For challenges involving cryptography: encryption or decryption.
23
votes
7answers
859 views
Decipher Neurotic Frogs
Decipher Neurotic Frogs
Now that Puzzling.SE has finally cracked my amphibian-obsessed cipher, let's write a program or function to decrypt it!
(If you want to look at the puzzle before having it ...
23
votes
20answers
784 views
PKCS#7 padding validation
In cryptography, PKCS#7 padding is a padding scheme which adds a number of bytes N ≥ 1, where the value of each added byte is equal to N.
For example, Hello, World!, which has 13 bytes, is the ...
4
votes
1answer
129 views
Write a VIC cipher decoder
You've received a coded message from your ally that you know is encoded with the VIC cipher. As you know that the VIC cipher is notoriously convoluted, you want to write a program that will decode the ...
11
votes
1answer
296 views
Write a VIC cipher encoder
The VIC cipher is one of the most complicated pencil and paper ciphers ever devised. Used in the 1950s by Soviet spy Reino Häyhänen, codenamed "VICTOR", its main principle is security through ...
45
votes
76answers
7k views
Print a Tabula Recta!
Print a Tabula Recta!
The Tabula Recta (sometimes called a 'Vigenere Table'), was created by Johannes Trithemius, and has been used in several ciphers, including all variants of Bellaso's Vigenere ...
6
votes
1answer
236 views
Help the poor Cryptographers - DLP Edition
Introduction and Motivation
I'm mainly active on Cryptography.SE and as such have already stumbled across the question: "How the hell am I supposed tools like cado-nfs to do stuff!?". They are super-...
15
votes
12answers
1k views
Shamir's Secret Sharing
Given n (the number of players), t (the threshold value), and s (the secret), output the n secrets generated by Shamir's Secret Sharing algorithm.
The Algorithm
For the purposes of this challenge, ...
6
votes
4answers
300 views
Time for some TEA!
Introduction
A while back I stumbled across the tiny encryption algorithm (TEA) and since then I have recommended it whenever special cryptographic security properties were un-needed and a self-...
-7
votes
13answers
493 views
Substitution cipher
A substitution cipher is an encoding method where each letter in the alphabet is replaced with a fixed, different one; for example, given the following substitution map:
abcdefghijklmnopqrstuvwxyz
...
2
votes
0answers
104 views
Crack the Cipher [duplicate]
Write a program or function that will break ciphers, using as few bytes as possible.
Task
Given an encrypted message and a dictionary as input, output the decrypted message. The messages will be ...
6
votes
2answers
255 views
ADFGX Cipher Encryption
ADFGX cipher encryption
The ADFGX cipher was used by the German army in WWI. Write a program to encipher with it!
It works as follows:
Create a 5x5 Polybius square with a mixed alphabet. The ...
27
votes
24answers
2k views
Atbash Self Palindromes
Consider the Atbash transformation:
A|B|C|D|E|F|G|H|I|J|K|L|M
Z|Y|X|W|V|U|T|S|R|Q|P|O|N
Where A ⇔ Z and L ⇔ O, e.g. There is an interesting property that some words share. When some strings ...
10
votes
3answers
309 views
Bacon's cipher: An Intro to Steganography
This little piggy went to market, this little piggy wrote some code...
Ah wait, we're not talking about that bacon, we're talking about Sir Francis Bacon! Specifically, the cipher Bacon devised in ...
16
votes
1answer
545 views
Convert ciphered Roman numerals to Arabic decimals
Write an algorithm to interpret a sequence of letters as a Roman numeral. (see roman numeral rules below)
Each distinct letter has a matching Arabic decimal value, no maximum. But you don't have ...
7
votes
1answer
214 views
An Affinity for Ciphers
Description
You, an armchair cryptanalyst, have noticed that the first letters of each word in your neighbor's mail (yes, you read it -- how else would you know if he's plotting something?) look very ...
33
votes
9answers
2k views
The Drunken Bishop
In public-key cryptography, a public key fingerprint is a short sequence of bytes used to identify a longer public key.
In SSH in particular they can be used to verify that a server is in fact the ...
7
votes
2answers
350 views
Crack a Vigenère Cipher
A Vigenère Cipher is encrypted by repeating a keyword to be the length of the plaintext, and summing each character in that text with the corresponding letter in the plaintext modulo 26. (using A=0,B=...
0
votes
0answers
72 views
Golf: Tabula Recta [duplicate]
From Chapter 18 of Significant Digits, a fan-fiction of the Harry Potter and the Methods of Rationality fan-fiction, we have the following cryptographic problem:
As it happened, the books were ...
12
votes
2answers
579 views
Cryptic Kicker //
Cryptic Kicker
A common but insecure method of encrypting text is to permute the letters of the alphabet. In other words, each letter of the alphabet is consistently replaced in the text by some ...
17
votes
3answers
461 views
A fiercer four-square cipher
Background
One-hundred and thirteen years ago, amateur cryptographer Félix Delastelle published his work on the four-square cipher, a digraphic substitution cipher that accepts two permutations of a ...
10
votes
5answers
344 views
CipherSaber encryption
Implement a CipherSaber encryption program, as described below. Guidelines:
The smallest entry, in bytes, wins.
However, in a departure from code-golf norms, you are welcome to post interesting ...
12
votes
20answers
3k views
Cryptographic hash golf (robbers)
This contest is over.
There are no remaining crackable answer in the cops challenge.
Companion thread of Cryptographic hash golf
As a reminder, here are the rules for robbers from the main ...
33
votes
23answers
4k views
Cryptographic hash golf
This contest is over.
Due to the nature of cops-and-robbers challenges, the cops challenge becomes a lot easier when the interest in the associated robbers challenge has diminished. Therefore, ...
