Gronsfeld Cipher

Yaser Rahmati | یاسر رحمتی

The Gronsfeld Cipher is a variation of the Vigenère Cipher but is simpler because it uses only digits (0-9) as the key. Each digit in the key indicates a shift for the corresponding letter in the plaintext.

Step 1: Understand the Key

  • The key for the Gronsfeld Cipher is a sequence of digits. Each digit represents a shift in the alphabet.

Example Key: 314

Step 2: Prepare the Plaintext

  • Write down the message you want to encrypt.

Example Plaintext: HELLO

Step 3: Repeat the Key

  • If the key is shorter than the plaintext, repeat the key to match the length of the plaintext.

Extended Key for "HELLO": Key: 31431

Step 4: Encrypt the Message

  • Shift each letter in the plaintext by the number of positions specified by the corresponding digit in the key.

Encryption Table:

  • A=0, B=1, C=2, ..., Z=25

  • If a digit is 3, you shift the letter three places forward in the alphabet.

Example Calculation:

  1. H with a shift of 3: H → I → J → K

  2. E with a shift of 1: E → F

  3. L with a shift of 4: L → M → N → O → P

  4. L with a shift of 3: L → M → N → O

  5. O with a shift of 1: O → P

Encrypted Message: KFPOP

Step 5: Decrypt the Message

  • To decrypt, reverse the shift using the same key.

Decryption Calculation:

  1. K with a shift of 3: K → J → I → H

  2. F with a shift of 1: F → E

  3. P with a shift of 4: P → O → N → M → L

  4. O with a shift of 3: O → N → M → L

  5. P with a shift of 1: P → O

Decrypted Message: HELLO

Summary of Example:

  • Plaintext: HELLO

  • Key: 314

  • Extended Key: 31431

  • Encrypted Text: KFPOP

  • Decrypted Text: HELLO

This step-by-step guide provides a basic overview of the Gronsfeld Cipher, allowing you to encrypt and decrypt messages using this technique.

Keywords

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