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README.md
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xorBasedOneTimePad
xorBasedOneTimePad
 
 
xorBasedOneTimePad.asm
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xorBasedOneTimePad.o
xorBasedOneTimePad.o
 
 

README.md

One-Time Pad

One-Time Pad (OTP) guarantees perfect secrecy.

It's Claude Shannon that introduced the concept in 1946.

Definition of perfect secrecy:

"A ciphertext maintains perfect secrecy if the attacker’s knowledge of the contents of the message is the same both before and after the adversary inspects the ciphertext, attacking it with unlimited resources. That is, the message gives the adversary precisely no information about the message contents." (source)

However, OTP is not practical for our everyday messages, as it requires a key as long as the plaintext and a new random key for each new message or data group.

Algorithme

The usual form of OTP is the following one: $C = P \oplus K$.

If you're not working on bits, you can use a variant that is the Vigenère cipher, however, the key should be at least as long as the message for both of them.

Vigenère variante:

Note

We use a latin alphabet, hence the modulo 26.

Encryption: $C = M + K \text{ (mod } 26)$

Decryption: $C = M - K \text{ (mod } 26)$

Code

I've coded a XOR based one time pad, and as you can see it's pretty straightforward, so I decided to do it in assembly (which is less straightforward).

Instruction

Note

on a Linux machine using NASM

  • Execute ./xorBasedOneTimePad
  • Re-build the executable:
    • nasm -f elf xorBasedOneTimePad.asm
    • ld -m elf_i386 -s -o xorBasedOneTimePad xorBasedOneTimePad.o

The script checks if a message is entered.

Verify if the key is at least as long as the message.

You can let the script create a random key using /dev/urandom.

It's a XOR based OTP so you can get a result with strange character.

Example of inputs:

Plaintext: hello
Key: FUZZY
Ciphertext: .0666

[OR]

Plaintext: jump
Key TEST
Ciphertext: >0>$

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