Breaking Vigenere

Split by the period and each stream is a Caesar cipher again, so the first attack of the module finishes this one.

CryptanalysisCiphertext onlydemo

Objectives

Split a ciphertext into cosets and solve each as an independent Caesar.
Assemble the recovered shifts into the keyword and decrypt the message.
Explain what a wrong period does to the cosets and why the attack then stalls.

33%

Split by the period, and each stream is a Caesar cipher again.

Vigenère keyword (the secret)

Plaintext

Period the attacker guessed

8 Caesar ciphers in parallel

coset	letters (first 20)	best shift	key letter
0	NYYZJWZVJOVMNZAJZGZD	21	V
1	PPPZCMNEZWVWQBZLTIKU	8	I
2	KGKZRKUUXSJCTZUGGHUK	6	G
3	WZVLHRVVSSXGXCQCWPVE	4	E
4	URRRUNFQJEBEUCQGGRQA	13	N
5	SHEVEXYXEVQIMEESWSIH	4	E
6	LZWVMZTFEFFVJTPKPWUR	17	R
7	PIXAIQLQHAVTTIXLPVXP	4	E

recovered key

VIGENERE

That is the keyword, from ciphertext alone. No known plaintext, no guess at the algorithm.

And the message

ciphertext

NPKWU SLPYP GZRHZ IYPKV REWXZ ZZLRV VAJCR HUEMI WMKRN XZQZN UVFYT LVEUV

key stream from the recovered keyword

VIGEN EREVI GENER EVIGE NEREV IGENE REVIG ENERE VIGEN EREVI GENER EVIGE

plaintext

SHESH OULDH AVEDI EDHER EAFTE RTHER EWOUL DHAVE BEENA TIMEF ORSUC HAWOR

Three centuries of “le chiffre indéchiffrable”, reduced to 8 independent problems of 25 candidates each. Each coset carries one fixed shift, so the same chi-square that broke Caesar on the first slide finishes the job here.

step 1/3

CPE 645 / CS 685

Breaking Vigenere

Kasiski Examination

Breaking Autokey