1. Four Square Cipher encoder

Text : ABCD efghi
Key1 : key
Key2 : word
Padding Character : z

Solution:

Key1 = keyabcdfghilmnopqrstuvwxz

Key2 = wordabcefghiklmnpqstuvxyz

Grid-1

	1	2	3	4	5
1	a	b	c	d	e
2	f	g	h	i	k
3	l	m	n	o	p
4	q	r	s	t	u
5	v	w	x	y	z

Grid-2

	1	2	3	4	5
1	k	e	y	a	b
2	c	d	f	g	h
3	i	l	m	n	o
4	p	q	r	s	t
5	u	v	w	x	z

Grid-3

	1	2	3	4	5
1	w	o	r	d	a
2	b	c	e	f	g
3	h	i	k	l	m
4	n	p	q	s	t
5	u	v	x	y	z

Grid-4

	1	2	3	4	5
1	a	b	c	d	e
2	f	g	h	i	k
3	l	m	n	o	p
4	q	r	s	t	u
5	v	w	x	y	z

Plaintext = abcdefghi
Split into pairs of letters (digraphs)
Pairs : ab cd ef gh i

Pad at end, if necessary
If one letter is left at the end, then pad to make it pair
Pairs : ab cd ef gh iz

Encryption :
Rule : If both letters are on the same row or column then reverse them, otherwise use Rectangle rule (Replace each letter with the one in its row, but in the other letter's column)

ab `=` a(G1,r1,c1), b(G4,r1,c2) `=>` e(G2,r1,c2), w(G3,r1,c1) `=` ew

cd `=` c(G1,r1,c3), d(G4,r1,c4) `=>` a(G2,r1,c4), r(G3,r1,c3) `=` ar

ef `=` e(G1,r1,c5), f(G4,r2,c1) `=>` k(G2,r1,c1), g(G3,r2,c5) `=` kg

gh `=` g(G1,r2,c2), h(G4,r2,c3) `=>` f(G2,r2,c3), c(G3,r2,c2) `=` fc

iz `=` i(G1,r2,c4), z(G4,r5,c5) `=>` h(G2,r2,c5), y(G3,r5,c4) `=` hy

Plaintext	: abcdefghiz
Ciphertext	: ewarkgfchy

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2. Four Square Cipher encoder

Text : hello world
Key1 : key
Key2 : word
Padding Character : z

Solution:

Key1 = keyabcdfghilmnopqrstuvwxz

Key2 = wordabcefghiklmnpqstuvxyz

Grid-1

	1	2	3	4	5
1	a	b	c	d	e
2	f	g	h	i	k
3	l	m	n	o	p
4	q	r	s	t	u
5	v	w	x	y	z

Grid-2

	1	2	3	4	5
1	k	e	y	a	b
2	c	d	f	g	h
3	i	l	m	n	o
4	p	q	r	s	t
5	u	v	w	x	z

Grid-3

	1	2	3	4	5
1	w	o	r	d	a
2	b	c	e	f	g
3	h	i	k	l	m
4	n	p	q	s	t
5	u	v	x	y	z

Grid-4

	1	2	3	4	5
1	a	b	c	d	e
2	f	g	h	i	k
3	l	m	n	o	p
4	q	r	s	t	u
5	v	w	x	y	z

Plaintext = helloworld
Split into pairs of letters (digraphs)
Pairs : he ll ow or ld

Encryption :
Rule : If both letters are on the same row or column then reverse them, otherwise use Rectangle rule (Replace each letter with the one in its row, but in the other letter's column)

he `=` h(G1,r2,c3), e(G4,r1,c5) `=>` h(G2,r2,c5), r(G3,r1,c3) `=` hr

ll `=` l(G1,r3,c1), l(G4,r3,c1) `=>` i(G2,r3,c1), h(G3,r3,c1) `=` ih

ow `=` o(G1,r3,c4), w(G4,r5,c2) `=>` l(G2,r3,c2), y(G3,r5,c4) `=` ly

or `=` o(G1,r3,c4), r(G4,r4,c2) `=>` l(G2,r3,c2), s(G3,r4,c4) `=` ls

ld `=` l(G1,r3,c1), d(G4,r1,c4) `=>` n(G2,r3,c4), w(G3,r1,c1) `=` nw

Plaintext	: helloworld
Ciphertext	: hrihlylsnw

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