Index Card Cipher

The Index Card Cipher is a manual polyalphabetic substitution cipher that uses a set of shuffled alphabets (cards) to encode plaintext. Each card represents a full alphabet in a predetermined, scrambled order. By arranging and cycling through these cards according to a numeric key sequence, the plaintext letters are substituted in a way that depends on both the card and the position of the letter, increasing encryption complexity.

Unlike simple substitution ciphers such as the Simple Substitution Cipher, the Index Card Cipher uses multiple alphabets and a rotation rule, making frequency analysis more difficult. The security relies on the secrecy of the cards’ shuffle order and the numeric key that determines the card sequence for encoding and decoding.

Index Card Cipher: Encoding

In this implementation, there are 3 predefined shuffled alphabets (cards):

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

To encode a message, the user selects a numeric key such as 2-1-3, which indicates the sequence of cards used for each consecutive block of letters. For example, using the input “CATENCODE” with key 2-1-3:

Plaintext: C  A  T  E  N  C  O  D  E
Key (cards): 2  1  3  2  1  3  2  1  3

Encoding with each card:
C → Card 2 → B
A → Card 1 → Q
T → Card 3 → M
E → Card 2 → C
N → Card 1 → F
C → Card 3 → E
O → Card 2 → S
D → Card 1 → R
E → Card 3 → T

Ciphertext: B Q M C F E S R T

Each letter is substituted according to the card corresponding to its position in the block. Blocks are processed sequentially, and letters beyond the block length wrap to the next set of cards.

Index Card Cipher: Decoding

Decoding reverses the process using the same 3 cards and the numeric key. Each ciphertext letter is mapped back to its original plaintext letter based on its assigned card:

Ciphertext: B Q M C F E S R T
Key (cards): 2  1  3  2  1  3  2  1  3

Decoding with each card:
B → Card 2 → C
Q → Card 1 → A
M → Card 3 → T
C → Card 2 → E
F → Card 1 → N
E → Card 3 → C
S → Card 2 → O
R → Card 1 → D
T → Card 3 → E

Plaintext: CATENCODE

Index Card Cipher: Notes

This version of the Index Card Cipher demonstrates polyalphabetic substitution using multiple pre-shuffled alphabets. By cycling through the cards according to a numeric key, repeated letters in the plaintext are less likely to map to the same ciphertext letters, providing enhanced security compared to a single-card substitution. Correct decryption requires both the exact card shuffles and the numeric key. Conceptually, it shares similarities with the Cadenus–Gronsfeld Cipher in its use of a controlled sequence of substitution tables.