Links

From the German BSI: Videos like the animation movie about bot networks, edited by the Dutch Government Computer Emergency Response Team (65 MB WMV file): https://www.bsi-fuer-buerger.de/BSIFB/DE/Service/Mediathek/Videos/video.html

Kryptografie.de describes a lot of classical and modern ciphers and geocaching functions in easily understandable words and often offers the possibility to encrypt or decrypt a text online. Many terms from cryptanalysis are also explained:
https://Kryptografie.de

Webpage to the book "The Code Book" by Simon Singh from the year 2000 and the corresponding cipher challenge:
https://simonsingh.net/cryptography/cipher-challenge

Website of the Cryptoclub of the University of Illinois at Chicago (UIC):
(very colourful, for kids; sadly, Flash required)
https://www.cryptoclub.org

The Spy Camp ("Spioncamp") of the University of Wuppertal offers proved and tested learning stations about cryptography and cryptology for teachers and pupils -- (German only):
https://ddi.uni-wuppertal.de/website/index-ddi.html

Secret Codes and the Enigma Machine (for pupils; site a little bit outdated):
https://motivate.maths.org/conferences/conf95/c95_resources.shtml

Website about the CODEBREAKERS.EU cryptology game (currently 5th edition):
(e.g. as a game for teams with three members, offering 3 levels a 6 tasks (lectures), supported by hints and "cryptologic bombs" to deal with the Enigma)
https://thecodebreakers.org

Website on the National Cipher Challenge NCC (University of Southampton, IBM, GCHQ):
https://www.cipherchallenge.org

Great simulations of machines which ran at Bletchley Park and in Germany during WW2:

Famous websites dealing with encrypting and breaking classical ciphers:

Further links at the website of the CrypTool portal (CTP):
https://www.cryptool.org/en/links

Further links at the website of the MysteryTwister cipher challenge (MTC3):
https://mysterytwister.org/home/links