updated rsa

Cryptography/RSA.md

@@ -24,6 +24,20 @@ for i in range (phi + 1, phi + foo):
 * \\( Cleartext = cipher ** e mod $\phi$ ) 
 
 ## Euklid
+
+Just a short excourse:  
+A greatest common divisior out of an example a = 32 and b = 14 would be the groups of the following divisors
+```sh
+a = 32, b = 24
+a = {1, 2, 4, 8, 16}
+b = {1, 2, 3, 8, 12}
+gcd(a,b) = 8
+```
+
+### Greatest Common Divisor (GCD)
+
+Two values are prime and have themselves and only `1` as a divisor are called coprime.
+To check if a and b have a greatest common divisor do the euclidean algorithm.
 ```python
 def gcd(a, b):
     if b == 0:
@@ -31,6 +45,12 @@ def gcd(a, b):
     return gcd(b, a % b)
 ```
 
+### Extended GCD
+
+#TODO
+
+
 ## Links
 
 * [Encryption+Decryption](https://www.cs.drexel.edu/~jpopyack/Courses/CSP/Fa17/notes/10.1_Cryptography/RSA_Express_EncryptDecrypt_v2.html)
+* [Extended GCD](http://www-math.ucdenver.edu/~wcherowi/courses/m5410/exeucalg.html)