# Psst! Secret codes and invisible inks
**URL:** https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230
**Category:** Miscellaneous and Personal Stuff I Must Share
**Created:** [June 9, 2009, 6:40pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230 "2009-06-09T18:40:42Z")
**Posts on this page:** 20
**Page:** 2
### Author:  [@constanze](https://boards.straightdope.com/u/constanze)
#### Post date: [June 10, 2009, 4:29pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/21 "2009-06-10T16:29:24Z")
> [@shantih](#):
>
> The second part is true, though; I’m American. My daughter’s school is a normal German Grundschule. Do you think I can convince them that English is a secret code and hold my sections in my better language? 🙂
No hope of that. Even 8-year olds know that English is a cool language of the modern music. Unless you know a Native American language or Hebrew or something exotic…
> [@](#):
>
> To be accurate, the project is “Die Drei Fragezeichen” (or better, “Die Drei ???”), but I thought Detective School would convey the right information without getting needlessly confusing.
Aha! And at the end of the week, you’ll take them to see the movie?
> [@](#):
>
> Interesting what you say about cheat sheets – it’s probably better if I don’t mention that to the kids!
The smart kids will figure it out by themselves once they go to secondary school. Tips on cheat sheets are shared around, and today, there are not only books on how to cheat, but the whole internet (where you can buy homework essays, after all! It’s not easy for the teachers to figure out all ways…)
But I think it would be better to talk about the ethical questions of spying and cheating, how much is fun and exercise, and where things start getting questionable.
I remember very well one of my teachers advocating writing cheat sheets (and then leaving them at home), because if you could condense the whole stuff that you had to know down to a small paragraph to write onto a small card, this meant you understood the subject and just needed the little details.
And for those students who are nervous about taking tests - many of them good students - it can be calming to know that there is a cheat sheet in case their mind goes blank, even if they never actually use it.
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### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [June 10, 2009, 4:33pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/22 "2009-06-10T16:33:17Z")
> [@constanze](#):
>
> But historically and technically, a one-time pad was a different track in the spy game, used extensivly in WWII by the Allies (I think without looking up details). Before, the research focused on making the codes harder and harder to break (and taking longer and longer) - resulting in things like Enigma.
> The one-time pad is the directly opposite approach: you mass-produce a pad with lots of rather simple codes - substitution - but after each message, you rip of and destroy the top paper and use the next one. It doesn’t matter that each code is easy to break, because if the enemy manages to decode one message, he still doesn’t know how the next message will be encoded, and there’s no chance of slipping a false message in once the enemy has broken the code. Only the people who have that specific pad can decode the message correctly.
>
> And surely you didn’t mean to suggest cutting holes in the book?
No, you take a sheet of paper and cut holes in it - using a page of the book as a template. You might have to hang the spine of the book off the edge of a light table to do this (shine light up through the page, with the paper overlaid, mark the letters in your message, then cut holes where you made the marks). Then the book acts as your one-time pad. If you agree ahead of time on what book is your reference, you can just send the pages with the holes marked with a page number, and the recipient can decode your messages. Since the position of the holes is unlikely to contain any information, this method is pretty secure (I say unlikely - there may be a cryptographic weakness, but I haven’t though about this in depth). As with a one-time pad, if the book is compromised, your code is 100% broken.
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### Author:  [@Markxxx](https://boards.straightdope.com/u/Markxxx)
#### Post date: [June 10, 2009, 4:34pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/23 "2009-06-10T16:34:05Z")
This might be a bit advanced but check out the [Paper Enigma Machine](http://mckoss.com/Crypto/Enigma.htm)
Enigma, for those who may not know, being the machine the Germans used in WWII to encode messages
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### Author:  [@constanze](https://boards.straightdope.com/u/constanze)
#### Post date: [June 10, 2009, 4:36pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/24 "2009-06-10T16:36:27Z")
> [@beowulff](#):
>
> No, you take a sheet of paper and cut holes in it - using a page of the book as a template. You might have to hang the spine of the book off the edge of a light table to do this (shine light up through the page, with the paper overlaid, mark the letters in your message, then cut holes where you made the marks). Then the book acts as your one-time pad. If you agree ahead of time on what book is your reference, you can just send the pages with the holes marked with a page number, and the recipient can decode your messages. Since the position of the holes is unlikely to contain any information, this method is pretty secure (I say unlikely - there may be a cryptographic weakness, but I haven’t though about this in depth). As with a one-time pad, if the book is compromised, your code is 100% broken.
