This is my second contribution to those who are riddled by how to write RSA keymaker problems.

Rsaegis.zip (file accompanying this tutorial) (5k).

__1. RSA Explanation__

Maybe you have heard that, if you want to break a RSA key, you should factorize the public key. But few know how RSA works, so even if he is told the public key and factorizes it, he will not know how to write a keymaker.

RSA works as follows : take two large primes, p and q, and
compute their product n = pq; n is called the modulus. Choose
a number, e, less than n and relatively prime to (p-1)(q-1), which
means e and (p-1)(q-1) have no common factors except

1. Find another number d such that (ed - 1) is divisible by (p-1)(q-1).
The values e and d are called the public and private exponents,
respectively. The public key is the pair (n, e); the private key
is (n, d). The factors p and q may be kept with the private key,
or destroyed.

Encryption : c = ( m ^ e ) % n

Decryption : m = ( c ^ d ) % n

Example :

Now we choose p=5, q=7, so n=5*7=35. And we choose e=11.

Since (p-1)(q-1)=4*6=24, we get d=11, for (11*11-1)%24=0. Note : d is not necessarily equal to e.

We encrypt a number, m=4, with:

c = ( 4 ^ 11 ) % 35 = 9

Here, m=4, n=35 and e=11 are the public part.

When we want to decrypt, we do :

m = ( 9 ^ 11 ) % 35 = 4

If you know how RSA works, it will not be hard to find n and e by yourself, and factorize n then try to find d, so you will be able to write your own keymaker. I offer SFXFactory v2.1 keymaker to provide you a live example of a RSA keymaker. Personally, I don't think SFXFactory is a good software for it is too big and slow to be a SFX creator. Maybe this will excite you : SFXFactory is from the same author of WinACE, an excellent archiver, which also uses RSA. If you want to write its keymaker, you can trace it step by step to find yourself what are n and e.

MIRACL comes with a good factorizer named FACTOR.EXE, but it only accepts decimal numbers, so you should first try to convert the hexadecimal n to decimal, and factorize it. Another problem is how to find d, since d is not unique and there is no direct method to compute it. MIRACL also offers some functions to get d. I prefer SSLeay, another cryptography library in both C and C++. It is well organized. But it doesn't bring you a factorizer.

In this keymaker, I don't include my BigNum library source code for it is actually lame and slow and unoptimized. Try to use MIRACL or SSLeay. If you want to make RSA keymakers, I have to warn you, that the main difficulty of a RSA keymaker is not the factorization. You will be probably in a trouble to recognize if a program uses RSA protection, and to find what is n and e.

__2. What is Radix-23?__

Another problem is, SFXFactory first decodes your registration code. So what is the decoding algorithm?. I call it Radix-23. What is it? Well, we usually call Radix-10 decimal, call Radix-16 hexadecimal. So now you maybe understand what is Radix-23. Hexadecimal has 16 numbers : 0~9 and A~F. Similarly, Radix-23 has 23 numbers. We can freely choose what they are. The author of SFXFactory chooses the following characters :

34679ACDEFHKLMNPQRSTWXY.

How we convert a decimal number to a hexadecimal number? We do as follows :

12345 / 16 = 771 ... 9 771 / 16 = 48 ... 3 48 / 16 = 3 ... 0 3 / 16 = 0 ... 3

So, 12345 = 0x3039. We convert decimal to Radix-23 the same way :

12345 / 23 = 536 ... 17 (R) 536 / 23 = 23 ... 7 (D) 23 / 23 = 1 ... 0 (3) 1 / 23 = 0 ... 1 (4)

So, 12345 = 43DR(23).

Best regards, Egis.

© 1998-2003 Egis, hosted by CrackZ. 11th February 2003.