Simulation Notes

Letter Mapping

A=0, B=1, ..., Z=25. Ex: H=7 (ord('H')-ord('A')).

Block Division

  • 4 letters/block.

  • Pad short final block with X (23).

  • Ex: "HELLOCRYPTOGRAPHY" → Blocks: HEL L, OCR Y, PTO G, RAPH Y (pad if needed).​

Encryption Formula

C=(P+K)mod  26C=(P+K)mod26

  • P = plaintext number.

  • K = key (usually 3).

  • C → letter (C + ord('A')).
    Ex: H(7) +3 =10 mod26=10 → K.​

Decryption Formula

P=(C−K)mod  26P=(C−K)mod26
Ex: K(10) -3 =7 mod26=7 → H.

Step-by-Step Example: "HELLO"

  1. Numbers: H=7, E=4, L=11, L=11, O=14.

  2. Block1: HELL → 7,4,11,11.

  3. Encrypt: 7+3=10(K), 4+3=7(H), 11+3=14(O), 11+3=14(O) → "KHOO".

  4. Decrypt back: K(10)-3=7(H), etc. → "HELL".​

Practice Exercise: "INTEGRATIVE"

  1. Numbers: I=8,N=13,T=19,E=4,G=6,R=17,A=0,T=19,I=8,V=21,E=4.

  2. Blocks: INTE GRAT IVE (pad? 10 letters → INTE, GRAT, IVE +X? Check length).

  3. Encrypt each +3 mod26.
    Expected ciphertext: Compute step-by-step (L Q W H J T D W L Y).