Null cipher

Key takeaways

Null cipher belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Null cipher to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Null cipher from memory before moving on to harder problems.

Reference excerpt

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. This is one of three categories of cipher used in classical cryptography along with substitution ciphers and transposition ciphers.

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.) 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 null cipher message, sent by a German during World War I:

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 "Pershing sails from N.Y. June I".

Following is a more complicated example from England's Civil War which aided Royalist Sir John Trevanian in his escape from a Puritan castle in Colchester: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.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:

Taking only every fifth word, one can reconstruct the hidden text which recommends a "hit" on someone:

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.

Dot concealment cipher A dot or pinprick concealment cipher is a common classical encryption method in which dot or pinprick is placed above or below certain letters in a piece of writing. An early reference to this appears in Aeneas Tacticus's book On the Defense of Fortifications. The Germans improved upon this, using a dot of invisible ink during World War I and World War II. In 19th-century England, pinpricks in newspapers were once a popular way to send letters with little or no cost. If dots were placed far apart, this cipher could be used effectively. The dots should be small and the null text must make sense in the context of the senders and their relationship. Both also must have agreed on the page, chapter, article, or section to be used, typically several. Another option is to have an indicator, such as the date in a newspaper, which shows which page the message is on. This version is less secure.

Acrostics

The acrostic puzzle is an extended form of null cipher, but not an anacrostic (which uses a set of lettered clues with numbered blanks representing the letters of the answer to figure out the second part, a long series of numbered blanks and spaces representing a message into which the answers for the clues fit). Hidden or otherwise unmentioned acrostics are a type of null cipher. This method can be used to secretly insult a famous or important individual. For example, Rolfe Humphries received a lifelong ban from contributing to Poetry Magazine after he wrote and tried to publish "a poem containing a concealed scurrilous phrase aimed at a well-known person", namely Nicholas Murray Butler.

Definition of a null Put simply, a null cipher is any cipher which involves a number of nulls, or decoy letters. As well as the methods shown above, a null cipher could be plaintext words with nulls placed in designated areas or even a plaintext message broken up in different positions with a null at the end of each word. However, a message with only a couple nulls (for example, one at the beginning and one at the end) is not a null cipher. A null cipher is technically only a term for hidden letters or words within an otherwise unimportant message, however, it is occasionally used for the entire category of concealment ciphers.

Usage In general, it is difficult and time-consuming to produce covert texts that seem natural and would not raise suspicion, but a null cipher is an option if one is unable to use an advanced encryption method and has ample time. If no key or additional encryption is involved, the security of the message relies entirely on the secrecy of the concealment method. In the present day, null ciphers are used by prison inmates in an attempt to have their messages pass inspection. Null ciphers are one of three major cipher types in classical cryptography (the other types being substitution and transposition), but they are less well known than the others. They are often marked as a subcategory of transposition ciphers, but that is not true, as transposition ciphers are scrambled messages.

See also Transposition cipher Substitution ciphers Acrostic Nulls Cipher Code Classical Cryptography Cryptanalysis Steganography

References

Worked examples

Example 1 — a first encounter with Null cipher

Start with the simplest possible case. Write down what Null cipher claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Null cipher before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Null cipher ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Null cipher

In research: Null cipher appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Null cipher in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Null cipher is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cardinal Richelieu, Classical ciphers, Steganography, so understanding it makes those chapters shorter.
In everyday life: Look for Null cipher outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Null cipher in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Null cipher means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Null cipher out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Null cipher in simple terms?

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.

Why does Null cipher matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Null cipher?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Null cipher.