Ah, I see. However, the normal book method mentioned above - where you write down the page number, line number, word number for each word you want to use - is far less conspicious (you can write them on a piece of paper as if it’s your grocery bill or telephone numbers or whatever) than sending a whole sheet with holes in it. And it takes more time to cut out holes instead of counting the words.
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### Author:  [@Shot\_From\_Guns](https://boards.straightdope.com/u/Shot_From_Guns)
#### Post date: [June 10, 2009, 5:12pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/25 "2009-06-10T17:12:11Z")
A simple code I learned in grade school was this:
[ul][li]Break up the message into pairs of letters.[/li][li]Add “X” to the end if you have an odd number.[/li][li]Swap the order of the letters in each pair.[/li][li]Randomly redistribute spacing and punctuation.[/ul][/li]
So,
THIS IS AN EXAMPLE SENTENCE. --\>
TH IS IS AN EX AM PL ES EN TE NC EX --\>
HT SI SI NA XE MA LP SE NE ET CN XE --\>
HTSI’S IN AXE, MALP’S EN EET CN’XE.
---
### Author:  [@beowulff](https://boards.straightdope.com/u/beowulff)
#### Post date: [June 10, 2009, 5:15pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/26 "2009-06-10T17:15:50Z")
> [@constanze](#):
>
> Ah, I see. However, the normal book method mentioned above - where you write down the page number, line number, word number for each word you want to use - is far less conspicious (you can write them on a piece of paper as if it’s your grocery bill or telephone numbers or whatever) than sending a whole sheet with holes in it. And it takes more time to cut out holes instead of counting the words.
Well, to be honest, I saw this in a movie as a kid (kid stumbles on secret code paper, needs to find the right book to decode, leading to buried treasure), and I always thought it was really cool.
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### Author:  [@Sunspace](https://boards.straightdope.com/u/Sunspace)
#### Post date: [June 10, 2009, 5:19pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/27 "2009-06-10T17:19:44Z")
> [@shantih](#):
>
> And as much as I love the idea of talking about different codes used over the ages, I’m leary of an American mother talking to German kids about a method used by Americans in World War II, albeit not against Germans (to my knowledge). I’m thinking it’s probably best to stick with the methods and let them learn the history when they’re older.
You could always introduce a [different](http://en.wikipedia.org/wiki/Lojban) [obscure](http://en.wikipedia.org/wiki/Toki_Pona) [language](http://en.wikipedia.org/wiki/L%C3%A1adan)…
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### Author:  [@JSexton](https://boards.straightdope.com/u/JSexton)
#### Post date: [June 10, 2009, 8:29pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/28 "2009-06-10T20:29:52Z")
> [@constanze](#):
>
> The one-time pad is the directly opposite approach: you mass-produce a pad with lots of rather simple codes - substitution - but after each message, you rip of and destroy the top paper and use the next one. It doesn’t matter that each code is easy to break,
These codes wouldn’t be easy to break. Just the opposite: they are unbreakable, as long as the pad itself remains secure. The pads consists of a very, very long string of random characters, and you encode your message using this string. Unlike encryption via a pass phrase, it doesn’t repeat, so there is no way to break it.
The weakness is security of the pad itself, plus the expense and preparation required to make and distribute them to the users.
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### Author:  [@constanze](https://boards.straightdope.com/u/constanze)
#### Post date: [June 10, 2009, 8:43pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/29 "2009-06-10T20:43:22Z")
> [@shantih](#):
>
> To be accurate, the project is “Die Drei Fragezeichen” (or better, “Die Drei ???”), but I thought Detective School would convey the right information without getting needlessly confusing.
Slight hijack out of curiosity: Are the ??? as well-known and popular among American kids as they are in Germany? Since the series was started by Hitchcok, and takes place in California (necessitating the use of a limousine through a contrivance, while German detective kids can take public transport or their bikes)…
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### Author:  [@constanze](https://boards.straightdope.com/u/constanze)
#### Post date: [June 10, 2009, 9:15pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/30 "2009-06-10T21:15:47Z")
> [@shantih](#):
>
> Heh. I’m not worried so much about the parents being squeamish as the parents knowing where I live and tracking me down to do me harm after their kids write all over the house with their own urine. I read about other bodily fluids being used as invisible inks, too, and I’m REALLY not going there, for my own sake!
