David Eisinger

Elsewhere > OTP: a Language-Agnostic Programming Challenge

Posted 2015-01-26 on viget.com

We spend our days writing Ruby and JavaScript (and love it), but we’re always looking for what’s next or just what’s nerdy and interesting. We have folks exploring Rust, Go, D and Elixir, to name a few. I’m personally interested in strongly-typed functional languages like Haskell and OCaml, but I’ve had little success getting through their corresponding animal books. I decided that if I was going to get serious about learning this stuff, I needed a real problem to solve.

Inspired by an online course on Cryptography, I specced out a simple one-time pad encryptor/decryptor, pushed it up to GitHub and issued a challenge to the whole Viget dev team: write a pair of programs in your language of choice to encrypt and decrypt a message from the command line.

The Challenge

When you exclusive or (XOR) a value by a second value, and then XOR the resulting value by the second value, you get the original value back. Suppose you and I want to exchange a secret message, the word “hi”, and we’ve agreed on a secret key, the hexadecimal number b33f (or in binary, 1011 0011 0011 1111).

To encrypt:

1. Convert the plaintext (“hi”) to its corresponding ASCII values (“h” becomes 104 or 0110 1000, “i” 105 or 0110 1001).

2. XOR the plaintext and the key:

   Plaintext: 0110 1000 0110 1001
   Key: 1011 0011 0011 1111
   XOR: 1101 1011 0101 0110
   

3. Convert the result to hexadecimal:

   1101 = 13 = d
   1011 = 11 = b
   0101 = 5 = 5
   0110 = 6 = 6
   

4. So the resulting ciphertext is “db56”.

To decrypt:

1. Expand the ciphertext and key to their binary forms, and XOR:

   Ciphertext: 1101 1011 0101 0110
   Key: 1011 0011 0011 1111
   XOR: 0110 1000 0110 1001
   

2. Convert the resulting binary numbers to their corresponding ASCII values:

   0110 1000 = 104 = h
   0110 1001 = 105 = i
   

3. So, as expected, the resulting plaintext is “hi”.

The Wikipedia page plus the project’s README provide more detail. It’s a simple problem conceptually, but in order to create a solution that passes the test suite, you’ll need to figure out:

• Creating a basic command-line executable
• Reading from STDIN and ARGV
• String manipulation
• Bitwise operators
• Converting to and from hexadecimal

As of today, we’ve created solutions in eleven twelve thirteen languages:

• C
• D
• Elixir
• Go
• Haskell
• JavaScript 5
• JavaScript 6
• Julia
• Matlab
• OCaml
• Ruby
• Rust
• Swift (thanks wasnotrice!)

The results are varied and fascinating – stay tuned for future posts about some of our solutions. In the meantime, we’d love to see how you approach the problem, whether in a new language or one we’ve already attempted. Fork the repo and show us what you’ve got!