Espionage science
This web page explores some of the science used in espionage as it happened in the past. Mainly, it will look at invisible inks and ciphers or codes, beginning with the invisible inks. These days, microdots, chips and other methods have made this sort of work fairly useless, and a lot of the most effective espionage uses public information in clever ways. Later, I may look at archaeology as a form of espionage through time, unravelling the secrets of the past. It was started in 2001 and taken up again in 2005.

Remember that a lot of the principles of surveillance can also be used to spy on wildlife, except that you don't have to be as secretive: birds don't mind if you point a large parabolic microphone at them to record their song! (I was once stopped by a ranger in a park, when he saw me carrying a 3-metre microphone rig, but once I showed him what it was for, he was happy.)

#### Activities

Invisible Ink 1
Invisible Ink 2
Invisible Ink 3
Invisible Ink 4
Invisible Ink 5
Invisible Ink 6
Invisible Ink 7
Substitution ciphers
Code challenge 1
Other pages on this site

## How to do it

Note: if you are looking for science fair or science project areas, this set of Web pages may help you with ideas for techniques you might use: read with a prepared mind! Alternatively, look at the projects collection which is part of this series.
Invisible Ink 1
What you need: A small bowl, some milk, and either a pipe cleaner, a toothpick, a small paint brush to use as your 'pen' — or try a quill pen. Steel pens were never used because they left fine scratches on the paper that could be seen in a slanting light.

Put a little milk in the bowl. Use your 'pen' to write a message with the milk on a piece of paper. Do not use too much milk or the paper will wrinkle. Let the milk message dry completely.

The person who gets your message must heat the paper so that the message reappears. This can be done with an iron or by holding the paper above a 100 watt light bulb — but watch out for burns.

Invisible Ink 2
What you need: A small bowl, some lemon juice (or a lemon, a knife, a board and a squeezer), and either a pipe cleaner, a toothpick, a small paint brush or a quill pen to write with.

You can write with lemon juice the same way as with milk and read it after it is dry by heating it. To read the message, heat the paper as you did with the message in milk.

There is another way to read an invisible message in lemon juice. You can read it by spraying the paper lightly with red cabbage water. Use a plant misting bottle to spray the red cabbage water on.

Invisible Ink 3
What you need: A small bowl, some vinegar, and either a pipe cleaner, a toothpick, a small paint brush or a quill pen to write with.

Write your message in vinegar and use red cabbage juice to read the message, as in the previous method.

Invisible Ink 4
What you need: 5 mL (1 teaspoon) of corn starch in 60 mL (1/4 cup) of water, some way of heating the starch solution gently: half a minute in a microwave is about right.

Stir the starch and let it cool. Write your message on paper and let it dry.

To read the invisible writing, wipe the surface of the paper with a sponge which has been wetted in iodine and water. The iodine mixture should be 10 drops of standard iodine solution in 60 mL (1/4 cup) of water. The message will show up dark purple on a light purple background.

Safety tip: Iodine can be used on cuts to kill germs because it is poisonous to living things. Drinking or eating iodine could make you sick. Wash up carefully afterwards.

Invisible Ink 5
What you need: Baking soda, sodium bicarbonate or sodium hydrogen carbonate (same thing, three names), water and grape juice concentrate

Mix about 1/4 cup (60 ml) of baking soda and 1/4 cup (60 ml) of water. Write with this mixture on paper, using a brush (you could use a steel pen, if you could find one, but it marks the paper — a quill pen would be better for this). Let it dry completely. To read the secret message, paint grape juice concentrate across the paper from top to bottom with a paint brush or a sponge.

Does it work with regular grape juice?

Tip: Grape juice stains. Be careful not to spill it.

Invisible Ink 6
I appear to have found this on the Web — it was in my notes, but I don't think I wrote it, so I make no claim to it. Also, I have not tried it yet. You are on your own, Secret Agent!

