* in general use more than one substitution alphabet
* makes cryptanalysis harder since have more alphabets to guess
* and because flattens frequency distribution (since same plaintext letter gets replaced by several ciphertext letter, depending on which alphabet is used)

Vigenère Cipher

* basically multiple caesar ciphers
* key is multiple letters long K = k_(1) k_(2) ... k_(d)
* ith letter specifies ith alphabet to use
* use each alphabet in turn, repeating from start after d letters in message

Plaintext: THISPROCESSCANALSOBEEXPRESSED
Keyword: CIPHERCIPHERCIPHERCIPHERCIPHE
Ciphertext: VPXZTIQKTZWTCVPSWFDMTETIGAHLH

* can use a Saint-Cyr Slide for easier encryption

* based on a Vigenère Tableau

ABCDEFGHIJKLMNOPQRSTUVWXYZ
A ABCDEFGHIJKLMNOPQRSTUVWXYZ
B BCDEFGHIJKLMNOPQRSTUVWXYZA
C CDEFGHIJKLMNOPQRSTUVWXYZAB
D DEFGHIJKLMNOPQRSTUVWXYZABC
E EFGHIJKLMNOPQRSTUVWXYZABCD
F FGHIJKLMNOPQRSTUVWXYZABCDE
G GHIJKLMNOPQRSTUVWXYZABCDEF
H HIJKLMNOPQRSTUVWXYZABCDEFG
I IJKLMNOPQRSTUVWXYZABCDEFGH
J JKLMNOPQRSTUVWXYZABCDEFGHI
K KLMNOPQRSTUVWXYZABCDEFGHIJ
L LMNOPQRSTUVWXYZABCDEFGHIJK
M MNOPQRSTUVWXYZABCDEFGHIJKL
N NOPQRSTUVWXYZABCDEFGHIJKLM
O OPQRSTUVWXYZABCDEFGHIJKLMN
P PQRSTUVWXYZABCDEFGHIJKLMNO
Q QRSTUVWXYZABCDEFGHIJKLMNOP
R RSTUVWXYZABCDEFGHIJKLMNOPQ
S STUVWXYZABCDEFGHIJKLMNOPQR
T TUVWXYZABCDEFGHIJKLMNOPQRS
U UVWXYZABCDEFGHIJKLMNOPQRST
V VWXYZABCDEFGHIJKLMNOPQRSTU
W WXYZABCDEFGHIJKLMNOPQRSTUV
X XYZABCDEFGHIJKLMNOPQRSTUVW
Y YZABCDEFGHIJKLMNOPQRSTUVWX
Z ZABCDEFGHIJKLMNOPQRSTUVWXY

can describe this cipher as:

given K = k(1) k(2) ... k(d)
then f(i) (a) = a + k(i) (mod n)