cipher-collection

Zayslash JavaScript collection of ciphers

This repository serves as a collection of knowledge and implementatioin of

Caesar Cipher

Introduction

The Caesar cipher is one of the earliest known and simplest ciphers. It is a type of substitution cipher in which each letter in the plaintext is 'shifted' a certain number of places down the alphabet. For example, with a shift of 1, A would be replaced by B, B would become C, and so on. The method is named after Julius Caesar, who apparently used it to communicate with his generals.

Mathematical Description

First we translate all of our characters to numbers, 'a'=0, 'b'=1, 'c'=2, ... , 'z'=25. We can now represent the caesar cipher encryption function, e(x), where x is the character we are encrypting, as:

e(x) = (x + k) (mod 26)

Where k is the key (the shift) applied to each letter. After applying this function the result is a number which must then be translated back into a letter. The decryption function is :

e(x) = (x - k) (mod 26)

References

http://practicalcryptography.com/ciphers/caesar-cipher/

Wikipedia has a good description of the encryption/decryption process, history and cryptanalysis of this algorithm
Simon Singh's 'The Code Book' is an excellent introduction to ciphers and codes, and includes a section on caesar ciphers.
Singh, Simon (2000). The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography. ISBN 0-385-49532-3.

Vigenre Cipher

Introduction

The Vigenère Cipher is a polyalphabetic substitution cipher. The method was originally described by Giovan Battista Bellaso in his 1553 book La cifra del. Sig. Giovan Battista Bellaso; however, the scheme was later misattributed to Blaise de Vigenère in the 19th century, and is now widely known as the Vigenère cipher.

Blaise de Vigenère actually invented the stronger Autokey cipher in 1586.

The Vigenère Cipher was considered le chiffre ind hiffrable (French for the unbreakable cipher) for 300 years, until in 1863 Friedrich Kasiski published a successful attack on the Vigenère cipher. Charles Babbage had, however, already developed the same test in 1854. Gilbert Vernam worked on the vigenere cipher in the early 1900s, and his work eventually led to the one-time pad, which is a provably unbreakable cipher.

Variants

There are several ciphers that are very similar to the vigenere cipher.

The Gronsfeld cipher is exactly the same as the vigenere cipher, except numbers are used as the key instead of letters. There is no other difference. The numbers may be picked from a sequence, e.g. the Fibonacci series, or some other pseudo-random sequence.

The gronsfeld cipher is cryptanalysed in the same way as the vigenere algorithm, however the autokey cipher will not be broken using the kasiski method since the key does not repeat. The best way to break the autokey cipher is to try and guess portions of the plaintext or key from the ciphertext, knowing they must both follow the frequency distribution of english text. Guessing how the plaintext begins is the easiest way of cracking the cipher.

 References

  http://practicalcryptography.com/ciphers/caesar-cipher/

 [1] Wikipedia has a good description of the encryption/decryption process, history and cryptanalysis of this algorithm
 [2] Kahn, D (1973) The CodeBreakers. Macmillan: New York
 [3] http://www.trincoll.edu/depts/cpsc/cryptography/vigenere.html has a good description of the algorithm along with cryptanalysis.
  Simon Singh's 'The Code Book' is an excellent introduction to ciphers and codes, and includes a section on ADFGVX ciphers.
 Singh, Simon (2000). The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography. ISBN 0-385-49532-3.

ROT13

Introduction

The ROT13 cipher is a substitution cipher with a specific key where the letters of the alphabet are offset 13 places. I.e. all 'A's are replaced with 'N's, all 'B's are replaced with 'O's, and so on. It can also be thought of as a Caesar cipher with a shift of 13.

The ROT13 cipher offers almost no security, and can be broken very easily. Even if an adversary doesn't know a piece of ciphertext has been enciphered with the ROT13 cipher, they can still break it by assuming it is a substitution cipher and determining the key using hill-climbing. The ROT13 cipher is also an Caesar cipher with a key of 13, so breaking it as a Caesar cipher also works.

The Algorithm

The ROT13 cipher is essentially a substitution cipher with a fixed key, if you know the cipher is ROT13, then no additional information is needed to decrypt the message. The substitution key is:

ABCDEFGHIJKLMNOPQRSTUVWXYZ

NOPQRSTUVWXYZABCDEFGHIJKLM

To encipher a message, find the letter you wish to encipher in the top row, then replace it with the letter in the bottom row. In the example below, we encipher the message 'ATTACK AT DAWN'. The first letter we wish to encipher is 'A', which is above 'N', so the first ciphertext letter is 'N'. The next letter is 'T', which is above 'G', so that comes next. The whole message is enciphered:

  References
  http://practicalcryptography.com/ciphers/classical-era/rot13/

Affine Cipher

Introduction

The Affine cipher is a special case of the more general monoalphabetic substitution cipher.

The cipher is less secure than a substitution cipher as it is vulnerable to all of the attacks that work against substitution ciphers, in addition to other attacks. The cipher's primary weakness comes from the fact that if the cryptanalyst can discover (by means of frequency analysis, brute force, guessing or otherwise) the plaintext of two ciphertext characters, then the key can be obtained by solving a simultaneous equation [1].

The Algorithm

The 'key' for the Affine cipher consists of 2 numbers, we'll call them a and b. The following discussion assumes the use of a 26 character alphabet (m = 26). a should be chosen to be relatively prime to m (i.e. a should have no factors in common with m). For example 15 and 26 have no factors in common, so 15 is an acceptable value for a, however 12 and 26 have factors in common (e.g. 2) so 12 cannot be used for a value of a. When encrypting, we first convert all the letters to numbers ('a'=0, 'b'=1, ..., 'z'=25). The ciphertext letter c, for any given letter p is (remember p is the number representing a letter):

c = ap + b (mod m)

The decryption function is:

p = a^-1(c-b) (mod m)

where a−1 is the multiplicative inverse of a in the group of integers modulo m.