Nihilist Cipher

Summary: Uses a Polybius square with a keyword, converting letters to two-digit numbers, then adds a numeric key (also derived from a keyword). The result is a series of two-digit numbers.

Example:

• Polybius Keyword: ZEBRA
• Square (I/J combined):

  1 2 3 4 5
1 Z E B R A
2 C D F G H
3 I K L M N
4 O P Q S T
5 U V W X Y

• Key Keyword: CIPHER
• Numeric Key: C=21, I=31, P=42, H=25, E=12, R=14

Encryption:

• Plaintext: HIDE
• Convert to numbers: H=25, I=31, D=22, E=12
• Key (repeating): CIPH = 21, 31, 42, 25
• Add modulo 100:
  • H: 25 + 21 = 46
  • I: 31 + 31 = 62
  • D: 22 + 42 = 64
  • E: 12 + 25 = 37
• Ciphertext: 46 62 64 37

Decryption (given both keys):

1. Build Polybius square with keyword ZEBRA
2. Convert key word CIPHER to numbers: 21, 31, 42, 25, 12, 14
3. Subtract key from ciphertext: 46-21=25, 62-31=31, 64-42=22, 37-25=12
4. Look up in Polybius square: 25=H, 31=I, 22=D, 12=E
5. Plaintext: HIDE

Additional Example:

• Polybius Keyword: ROYAL
• Square:

  1 2 3 4 5
1 R O Y A L
2 B C D E F
3 G H I K M
4 N P Q S T
5 U V W X Z

• Key Keyword: GOLD
• Numeric Key: G=31, O=12, L=15, D=23

Encryption of "SAFE":

• Convert to numbers using ROYAL square:

  • S = row 4, col 4 -> 44
  • A = row 1, col 4 -> 14
  • F = row 2, col 5 -> 25
  • E = row 2, col 4 -> 24

• Add key (repeating GOLD = 31, 12, 15, 23):

  • S: 44 + 31 = 75
  • A: 14 + 12 = 26
  • F: 25 + 15 = 40
  • E: 24 + 23 = 47

Ciphertext: 75 26 40 47

Decryption:

1. Build ROYAL square (shown above)
2. Convert GOLD to numbers: 31, 12, 15, 23
3. Subtract: 75-31=44, 26-12=14, 40-15=25, 47-23=24
4. Look up: 44=S, 14=A, 25=F, 24=E
5. Plaintext: SAFE