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Vigenère Cipher

Encode and decode text with the Vigenère polyalphabetic cipher. Includes step-by-step visualization and Vigenère square.

Plaintext

Keyword

Ciphertext

RIJVS UYVJN

Step-by-Step

#	Plain	Key	Shift	Result
1	H	K	+10	R
2	E	E	+4	I
3	L	Y	+24	J
4	L	K	+10	V
5	O	E	+4	S
6	W	Y	+24	U
7	O	K	+10	Y
8	R	E	+4	V
9	L	Y	+24	J
10	D	K	+10	N

Index of Coincidence (IC)

0.0889

English text ≈ 0.0667 | Random text ≈ 0.0385. A high IC on ciphertext suggests a short key or monoalphabetic cipher.

Vigenère vs. Caesar (ROT-N)

A Caesar cipher uses a single shift for all letters (e.g., ROT-13). The Vigenère cipher uses a repeating keyword where each letter defines a different shift, making simple frequency analysis ineffective.

However, if the key length is known, each column can be attacked as a separate Caesar cipher. The Kasiski method and Index of Coincidence help estimate key length.

#	Plain	Key	Shift	Result
1	H	K	+10	R
2	E	E	+4	I
3	L	Y	+24	J
4	L	K	+10	V
5	O	E	+4	S
6	W	Y	+24	U
7	O	K	+10	Y
8	R	E	+4	V
9	L	Y	+24	J
10	D	K	+10	N

#	Plain	Key	Shift	Result
1	H	K	+10	R
2	E	E	+4	I
3	L	Y	+24	J
4	L	K	+10	V
5	O	E	+4	S
6	W	Y	+24	U
7	O	K	+10	Y
8	R	E	+4	V
9	L	Y	+24	J
10	D	K	+10	N

#	Plain	Key	Shift	Result
1	H	K	+10	R
2	E	E	+4	I
3	L	Y	+24	J
4	L	K	+10	V
5	O	E	+4	S
6	W	Y	+24	U
7	O	K	+10	Y
8	R	E	+4	V
9	L	Y	+24	J
10	D	K	+10	N