One of the methods used to encrypt a message is to use the message itself. We begin by providing a secret number that is used for the offset of the first letter and is known only to sender and receiver. This starting value is called a seed. For example, if we encode the word, CAT, we begin by finding the alphabetic value of each letter: 3, 1 and 20; and then use the value of the previous letter as an offset, with the first letter using the seed value.

Suppose the seed is 5:

C (3) (3 + 5[seed] = 8) H

A (1) (1 + 3[1st letter] = 4) D

T (20) (20 + 1[2nd letter] = 21) U

So the encoded message is HDU. It should be clear how, using the seed 5, it is possible to decode it easily.

Decode this message, without being told the seed:

X B M W W N B G C U A O O M W N W H C V R Y L S W A Q

That would certainly have been easier Charlie!!  Alas, I've had to rely on good old trial and error (working back from the final Q initially, then forward from X), and discovered the initial 'seed' here is 4.  Some of the following (*) require 'rolling back' alpha numerically through A to Z, Y, X and so on.)

 

X (24) – 4 = 20, being the first letter in the solution, T

B (2) – 20 (*) = 8 = H

M (13) – 8 = 5 = E

W (23) – 5 = 18 = R

W (23) – 18 = 5 = E

N (14) – 5 = 9 = I

B (2) – 9  = 19 = S

G (7) – 19 (*) = 14 = N

C (3) – 14 (*) = 15 = O

U (21) – 15 = 6 = F

A (1) – 6 (*) = 21 = U

O (15) – 21 (*) = 20 = T

O (15) – 20 (*) = 21 = U

M (13) – 21 (*) = 18 = R

W (23) – 18 = 5 = E

N (14) – 5 = 9 = I

W (23) – 9 = 14 = N

H (8) – 14 (*) = 20 = T

C (3) – 20 (*) = 9 = I

V (22) – 9 = 13 = M

R (18) – 13 = 5 = E

Y (25) – 5 = 20 = T

L (12) – 20 (*) = 18 = R

S (19) – 18 = 1 = A

W (23) – 1 = 22 = V

A (1) – 22 (*) = 5 = E

Q (17) – 5 = 12 = L   

 

This encryption of a familiar (anonymous) quote, is 'THERE IS NO FUTURE IN TIME TRAVEL'

Edited on January 19, 2009, 7:53 pm

Posted by rod hines on 2009-01-19 19:49:43