This challenge is a puzzle about a mysterious ciphertext, which is also protected by a clever disguise. Can you crack it?

The Syllabary cipher seems to be just another substitution cipher, but is it? Find out and solve its mystery!

The legendary Merkle-Hellman Knapsack cryptosystem is not suitable for hiking, but it is considered a pioneer of asymmetric cryptography. Can you crack the ciphertext?

This challenge is about one of the oldest polyalphabetic ciphers. Can you crack this centuries-old puzzle?

The Wheatstone Cryptograph is a simple device that resembles a clock with two hands. For each hand there is a ring of symbols. Are you able to decrypt the given ciphertext?

This series deals with a grid — a basic tool cryptographers use to separate sequences of data into columns and rows. In this challenge you are given three grids where the first grid gives you hints to reveal the plaintext of the second grid. Are you able to find the right route?

This series deals with a grid — a basic tool cryptographers use to separate sequences of data into columns and rows. In this challenge you are given four grids where the first grid gives you hints to reveal the plaintext of the second grid. Are you able to find the right route?

This series deals with a grid — a basic tool cryptographers use to separate sequences of data into columns and rows. In this challenge you are given two grids where the first grid gives you hints to reveal the plaintext of the second grid. Are you able to find the right route?

Lattice-based cryptography schemes are relevant for current post-quantum cryptography research. This challenge series accompanies the basic theory from a chapter of the CrypTool Book called "LIGHTWEIGHT INTRODUCTION TO LATTICES". This part of the challenge series uses vectors to hide a famous quote in modern art. Can you reveal it?

Lattice-based cryptography schemes are relevant for current post-quantum cryptography research. This challenge series accompanies the basic theory from a chapter of the CrypTool Book called "LIGHTWEIGHT INTRODUCTION TO LATTICES". In this part of the challenge series we introduce systems of linear equations to find a hidden message in a picture.

This part of the challenge series is a warm-up with homophonic substitution. In a homophonic substitution cipher, there are more than one ciphertext symbol for each plaintext symbol. The key is a mapping.

Solving this challenge is not as hard as finding the needle in a haystack, isn't it?

Three messages were delivered — two encrypted, one in clear. Can you solve the puzzle?

The autokey cipher uses only a short keyword and the plaintext to encrypt messages. Are you able to break the given ciphertext?

What do you know about modular sequences? Work it out.

How many experience points does Thomas need to reach the 200th level of his new computer game?

Little Anna has forgotten how to decrypt her ciphertext. Can you come up to her pre-Christmas wish?

Are you able to crack the password of Radomil and find the missing 5 digits?

This series consists of five parts which are based on each other and that will be getting more and more complicated with each part. This part is the actual challenge and tells you the New Year's greeting of the author.

This series consists of five parts which are based on each other and that will be getting more and more complicated with each part. Part 4 contains a little New Year's greeting of the author and part 5 teaches us something about the weaknesses of this method. Part 3 adds another modification.

This series consists of five parts which are based on each other and that will be getting more and more complicated with each part. Part 4 contains a little New Year's greeting of the author and part 5 teaches us something about the weaknesses of this method. Part 2 modifies the encryption of part 1 a little bit.

This series consists of five parts which are based on each other and that will be getting more and more complicated with each part. Part 4 contains a little New Year's greeting of the author and part 5 teaches us something about the weaknesses of this method. Part 1 is the easiest part of this series.

Both images in this challenge seem to contain no information at all. Or do they trick us? Find the secret message.

The story of the simple bookseller Paul from Eisleben and his beloved from the near Castle Mansfeld will be continued: A second postcard has been found. Can you decipher the encrypted message?

The challenges for the simplified GRANIT cipher, a method that can be done manually, serve as an exercise and use an intentionally simplified version of GRANIT. Part 1 is a ciphertext-only challenge for which both permutation keys are known.

How are the numbers interconnected? Can you find the three missing numbers?

A friend of yours has given you the following text which contains only X and Y characters. He claims that this text contains a codeword. Can you find it?

How are the numbers linked? Can you find the two missing numbers?

How are the numbers linked? Can you find the two missing numbers?

This is the beginner's challenge in a series about M-138, a manual cipher of the US army which has been relatively save at the beginning of world war 2nd. Part 1 of the series is a ciphertext-only challenge whose key is largely known.

You can find a short encrypted note at the end of the novel "Die achte Offenbarung" by Karl Olsberg. Decrypt the secret message!

This challenge is based on the Heartbleed bug in OpenSSL discovered in April 2014. Attack a server which is specifically prepared to be vulnerable to the Heartbleed bug. Please note that it is necessary to solve Part 1 first. !!! We had to take off this challenge, as our firewall doesn't allow flawed servers any more -- even if this flaw was offered by will for training and within a sandbox. !!!

This challenge is based on the Heartbleed Bug in OpenSSL discovered in April 2014. Attack a server provided by the group for Privacy and Compliance with the Research Institute Cyber Defence (CODE) at Bundeswehr University Munich, which is specifically prepared to be vulnerable to the Heartbleed bug. !!! We had to take off this challenge, as our firewall doesn't allow flawed servers any more -- even if this flaw was offered by will for training and within a sandbox. !!!

In this part of this series of Enigma challenges, a plaintext message consisting of three letters which are repeated several times has been encrypted by the Enigma I. What are the first three letters of the plaintext?

In this part of this series of Enigma challenges, a plaintext message consisting of three letters which are repeated several times has been encrypted by the Enigma I. What was the setting of the Enigma rotors?

In this part of this series of Enigma challenges, a plaintext message consisting of two letters which are repeated several times has been encrypted by the Enigma I. What was the setting of the Enigma rotors?

In this part of this series of Enigma challenges, a plaintext message consisting of one letter which is repeated several times has been encrypted by the Enigma I. What was the setting of the Enigma rotors?

Two scatterbrained professors exchange messages that have been encrypted with the Trifid cipher. Unfortunately, one of them lost the second of three layers. Will he nevertheless be able to decrypt the message he received from his colleague?

In this part of this series of Enigma challenges, a plaintext message consisting of one letter which is repeated several times has been encrypted by the Enigma I. What is the letter we are looking for?

You find a note with strange characters and an unsolved Sudoku on your brother's desk. You are curious and would like to find out what your brother is working on. Can you decrypt the note using the Sudoku?