Comments on: Autokey

By: Adam Thomas

Adam Thomas — Thu, 17 Dec 2009 13:30:17 +0000

The solution I posted above is implemented in C.

By: Adam Thomas

Adam Thomas — Thu, 17 Dec 2009 13:25:51 +0000

[sourcecode lang="C"] /* * Obtaining the encrypted character from the dictionary: * a - plain text character * b - cipher key character * x - first character in dictionary ('A') * z - size of dictionary (26) * * y = x + (a + b - 2 * x) % z */ char* encrypt(const char *txt, const char* key, char *enc) { unsigned i, len; for (i = 0, len = strlen(txt); i < len; i++) { enc[i] = 'A' + (txt[i] + key[i] - 2 * 'A') % 26; } return enc; } /* * Obtaining the decrypted character from the dictionary: * a - encrypted character * b - cipher key character * x - first character in dictionary ('A') * z - size of dictionary (26) * * y = x + (a - b + z) % z */ char* decrypt(const char *enc, const char *key, char *dec) { unsigned i, len; for (i = 0, len = strlen(enc); i < len; i++) { dec[i] = 'A' + (enc[i] - key[i] + 26) % 26; } return dec; } [/sourcecode]

By: programmingpraxis

programmingpraxis — Tue, 15 Dec 2009 21:09:38 +0000

Fixed!

By: gramie

gramie — Tue, 15 Dec 2009 21:03:58 +0000

> invented by Blaise de Vigenère in 1586, over five hundred years ago

Wow, I really must have overslept this morning! I could have sworn it was only 2009!

By: agilefall

agilefall — Sat, 05 Dec 2009 20:49:08 +0000

Here is a scala version:

class Crypt(key: String) {
  private def convert(s: String, converter: (Char, Char)=>Int) = {
    new String(
      (0 until plain.length).map(
        ix => (converter(s(ix),key(ix % key.length)) % 26 + ’A').toChar
      ).toArray)
  }

  def encrypt(plain: String) = convert(plain, (a: Char, b: Char)=> a + b)
  def decrypt(encrypted: String) = convert(encrypted, (a: Char, b: Char) => a - b + 26)
}

By: Autokey | Andrew Ferguson

Autokey | Andrew Ferguson — Sat, 05 Dec 2009 02:09:15 +0000

[...] programming puzzles from time to time. I’m also going to try work these problems using Perl. Today’s assignment is to make a simple Autokey Cipher, which I accomplished over my lunch [...]

By: Gierad

Gierad — Fri, 04 Dec 2009 21:20:55 +0000

Here's a quick PHP Solution: [code lang="php"] function encrypt($str, $key) { $out = ""; for ($i=0; $i

By: Gierad

Gierad — Fri, 04 Dec 2009 21:19:30 +0000

Here’s a quick PHP Solution

function encrypt($str, $key) { $out = ""; for ($i=0; $i { $base = ord($str[$i]) - 65; // take index of base string $offset = ord($key[$i]) - 65; // take index of key $ascii = ($base + $offset) % 26; // add indices and wrap around 26 $out .= chr(65 + $ascii); // add 65 to indices to display ASCII } return $out; }


function decrypt($str,$key)

{

	$out = "";

	for ($i=0; $i
	{

		$base = ord($str[$i]) - 65;	// take index of base string

		$offset = ord($key[$i]) - 65;   // take index of key
		// Perform reverse "wrapping"

		$ascii = ($base - $offset);

		if ($ascii < 0) $ascii = 26 + $ascii;

$out .= chr(65 + $ascii); // add 65 to indices to display ASCII } return $out; }

By: Skrud

Skrud — Fri, 04 Dec 2009 17:22:17 +0000

Here's a quick Python solution. [sourcecode lang="python"] def encrypt(cleartext,key): ciphertext = "" key = key + cleartext i = 0 while i < len(cleartext): ciphertext += __vig( cleartext[i], key[i] ) i += 1 return ciphertext def decrypt(ciphertext,key): cleartext = "" i = 0 while i < len(ciphertext): cleartext += __vig( ciphertext[i], key[i % len(key)], True ) key += cleartext[-1] i += 1 return cleartext def __rot(l,d): # Rotate the letter l by the specified number of degrees (assume uppercase) return chr((((ord(l) - ord('A')) + d) % 26) + ord('A')) def __vig(l,k,inverse=False): # Determine the index of key-letter 'k' relative to 'A' and use that # as the number of degrees to rotate by. degree = ord(k) - ord('A') if inverse: degree = -degree return __rot(l, degree) [/sourcecode]

By: programmingpraxis

programmingpraxis — Fri, 04 Dec 2009 16:20:34 +0000

Of course it is possible. But one-time pads are generally more useful for diplomats than spies. A spy would rather memorize a keyword, or a system of generating a keyword from the daily newspaper, or carry an innocent book for a book cipher, than carry an incriminating one-time pad. I know Kahn says that in his book, though I can’t cite a specific reference.