math to work out? Every example I've come across on the internet shows the process with encrypting one byte. I've written a large number C++ class and I believe its working. Now, when I try the examples on the internet it works for one byte only. When I try encrypting two bytes at a time it breaks down and the decryption process spits out something other than the original message. Anyhow, you guys have any idea what the problem may be?

P.S. If you want to see my code I'll post it but it is extremely long.

If you understand the algorithm and believe you duplicated it correctly, I would apply the same algorithm using an accepted well known and well-tested bignum library.

The most likely candidate for your problem is your bignum class.

I see what you're saying jim, but I'm pretty sure the problem is not my class. When I work out the examples on the internet with a calculator program the same thing happens. After going through the calculations with the calculator program it spits out the same thing as my code. Something's not stirring the cool-aid here. Maybe you can answer this: Do most programs that encrypt files do so one byte at a time?

It depends on the encryption method. Encryption can be classified as either stream or block based.

Hi fpmurphy. Thanks for the input. You wouldn't by chance know what happens when you calculate your keys from really small prime numbers and then try encrypting data blocks larger than the keys would you? Cause the math is not working out for me. When I calculate it by hand with a calculator program the decrypted data is not the same as what was encrytped.

---------- Post updated at 11:15 AM ---------- Previous update was at 10:17 AM ----------

Nevermind guys, I found the answer to my question. The answer can be summed up in this sentence from a professor at Purdue:

"Since our message integer M must be smaller than the modulus n, obviously our
block size cannot equal the modulus size."

You can break the message in smaller blocks and encrypt those. That's typically what happens with block cyphers.

If you want to use an asymmetric key pair like RSA to encrypt a whole message, then, yes, the message must be shorter than the modulus.