Operation Modes For Polyalphabetic Ciphers
Examples are given for the encryption of the text MESSAGE using
the key word KEY.
Repetitive Key
As it's name implies, the key is simply repeated:
| Plain text |
M |
E |
S |
S |
A |
G |
E |
| Key |
K |
E |
Y |
K |
E |
Y |
K |
| Cipher Text |
W |
I |
Q |
C |
E |
E |
O |
Key Progression
The key is repeated, but at each iteration the letters of the key are replaced
with there successors:
| Plain text |
M |
E |
S |
S |
A |
G |
E |
| Key |
K |
E |
Y |
L |
F |
Z |
M |
| Cipher Text |
W |
I |
Q |
D |
F |
F |
Q |
Autoclave
The key is not repeated. Instead the letters of the plain text are appended
to obtain a key that is long enough to encipher the plain text:
| Plain text |
M |
E |
S |
S |
A |
G |
E |
| Key |
K |
E |
Y |
M |
E |
S |
S |
| Cipher Text |
W |
I |
Q |
E |
E |
Y |
W |
PT Interrupted
This is basically the same as the repetitive key mode. If a plain text
letter is found in a given group of letters, the repeated use of the key
word is interrupted, e.g. encryption will continue at the begin of the
key word. This scheme is only available for the generic Polyalphabetic
substitution cipher.
CT Interrupted
This is basically the same as the repetitive key mode. If a cipher text
letter is found in a given group of letters, the repeated use of the key
word is interrupted, e.g. encryption will continue at the begin of the
key word. This scheme is only available for the generic Polyalphabetic
substitution cipher.