Ciphers

Running on from the previous article about codes, I thought I'd write some more, this time about ciphers.

A cipher differs from a code, in that a cipher replaces letters, or numbers, with another, whereas a code can replace many letters or words with just one. For example, the phrase "Meet me tonight" can be replaced by the code 1254, or "blue", or whatever. If we encypher the message, however, by replacing the individual letters with the key to the right on the keyboard we get "<rry ,r ypmohjy". This is known as a substitution cipher. The other type of cipher is called transposition. This is simply taking the letters of the message and re-arranging them, as in an anagram:

M	M	N	T
E	E	I	X
E	T	G	Y
T	O	H	Z

Reading the letters vertically produces the message, with a little extra letters to make a square number. Horizontally produces the code, MMNT EEIX ETGY TOHZ.

Digital Ciphers

Instead of working on letters of the alphabet, digital ciphers work on bits, or blocks of bits, usually using an XOR function. For example, convert the message to ASCII:

M	e	e	t		m	e
77	101	101	116	32	109	101
1001101	1100101	1100101	1110100	0100000	1101101	1100101

The XOR function changes the bits such that unequal bits produce a 1, while equal bits produce a 0. If we now use an ASCII key phrase:

Plaintext	M	e	e	t		m	e
ASCII (Decimal)	77	101	101	116	32	109	101
Binary	1001101	1100101	1100101	1110100	0100000	1101101	1100101
Keytext	S	e	n	d		t	h
ASCII (Decimal)	83	101	110	100	32	116	104
Binary	1010011	1100101	1101110	1100100	0100000	1110100	1101000
XOR Result	0011110	0000000	0001011	0010000	0000000	0011001	0001101
Decimal	30	0	11	16	0	25	13

The result of XORing is a series of mostly unprintable ASCII characters, but the stream now bears no relation to the original message. For example, the three "e"'s in the message are now all different, 0, 11, and 13.

The mechanism above is roughly how a Stream Cipher works. Instead of a key phrase, the cipher generates an endless stream of seemingly random bits which are XOR'd with the plaintext stream to produce the cipher stream.