Links
Articles on MysteryTwister
wikipedia.org: Wikipedia on MTC3
Cryptography Websites and Challenges (English)
cryptool-online.org: Website on cryptography (CrypTool)
bund.de: Information on IT security (Federal Office for Information Security)
certicom.com: ECC Challenge (Certicom)
cia.gov: Website on cryptography for children (CIA)
cipherchallenge.org: Website of the National Cipher Challenge (University of Southampton)
cryptoclub.org: Website on cryptography for children (Crypto Club, University of Chicago)
cryptohack.org: Platform for learning modern cryptography (Cryptohack)
cryptopals.com: Matasano Crypto Challenges (Cryptopals)
fbi.gov: Cryptography challenge (FBI)
latticechallenge.org: Lattice challenge for mathematicians (University of Darmstadt)
nsa.gov: Website on cryptography for students (NSA)
simonsingh.net: Website accompanying the book "The Code Book" (Simon Singh)
telenet.be: Enigma Cipher Challenge (Dirk Rijmenants)
Cryptography Websites and Challenges (German)
cryptool-online.org: Website on cryptography (CrypTool)
blinde-kuh.de: Secret languages in a search engine for kids (Blinde Kuh)
bund.de: Informationen über IT-Sicherheit (Bundesamt für Sicherheit in der Informationstechnik)
krypto-im-advent.de: An online Advent calender for children to discover cryptography (KiA)
sat1.de: Tips on internet security (Sat1)
scienceblogs.de: Blog about cryptography (Klaus Schmeh)
sueddeutsche.de: Codebreaker Quiz (Sueddeutsche Zeitung)
Github/Gitlab Repositories
github.com/pFarb: A list of cryptographic papers, articles, tutorials, and how-tos
github.com/sobolevn: A list of cryptographic resources and links
gitlab.com/obxium: A list of cryptographic resources related to arts