Autokey Cipher / Autoclave Cipher Examples (encoder / decoder)

1. Autokey Cipher / Autoclave Cipher encoder

Text : ABCD efghi
Key : bcd
Alphabet : abcdefghijklmnopqrstuvwxyz

Solution:

The Autokey Cipher is a polyalphabetic substitution cipher that encrypts a message using a keyword and a portion of the plaintext itself to create non-repeating key

Alphabet = abcdefghijklmnopqrstuvwxyz

Letter	a	b	c	d	e	f	g	h	i	j	k	l	m	n	o	p	q	r	s	t	u	v	w	x	y	z
Number	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25

Each letter is shifted down based on the key letter

Plaintext	A	B	C	D	e	f	g	h	i
Text Value (2)	0	1	2	3	4	5	6	7	8
Key	b	c	d	a	b	c	d	e	f
Key Value (4)	1	2	3	0	1	2	3	4	5
Addition (2) + (4) Mod 26	1	3	5	3	5	7	9	11	13
Ciphertext	B	D	F	D	f	h	j	l	n

Plaintext	: ABCD efghi
Key	: bcd
CurrentKey	: bcdabcdefghi
Ciphertext	: BDFD fhjln

2. Autokey Cipher / Autoclave Cipher encoder

Text : hello world
Key : bcd
Alphabet : abcdefghijklmnopqrstuvwxyz

Solution:

The Autokey Cipher is a polyalphabetic substitution cipher that encrypts a message using a keyword and a portion of the plaintext itself to create non-repeating key

Alphabet = abcdefghijklmnopqrstuvwxyz

Letter	a	b	c	d	e	f	g	h	i	j	k	l	m	n	o	p	q	r	s	t	u	v	w	x	y	z
Number	0	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25

Each letter is shifted down based on the key letter

Plaintext	h	e	l	l	o	w	o	r	l	d
Text Value (2)	7	4	11	11	14	22	14	17	11	3
Key	b	c	d	h	e	l	l	o	w	o
Key Value (4)	1	2	3	7	4	11	11	14	22	14
Addition (2) + (4) Mod 26	8	6	14	18	18	33=7	25	31=5	33=7	17
Ciphertext	i	g	o	s	s	h	z	f	h	r

Plaintext	: hello world
Key	: bcd
CurrentKey	: bcdhelloworld
Ciphertext	: igoss hzfhr

This material is intended as a summary. Use your textbook for detail explanation.
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1. Examples