What you need: Lemon juice, runny honey and glycerine
My notes say the mix should be about 4:4:1 lemon:glycerine:honey. Glycerine can be bought at any pharmacy, and it is safe enough — as a rule, never drink anything in the lab. Mix well and store in the refrigerator.

1. Dip a cotton swab into the ink and use it to write the secret message on a piece of paper.

2. Allow the message to dry completely.

3. To read the invisible ink, spray the message with the red cabbage water solution.

Invisible Ink 7
Some notes from the CIA: this material is taken from that link, and refers to historical methods of concealing messages, in this case in the American War of Independence. The 'Patriots' are American agents.

Secret Writing

While serving in Paris as an agent of the Committee of Secret Correspondence, Silas Deane is known to have used a heat-developing invisible ink, compounded of cobalt chloride, glycerine and water, for some of his intelligence reports back to America. Even more useful to him later was a "sympathetic stain" created for secret communications by James Jay, a physician and the brother of John Jay. Dr. Jay, who had been knighted by George III, used the "stain" for reporting military information from London to America. Later he supplied quantities of the stain to George Washington at home and to Silas Deane in Paris.

The stain required one chemical for writing the message and a second to develop it, affording greater security than the ink used by Deane earlier. Once, in a letter to John Jay, Robert Morris spoke of an innocuous letter from "Timothy Jones" (Deane) and the "concealed beauties therein," noting "the cursory examinations of a sea captain would never discover them, but transferred from his hand to the penetrating eye of a Jay, the diamonds stand confessed at once."

Washington instructed his agents in the use of the "sympathetic stain," noting in connection with "Culper Junior" that the ink "will not only render his communications less exposed to detection, but relieve the fears of such persons as may be entrusted in its conveyance." Washington suggested that reports could be written in the invisible ink "on the blank leaves of a pamphlet. . . a common pocket book, or on the blank leaves at each end of registers, almanacks, or any publication or book of small value."

Washington especially recommended that agents conceal their reports by using the ink in correspondence: "A much better way is to write a letter in the Tory stile with some mixture of family matters and between the lines and on the remaining part of the sheet communicate with the Stain the intended intelligence."

Even though the Patriots took great care to write sensitive messages in invisible ink, or in code or cipher, it is estimated that the British intercepted and decrypted over half of America's secret correspondence during the war.

Substitution ciphers
Substitution ciphers replace one letter with another. The simplest form is supposed to date from the time of Julius Caesar, and it involves replacing each letter by another letter, perhaps three along in the alphabet.

The computer in the film 2001: A Space Odyssey is called HAL, and some people think this was a substitution cipher for IBM. Think about it . . .

Code challenge 1
Here is a common sentence typed by typists when they are learning to type. It is hard to analyse the frequencies, because most of the letters of the alphabet are used.

wkhtx lfneu rzqir amxps hgryh uwkho dcbgr j

Here is another sentence often used by typists.

wxfrb cqncr vnoxa juupx xmvnw cxlxv ncxcq njrmx ocqny jach

I used a spreadsheet to generate the substitution ciphers above. Here, I will give you just a few hints, but the rest will be up to you.

In my spreadsheet, cell A1 is the offset, the number of characters that are different. For example, if I have 5, an A will become F, B will become G and so on. Then in the rest of column A, I type the letters of the sentence to be coded, leaving out the spaces. That means your sentence becomes a column of letters, starting in A2.

I use all lower case letters. There is an operator in spreadsheets called CODE that converts letters to numbers: A is 65, a is 97, Z is 90, z is 122. I enter in cell B2 this formula: =CODE(A2). Now I need to increase all of these values, so in cell C2 I put the formula =B2+A\$1. (The \$ is a special trick that means when we copy this formula down later, we will keep looking at cell A1 -- this is called absolute addressing. Don't worry about it, just do it!)

