ran-jit / codes-and-cyphers

Codes-and-Ciphers

Codes and cyphers have been used to protect secrets since ancient times. In 1882, Frank Miller described the use of a one-time pad: an encryption technique that cannot be cracked if used correctly. The technique involves the use of a key and plaintext that is to be coded. Each character of the plaintext and the key are paired up using modular addition.

Example: Suppose Alice wants to send the following message to Bob: The enemy will attack at dawn!

If they agree on the key venturesity, then you pair up the characters as follows: The enemy will attack at dawn! Venturesityventuresityve!

The coding will be done as follows:

• T h e e n e m y w i l l a t t a c k a t d a w n! Message
• 20 8 5 5 14 5 13 25 23 9 12 12 1 20 20 1 3 11 1 20 4 1 23 14! Message
• [phone] 20 25 22 5 14 20 21 18 5 19 9 20 25 22 5! Key
• 42 13 19 25 35 23 18 44 32 29 37 34 6 34 40 22 21 16 20 29 24 26 45 19! Message + Key
• 17 14 20 26 10 24 19 19 7 4 12 9 7 9 15 23 22 17 21 4 25 1 20 20! Message + Key (mod 26) + 1
• Qntzjxssgdligiowvqudyatt!

So Alice would send Qntzjxssgdligiowvqudyatt! to Bob. If Bob knows the key, he would decode it as follows:

• Q n t z j x s s g d l i g i o w v q u d y a t t! ciphertext
• 17 14 20 26 10 24 19 19 7 4 12 9 7 9 15 23 22 17 21 4 25 1 20 20! Ciphertext
• -22 5 14 20 21 18 5 19 9 20 25 22 5 14 20 21 18 5 19 9 20 25 22 5! Key
• -5 9 6 6 -11 6 14 0 -2 -16 -23 -13 2 -5 -5 2 4 12 2 -5 5 -24 -2 15! Ciphertext-key
• 20 8 5 5 14 5 13 25 23 9 12 12 1 20 20 1 3 11 1 20 4 1 23 14! Ciphertext-key (mod 26)-1
• Theenemywillattackatdawn!

Sample input 1:

• Key: venturesity
• Is message plaintext (Y/N): Y
• Plaintext: The enemy will attack at dawn!

output:

• Ciphertext: Qntzjxssgdligiowvqudyatt!

Sample input 2:

• Key: venturesity
• Is message plaintext (Y/N)?: N
• Ciphertext: Qntzjxssgdligiowvqudyatt!

output:

• Plaintext: The enemy will attack at dawn!