# Tagged Questions

For challenges involving cryptography: encryption or decryption.

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### 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-...
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### English to Caesar cipher [duplicate]

This code will crypt/decrypt message from English to Caesar cipher. Input In Input you will write a text like "Hello". Outpot The programm will return the input message crypted. "Hello" will ...
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### 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, ...
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### 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-...
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### 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 ...
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### 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 ...
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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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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=...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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, ...
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### 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 ...
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### 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,...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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: ...
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### 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 ...
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### 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. (...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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[...
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### 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 ...
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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 ...
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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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 = ...
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### 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 ...