Editing Null cipher (section)

== Classical cryptography ==
In [[classical cryptography]], a ''null'' is an extra character intended to confuse the [[cryptanalyst]]. In the most common form of a null cipher, the [[plaintext]] is included within the [[ciphertext]] and one needs to discard certain characters in order to decrypt the message (such as first letter, last letter, third letter of every second word, etc.)<ref name=":0" /> Most characters in such a cryptogram are nulls, only some are significant, and some others can be used as pointers to the significant ones.<ref name=":2">{{Cite book |last=Gaines |first=Helen F. |title=Cryptanalysis: A Study of Ciphers and Their Solution |publisher=Courier Corporation |year=2014 |isbn=9780486800592 |pages=}}</ref>

Here is an example null cipher message, sent by a German during [[World War I]]:<ref>{{cite book |last1=Kahn |first1=David |title=The Codebreakers: The Comprehensive History of Secret Communication from Ancient Times to the Internet |date=5 December 1996 |publisher=Simon and Schuster |isbn=978-1-4391-0355-5 |url=https://books.google.com/books?id=3S8rhOEmDIIC&pg=PT547 |access-date=12 October 2021 |language=en}}</ref>

{{blockquote|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.}}

Taking the first letter of every word reveals the hidden message "[[John J. Pershing|Pershing]] sails from N.Y. June I".

Following is a more complicated example from [[English Civil War|England's Civil War]] which aided Royalist Sir John Trevanian in his escape from a Puritan castle in [[Colchester]]:<ref name=":1">{{Cite book |last=Janeczko |first=Paul B. |title=Top Secret: a Handbook of Codes, Ciphers, and Secret Writing |publisher=Candlewick Press |year=2004 |isbn=9780763629724}}</ref><blockquote>WORTHIE SIR JOHN,

HOPE, THAT IS YE BESTE COMFORT OF YE AFFLICTED, CANNOT MUCH, I FEAR ME, HELP YOU NOW. THAT I WOULD SAY TO YOU, IS THIS ONLY: IF EVER I MAY BE ABLE TO REQUITE THAT I DO OWE YOU, STAND NOT UPON ASKING ME. TIS NOT MUCH THAT I CAN DO; BUT WHAT I CAN DO, BEE YE VERY SURE I WILL. I KNOW THAT, IF DETHE COMES, IF ORDINARY MEN FEAR IT, IT FRIGHTS NOT YOU, ACCOUNTING IT FOR A HIGH HONOUR, TO HAVE SUCH A REWARDE OF YOUR LOYALTY. PRAY YET YOU MAY BE SPARED THIS SOE BITTER, CUP. I FEAR NOT THAT YOU WILL GRUDGE ANY SUFFERINGS; ONLY IF BIE SUBMISSIONS YOU CAN TURN THEM AWAY, TIS THE PART OF A WISE MAN. TELL ME, AN IF YOU CAN, TO DO FOR YOU ANYTHINGE THAT YOU WOLDE HAVE DONE. THE GENERAL GOES BACK ON WEDNESDAY. RESTINGE YOUR SERVANT TO COMMAND.</blockquote>The third letter after each punctuation reveals "Panel at East end of Chapel slides".

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:<ref name=fbi>{{Cite web|url=https://www.fbi.gov/news/stories/breaking-codes-to-stop-crime-part-1|title=Cryptanalysts: Breaking Codes to Stop Crime, Part 1|date=March 21, 2011|website=[[Federal Bureau of Investigation]]|language=en-us|access-date=2018-09-30}}</ref>

{{poem quote|SALUDOS LOVED ONE
&nbsp;&nbsp;&nbsp;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:

{{poem quote|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}}

Historically, users of concealment ciphers often used [[Substitution cipher|substitution]] and [[transposition cipher]]s on the data prior to concealment. For example, [[Cardinal Richelieu]] is said to have used a [[Grille (cryptography)|grille]] to write secret messages, after which the blank spaces were filled out with extraneous matter to create the impression of a continuous text.<ref name=":2" />