December Cryptography Challenges
31 Days • 31 Ciphers
Cryptography Challenges for Cybersecurity and CTF Practice
31 days of cryptography challenges to master cipher-breaking this December
Daily Cryptography Challenges
New cryptography challenges unlock each day throughout December. Decrypt the ciphers and submit your flags!
The Cipher
In 58 BC, Julius Caesar encrypted his military communications using a substitution cipher. Your mission: intercept and decrypt an encoded message from his campaign in Gaul. The cipher shifts each letter by a fixed number of positions in the alphabet. Intelligence suggests the shift value is 3.
Mirror Mirror
A spy has sent you an urgent message, but it appears scrambled. Upon closer inspection, you realize the text has been reversed—a technique used by ancient Greek cryptographers. Your task is to restore the message to its original form and reveal the hidden intelligence.
The Enigma
You've intercepted a data transmission from an unknown source. The message appears to be encoded in Base64—a binary-to-text encoding scheme commonly used to safely transmit data across networks. Decode this transmission to uncover the flag hidden within the seemingly random string of characters.
Secrets
A double agent has delivered an encrypted hex string. Intelligence reports suggest it was encrypted using the XOR (exclusive OR) cipher with a single repeating byte as the key. XOR encryption is symmetric—the same key both encrypts and decrypts. Your mission: brute force the key space (0-255) to reveal the secret.
The Vault
You've breached a secure vault and discovered a message protected by the Vigenère cipher—a polyalphabetic substitution cipher once considered 'le chiffre indéchiffrable' (the indecipherable cipher). Unlike Caesar's simple shift, Vigenère uses a keyword to shift each letter by different amounts. The keyword has been compromised: 'CIPHER'. Decrypt the vault's contents before time runs out.
Random
I have the link and you have the skills. Break into my encrypted message if you can.
Jupiter
The largest planet in our solar system holds many secrets. Decode the encrypted message to uncover the flag hidden within the swirling storms of Jupiter.
Ultra Tiger
Maybe tiger wants to connect to the remote server using vnc or ssh? Find the encrypted message and decrypt it to get the flag.
Morse
A secret agent has sent you a message in Morse code. Your task is to decode the message and reveal the hidden flag. Remember, in Morse code, dots (.) represent short signals and dashes (-) represent long signals. Spaces separate letters, and slashes (/) separate words.
deadshark
Fish is friend. Find the encrypted message and decrypt it to get the flag.
SMS PDU
A mysterious SMS message has been intercepted in PDU format. Your task is to decode the PDU string to reveal the hidden flag. SMS PDU (Protocol Data Unit) is a format used to encode SMS messages for transmission over mobile networks. Decode the PDU to uncover the secret message.
Broken Block Cipher
An AES-CBC encrypted message was leaked along with its IV and key. Decrypt the ciphertext to retrieve the flag.
Token of Trust
You intercepted a JSON Web Token (JWT). The signature uses HMAC-SHA256 with a weak secret. Crack the secret to reveal the embedded flag.
Too much layer
Peel the onion! My friend don't cry. The message is easy to find if you know how to look for it.
Windows Relic
An NTLM hash was extracted from a compromised Windows system. Crack it to obtain the flag.
Modular Lock
A digital lock opens only if you find the smallest positive number that satisfies all given modular conditions.
Shifting Pattern
A sequence appears to follow a simple pattern, but only one operation truly changes between terms. Find the missing value.
Affine Mistake
A message was encrypted using an Affine Cipher. Recover the original plaintext.
Packed Secrets
The message was compressed and encoded multiple times to evade detection. Carefully unpack each layer to recover the original flag.
Bacon's Secret
Francis Bacon invented a steganographic cipher in 1605 that hides messages using only two symbols. Decode the following binary-like sequence to reveal the hidden flag.
Tap Code
The Tap Code is a simple way to encode messages using a series of taps. Each letter is represented by two numbers corresponding to its row and column in a 5x5 grid. Decode the following sequence of taps to reveal the hidden flag.
Homophonic Havoc
A homophonic substitution cipher where common letters have multiple possible ciphertext values to flatten frequency distribution.
Keyboard Shift
The author typed the message with their hands shifted one key to the right on a QWERTY keyboard.
Leetspeak Relic
A hacker from the early 2000s left behind a message written in classic leetspeak.
Emoji Cipher
A playful developer encoded letters using emojis instead of characters.
Mixed Radix
The data uses different number bases per chunk.
SMS
Send me SMS.
Cron Mystery
A cron schedule hides a number-based flag.
Alphabet Wheel
Letters were arranged in a circular alphabet, but indexing starts at the wrong place.
Alphabet Modulo
Alphabet indices were reduced modulo a fixed number.
Mirror Text or Word
The text was written using mirrored Unicode characters and reversed. Restore it by reversing the string and mapping characters to their visual Latin equivalents.