Null Cipher

The Null Cipher is a steganographic technique that hides a secret message within an ordinary-looking text. Most letters in the text are "nulls" and do not convey information, while only certain letters—determined by a prearranged rule—carry the hidden message. Its security relies on subtlety: without knowledge of the extraction rule, the cover text appears completely normal.

The sender blends the secret letters into a cover text according to the agreed-upon rule. The recipient, knowing the rule, can extract the message by selecting only the letters in the predetermined positions.

Null Cipher: Encoding

To encode a message, the secret letters are blended into a cover text so that the agreed-upon positions contain the message. For example, to hide the message "PERSHINGSAILSFROMNYJUNEI" using the "1st" letter of each word:

Secret Message: P E R S H I N G S A I L S F R O M N Y J U N E I
Rule: first letter of each word in the cover text

Cover Text (crafted so the secret letters fall in the first position of each word):
PRESIDENT'S EMBARGO RULING SHOULD HAVE IMMEDIATE NOTICE.
GRAVE SITUATION AFFECTING INTERNATIONAL LAW.
STATEMENT FORESHADOWS RUIN OF MANY NEUTRALS.
YELLOW JOURNALS UNIFYING NATIONAL EXCITEMENT IMMENSELY

Explanation:
Each first letter of the words above spells out the secret message:
PRESIDENT'S → P
EMBARGO     → E
RULING      → R
SHOULD      → S
HAVE        → H
IMMEDIATE   → I
NOTICE      → N
GRAVE       → G
SITUATION   → S
AFFECTING   → A
INTERNATIONAL → I
LAW         → L
STATEMENT   → S
FORESHADOWS → F
RUIN        → R
OF          → O
MANY        → M
NEUTRALS    → N
YELLOW      → Y
JOURNALS    → J
UNIFYING    → U
NATIONAL    → N
EXCITEMENT  → E
IMMENSELY   → I

Null Cipher: Decoding

To decode, the recipient uses the agreed-upon rule to select the secret letters from the cover text. Applying the "1st" letter rule to the above text reveals the hidden message:

Cover Text:
PRESIDENT'S EMBARGO RULING SHOULD HAVE IMMEDIATE NOTICE.
GRAVE SITUATION AFFECTING INTERNATIONAL LAW.
STATEMENT FORESHADOWS RUIN OF MANY NEUTRALS.
YELLOW JOURNALS UNIFYING NATIONAL EXCITEMENT IMMENSELY

Rule: take the first letter of each word

Decoding:
PRESIDENT'S → P
EMBARGO     → E
RULING      → R
SHOULD      → S
HAVE        → H
IMMEDIATE   → I
NOTICE      → N
GRAVE       → G
SITUATION   → S
AFFECTING   → A
INTERNATIONAL → I
LAW         → L
STATEMENT   → S
FORESHADOWS → F
RUIN        → R
OF          → O
MANY        → M
NEUTRALS    → N
YELLOW      → Y
JOURNALS    → J
UNIFYING    → U
NATIONAL    → N
EXCITEMENT  → E
IMMENSELY   → I

Plaintext: PERSHINGSAILSFROMNYJUNEI

Null Cipher: Notes

The Null Cipher does not rely on mathematical encryption but on shared knowledge of the extraction rule. Without knowing the rule, the message remains hidden in plain sight. This example demonstrates how German agents used first-letter null ciphers during World War I to transmit the message "PERSHINGSAILSFROMNYJUNEI" covertly amidst ordinary-looking text.