Well, in every of the maybe half-a-dozen non-fiction children’s books I read at around age 10 of the “history of codes and how to make your own/ detectives handbook” type, they said in the first sentence on invisible ink: the oldest known and most widely used because they are easy to get inks are onion juice, milk and urine. Which meant that I never tried them, because a) everybody who knew anything about codes would know this, including every adult, b) because the chemical reaction was irreversible, it was a pretty bad method of writing codes, because once the message had been deciphered, you had to destroy the letter.
Besides, German parents aren’t that easily squicked out or squeamish about bodily functions as Americans.
> [@](#):
>
> And as much as I love the idea of talking about different codes used over the ages, I’m leary of an American mother talking to German kids about a method used by Americans in World War II, albeit not against Germans (to my knowledge). I’m thinking it’s probably best to stick with the methods and let them learn the history when they’re older.
Um, given the widespread fascination and adoration for the “Indianer” (Native Americans) among German children (partly because of Karl May romantizing them), which leads many kids to read non-fiction books about the real history of the Indians, they likely already have a bad view about what the white Americans did to the Indians, regardless of what you tell them.
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### Author:  [@Shot\_From\_Guns](https://boards.straightdope.com/u/Shot_From_Guns)
#### Post date: [June 10, 2009, 9:30pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/31 "2009-06-10T21:30:17Z")
> [@constanze](#):
>
> Um, given the widespread fascination and adoration for the “Indianer” (Native Americans) among German children (partly because of Karl May romantizing them), which leads many kids to read non-fiction books about the real history of the Indians, they likely already have a bad view about what the white Americans did to the Indians, regardless of what you tell them.
I think the concern was more for, “Hey, here’s an example of how we beat the pants off people allied with your evil great-grandparents in the last World War.”
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### Author:  [@constanze](https://boards.straightdope.com/u/constanze)
#### Post date: [June 10, 2009, 10:24pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/32 "2009-06-10T22:24:51Z")
> [@Shot\_From\_Guns](#):
>
> I think the concern was more for, “Hey, here’s an example of how we beat the pants off people allied with your evil great-grandparents in the last World War.”
Only we don’t teach history like that. We tell the facts and how things are connected. In this case, it would be either the bare bones - that the US army, when looking for code experts, went to the Navajos and asked them; that the Navajo developed a substitution of letter=word, so even other Navajos couldn’t understand the code; and that the Japanese, despite their efforts, couldn’t break it.
Or you give the larger picture: that the Navajo, like other Native American tribes, were forcefully taught English in mission and govt. schools by forbidding them to speak their own language, because everything that was part of their own culture was looked down on (and how this is still a problem of self-esteem in Indians today), going as far as to brushing kids mouth out with soap or making them eat soap for speaking their own language; that even after WWII, Indians didn’t have full human rights in US society (like the blacks, who were also disenfranchised); that the soldiers that had to protect the code talkers from being captured by the Japanese as shown in the movie “Wind talkers” were ordered to shoot the code talkers if capture was likely; that for years afterwards, the code talkers themselves were strictly forbidden to mention the code at all, while the superior officers blabbed about it widely.
Who was having war with whom is of secondary importance to the facts of the story itself, which is interesting from a how-to point (using a language not directly, but indirectly, and using it on top of military code words) and that it was never broken is interesting linguistically.
How effective the use of an unbreakable code actually was in the overall scope of the tactics and battles fought would be an interesting research for older students of military tactics. Or for a discussion on the advantages of having breakable codes for some stuff to deceive the enemy. The advantages and disadvantages of unbreakable code vs. difficult to break code. (Was the time the Allied spent solving Enigma and trying to get the machine worth the effort?)
When the US broke the Japanese code early, one of the problems was to decide how much intercepted information they could use without making it obvious to the enemy that they had broken the code - which would have meant that the Japanese would switch to a new, possibly more difficult, code. (Similar to the problems of keeping a known enemy spy at large, as the British did, and feeding him misinformation, instead of simply arresting them, as Hoover did, which only meant that a new spy would come along and have to be found painstackingly again).
