diff --git a/20240111/diffie-hellman.txt b/20240111/diffie-hellman.txt
new file mode 100644
index 0000000000000000000000000000000000000000..1f56ea1b60d54b260783bf6a6003f649be8caf66
--- /dev/null
+++ b/20240111/diffie-hellman.txt
@@ -0,0 +1,14 @@
+Diffie-Hellman-Schlüsselaustausch
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Alice                    Bob
+
+ <------ gemeinsam: e --->
+
+geheim: a               geheim: b
+
+  A = e^a -------------->
+
+ <-------------------- B = e^b
+
+B^a = e^(b·a)          A^b = e^(a·b)
+      ~~~~~~~                ~~~~~~~  <----- gemeinsamer geheimer Schlüssel
diff --git a/20240111/hashes-01.txt b/20240111/hashes-01.txt
new file mode 100644
index 0000000000000000000000000000000000000000..c19ab40a0bf87409f5d1933d4c2b3c18f142dc85
--- /dev/null
+++ b/20240111/hashes-01.txt
@@ -0,0 +1,10 @@
+cassini/home/peter/bo/2023ws/dbs/20240111> sha256sum
+Hallo. Dies ist ein Test.
+565fd769eb3b7df2c4f6e65f4ed8cc977d93d6eebcea67b1de09cbad37911521  -
+cassini/home/peter/bo/2023ws/dbs/20240111> sha256sum
+Hallo. Dies ist ein Test.
+565fd769eb3b7df2c4f6e65f4ed8cc977d93d6eebcea67b1de09cbad37911521  -
+cassini/home/peter/bo/2023ws/dbs/20240111> sha256sum
+Hallo. Dies ist ein test.
+f6d865709ef59cb882ad71695a7af2a248040a8d44856bfc84a0821f71070af7  -
+cassini/home/peter/bo/2023ws/dbs/20240111>
diff --git a/20240111/mac-01.txt b/20240111/mac-01.txt
new file mode 100644
index 0000000000000000000000000000000000000000..e757dd976e771a6d501d1721f6eb39b172725201
--- /dev/null
+++ b/20240111/mac-01.txt
@@ -0,0 +1,9 @@
+Message Authentication Code (MAC)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Nachricht: Dies ist ein Test.
+
+Hash-Wert: 8a8c0ba357659d8ad10cc6c79048a07b440c053c50a1a83466440e5a22a319e0
+
+mit geheimem Schlüssel verschlüsseln --> MAC
+
+Empfänger kennt denselben geheimen Schlüssel --> entschlüsseln, mit Hash vergleichen
diff --git a/20240111/man-in-the-middle.txt b/20240111/man-in-the-middle.txt
new file mode 100644
index 0000000000000000000000000000000000000000..ce21bc6fd913237565e33fe5b34360096e1a73fb
--- /dev/null
+++ b/20240111/man-in-the-middle.txt
@@ -0,0 +1,41 @@
+Diffie-Hellman-Schlüsselaustausch
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Alice                    Bob
+
+ <------ gemeinsam: e --->
+
+geheim: a               geheim: b
+
+  A = e^a -------------->
+
+ <-------------------- B = e^b
+
+B^a = e^(b·a)          A^b = e^(a·b)
+      ~~~~~~~                ~~~~~~~  <----- gemeinsamer geheimer Schlüssel
+
+Man-In-The-Middle-Angriff
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Alice                          Mallory                              Bob
+                                                                                                                       
+ <------ gemeinsam: e--------->      <------------ gemeinsam ------->
+                                                                                                                       
+geheim: a                         m                                geheim: b
+                                                                                                                       
+  A = e^a --------------------> A    m ---------------------------->
+
+ <----------------------- M = e^m    B <------------------------- B = e^b
+                                                                                                                       
+M^a = e^(m·a)                                                     M^b = e^(m·b)
+
+Gegenmaßnahme: Sorgfalt beim Schlüsselaustausch
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Öffentlicher Schlüssel von Alice an Bob:
+
+Alice                    Bob
+
+  A = e^a -------------->
+
+Hash-Wert des Schlüssels ("Fingerprint") über separaten Kanal austauschen
+z.B. Telefonat: Stimme erkennen -- Achtung: KI kann Stimme fälschen
+z.B. Visitenkarte
diff --git a/20240111/random-01.c b/20240111/random-01.c
new file mode 100644
index 0000000000000000000000000000000000000000..a3a48cfc04fa79dd382feee65cb4721b087532d9
--- /dev/null
+++ b/20240111/random-01.c
@@ -0,0 +1,10 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+int main (void)
+{
+  for (int i = 0; i < 10; i++)
+    printf ("%d ", rand ());
+  printf ("\n");
+  return 0;
+}
diff --git a/20240111/random-02.c b/20240111/random-02.c
new file mode 100644
index 0000000000000000000000000000000000000000..dd8a2e8b4c85fa306ba57c82cdc4ca5a626484a3
--- /dev/null
+++ b/20240111/random-02.c
@@ -0,0 +1,10 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+int main (void)
+{
+  for (int i = 0; i < 10; i++)
+    printf ("%d ", rand () % 26);
+  printf ("\n");
+  return 0;
+}
diff --git a/20240111/random-03.c b/20240111/random-03.c
new file mode 100644
index 0000000000000000000000000000000000000000..6f32d19627c76cdeccefd8b3c3eee76402b79ea5
--- /dev/null
+++ b/20240111/random-03.c
@@ -0,0 +1,11 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+int main (void)
+{
+  srand (42);
+  for (int i = 0; i < 10; i++)
+    printf ("%d ", rand () % 26);
+  printf ("\n");
+  return 0;
+}
diff --git a/20240111/random-04.c b/20240111/random-04.c
new file mode 100644
index 0000000000000000000000000000000000000000..f1d322840dcafb4b9a0f41cb19f83b99293c3ee4
--- /dev/null
+++ b/20240111/random-04.c
@@ -0,0 +1,12 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int main (void)
+{
+  srand (getpid ());
+  for (int i = 0; i < 10; i++)
+    printf ("%d ", rand () % 26);
+  printf ("\n");
+  return 0;
+}