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# Need help with RSA Encryption

Dan Bolens
Greenhorn
Posts: 25
Hey guys,

So my latest project involves RSA encryption and decryption. I build the bare bones of the program, but now need to change some variable elements to complete the assignment, and I'm having some trouble.
My code is currently this:

Right now, all variables are specifically declared, but I need them to be generated.

1. P and Q need to be randomly generated primes (between 3 and 128)
2. Use Z ((P-1) * (Q-1)) to randomly generate an e which:
A. 1 < E < Z
B. E and phiPQ are relatively prime (relatively prime means that gcd(E, phiPQ) == 1)
C. There may be several candidates for E, so pick the smallest one
and finally,
3. Generate D ((DE-1) is evenly divisible by phiPQ)

I don't necessarily want the code here, but I'm just not sure how I can take what I have above and now modify it to accept what needs to be generated.

Campbell Ritchie
Sheriff
Posts: 50738
83
You can’t do all that lot at once, so don’t even try. One bit at a time. Do you know how to generate prime numbers? If not google for Sieve of Eratosthenes. Use that to remove non‑primes from an array with 128 elements.
Then you can create a new prime number array and use a Random object to select from that array. If you use that in high‑security settings, there is a SecureRandom class. You might be able to use methods of the BigInteger class to find prime numbers.

Shankar murthy
Greenhorn
Posts: 1

This program encrypts the text in plaintext.txt file and saves the cipher text in encrypted.txt file. Similarly decryption is done using encrypted.txt file and plain text is saved in decrypted.txt file

Campbell Ritchie
Sheriff
Posts: 50738
83
Welcome to the Ranch

I have changed the red text to black because many people find coloured text hard to read.