# Thread: Any Cryptology/Cryptography experts here?

1. ## Any Cryptology/Cryptography experts here?

I just completed the following worksheet for class, but can someone check my work?

Suppose you want to receive private messages. To set up a public key system, you pick 2 primes p and q and set n=pq. Then you choose a positive integer e relatively prime with phi (n)=(p-1)(q-1). Suppose you choose p=101 and q=29, so n=2929 and phi(n)=2800.

Next you choose e=2291 which is relatively prime to phi(n)=2800. The pair (e,n)=(2291, 2929) is your enciphering key.

You now make public (only) your enciphering key. Of course, p and q need to be much larger in real life, so that n will be hard to factor.

Now suppose I have your enciphering key and decide to send you an encrypted message.

Given the value of n, the block size should be m=2 (2 letters per numerical block).

I use the key to encipher a message as follows.

First I translate to a numerical message.

i f t h i s i s m y m e s s a g e 
0805 1907 0818 0818 1224 1204 1818 0006 1426

Here we have used the translation table:

A b c d e f g and so on. to z and then 
00 01 02 03 04 05 06 25 26

We added the symbol - since all messages need to have length divisible by m (2 in this case) so we can add a - to make an odd length even.

To encode the first block of message above, I raise 805 to the power e=2291 and take the result mod n=2929 obtaining 1372.

The first couple of blocks of the encrypted example message are 1372 937 2313

Ok, now I am sending you the following message using your enciphering key:

230 1989 362 653 516 1069 2373 571 724 1420

I figured out the above message, but now I am supposed to send back a message of my own using enciphering key: (e,n)=(7, 3131) using block length m=2.

Heres what I came up with. Can anyone decode it to let me know I did it correctly:

2891 1202 2045 2069 1188 0072 2337 2863 0919 2891 2691 0102 0880 1581 2691 1931 0571

2. is this RSA chryptography?

3. Originally posted by Johnny Knoxville
is this RSA chryptography?
Yes

4. I did this just last week, but differently, we found the inverse of E using a coprime formula and then used 2 other formulas to encode and decode the message, so i'm not sure

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