15
votes
4answers
812 views
Shifty XORyption
Write a program or function (or set of programs/functions) to encrypt and decrypt data given the following specification:
Encryption
Calculate an XOR hash of the input by XOR-ing every byte with ...
7
votes
1answer
416 views
Decrypting a block of AES
Today your goal is to decrypt a secret message using AES. Given a ciphertext and key you will decrypt and print the message.
Your program may be in any language. It will be run with input on stdin, ...
1
vote
0answers
60 views
Solve a Date Shift Cipher [duplicate]
Introduction
In this challenge you will be solving date shift ciphers. Here is a general overview of what a date shift cipher is. To encode a message, you first must make a key. A key in a date shift ...
11
votes
6answers
890 views
Solve a Diagonal Burrows-Wheeler transform
Introduction
In this challenge you will be solving diagonal Burrows-Wheeler transforms. Here is a general overview of what a diagonal Burrows-Wheeler transform is. To encode a message, you first must ...
6
votes
2answers
266 views
Key length of Vigenère cipher
The Vigenère cipher is a substitution cipher where the encryption of each letter of the plaintext depends on a different character in a keyword. This stops you using simple methods of frequency ...
-8
votes
2answers
401 views
Challenge: Crack the code [closed]
I have created a simple encryption algorithm. Your challenge, if you choose to accept it, is to re-produce the algorithm that encodes one string into it's encoded counterpart and then back again by ...
3
votes
3answers
483 views
Compress and Decompress sentence using vowel based encryption cipher [closed]
The aim is to parse a sentence and compress/encrypt it according to the rules below and then decrypt it using the key:
Remove all letters in a word before the first vowel is encountered. Example: ...
7
votes
2answers
592 views
2nd byte bias of RC4
Short version
RC4, designed in 1987, is one of the most famous stream ciphers. This question asks you to practically demonstrate its 2nd byte bias (for a theoretical proof, see section 3.2 of this ...
22
votes
13answers
2k views
What ROT is this? - decrypt ROT-n
Here are the letters of the English alphabet in order by frequency:
e t a o i n s h r d l c u m w f g y p b v k j x q z
That is, e is the most frequently used letter, and z is the least common. (...
18
votes
13answers
3k views
Write a Playfair encryption program
Write a program that takes two lines of input and uses the first as a key phrase to encrypt the second according to the Playfair encryption technique.
Wikipedia describes Playfair encryption in some ...
23
votes
4answers
2k views
Implement ROT-47… in ROT-47
Challenge: Implement ROT-47 in code that works as both itself and as the ROT-47 version of itself.
Scoring:
Your score is calculated as a percentage of used, ROT-47 eligible bytes in total of both ...
3
votes
3answers
439 views
Build an RSA encoder
Your task is to build a function in any language that takes a message m, an encryption e, and a modulus k (all positive integers), and takes m to the power of e modulo k. Your solution must not be a ...
3
votes
2answers
2k views
Rail fence cipher
Write a program which reads a string and a key and encodes the string into a rail-fence cipher using that key.
Similarly, write a program for reverse function, ie, deciphering a rail fence using a ...
14
votes
7answers
2k views
C: replace AES FIPS-197 SubBytes table by constant-time code
In FIPS-197 (the Advanced Encryption Standard, known as AES), it is made heavy use of SubBytes, which could be implemented as
unsigned char SubBytes(unsigned char x) {
static const unsigned char t[...
4
votes
2answers
696 views
Implement DES key expansion
I wanted to make a challenge to do the full DES encryption algorithm, but that may be a bit too involved to get a ton of participation. This challenge is to simply generate the 16 48-bit subkeys used ...
33
votes
3answers
2k views
Underhanded Password Hashing [closed]
In the spirit of the Underhanded C Contest, I'm starting an Underhanded Code Contest. The objective of this contest is to straightforwardly implement some code, while subtly hiding a nefarious bug ...
12
votes
12answers
2k views
Implement a One-Time Pad
Background
A one-time pad is a form of encryption that has been proven impossible to crack if used properly.
Encryption is performed by taking a plaintext (comprised of only letters A-Z) and ...
5
votes
4answers
515 views
Implement Oplop
Oplop is an algorithm to generate website specific passwords based on a master password and a keyword.
It is described here: http://code.google.com/p/oplop/wiki/HowItWorks
There's an online ...
12
votes
6answers
2k views
Implement Rijndael's S-box
Rijndael's S-box is a frequently used operation in AES encryption and decryption. It is typically implemented as a 256-byte lookup table. That's fast, but means you need to enumerate a 256-byte ...
15
votes
5answers
2k views
Write Buggy Code
Now is the time to show off your abilities to write bad code. I am trying out a new sort of programming puzzle, most similar, I think, to the underhanded C contest. The main difference is that this is ...
6
votes
3answers
528 views
Break the broken hash [closed]
Some time ago, I found this unused hash function (written in Java) in our codebase:
long hashPassword(String password)
{
password = password.toUpperCase();
long hash = 0;
int multiplier = ...
10
votes
7answers
3k views
Implement Shamir's Secret Sharing reconstruction
Shamir's secret sharing scheme is a simple way of protecting a secret by splitting it into several parts needed to reconstruct it.
Your task is to implement Shamir's Secret Sharing reconstruction ...
12
votes
1answer
697 views
Break the broken cipher
I have designed a simple random generator that cycles two numbers in a chaotic way using a multiply and modulus method. It works great for that.
If I were to use it as a cipher generator it would ...
11
votes
2answers
2k views
Decryption by pattern analysis
You are given an encrypted string, encrypted using a very simple substitution cipher.
Problem
You do not know what the cipher is but you do know the ciphertext is English and that the most frequent ...
