Cryptography Reference
In-Depth Information
TABLE 1.17
Plaintext
T ORAT ORATORA
Key
ANDGODS AI
DL E
Ciphertext
T BUGHRJ
ABRDE
1.12
BREAKING AUTO-KEY AND RUNNING KEY VIGENERE CIPHERS
Though the auto-key Vigenere and the running key Vigenere evade the problem of the
repeating key, they are still vulnerable to frequency analysis. This is because plaintext is being
used for the key. Even though this plaintext never repeats, it still provides information. This
is because high frequency letters in the key will often encipher high frequency letters in the
message. This information is often enough to recover messages.
1.13
THE ONE-TIME PAD
One solution to thwarting frequency analysis on polyalphabetic substitution ciphers was to
use a truly random key that would never repeat. Such a key was called a one-time pad.
These were notebooks consisting of sheets with tables of random numbers on them. The ran-
dom numbers were used as shift values. Each sheet in the pad was different from every
other, and each sheet was used only once. Encrypting using a one-time pad would look
something like Table 1.18.
Using this particular sheet from a one-time pad, the ciphertext message
NHTAB
FJTAUCDHZL
is produced from the plaintext message
ENGAGE WARP DRIVE.
If the message does not fill out the sheet, the rest of the sheet is ignored. After the sheet
is used, it is destroyed. The recipient of the message would also have an identical one-time
pad. The messages are numbered, so the recipient would know which sheet to use. They
would use the same shift values to shift back to the plaintext.
The one-time pad is the ultimate cipher, if used properly. In terms of ciphertext analy-
sis, it is totally secure. In fact, it is the most secure cipher possible. There is no way an ana-
Plaintext letter
Shift value
Ciphertext letter
E NGAGE WARPDRI VE
9 20130 211 13199 5 2512254 7 250 8 8 7 24
NHT ABF J T AUCDHZ L
Plaintext letter
Shift value
Ciphertext letter
2
6
18
16
10
23
5
11
12
13
6
22
22
17
3
8
0
0
19
4
15
TABLE 1.18
 
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