Continued...
Cryptography Basics
Say you have a message that you want to convey to someone else, and you want to encode it so that no one else can read it. How can this be done? There are many methods that have been used in the past.
One of the first methods used a substitution of letters. It is believed that Caesar used this system. For example, the letter A could be substituted by F, B by G, and so on.
Or, a completely random substitution system could be used.
Letter |
Substitute |
|
Letter |
Substitute |
|
A |
X |
|
N |
C |
B |
L |
|
O |
Z |
C |
K |
|
P |
T |
D |
Q |
|
Q |
F |
E |
D |
|
R |
V |
F |
R |
|
S |
G |
G |
N |
|
T |
M |
H |
A |
|
U |
U |
I |
W |
|
V |
H |
J |
B |
|
W |
O |
K |
Y |
|
X |
J |
L |
E |
|
Y |
P |
M |
S |
|
Z |
I |
Unfortunately, this system quickly falls prey to analysis.
There are well known tables of the frequency of occurance of each letter in the English language. Given a large enough amount of ciphertext, it would be possible to determine at least some of the more commonly used letters. By examining the partially decoded text, it would then be possible to make
guesses at the other letters, by looking at partially decoded words.
Frequency, expressed per 100 letters:
13 9 8 8 7 7 7 6 6 4 4 3 3 3 3 2 2 2 1 1 1 - - - - -
E T A O N I R S H L D C U P F M W Y B G V K Q X J Z
A slightly more advanced system could change the substitution used throughout the message by a known algorithm. Unfortunately this method can also be sucessfully attacked, given enough ciphertext. With the computing power available today, any such system could easily be broken, even using a personal computer.
Today there are encryption methods employed by computers, which make use of complex encoding methods using large numbers as keys. These systems too can be broken, given enough computing power. And the National Security Agency is the world's largest buyer of supercomputers!
One Time Pads
One very secure method of encrypting a document is by the use of a One Time Pad. The pad (which may actually more closely resemble a book due to it's size!) contains a listing of random numbers used to encrypt the text. A similar reverse pad is employed by the recipient to decode the message.
Example:
Plaintext:
R A D I O H A B A N A I S B O R I N G
Equivalent:
18 1 4 9 15 8 1 2 1 14 1 9 19 2 15 18 9 14 7
Using the table of truly random numbers from the one time pad:
47693 94573 18483 59384 51839 47263 58347 21634 59347 73633
04732
38483 63933 74342 03843 37549 45839 59843 94784 83744
28483 93843
47539 72384 19383 94833 03484 58393 ...
Add the cipher equivalent to the random key:
R A D I O.....
18 1 4 9 15
47693 94573 18483 59384 51839
----- ----- ----- ----- -----
47711 94574 18487 59393 51854
Transmit new cipher text:
47711 94574 18487 59393 51854.....
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