T9 AKA Phone Code

The T9 / Phone Code Cipher is based on the multi-tap input method used on traditional numeric keypads. Each digit key corresponds to multiple letters, and the number of presses determines which letter is selected. Spaces between words are preserved.

Key Mapping (Multi-tap):
2 → A(1) B(2) C(3)
3 → D(1) E(2) F(3)
4 → G(1) H(2) I(3)
5 → J(1) K(2) L(3)
6 → M(1) N(2) O(3)
7 → P(1) Q(2) R(3) S(4)
8 → T(1) U(2) V(3)
9 → W(1) X(2) Y(3) Z(4)

T9 / Phone Code Cipher: Encoding

To encode a message, replace each letter with the key digit repeated for the number of presses needed to reach the letter. For example:

Plaintext: CATENCODE

Encoding Steps:
C → 2 pressed 3 times → 222
A → 2 pressed 1 time  → 2
T → 8 pressed 1 time  → 8
E → 3 pressed 2 times → 33
N → 6 pressed 2 times → 66
C → 2 pressed 3 times → 222
O → 6 pressed 3 times → 666
D → 3 pressed 1 time  → 3
E → 3 pressed 2 times → 33

Ciphertext: 222 2 8 33 66 222 666 3 33

T9 / Phone Code Cipher: Decoding

To decode, split the digits by repeated sequences corresponding to key presses. Each sequence maps back to the letter selected by the number of presses:

Ciphertext: 222 2 8 33 66 222 666 3 33

Decoding Steps:
222 → C
2   → A
8   → T
33  → E
66  → N
222 → C
666 → O
3   → D
33  → E

Plaintext: CATENCODE

T9 / Phone Code Cipher: Notes

- Each digit represents a key on a phone keypad. The number of times the key is pressed selects the intended letter. - Spaces are preserved between words for readability. - This cipher demonstrates numeric multi-tap substitution, different from predictive T9 text input. - While modern smartphones rarely use multi-tap input, this method illustrates the historical T9 keypad encoding and is useful for educational or cryptographic exercises.