It’s possible to discuss the technical difficulties of the Allies when bombing the water dams in the Rhine region seperatly from the ethical view and the consequences of succeeding, or the differences between the russian T 34 tank and the German tank approach, from a purely technical standpoint, and discuss the ethical and larger implications seperately.
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### Author:  [@Left\_Hand\_of\_Dorkness](https://boards.straightdope.com/u/Left_Hand_of_Dorkness)
#### Post date: [June 10, 2009, 11:36pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/33 "2009-06-10T23:36:48Z")
Dang. I had a German kid in my class this year, true, and he was smart as a whip, but I’m still thinking some of y’all are going way beyond the abilities of your **average** eight-year-old. I’m interested in hearing how this works, and if any of the activities are especially successful or less than successful: next year I’d love to end the year with a more involved unit on codes.
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### Author:  [@Shot\_From\_Guns](https://boards.straightdope.com/u/Shot_From_Guns)
#### Post date: [June 11, 2009, 1:57pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/34 "2009-06-11T13:57:20Z")
> [@constanze](#):
>
> Only we don’t teach history like that. We tell the facts and how things are connected.
I didn’t think you did–I was just pointing out what I thought some people were seeing the issue as.
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### Author:  [@Quercus](https://boards.straightdope.com/u/Quercus)
#### Post date: [June 11, 2009, 6:03pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/35 "2009-06-11T18:03:52Z")
> [@beowulff](#):
>
> No, you take a sheet of paper and cut holes in it - using a page of the book as a template. You might have to hang the spine of the book off the edge of a light table to do this (shine light up through the page, with the paper overlaid, mark the letters in your message, then cut holes where you made the marks). Then the book acts as your one-time pad. If you agree ahead of time on what book is your reference, you can just send the pages with the holes marked with a page number, and the recipient can decode your messages. Since the position of the holes is unlikely to contain any information, this method is pretty secure (I say unlikely - there may be a cryptographic weakness, but I haven’t though about this in depth). As with a one-time pad, if the book is compromised, your code is 100% broken.
Hey, I can do needlessly pedantic with the best of them! Let’s go!
This would only be a one-time pad if you printed up a book especially for this, with only two copies (one for the sender, one for the receiver). Then, even if the enemy intercepted a message, they’d have no way of knowing what letters were underneath the holes, even if they knew exactly how you were encoding the message. That’s a key characteristic of a one-time pad.
But if it’s a common book that the enemy can get a copy of, then this isn’t a one-time pad at all. If they have the book they can read any intercepted message just as easily as the intended recipient.
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### Author:  [@Full\_Metal\_Lotus](https://boards.straightdope.com/u/Full_Metal_Lotus)
#### Post date: [June 11, 2009, 7:12pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/36 "2009-06-11T19:12:23Z")
A variation on the “Page with holes” method of the Book Code is to use a sheet of tracing paper, and simply circle the words, letters, etc as required.
The sheet is sent to the code recipient, who knows what page of what book to place the marked tracing paper over.
Anyone recieving the “message” without knowledge of the correct book, page, edition etc will only have a sheet of peper with some oddly placed circles on it.
regards
FML
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### Author:  [@Crowbar\_of\_Irony\_3](https://boards.straightdope.com/u/Crowbar_of_Irony_3)
#### Post date: [June 12, 2009, 11:05am UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/37 "2009-06-12T11:05:25Z")
Ahhh those were the days. I wonder if they still sell those books, but when I was young there were all sort of spy books with all sort of ciphers and codes. Here are a few.
Write with wax, then scatter dirt on it so that the dirt will stick.
On a notepad, write heavily on the first sheet so that there’s an imprint on the second sheet which can be revealed by shading the region
A few easy re-arrangement. Break words into groups of five then swap the first and last letter. “Can you read this” becomes “cany oure adth is” and then “yanc eouy…” there are a lot of other patterns of the same vein.
Then there is this when you take two identical rod; wrap a long strip of paper over one and write on it and the message can only be assembled if you wrap the paper over rod of the same thickness.
Then there is also the “off by one key” keyboard code. You shift all alphabets by one key to the right on the keyboard. So Q becomes W, E becomes R and so on.
