A null cipher, also known as concealment cipher, is an ancient form of encryption where the plaintext is mixed with a large amount of non-cipher material. Today it is regarded as a simple form of steganography, which can be used to hide ciphertext.
In classical cryptography, a null is intended to confuse the cryptanalyst. In a null cipher, the plaintext is included within the ciphertext and one needs to discard certain characters in order to decrypt the message. Most characters in such a cryptogram are nulls, only some are significant, and some others can be used as pointers to the significant ones.
Here is an example implementation of a null cipher. Stringing together the first letter of every third word of the following covertext reveals "Wikipedia" as the hidden message:
- It's important we allow anyone interested in gaining knowledge access to information which is published freely. There exists a website devoted to this idea, and you are on it!
A similar technique is to hide entire words, such as in this seemingly innocent message written by a prison inmate but deciphered by the FBI:
- SALUDOS LOVED ONE SO TODAY I HEARD FROM UNCLE MOE OVER THE PHONE. HE TOLD ME THAT YOU AND ME GO THE SAME BIRTHDAY. HE SAYS YOUR TIME THERE TESTED YOUR STRENGTH SO STAY POSITIVE AT SUCH TIMES. I'M FOR ALL THAT CLEAN LIVING! METHAMPHETAMINES WAS MY DOWN FALL. THE PROGRAM I'M STARTING THE NINTH IS ONE I HEARD OF A COUPLE WEEKS BEFORE SEPTEMBER THROUGH MY COUNSELOR BARRIOS. BUT MY MEDICAL INSURANCE COVERAGE DENIES THEY COVER IT. I'M USING MY TIME TO CHECK AND IF THE INSURANCE AGENT DENIES STILL MY COVERAGE I'M GETTING TOGETHER PAPERWORK SAYING I TESTED FOR THIS TREATMENT REQUIRED ON THE CHILD CUSTODY. THE NINTH WILL MEAN I HAVE TESTED MY DETERMINATION TO CHANGE. ON THE NEXT FREE WEEKEND THE KIDS ARE COMING, BUT FIRST I GOTTA SHOW CAROLINA I'M STAYING OUT OF TROUBLE WAITING TO GET MYSELF ADMITTED ON THE PROGRAM. THE SUPPORTING PAPERWORK THAT THE FAMILY COURTS GOT WILL ALSO PROVE THERE'S NO REASON NEITHER FOR A WITNESS ON MY CHILDREN'S VISITS. OF COURSE MY BRO HAS HIS MIND MADE UP OF RECENT THAT ALL THIS DRUG USAGE DON'T CONCERN OUR VISITS. I THINK THAT MY KIDS FEEL I NEED THEIR LOVE IF I'M GONNA BE COOL. GUILTY FEELINGS RISE ON ACCOUNT OF THE MISTAKES I COULD WRITEUP. FOR DAYS I'M HERE. HE GOT A GOOD HEART. SHOULD YOU BE HAVING PROBLEMS BE ASSURED THAT WHEN YOU HIT THE STREETS WE'LL BE CONSIDERING YOU...
Taking only every fifth word, one can reconstruct the hidden text which recommends a "hit" on someone:
- TODAY MOE TOLD ME HE TESTED POSITIVE FOR METHAMPHETAMINES THE NINTH OF SEPTEMBER BUT DENIES USING AND DENIES GETTING TESTED ON NINTH
- TESTED ON THE FIRST
- I'M WAITING ON PAPERWORK
- GOT NO WITNESS OF HIS RECENT USAGE
- I FEEL IF GUILTY OF WRITEUP HE SHOULD BE HIT
Other options include positioning of the significant letters next to or at certain intervals from punctuation marks or particular characters.
Historically, users of concealment ciphers often used substitution and transposition ciphers on the data prior to concealment. For example, Cardinal Richelieu is said to have used a grille to write secret messages, after which the blank spaces were filled out with extraneous matter to create the impression of a continuous text.
In general, it is difficult and time-consuming to produce covertexts that seem natural and would not raise suspicion. If no key or actual encryption is involved, the security of the message relies entirely on the secrecy of the concealment method. Null ciphers in modern times are used by prison inmates in an attempt to have their most suspicious messages pass inspection.
- Gordon, Adam (2015). Official (ISC)2 Guide to the CISSP CBK - Fourth Edition. (ISC)2 Press. p. 349. ISBN 978-1939572066.
- Gaines, Helen F. (2014). Cryptanalysis: A Study of Ciphers and Their Solution. Courier Corporation. pp. 4–5. ISBN 9780486800592.
- "Cryptanalysts: Breaking Codes to Stop Crime, Part 1". Federal Bureau of Investigation. March 21, 2011. Retrieved 2018-09-30.
- Mark Adabi (February 5, 2018). "An Atlanta jail intercepted a letter from an inmate who was using a secret code to orchestrate a murder". Business Insider. Retrieved 2018-09-30.