Tagged Questions

For challenges involving cryptography: encryption or decryption.

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-6
votes
2answers
204 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
383 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
324 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 ...
20
votes
12answers
1k 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. ...
16
votes
13answers
1k 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
360 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 ...
2
votes
2answers
1k 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 ...
11
votes
7answers
915 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 ...
3
votes
2answers
508 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 ...
30
votes
3answers
1k views

Underhanded Password Hashing

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 ...
11
votes
12answers
1k 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 ...
4
votes
4answers
479 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 ...
8
votes
5answers
1k 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 ...
4
votes
3answers
437 views

Break the broken hash

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 = ...
9
votes
7answers
2k 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 ...
10
votes
2answers
624 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 ...
10
votes
2answers
966 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 ...