Playfair Cipher

Invented by Sir Charles Wheatstone (1802-1875). (Wheatstone also invented other useful things, most notably ticker tape.) Made popular by Lyon Playfair. Idea: use a 5-by-5 grid based on a keyword, encrypt two letters at a time. Protects against single-letter frequency analysis, but still susceptible to digraph analysis, since pairs of plain letters will frequently appear as same pairs of cipher letters. Small change in plaintext can change ciphertext substantially (depending on size of change - odd number of letters added or removed from plaintext make bug difference). It is still really a monoalphabetic substitution cipher - but one that applies to an alphabet consisting of digraphs rather than single letters. (See Singh, Appendix E, for more details.)

Examples:

keyword: HELLO
(remove dupes)

  key grid:
H  E  L  O  A
B  C  D  F  G
IJ K  M  N  P
Q  R  S  T  U
V  W  X  Y  Z

plaintext: test
written as digraphs:  te st
becomes
ciphertext:           RO TU


Example: repeated plaintext letters or odd number of letters

(using the same grid)

plaintext:                 hammer
digraphs:                  ha mm er
add x to break repeat:     ha mx me r
add x to make even length: ha mx me rx
ciphertext digraphs:       EH SL KL SW
ciphertext                 EHSLKLSW


Example:
keyword: SCHOOL

  key grid:
S  C  H  O  L
A  B  D  E  F
G  IJ K  M  N
P  Q  R  T  U
V  W  X  Y  Z

ciphertext:          MDZTKFDTEFHG
digraphs:            MD ZT KF DT EF HG
plaintext digraphs:  ke yu nd er de sk
plaintext:           key under desk


Example:
keyword: WISEGUY

  key grid:
W  IJ S  E  G
U  Y  A  B  C
D  F  H  K  L
M  N  O  P  Q
R  T  V  X  Z

ciphertext:          ISPTPITZ
digraphs:            IS PT PI TZ
plaintext digraphs:  wi nx ne rx
plaintext:           winner

ADFGVX Cipher

(See Singh, Appendix F for background and details.) Examples: (using grid from book:)

	A	D	F	G	V	X
A	8	p	3	d	1	n
D	l	t	4	o	a	h
F	7	k	b	c	5	z
G	j	u	6	w	g	m
V	x	s	v	i	r	2
X	9	e	y	0	f	q

plaintext:              f  i  g  u  r  e  8
ciphertext digraphs:    XV VG GV GD VV XD AA
keyword: CAT
cipher digraphs written in 3 columns:
  2 1 3           1 2 3
  C A T           A C T
  -----   -->  -----
  X V V           V X V
  G G V           G G V
  G D V           D G V
  V X D           X V D
  A A             A A 

ciphertext generated from reading down columns in order:

  VGDXAXGGVAVVVD