Now we have a problem, because some of the numbers will be too high, because we have gone beyond the end of the alphabet. All of my numbers need to be in the range from 97 to 122. So now I add a new formula in cell D2: =IF(C2<123,C2,(C2-26)).

Once again, don't worry about this too much, it is a handy bit of formula that turns numbers outside the range back into numbers inside the range. And now, we are ready to convert the number back to a letter

In cell E2, we add the formula =CHAR(D2). Now we are ready to copy down cells B2 to E2. Highlight those cells, then drop down to, say, row 100 and use the command Edit — Fill — Down to copy those formulas. Now if you type a sentence, starting a A2 and working down, the code will appear in column E

(I actually get a bit more complicated, using the =CONCATENATE command to pull all of the letters into a single string, but I will leave that for you to play with.)

## And now for some help

Explaining Invisible Ink 1
Milk is an organic product, meaning it comes from a living thing. When it is heated, it burns at a lower temperature than the paper and turns brown. The invisible message reappears in brown.

Explaining Invisible Ink 2
The writing appears gradually because heat causes a chemical change in the lemon juice. The "invisible ink" chars at a lower temperature than the paper, so the writing appears faint and brown.

Red cabbage water is an indicator for acids and bases. Since lemon juice is an acid, the red cabbage water interacts with the dried lemon juice and turns a different color. The invisible message reappears.

Explaining Invisible Ink 3
The writing appears gradually because heat causes a chemical change in the vinegar. The "invisible ink" chars at a lower temperature than the paper, so the writing appears faint and brown.

Vinegar is also an acid. The red cabbage water will turn a different color where the vinegar dried.

Explaining Invisible Ink 4
Iodine reacts with starch. Since the dry corn starch message has a lot of starch, it turns dark purple. The paper has some starch in it also, so it turns colour too.

Why it may not work: Some types of paper may contain starch as a filler, in which case you will not be able to read the message. This is a good way to discover the value of thinking ahead — and there is also a project idea here. Newspaper should work well.

Explaining Invisible Ink 5
The acid grape juice interacts with the alkaline baking soda to produce a different color making the secret message appear. Perhaps you can explore a few other fruit juices, to see if any of those work? I haven't tried this one, but I think you will need to avoid flooding the page with grape juice, as that may wash the message awa, so use a damp brush.

Explaining Invisible Ink 6
To make the solution:
a. Carefully chop a large red cabbage into small pieces with a kitchen knife.
b. Simmer the cabbage pieces in hot water until it turns into a deep shade of purple.
c. Allow the water to cool, and refrigerate it when not in use.

Be careful with the red cabbage solution, because it can stain your clothes! Be sensible around sharp knives and boiling water.

Explaining Invisible Ink 7
Cobalt chloride, used as an invisible ink, is almost colorless in dilute solution when applied to paper. Upon heating it undergoes dehydration and turns blue, becoming colorless again when the heat is removed and water is taken up. You can make your own recipe for this, with a bit of experimentation.

Substitution ciphers
The trick with these ciphers is to do a quick count of frequencies for the different letters. That means longer passages are easier to decode, because e, r, s and t will all be common. If you find three consecutive letters that are common, they are likely to be r, s and t. Try making two slips of paper like this:
 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Now all you have to do is slide these past each other, until the equivalent letters match up, then you can read the message off.

These are substitution ciphers

I haven't finished this one yet. Work it out yourself!

This file is http://members.ozemail.com.au/~macinnis/scifun/spysci.htm, first created on July 23, 2001. Last recorded revision (well I get lazy and forget sometimes!) was on May 17, 2005.
Worried about copyright? You need to go look at my non-copyright notice. Well, maybe you don't, but do it anyhow . . .

©The author of this work is Peter Macinnis -- macinnis@ozemail.com.au, who asserts his sole right to the product as it is packaged here, recognising that many of the ideas are common. Any non-profit educational or home use is completely acceptable without let or hindrance. Copies of this whole file or scifun site may be made and stored or printed for personal or educational use.