For subtle messages, there is the “read the first letter of each word” or “one letter of each paragraph” or some sort of arrangement.
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### Author:  [@shantih](https://boards.straightdope.com/u/shantih)
#### Post date: [June 12, 2009, 1:42pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/38 "2009-06-12T13:42:01Z")
Well, I met with the other parents working on this project, and I’ve worked out a plan for my part. As it turns out, the kids will be solving a case on the fourth project day, so I only needed to plan ideas for three days, which was unexpectedly painful – I found so many cool codes and tricks that I could have easily filled the week! However, our junior detectives will be kept busy learning how to craft periscopes and make hidden compartments in the center of old books, taking and dusting for fingerprints, and making casts of footprints, as well as doing activities meant to sharpen their powers of observation and memories.
So, here is what I’m planning for each day. The first day is for the easiest things, as I won’t be there myself and want to keep it simple for the ones leading the groups; it also gives the kids a first taste of what they can do and a foundation for the next days. Each day includes at least one code that we’ll only go into if time allows.
Day 1
Invisible ink: writing messages with lemon juice and setting out to dry.
Secret code: write out a message, remove the vowels, and write the result backwards. Example: “This is a secret message” becomes “Sht s trcs gssm.”
If time allows: First Letter Code, in which the message is hidden in the first letters of your sentence(s).
Day 2
Invisible ink: revealing the messages of the first day by ironing them or holding them over a light bulb. Writing new secret messages in lemon juice between the double-spaced lines of a harmless text or on the envelope of a letter, and indicating where to look by the phrasing of the header (“Dear Reader” to look on the letter itself, and “Hello Reader” to look on the envelope). (Thanks, **Otanx**!)
Secret code: [Code wheels](http://home.howstuffworks.com/make-games13.htm), plus the Ruler Code: hold a ruler against a piece of paper and write each letter of your message over the centimeter marks. Fill in the space between letters with random letters. Decode by holding a ruler below the message.
If time allows: Letter Pairs Code, as described by **Shot by Guns** above. (Thanks, **SbG**!)
Day 3
Invisible ink: Turn an innocent letter into a secret message by crossing off words or by underlining words or putting dots above letters with the invisible ink.
Secret code: [Grid Code](http://www.nsa.gov/kids/home.shtml) and Code Stick (wrapping the paper around a stick, as described above by several kind and clever Dopers).
If time allows: [Rail Fence Code](http://www.nsa.gov/kids/home.shtml)
I’m still trying to find a way to work in the trick of writing a brief message on a tiny sheet of paper which is then hidden on the underside of a stamp or a sticker, but I have to let the others have a bit of time for their own parts, I suppose.
I really want to thank everyone for their input! **constanze** , I’m particularly grateful to you for the many fascinating ideas! Oh, and I can’t really speak for other Americans as to how popular the 3 ??? are in the States, since I’ve only had kids on this side of the Atlantic. My kids are Germericans, as my aunt puts it, so they get their own interest in these books and characters through their friends.
If you all like, I’ll update at the end of next week with how it all went. It would be bad manners to put that post in code, right? 🆒
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### Author:  [@Shot\_From\_Guns](https://boards.straightdope.com/u/Shot_From_Guns)
#### Post date: [June 12, 2009, 3:53pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/39 "2009-06-12T15:53:38Z")
So exciting! Can’t wait to hear how it goes–and if you can post pictures, even better! 😃
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### Author:  [@Scuba\_Ben](https://boards.straightdope.com/u/Scuba_Ben)
#### Post date: [June 12, 2009, 5:23pm UTC](https://boards.straightdope.com/t/psst-secret-codes-and-invisible-inks/499230/40 "2009-06-12T17:23:08Z")
Just for completeness – after the kids have their fun creating simple substitution cyphers, give them some cryptanalysis instruction, and have them **crack each other’s secret messages**.
Once they get over the Ooh, Shiny! of their own cyphers, they’ll be amazed at how easy it is – with techniques at least 500 years old – to crack these encryption systems.
ETA:
> [@CrazyChop](#):
>
> For subtle messages, there is the “read the first letter of each word” or “one letter of each paragraph” or some sort of arrangement.
How instructive. Optional potentially advanced lesson!
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